The University of Arizona

Monthly Archive | CLIMAS

Monthly Archive

Tourism and Recreation

Monday, September 15, 2008

In the Southwest, climate is an important natural resource and a draw for tourists. Many people come to the region to take advantage of its warm, mild winters, to boat or kayak in the lakes and rivers, or to enjoy snow sports in the higher elevations. In addition, the natural beauty of the Southwest’s national parks attracts approximately 35 million visitors who spend an estimated $1.3 billion in the region annually (Figure 1). Regional projections suggest that temperatures will rise in the Southwest and precipitation will decrease. Consequently, climate change could affect tourism and recreation in the Southwest by: 

  • Changing the length of tourism or recreation seasons
  • Changing ecological systems
  • Changing the physical environment
  • Changing economic livelihoods

Changing the length of tourism or recreation seasons

Depending on location and activity, changes in season lengths could be positive or negative. Warm weather activities such as boating or camping might see an increase in the length of their operating seasons. Water activities, such as rafting and kayaking, could be affected by changes in seasonal precipitation patterns. Winter activities such as skiing or snowmobiling could be negatively affected by shorter, warmer winter seasons, as well as decreased precipitation. For example, the Arizona Snowbowl ski facility near Flagstaff is open an average of 96 days per season. However during the extremely dry winters of 2001–02 and 2005–06, the area was only open 4 and 16 days, respectively.

Changing ecological systems

Many tourist destinations and outdoor recreation activities are based on the presence of unique wildlife and plants. Dominant plant and animal populations could shift due to changes in climate, from wildfire, drought, or changes in precipitation and temperature. Several tourist and recreation destinations in the Southwest, including Saguaro National Park and Organ Pipe Cactus National Monument, offer unique desert ecosystems as primary attractions. Yet projections indicate these ecosystems are likely to shrink or migrate as the region warms.


Changing the physical environment

If natural features such as lakes or rivers are modified as a result of climate change, water-based tourism and recreation will be affected. Below-average water levels in lakes and reservoirs could reduce the number of boaters. Statistical analysis of tourism at Lake Powell suggests that for every 1 percent drop in reservoir levels, visits fall by 5 percent.1 If numbers are comparable for other reservoirs in the Southwest, such as Lake Mead or Elephant Butte, dry conditions will magnify decreases in tourism and economic impacts on local communities (Figure 2).

Increased frequency of extreme climate events such as drought, floods, wildfires, or severe storms could potentially damage tourism infrastructure, create hazardous conditions, and keep tourists away. For example, Sunset Crater National Monument in Arizona had a drop of 12,000 visits in July 2002, in part because of extreme drought and forest fire conditions (Figure 3). As a result, the local economy suffered an estimated loss of over $225,000.




Changing economic livelihoods

Many cities and recreation areas in the Southwest are economically dependent on tourism. Increased frequency of extreme climate events such as wildfires, drought, floods, or severe storms could damage tourism infrastructure, create hazardous conditions, and keep tourists away. For example, Morehouse and colleagues found that visits to New Mexico’s Bandelier National Monument were reduced by 7 percent during a year of extreme drought and by 21 percent due to the Cerro Grande fire in 2000.

Changes to the physical environment also have significant impacts on local tourism industries. Between 1999 and 2003, lake levels fell 5.4 percent at Lake Powell and 2.1 percent at Lake Mead. This drop in lake levels contributed to a drop of more than half a million visits to the Glen Canyon National Recreation Area in 2003, bringing a loss of $32.1 million in visitor spending, 758 jobs, and $13.4 million in personal income. At the Lake Mead National Recreation Area, lower lake levels contributed to approximately 900,000 fewer visits, with a $28.1 million loss in visitor spending, 680 lost jobs, and a $9.6 million loss in personal income.

Some recreation industries are investigating ways to deal with environmental changes that affect numbers of visitors. Ski resorts in Arizona, for example, are especially vulnerable to variability between snow seasons. Projections indicate that snowpack may decline as winters become warmer, especially at lower altitudes.

When there is below-average snowfall, ski resorts may rely on more expensive snowmaking techniques. Two resorts operate in Arizona, both of which help support surrounding communities through jobs, visitor spending, and tax revenues. Sunrise Park Resort in the White Mountains has snowmaking capability for 10 percent of its runs to augment the natural snowfall. This technique helps level out seasonal variability in snowfall, as long as temperatures remain cool enough to transform water into snow. But snowmaking has also created controversy in the Southwest. In August 2008, Arizona Snowbowl near Flagstaff received federal approval for a plan to make snow from treated wastewater. However, many regional environmental groups and Native American communities have opposed this plan for ecological, cultural, and spiritual reasons. Coming up with logical and economically viable human adaptations to climate change impacts can be quite complex.


  1. Ponnaluru, Srinivasa. 2005. Visitation to the National Parks of the Southwest: The influence of economic and climate variables. MS Thesis. Department of Agricultural and Resource Economics, University of Arizona.

  2. USDA. 2005. Final environmental impact statement for Arizona Snowbowl facilities improvements. Vol. 1. Coconino National Forest, Coconino County, Arizona.

  3. Finch, Deborah M. (ed.). 2004. Assessment of grassland ecosystem conditions in the Southwestern United States. Vol. 1. Gen. Tech. Rep. RMRS-GTR-135-vol. 1. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

  4. Morehouse B., G. Frisvold, R. Bark-Hodgins. 2007. How can tourism research benefit from multi-disciplinary assessments of climate change? Lessons from the U.S. Southwest. In Matzarakis, A., C.R. de Freitas, and D. Scott. (eds.) Developments in Tourism Climatology. pp.274-281.


Figure 1. 2007 visits to Southwest national parks and attractions.
| Enlarge This Figure |
Credit: Gigi Owen, CLIMAS, The University of Arizona

Figure 2. Elephant Butte Reservoir.
| Enlarge This Figure |
Credit: George Frisvold, Agricultural and Resource Economics, The University of Arizona

Figure 3. July 2002 Fire restrictions affected visits to Sunset Crater National Park near Flagstaff, Arizona.
| Enlarge This Figure |
Credit: George Frisvold, Agricultural and Resource Economics, The University of Arizona

Streamflow: Natural Variability and Human-Caused Changes

Monday, September 15, 2008

Current observations suggest that climate change is altering streamflows in ways that negatively impact water supply for southwestern populations. Many climate models suggest that these changes will worsen as the climate warms, accentuating the natural variability inherent in river flows. Since water is one of the most vital resources in the arid Southwest, the consequences of reduced streamflows and changes in the timing of peak river flows will impact water consumption, agriculture production, economic growth, recreation opportunities, and electricity generation, among other vital services.

Extensive research and computer models now provide a coherent picture of the following topics:

  • Natural Variability
  • Streamflow-related observations
  • Streamflow-related predictions
  • Climate change impacts on streamflows

Natural variability


Major rivers in Arizona and New Mexico are fed by the winter precipitation that falls in the Rocky Mountains. The Colorado River, which supplies New Mexico, Arizona and five other states and Mexico with water, obtains 70 percent of its water from the melting snows of Utah, Colorado, and Wyoming. As a result, the amount of snow, the timing of melt, and the speed at which it melts is of central importance to water managers and users of the Colorado River and most other major western rivers.

The amount of water that would naturally flow in the Colorado River has been estimated since 1906 at Lees Ferry, a historical landmark located approximately 12 miles south of Glen Canyon Dam and the town of Page, Arizona. This record demonstrates large year-to-year variability and captures the drought in the 1950s and the on-going drought (Figure 1). However, even 100 years of data is insufficient to observe the breadth of natural variability.


An extended record dating back to 762 AD that was pieced together from tree-rings reveals valuable information: streamflows in the Colorado River have been even more variable than those recorded since 1906; there have been longer, more severe droughts than those observed; and the current allocation of water exceeds the 100-year historical average(Figure 2).

In addition, a recent tree-ring study suggests that streamflows in the upper Rio Grande mimic those in the Colorado River at Lee’s Ferry. This suggests that large-scale climate changes impact both river basins in similar ways.

Observations and tree-ring analysis demonstrate that rivers naturally experience streamflow fluctuations and that modern southwestern populations have not experienced the brunt of this variability. Many scientists also believe that global warming will not only amplify the natural variability, but will also reduce snowfall, which is critical for southwestern streamflows. Rivers such as the Colorado that are over-allocated and dominantly supplied by snowmelt are especially vulnerable to future climate change.

Streamflow-related observations

Increases in temperature and alterations in precipitation patterns are driving the following observed changes in Southwest streamflows:

  • The major pulse of spring snowmelt is currently occurring earlier than it did during the mid-20th century.
  • In many rivers, the date when 50 percent of the average annual streamflow passes a measurement station has advanced by 10 to 30 days over the observational period of 1948 to 2000—the center of water mass is currently occurring earlier than the mid-20th century.
  • A greater fraction of winter precipitation now falls as rain rather than snow. For example, the fraction of annual precipitation falling as rain rather than snow increased at three quarters of the weather stations located in the western mountain ranges between 1949 and 2004.

Streamflow-related projections

  • Models generally project substantial declines in the average annual runoff in the Southwest due to higher evaporation, particularly in the summer months.
  • Climate models suggest that average temperatures will continue to rise, causing many Southwest rivers to experience earlier peak streamflows.
  • Models suggest that winter and early spring flows will increase, which will elevate the risk of flooding.
  • As a result of higher temperatures, evapotranspiration by vegetation and evaporation in streams, lakes, and reservoirs will increase.
  • Although precipitation predictions are less certain, models suggest that even moderate increases in precipitation will not offset the negative impacts to the water supply caused by temperature increases—some models project declines in Arizona streamflows between 20 and 40 percent, while New Mexico streamflows may experience reductions between 10 and 20 percent.

Climate change impacts on streamflows

  • Declines in the amount of snow will reduce annual streamflows.
  • Increases in the frequency and extent of droughts will also reduce streamflows.
  • Higher temperatures will increase evaporative losses in reservoirs and streamflows.
  • Higher temperatures and longer growing seasons (as a result of higher temperatures) will increase water consumption for domestic use, irrigation, and energy production.
  • Periodic increases in winter and early spring streamflows as well as higher frequency of intense storms will produce more extreme floods.
  • Earlier streamflows will cause reservoirs to fill earlier in the year. Consequently, reservoirs will be drained to avoid floods, wasting limited water that normally helps satisfy human demand.
  • An advance in the timing of snowmelt will increase the length of the summer dry season, altering water and wildfire management and ecosystem interactions.


  • Christensen C. S., et al. 2004. The effects of climate change on the hydrology and water resources of the Colorado River basin. Climate Change, 62:337-363.

  • U.S. Bureau of Reclamation. 2008. Colorado River Basin Natural Flow and Salt Data Current Natural Flow and Salt Data.

  • (Last accessed on August 11, 2008).

  • Meko, D. M., et al. 2007. Medieval drought in the upper Colorado River Basin. Geophysical Research Letters, 34: L10705, doi:10.1029/2007GL029988.

  • Personal communication with Connie Woodhouse, powerpoint presentation for Border-Area climate Change Impacts and Water Sector Adaptation Workshop, April 1-2, 2008

  • Knowles, N., M. D. Dettinger and D. R. Cayan. 2006. Trends in snowfall versus rainfall in the western United States. Journal of Climate, 19:4545-4559.

  • Field, C.B., et al. 2007. In: North America. In Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Parry, M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson (eds).] Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

  • Stewart, I.T., D. R. Cayan, and M. D. Dettinger. 2005. Changes toward earlier streamflow timing across western North America. Journal of Climate, 18:1136-1155.

  • Stewart, I. T., D. R. Cayan, and M. D. Dettinger. 2004. Changes in snowmelt runoff timing in western North America under a “business as usual” climate change scenario. Climatic Change, 62:217–232.

  • Adger, N., et al. 2007. Summary for policy makers, In Climate Change 2007: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, [Parry, M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson (eds.)] pp.7-22. Cambridge University Press, Cambridge, UK and New York City, NY, USA.

  • Backlund, P., et al. 2008. The effects of climate change on agriculture, land resources, water resources, and biodiversity: Introduction. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Washington, DC., USA, pp. 362

  • Milly, P.C.D, et al. 2008. Stationary is dead: Whither water management. Science, 318:573–574.

Images and Figures 

Image 1. The Rio Grande with the Sandia Mountains in the background, near Albuquerque, New Mexico. Photo taken in September.
Credit: ©Jill Fromer,

Figure 1. Instrumental record of Colorado River streamflows. The naturalized flow of the Colorado River has varied between 5.3 and 24.0 million acre-feet per year at Lees Ferry near Page, Arizona. Naturalized flow is an estimate of the River's flow that would occur in absence of human diversions or withdrawals. One acre-foot is equal to approximately 326,000 gallons.
| Enlarge This Figure |
Credit: Zack Guido, CLIMAS, The University of Arizona

Figure 2. Time series plot of past Colorado River flows.
| Enlarge This Figure |
Credit: Dave Meko, Laboratory of Tree-Ring Research, The University of Arizona


Mountain Snowpack in the West and Southwest

Monday, September 15, 2008

Mountain snow plays a critical role in the hydrological cycle for most western states, stockpiling water during the winter and releasing it to the streams in the spring and early summer, which helps meet increasing demands for water.

A thick blanket of fog hovers over the Salt River in central Arizona following a winter storm that delivered a fresh coating of snow to the surrounding mountains.
Credit: ©Ron Adcock,


In the West, as much as 70 percent of the region’s precipitation falls during winter. Arizona and New Mexico are critically dependant on this winter precipitation. The region’s two main water lifelines, the Colorado River and the Rio Grande, tap the winter snows in the Rocky Mountains for approximately 70 percent of their annual water flow.

Many water and land managers as well as citizens, farmers, and ranchers are interested in three critical questions: How has the snowpack been changing, what are the likely future changes, and what are the effects of these changes? To address these concerns and to better understand changes in snowpack from natural climate variability and human-spurred change, research has focused on clarifying:

  • Observed snowpack-related trends
  • Changes to snowpack-related trends
  • Impacts of observed and predicted snowpack changes

Observed snowpack-related trends

Based on long-term monitoring of snow (Figure 2) the general picture for the 11 states in the western U.S. is clear. In comparison to time periods before 1950, winter snowpack is melting earlier in the year, rain is replacing some snow storms, and the April snowpack now contains less water. These changes are affecting streamflows (see table 1 for recent research on snowpack and streamflow). According to observations, dates of the first major spring snowmelt as well as the peak in streamflow are occurring earlier in the year by as much as four weeks1. Although these trends are influenced by natural climate variability, such as El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation, the increasing temperature attributed to global warming has played a significant, if not dominant, role.

In the Southwest, however, the picture is not as sharp because most scientific research has focused on the whole western region and not specifically on the mountains that contribute water to the Southwest. In the headwaters of the Colorado and Rio Grande, increases as well as decreases in snowpack have been observed in spite of the overarching rise in temperature. This variability occurs for several reasons. First, the Rocky Mountains crest at higher elevations, which typically make them colder.

Table 1. Recent Publications on Snowpack and Streamflows in the West and Southwest.

VIC = Variable Infiltration Capacity; PDO = Pacific Decadel Oscillation; ENSO = El Niño Southern Oscillation

Publication Title Data Type Used    
  Snow Streamflow Other

Effects of Temperature and Precipitation Variability on Snowpack Trends in the Western United States

Snow water equivalent on March 1, April 1, and May 1 via VIC model


PDO, ENSO influence

Trends in Snowfall versus Rainfall in the Western United States

Daily snow water equivalent


PDO, ENSO influence

Declining Snowpack in Western North America

Snow water equivalent on April 1


PDO, ENSO influence

Seasonal Shifts in Hydroclimatology over the Western United States

Snow water equivalent in March, April, May

Runoff timing, averages

PDO, ENSO influence















Recent warming, therefore, has not been sufficient to raise winter temperatures above the melting point on a consistent basis. Second, some of the analyses of snowpack and streamflow suffer from an observational record that begins in the major regional drought of the 1950s. An outcome of analyzing precipitation trends from data that begin in a period of drought is that the precipitation can appear to increase over the record when in reality no increase is occurring. Third, some areas may have experienced an increase in snowfall, which masked any reductions in snowpack due to warmer temperatures.

Even though variability exists in snowpack amounts, streamflows in the Colorado River, Rio Grande, and several other southwestern rivers appear to be peaking earlier in the year1. Since streamflows integrate upstream changes in snowpack, earlier streamflows suggest that the spring temperatures are warmer than in the past.

Snow in the mountains of Arizona and New Mexico also contributes to the flows of major rivers such as the Colorado, Rio Grande, Salt, and Gila. In Arizona, snowpack is monitored at numerous locations (Figure 3). At several of these sites, recent unpublished research suggests that the water content in the spring snowpack has decreased over the 1975 to 2004 period.

Predicted changes to snowpack-related trends

Figure 1. Snow monitoring sites in the Western U.S. with at least 20 years of data.
| Enlarge This Figure |
Credit: Joe Abraham, Institute of the Environment, The University of Arizona


Climate models suggest that the West will likely warm between 3.5 and 9 degrees F (2 to 5 degrees C) over the next century. The future estimates of precipitation, however, are less certain. Temperature changes will likely continue to drive losses in the snowpack in many western regions and may even accelerate the melting, with faster losses in the mountains of the milder climates like the Pacific Northwest and slower losses in the colder mountains of the northern Rockies.

Christensen and Lettermaier (2006) derived future scenarios for the Colorado River basin by downscaling temperature and precipitation values from global climate model (GCM) simulations. The authors found that the content of water contained in April snowpack declined by approximately 38 percent by the end of the 21st century, while the annual runoff declined by approximately 10 percent. This scenario, or more severe ones, would impose huge challenges for future water management and economic growth.

GCMs also suggest the storm tracks will move north, decreasing precipitation in the Southwest. This projection may already be occurring. Authors of a peer-reviewed article published in Geophysical Research Letters in 2008 conclude the winter storm track in the western U.S. shifted north between 1978 and 1998, particularly in the late winter.5 As a result, fewer winter storms brought rain and snow to southern California, Arizona, Nevada, Utah, western Colorado, and western New Mexico.

Impacts of observed and predicted changes

Continued warming in the Southwest undoubtedly will occur, most likely intensifying observed changes to the snowpack. As a result of earlier melting snowpacks, earlier peaking flows in rivers, and more winter rains instead of snow, many researchers have stressed that drought, water supplies, fires, floods, and phenology will be affected.

Earlier snow melt and streamflows may accentuate the stress on plants and increase the risk of wildfire by lengthening the dry season between the end of winter and the monsoon rains. In fact, recent research has linked earlier streamflows to an increase in wildfires in the West.

Earlier elevated streamflows may also increase spring flood risk. Historically, snowmelt occurs as water demand by people ramps up, draining reservoirs as they fill. When streamflows are elevated earlier in the year, reservoirs fill more quickly. In the event that spikes occur in river flows when the reservoirs are at full capacity, floods occur. Additionally, the mismatch in water demand and delivery to the reservoirs may decrease the total amount of water stored, a costly impact for a region precariously balancing supply and demand.

Delays in the timing of plant lifecycle events, such as plant bud bursts, have also been linked to decreases in snowpack7. These delays cause additional imbalances in animal and plant interactions, such as decreases in honey production.



  1. Stewart, I. T., D. R. Cayan and M. D. Dettinger. 2005. Changes toward earlier streamflow timing across western North America. Journal of Climate, 18:1136–1155.
  2. Cubasch, U., et al. 2001. Projections of future climate change. In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change,[Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. pp. 525-582. Cambridge University Press, Cambridge, UK and New York City, NY, USA.
  3. Mote, P. W., A. F. Hamlet, M. P. Clark, and D. P. Lettenmaier. 2005. Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society, 86:39–49.
  4. Christensen, N. S., and D. P. Lettenmaier. 2007. A multimodel ensemble approach to assessment of climate change impacts on the hydrology and water resources of the Colorado River Basin. Hydrology and Earth Systems Science, 11:1417–1434.
  5. McAfee, S. A. and J. L. Russel. 2008. Northern Annular Mode impact on spring climate in the western United States. Geophysical Research Letters, 35: L17701, doi: 10.1029/2008GL034828.
  6. Westerling, A.L., H.G. Hidalgo, D.R. Cayan and T.W. Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313:940–943.
  7. Inouye, D. W., M. A. Morales and G. J. Dodge. 2002. Variation in timing and abundance of flowering by Delphinium barbeyi Huth (Ranunculaceae): The roles of snowpack, frost, and La Niña, in the context of climate change. Oecologia, 130:543–550.
  8. Hamlet, A. F., P. W. Mote, M. P. Clark and D. P. Lettenmaier. 2005. Effects of temperature and precipitation variability on snowpack trends in the western United States. Journal of Climate, 18:4545–4561.
  9. Knowles, N., M.D. Dettinger and D.R. Cayan. 2006. Trends in snowfall versus rainfall in the western United States. Journal of Climatology, 4545–4559.
  10. Regonda, S.K., B. Rajagopalan, M. Clark and J. Pitlick. 2005. Seasonal cycle shifts in hydroclimatology over the western United States. Journal of Climatology, 18(2):372–384.

Invasive Species

Monday, September 15, 2008

Many types of invasive plant and animal species have expanded and will continue to expand into new habitats in the Southwest. An invasive species is a plant, animal, or microbe that adversely affects the native ecosystem upon introduction to a new community. Invasive species are well-adapted to encroach upon new territory, and invaders compete with native species for resources like water and soil nutrients. Many invasive species are so well-adapted to diverse conditions that they can outcompete their native counterparts, leading to environmental damage and decreased biodiversity. Regional impacts of climate change, including warmer temperatures, decreased precipitation, and increased levels of carbon dioxide will affect how and where invasive species migrate and colonize.

Figure 1. A large stand of buffelgrass just west of Soldier Canyon.
| Enlarge This Figure |
Credit: Aaryn Olsson, Arizona Remote Sensing Center, The University of Arizona

Invasive species may benefit from climate change because of their ability to adapt. Temperatures and increased carbon dioxide levels in the Southwest may help some invasive species expand into new areas, requiring action from land managers, farmers, ranchers, and those involved in outdoor recreation. Some of these actions include prevention, monitoring, and eradication. Invasive species control requires understanding:

  • How invasive species migrate and colonize
  • Observed impacts of invasive species
  • Climate change implications for invasive species

Migration and colonization

Carriers, most often humans, bring invasive species into a new area, either accidentally (through ship cargo, seed stock, livestock, or travelers) or purposefully (for food, fuel, forage, logging, biological control, recreation, or medicinal uses). Not all species introduced to an ecosystem will survive. Only 10 percent of non-native plant species are estimated to give rise to a steady population because they need the right germinating conditions, like soil type and climate.1 Colonization also depends on how many individual seeds or animals survive the transportation process. Plants with a wide geographic distribution, for example, will do better because they are more tolerant of climate and soil variations.

Certain traits enable plants to establish themselves and outcompete others for resources. These include fast-growth, a short juvenile period, bigger leaf size, and taller stem height than other plants. Invasive species tend to do well in ecosystems disturbed by natural causes, such as fires, or areas damaged by human activity, such as logging, ranching, farming, and urban expansion.

Observed invasive species impacts

The Southwest suffers from many types of invasive species outbreaks, including plants (like buffelgrass, cheatgrass, saltcedar, and red brome), animals (like bullfrogs, cowbirds, quagga mussels, and crayfish), and diseases (such as West Nile virus, rabies, and dengue fever). Invasive plants can alter the landscape by overtaking native species, facilitating fire outbreaks, and altering the food supply for herbivorous animals and insects. Buffelgrass was introduced to the region for cattle feed in the mid-1900s, but has since traveled from ranchlands into the desert ecosystem. Subsequently, these grasses have increased fuel loads in the Sonoran Desert, a region where native plants are not adapted to frequent fires. Buffelgrass-induced fires tend to be faster, longer-lasting, and hotter, and cause more plant and animal deaths than fires involving only native plants.2 After a fire, buffelgrass seeds sprout quickly, often within a few days, while many native desert plants, like saguaro cactus or palo verde trees, take months to years to re-establish themselves.

Figure 2. The known extent of salt cedar invasion in the U.S. The vertical line marks the 100th meridian, west of which rainfall drops below 45 cm yr−1 and agriculture becomes highly dependent on irrigation.
| Enlarge This Figure |
Credit: Allen Press Publishing Services

Invasive species also cause economic losses. Salt cedar, introduced in the mid-1900s to combat soil and wind erosion, has now spread to nearly all perennial drainage systems in the western arid and semi-arid U.S (see Figure 2). Research by Erika Zavaleta shows that saltcedar stands on average consume 3,000 to 4,600 m3 per hectare per year more water than regional native vegetation, leaving less available water for native plants and human consumption. In total, saltcedar in the western U.S. annually costs between $133 to 285 million in lost ecosystem services, such as irrigation water, municipal water, hydropower, and flood control.3

Management for invasive species includes using pesticides or herbicides, labor-intensive species removal, and restoration of native vegetation. Removing invasive species from agricultural or ranchlands, national and state parks, and urban landscapes requires time and money. But if uncontrolled, invaders will overtake these lands, often causing irreversible ecological damage.

Climate change implications

Currently, there is little scientific consensus on the precise impacts of climate change on invasive species, but many hypotheses exist regarding changes to dispersal and introduction patterns, colonization patterns, distribution and establishment patterns, and landscape and management practices.

As land conditions change due to climate variations, invasive species may be able to colonize new geographic areas. Climate change may allow for new travel pathways between previously separate geographic areas, increasing the potential for species interchange. Extreme weather or altered atmospheric circulation patterns could also increase dispersal to new geographical areas.4 Native species that are unable to adapt to changing conditions may die off, opening up resources for non-native plants and animals. Invasive species are more likely to have traits that help them tolerate or adapt to change, such as short juvenile periods or creative dispersal methods, and outcompete native species for soil, water, and nutrients.

As temperatures rise, landscapes currently protected against invasives by cold temperatures could become targets for invading species. Changing precipitation patterns may also allow invasive species to colonize new areas. Increased rain could allow expansion into new regions, while decreased rain could cause invasive species to die off.4

Increased levels of carbon dioxide encourage growth in some plants and decrease the necessary amount of water intake, which could put invasive plants at even more of an advantage over native plants. While both native and invasive plants will respond to increased carbon dioxide, research by Stanley Smith and colleagues suggests that some exotic species may benefit more. Red brome, an exotic grass found in the Southwest, will increase in plant size more than native grasses do under higher levels of atmospheric carbon dioxide. Red brome populations will also increase in density while native grasses populations do not.5

Invasive plants may also respond differently to management practices, like pesticide applications or removal programs, if under the influence of increased carbon dioxide. Common pesticides like glyphosate lose effectiveness on plants growing with increased carbon dioxide in the atmosphere.6 Land managers may therefore need to reassess their techniques to control invasive species.

  1. Theoharides, K. and J. Dukes. 2007. Plant invasion across space and time: factors affecting nonindigenous species success during four stages of invasion. New Phytologist 176 (2): 256-273.
  2. Esque, T., et al. 2007. Buffelgrass fuel loads in Saguaro National Park, Arizona, increase fire danger and threaten native species. Park Science, 24.
  3. Zavaleta, E. 2000. The economic value of controlling an invasive shrub. Ambio, 29(8): 462-467.
  4. Hellmann, J., J. Byers, B. Bierwagen, and J. Dukes. 2008. Five potential consequences of climate change for invasive species. Conservation Biology, 22(3): 534-543.
  5. Smith, S.D. et al. 2000. Elevated CO2 increases productivity and invasive species success in an arid ecosystem. Nature, 408:79–82.
  6. Backlund, P., et al. 2008. The effects of climate change on agriculture, land resources, water resources, and biodiversity in the United States. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Washington, D.C.

Human Health

Monday, September 15, 2008

The World Health Organization (WHO) estimated in 2000 that 150,000 human deaths per year are tied to climate change.1 WHO researchers project the risk of death from global climate change impacts will more than double by 2030.

In general, health risks are greater for older and younger segments of the population.
Credit: ©Elena Korenbaum,

Health risks are linked to climate change through direct and indirect cause-and-effect chains, and depend on factors such as a person’s age, health condition, economic status, access to quality healthcare programs, and exposure to the elements. In the Southwest, regional health risks linked to climate change include extreme heat conditions, poor air quality, and increased food-, water-, fungal- and animal-borne diseases.

Climate change may impact health risks due to:

  • Societal factors that increase health risk
  • Heat
  • Precipitation
  • Air quality
  • Malnutrition
  • Disease

Societal factors that increase health risk

Age or health condition: Older adults are more sensitive to temperature extremes and are more likely to have pre-existing medical conditions that could worsen due to climate change-related factors. Young children are also at a higher risk because they have a small body mass-to-surface ratio and faster breathing rates relative to body size, which makes it harder for them to cool off. People with compromised immune systems or other pre-existing medical conditions are also at higher risk of suffering negative health effects.

Economics and access: People with inadequate access to health care may be less able to cope with negative health consequences. Impoverished people, including those who have inadequate access to shelter, food, water, or cooling systems, will have a harder time adjusting to extreme weather. Migrants or new residents who are not acclimated to weather patterns have a lower awareness of local disease vectors, fewer social support networks, and reduced access to health care.

Exposure: Outdoor laborers will be directly subjected to changing weather patterns and weather extremes. Children and adults who participate in outdoor recreation will also be at higher risk. Undocumented migrants, particularly those crossing the Southwest desert, are often subjected to severe climate patterns while traveling to their desired destination. Residents in urban areas are exposed to temperatures between 2 and 10 degrees warmer than nearby rural areas as a result of the urban heat island effect.


Extreme heat, including heat waves and higher maximum temperatures, can directly cause heat stroke, heat exhaustion, or hyperthermia. Extreme heat also affects health by making chronic conditions worse.


If regional precipitation declines as projected, reservoir levels will drop, reducing the availability of water. Future changes in the intensity and frequency of storms, floods, and severe precipitation events will also hinder sanitation and water delivery infrastructure.

Air quality

Ground-level ozone formation, mainly created by burning fuels, increases at higher temperatures. Breathing ground-level ozone can result in lung inflammation or decreases in lung function, which may increase risk of asthma or premature mortality.

Climate projections indicate an earlier start for the pollen season for many North American plant species, lengthening the allergy season for those who suffer allergy attacks. Wildfires are projected to occur more often as temperatures increase and rainfall decreases, especially in arid areas, contributing to bad air quality. Smoke from wildfires can aggravate or cause asthma and chronic obstructive pulmonary disorder (COPD).


Nearly 800 million people in the world suffer from the lack of an adequate food supply. Drought and temperature changes have direct impacts on crop yield and crop failure that can make less food available for people. Southwest agriculture could suffer as the climate becomes hotter and drier.


Water, food, plants, fungi, and animals are capable of spreading disease to humans, and all are directly tied to climate or exhibit distinct seasonal patterns.

Food-borne disease: Incidence of food-borne disease associated with fresh produce is growing. More intense storm events may result in contamination of food crops with feces from nearby livestock or feral animals.

Figure 1. In a matter of a few years West Nile Virus spread across the entire U.S.. Maps show cases of West Nile Virus documented by the Centers for Disease Control and Prevention.
| Enlarge This Figure |
Credit: Centers for Disease Control and Prevention

Water-borne disease: Untreated water can contain protozoa, parasites, bacteria, or viruses that cause a variety of diseases. Increased storm events may cause runoff of untreated water into the supply of human drinking water. These events particularly impact certain populations, including over 100,000 residents of colonias in New Mexico, who already have limited access to clean water. Colonias are areas of high poverty that lie outside city limits, usually along the U.S.-Mexico border, and often lack public infrastructure like plumbing, garbage service, or roads.3 These communities are extremely vulnerable to drought-induced health effects, such as increased disease transmission through water-borne vectors due to poor water quality.

Animal vectors: The reproduction and survival rates of protozoa, bacteria, viruses and their vector organisms (e.g. mosquitoes, mice) are affected by temperature variations. The changing climate will also affect vegetation, bringing invasive species and pathogens to new areas. In the Southwest, diseases associated with animal vectors include Hantavirus pulmonary syndrome and West Nile virus.

Hantavirus pulmonary syndrome is transmitted to humans through rodent droppings. The Southwest has the highest number of cases in the U.S., which researchers have linked to climatic variations.4

Two types of mosquitoes common to the Southwest carry West Nile virus, which causes fever-like symptoms in humans. Changes in temperature, humidity, vegetation, and land cover will affect the number and distribution of these mosquitoes.

Fungal pathogens: Spores from fungi can also cause diseases in humans when inhaled, such as valley fever. The range distribution of these fungi could be connected to climate change.


  1. World Health Organization. 2002. The world health report 2002: reducing risks, promoting healthy life. World Health Report, World Health Organization, Geneva, Switzerland.
  2. McMichael, A., et al. 2004. Global climate change. In Ezzati, M., A. Lopez, A. Rodgers, and C. Murray. (eds.) Comparative quantification of health risks: Global and regional burden of disease attributable to selected major risk factors. pp. 1543-1649. World Health Organization, Geneva.
  3. Liverman D. and R. Merideth. 2002. Climate and society in the US Southwest: the context for a regional assessment. Climate Research, 21: 199-218.
  4. Eisen, R.J., et al. 2007. A spatial model of shared risk for plague and hantavirus pulmonary syndrome in the southwestern United States. American Journal of Tropical Medicine and Hygiene,77(6): 999-1004.
  5. Comrie, A. 2005. Climate factors influencing coccidioidomycosis seasonality and outbreaks. Environmental Health Perspectives, 113: 688-692.

Groundwater in the Arid Southwest

Monday, September 15, 2008

Groundwater plays a critical role in human development and in maintaining important natural ecosystems in the arid Southwest. It helps sustain the flow of streams and rivers and maintains riparian and wetland habitats that are vital to plants, animals, and people. Groundwater also provides drinking water to urban and rural communities and supports agriculture and industry, all of which have helped enable rapid population growth in Arizona and New Mexico. Population expansion, however, has not been without its consequences. It has led to increasing groundwater withdrawals that are outpacing the rate at which the vital resource is naturally replenished. As a result, the region’s groundwater resources are among the most overused in the United States.

Figure 1. Historic aquifer water-level declines in the Southwest.
| Enlarge This Figure |
Credit: S. A. Leake1, United States Geological Survey

In less than a century, the volume of water in underground reservoirs that accumulated over thousands of years has been reduced by urban development and agriculture. These reductions in water storage now require active management to prevent the depletion of these aquifers.

The challenge to water managers and users is grand and urgent: balance water consumption with sustainable supplies in a region where future demand will likely increase, and future supply will likely decrease. Sustainable water use requires integrating knowledge of numerous factors, including the Southwest’s limited water supplies, pressures imposed by an increasing population, and high-impact changes in the hydrologic cycle driven by climate change. Proper management by private and public interests will require understanding:

  • Groundwater use in the Southwest
  • Climate and the groundwater balance
  • Impacts of climate and society changes on groundwater resources

Groundwater use in the Southwest

Changes in groundwater supplies are influenced predominantly by climate and human consumption. In the absence of human use, the volume of groundwater generally expands as water from streams, rivers, and precipitation seep into the ground in quantities that exceed what is withdrawn by vegetation or lost to evaporation. With the expansion of human development, however, groundwater has been increasingly withdrawn from wells to support agricultural, industrial, and municipal demand at rates that have outpaced natural recharge. As a result, the depth of the water level below ground has declined in many aquifers, in some areas by more than 200 feet (or 60 meters; see Figure 1).

Figure 2. Single-family water use in southwestern cities.
| Enlarge This Figure |
Credit: Zack Guido, CLIMAS, The University of Arizona

Groundwater alone cannot satisfy the region’s water demand. Water in rivers, such as the Colorado and Rio Grande, is considered sustainable and is diverted to help meet demand and reduce the overuse of groundwater. The Central Arizona Project (CAP) diverts Colorado River water to areas in Arizona. In 2003, CAP channeled 3.7 billion gallons of water to users (1.15 million acre-feet). That amount, however, only supplied about 15 percent of Arizona’s 2.5 trillion gallons used in 2003, a conservative estimate that would fill approximately 7.8 million acres with water one-foot deep. CAP water and water from other rivers such as the Salt and Verde accounted for approximately 55 percent of the total water used in 2003, while groundwater provided about 45 percent.

Both groundwater and surface water supply residents with water. In the larger cities in the Southwest, the volume used by single family homes ranges from less than 100 gallons per day to nearly 250 gallons (Figure 2). Agriculture, and not residential water use, is the biggest drain on groundwater. In 2000, about 80 percent of total groundwater withdrawn in Arizona and parts of other western states, including New Mexico, was for irrigation.

Climate and the groundwater balance

Figure 3. Relationship between water demand and climate in Tucson, Arizona. Mostly notable is how demand drops as the summer monsoon begins. Maximum daily potable water demand (red) in Tucson Water utility’s service area (based on 2005-2007 data), and average daily precipitation at Tucson International Airport (blue).
| Enlarge This Figure |
Credit: Zack Guido, CLIMAS, The University of Arizona

Groundwater supplies fluctuate when inflows into the systems do not balance outflows. The inflows into aquifers are driven by rain, snow, and streamflow, while the outflows are dominated by evapotranspiration—the process by which vegetation uses water in the soil for plant growth—evaporation, and well pumping.

Climate and water use are intimately connected. When precipitation declines, the inflows typically drop, while pumping may increase. When temperature increases, evaporation generally increases—reducing streamflow and the amount of water in reservoirs—while people generally pump more water. In Tucson, for example, potable water use is lowest during mid-winter but increases when temperatures begin to rise and winter precipitation wanes (Figure 3). Water use peaks before the major thrust of summer thunderstorms and then nose-dives during the height of the monsoon season.

In the Southwest, although about half of the region’s precipitation falls in the summer, it is the winter rains that are primarily responsible for replenishing groundwater. The long-lasting winter storms enhance infiltration, while the cooler temperatures reduce evaporation and the plants demand less water.

Impacts of climate and society changes on groundwater resources

Future scenarios for the Southwest paint a region with warmer temperatures, a larger population, and perhaps less rain. Global climate models (GCMs) project the West will warm between 3.5 and 9 degrees F (2 to 5 degrees C) by the end of this century, while an additional 4.8 million people will inhabit Arizona and New Mexico by 2030. And although GCMs do not present a clear image of future precipitation, some models suggest a 10 to 15 percent decrease. The confluence of these changes will impact snowpack, evaporation, energy consumption, pumping, and winter storms, all of which impact groundwater supplies.

Air temperatures in the Southwest have increased in the past century and have been linked to reductions in snowpack. Future climate change will likely continue to diminish snowpack at higher elevations and therefore reduce groundwater recharge. Temperature increases will also hasten the evaporation of soil moisture after rain and snow storms, reducing the amount of water that seeps into aquifers.

Warmer temperatures may also bump up water consumption in the winter, and therefore groundwater withdrawals, as higher temperatures often correlate with increased water use. Higher summer temperatures also increase energy consumption as people run air conditioners to keep homes and buildings at comfortable temperatures. Because about 8.0 gallons of water are needed to generate one kilowatt of energy in Arizona, greater energy consumption demands more water, a fraction of which will be from groundwater.

Scenarios that depict decreases in winter precipitation are significant because winter precipitation is considered to be more important in replenishing groundwater than summer rain. Declines in winter precipitation have already been observed and linked to human-caused climate change. Research in 2008 concluded the winter storm tracks in the western U.S. shifted north between 1978 and 1998, delivering fewer rain and snow events to Arizona and western New Mexico, among other western states.

In addition to winter precipitation, summer monsoonal rains provide a portion of the total groundwater recharge. Although it is not well-known how climate change will alter future monsoon seasons, current water consumption in Tucson declines when the monsoon rains ramp-up. This suggests that if the monsoon season becomes shorter and less intense, future water consumption may increase. The converse is also true but would have a positive effect on groundwater supplies.

Regardless of climate change, population growth will affect water consumption. Expected growth will increase water demand, which will be partially satisfied by groundwater. It will also raise energy consumption, causing a simultaneous increase in water use.


  1. Leake, S.A., A.D. Konieczki, and J.A.H. Rees. 2000. Desert basins of the Southwest. U.S. Geological Survey Fact Sheet 086-00, 4 p.
  2. Arizona Department of Water Resources. 2006. Arizona Water Atlas: Introduction. Vol 1. AZ Water Atlas (Last accessed October 2008).
  3. Stonestrom, D.A. and J.R. Harrill. 2007. Ground-water recharge in the arid and semiarid southwestern United States-climatic and geologic framework. U.S. Geological Survey Professional Paper 1703-A, 27 p.
  4. Cubasch, U., et al. 2001. Projections of future climate change. In Climate change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. pp. 525-582. Cambridge University Press, Cambridge,UK and New York City, NY, USA.
  5. IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge, UK and New York City, NY, USA: Cambridge University Press.
  6. Torcellini, P., N. Long, and R. Judkoff. 2003. Consumptive water use for U.S. power production. Technical Report NREL/TP-550-33905. Golden C: National Renewable Energy Laboratory.
  7. McAfee, S.A. and J.L. Russel. 2008. Northern Annular Mode impact on spring climate in the western United States. Geophysical Research Letters, 35, L17701, doi: 10.1029/2008GL034828.

Climate and Floods in the Southwest

Monday, September 15, 2008

Despite the Southwest’s arid climate, rivers in the region are not immune to overflowing their banks and flooding city streets, farms, and desert. Climate change likely will increase both flash floods and regional floods, making the region’s growing population more susceptible to losses of life and property.

Summer flood event in Tucson, Arizona
Credit: Ashley Coles, The University of Arizona

One example of destructive flooding occurred during the winter of 1992–93, when vigorous storm activity drenched Arizona. Storms from the western Pacific built up snow packs in the higher elevations and saturated soils throughout the state. Streams and rivers topped their banks—peak flows on the Salt and Verde rivers surpassed their historical records. On the Gila River, billions of gallons of water poured over the Painted Rock Dam, flooding an estimated 10,000 acres of farmland and forcing several thousand people from their homes. Newspapers declared the economic burden of the flooding to be in the neighborhood of 50 million dollars.

How climate change alters future floods remains an open-ended question. However, recent observations and research are contributing to a better understanding of flooding in the Southwest, including:

  • Causes of floods in the Southwest
  • Trends in extreme precipitation and flooding in the Southwest
  • Effects of future warming on floods

Causes of floods in the Southwest

Flooding causes more deaths in the United States than any other weather-related hazard except severe heat. In Arizona and New Mexico, floods killed 57 people between 1995 and 2006, while hundreds of others needed swift water rescues. The economic price tag is also high, costing Arizona, New Mexico, Colorado, and Utah approximately $5 billion between 1972 and 2006.

Figure 1. Space-time domain of weather, climatic, and flooding events.
| Enlarge This Figure |
Credit: Katie Hirschboeck, The University of Arizona

Storms may produce flooding on one river while not on another. The link between precipitation and flooding is complex and controlled by several factors, including soil moisture, the intensity and duration of the precipitation event, and the area and steepness of the watershed.

Flooding in the Southwest primary results from three storm types: winter frontal storms that cover a large in area and frequently occur between late November and mid-March; dissipating tropical cyclones moving north from the Pacific Ocean that occur between late summer and early fall; and convective thunderstorms that form during the monsoon season between late June and mid-September.3 These storms also influence the duration of flooding, with more regional events causing long-lasting floods (Figure 1).

Regional floods often accompany slow-moving, low-pressure weather systems such as decaying tropical storms or winter frontal systems. Regional floods also occur when rain combines with melting snow. In these circumstances, frozen ground acts as an impervious layer and both rain and snowmelt flow quickly and in greater quantities into rivers, much like the flooding in the winter of 1993.

Flash floods can occur within seconds to hours after the onset of a rain storm. They can be deadly because they rapidly increase water levels and flow with a devastating swiftness. Urban areas are particularly susceptible to flash floods because a high percentage of the surface area is covered by impermeable streets and buildings; runoff occurs very rapidly because the water cannot sink into the ground. Mountainous areas also experience flash floods, as steep topography funnels water into canyons and gullies—a flash flood in Sabino Canyon in Tucson, Arizona, killed eight people in 1981.

Trends in extreme precipitation and flooding in the Southwest

The largest flood in the 75-year instrumental record of Sabino Creek in Tucson occurred in July 2006. During this event, the swollen waters of Sabino Creek joined the already raging Rillito River, typically an ephemeral wash carving through Tucson’s urban landscape, and flooded roads and bridges, destroying property.

Figure 2. Trends in the frequency of precipitation events in which precipitation over a 7-day duration exceeds a 1-year recurrence interval for each particular climate division. Blue circles are positive trends, red circles are negative trends. The magnitude of the trend is linearly proportional to the radius of the circle. A 100% trend is equal to a doubling of the average yearly frequency, from 1 to 2 events per year. Check marks indicate positive trends with a local significance at the 5% level.
| Enlarge This Figure |
Credit: Kenneth Kunkel5, Desert Research Institute

The magnitude of floods like these seems to be increasing, according to analysis of historic data for Sabino Creek. The analysis shows that the volume of water for large floods measured at the mouth of the creek has risen consistently over the past 75 years. The authors suggest that climate change is likely the cause for the increase in the size of floods because land change has not occurred in this basin during the period of record. This trend will likely continue if global warming increases atmospheric moisture as projected.

In a broader study that analyzed precipitation records for the U.S. and Canada, the authors found increasing trends for extreme precipitation events in the Southwest from 1931 to 1996 (Figure 2). In this study, the authors investigated the number of events in which the seven-, three-, and one-day precipitation totals for each climate station surpassed a threshold. The threshold was defined as the total precipitation in seven-, three-, and one-day events that would occur only once per year. The authors found that the number of events exceeding the threshold has been increasing in most climate divisions. In southwestern New Mexico, for example, the number of events per year has nearly doubled from one to two since 1931.

Effects of future warming on floods

While the Southwest is expected to become warmer and drier, it paradoxically is likely to see more flooding. This relates in part to the fact that warm air holds more moisture than cooler air. The frequency of floods is also influenced by the rate of snowmelt in the winter and spring, the character of the summer monsoon, and the incidence of tropical hurricanes and storms in the fall.

Snowpack measurements suggest that rising temperatures are melting winter snow progressively earlier in the year and causing streamflows to deliver water to reservoirs and users in greater quantities earlier in the spring season. Historically, snowmelt has occurred at the same time people ramp up their water consumption, which has drained reservoirs as they fill. When streamflows become elevated earlier in the year, however, reservoirs fill more quickly. Earlier future streamflows will likely increase the chance that spikes in river flows occur when the reservoirs are at full capacity, and therefore increase the probability of these types of floods.

Average air temperatures are rising, and it is likely that continued warming will accentuate the temperature difference between the Southwest and the tropical Pacific Ocean, enhancing the strength of the southwesterly winds that carry moist air from the tropics into the Southwest during the monsoon. This scenario may increase the monsoon’s intensity, or its duration, or both, in which case floods will occur with greater frequency.

Hurricanes and other tropical cyclones are projected to become more intense in the future.6 Since Arizona and New Mexico typically receive 10 percent or more of their annual precipitation from tropical storms, it is likely that this change will also increase flooding.


  1. National Oceanic and Atmospheric Administration: Office of Climate, Water, and Weather Services. 2009. Weather Fatalities. (last accessed on April 21, 2009).
  2. Changnon S.A. 2008. Assessment of flood losses in the United States, Journal of Contemporary Water Research & Education, 138:38-44.
  3. House, P.K. and V.R. Baker. 2000. Paleohydrology of flash floods in small desert watersheds in western Arizona. Water Resources Research, 37:1825-1839.
  4. Desilets, D. and S.L. Desilets. 2006. Magnitude of flash floods on the rise in the Sabino Creek. Arizona Geophysical Union, Fall Meeting 2006, abstract with programs: H21B-1369.
  5. Kunkel, K.K., and K. Andsager. 1999. Long-term trends in Extreme Precipitation Events over the Conterminous United States and Canada. Journal of Climate, 12:2515–2527.
  6. Trenberth, et al. 2007. Summary for policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)].Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  7. Lenart, M. 2006. East Pacific hurricanes bring rain to Southwest. In Lenart, M. (ed.) Global warming in the Southwest: Projection, observations, and impacts. University of Arizona, Climate Assessment of the Southwest, Tucson, Arizona.

Drought and People

Monday, September 15, 2008

Water is a critical natural resource for people in the arid Southwest, as the region is highly susceptible to drought. Climate change impacts on southwest drought could thus have profound implications for society. Global climate projections indicate the future holds higher annual temperatures and less winter precipitation for the Southwest. These warmer temperatures may intensify the impact of drought on residents of the Southwest: recent research links the regional drought that peaked in intensity in summer 2002 (Figure 1) with more severe impacts on water, land, and people, than previous, drier droughts, due to warmer temperatures.

Figure 1. Peak drought intensity. The peak intensity of the drought that began in the late 1990s occurred in 2002 and 2003. Using data from the U.S. Drought Monitor maps archive, this figure shows how the drought affected the entire Southwest. Extreme drought conditions very likely extended well into Mexico despite lack of data available to the U.S. Drought Monitor at that time.
| Enlarge This Figure |
Credit: Joe Abraham, Institute of the Environment, The University of Arizona

More intense periods of drought will affect many aspects of society, including:

  • Water supply
  • Urban populations
  • Regional economy

Water Supply

Climate change will impact the water supply of the region in several ways. Drier winters will diminish the natural recharge of the Southwest’s underground aquifers. Reduced snowfall in the Rockies and other mountain ranges in Arizona and New Mexico means less water flowing into the region’s reservoirs, on average. A shift towards earlier snowmelt due to warmer temperatures will extend the dry season before the summer monsoon, stressing water supplies and increasing demand for water during this period. With higher temperatures, evaporation rates will increase, affecting lakes, reservoirs, soil moisture, and plants.

Phoenix and Tucson store excess water allocations in Lakes Mead and Powell on the Colorado River. This water buffers the cities against short- and long-term droughts that impact regional water resources. But over time, climate change and increasing demand for water may reduce this excess water.

Water storage serves another purpose: it produces hydroelectric power. Together, Lakes Powell and Mead can produce up to 10,000 gigawatt-hours of energy; combined with other hydroelectric power plants, these reservoirs produce between 6.5 to 8 percent of Arizona’s electricity. Projected regional climate changes may reduce the amount of power these reservoirs can produce. For example, climate models forecast a 10 to 18 percent reduction in Colorado River streamflow that could drain water storage by 35 to 40 percent, reducing hydroelectric productivity by 45 to 56 percent.5 Less hydropower production due to low reservoir levels means less energy available for the quickly growing Southwest population.

Urban populations

Figure 2. Projected population growth for Arizona and New Mexico, 2010 - 2030.
| Enlarge This Figure |
Credit: Joe Abraham, Institute of the Environment, The University of Arizona

A stressed water supply plus population growth translates into more competition and higher costs for water. The Southwest is one of the fastest growing regions in the country. In the 1990s, Arizona’s population increased by 40 percent and New Mexico's by 20 percent while the national average during that decade was 13 percent.8 Projections through 2030 show population increases of 66 percent for Arizona6 and 33 percent for New Mexico (Figure 2).

These population estimates combined with regional climate projections may mean that all residents will not receive adequate water supplies, particularly in areas where demand challenges supply. Some people, including more 100,000 residents of colonias in New Mexico, already have limited access to clean water and would be unable to afford to pay more for it. Colonias are areas of high poverty that lie outside city limits, usually along the U.S.-Mexico border, and often lack public infrastructure like plumbing, garbage service, or roads.8 These communities are extremely vulnerable to drought-induced health effects, such as increased disease transmission through water-borne vectors due to poor water quality.

Regional economy

All economic sectors depend on a steady water supply to keep their industries and employees functioning. Drought can impact service industries, retail sales, and general trade, all of which affect the regional economy. However, some sectors in the Southwest are more susceptible than others to the impacts of drought, especially rain-fed agriculture and outdoor tourism and recreation.

Climate change is projected to worsen wildfire risk in the Southwest that in turn impacts rural economies.
Credit: U.S. Forest Service

Agriculture in the Southwest already competes with other sectors for water and land, regardless of regional drought. In Arizona, agriculture currently uses about 80 percent of the state’s water. Limited supplies due to drought conditions will exacerbate water conflicts between urban populations, commerce and industry, energy producers, and natural environments. Water may shift from agricultural use to urban use as the demand for water in the Southwest continues to grow.

The agricultural sector in the Southwest is dependent upon a regular water supply, and will suffer economically from intensified drought and climate change. Drought not only implies direct effects on crops or livestock, but also a myriad of indirect effects. For farmers, drought might mean increased water costs, a reduction in crop yields or crop quality, increased threat of pests, weeds, or diseases signifying higher management costs for pesticides, herbicides, and fertilizers. For ranchers, drought may lead to animal mortality or disease, increased threat of predation, cattle sell offs, a reduction in livestock forage, and higher costs for supplemental feed or water. Retailers who provide services and goods to farmers and ranchers could also face reduced business, creating a ripple effect towards a weak economy.

Crops that are particularly weather sensitive will be more vulnerable to drought and climate change. For example, production of wheat and sorghum in New Mexico (which occupies 42 percent of the state’s agricultural land) declined during drought events in the 1990s. Irrigated agriculture in the Southwest has shifted from crops like cotton to more water-intensive vegetables and cattle feed like alfalfa. Not only do these particular crops demand a lot of water, but they also lose a lot of water to evaporation through storage and transportation and through the soil and plants themselves.

Tourism and recreation will also experience economic impacts due to increased drought frequency and severity. Levels of lakes and rivers will decrease, as will snowfall in the mountains. Trees will suffer with less moisture, becoming susceptible to pests and disease; this can lead to increased wildfires and wildlife habitat degradation. These lakes, snowfall, mountains, and other natural features provide the basis for outdoor tourism and recreation activities, including water sports, skiing, hiking, and camping, as well as local economies that support these activities. Studies show the number of visits to national parks in the Southwest decrease due to side effects of drought, such as lower lake levels and wildfires. People with jobs in tourism and recreation industries may suffer economic effects with decreased visitation. For example, the 5.4 percent drop in water level at Lake Powell from 1999 to 2003 contributed to half a million fewer visits to the Glen Canyon National Recreation Area in 2003. This plummet in visitors brought a loss of $32.1 million in spending in the area, along with 758 jobs and $13.4 million in personal income.


  1. Breshears, D., et al. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences, 102:15144–15148.
  2. Jacobs, K. and J. Holway. 2004. Managing for sustainability in an arid climate: Lessons learned from 20 years of groundwater management in Arizona, USA. Hydrogeology Journal, 12:52-65.
  3. Barnett, T. and D. Pierce. 2008. When will Lake Mead go dry? Water Resources Research, 44:W03201.
  4. Energy Information Administration. 2006. Table 5. Electric Power Industry Generation by Primary Energy Source, 1990 Through 2007. (Last accessed October 2008).
  5. Christensen, N., et al. 2004. The effects of climate change on the hydrology and water resources of the Colorado River Basin. Climatic Change, 62: 337-363.
  6. Arizona Department of Commerce. 2006. State and County Projections in Detail. (Last accessed October 2008).
  7. Bureau of Business and Economic Research. 2004. Population Projections by County, 1990-2030. (Last accessed October 2008).
  8. Liverman D. and R. Merideth. 2002. Climate and society in the US Southwest: The context for a regional assessment. Climate Research, 21:199-218.
  9. Ponnaluru, S. 2005. Visitation to the national parks of the Southwest: The influence of economic and climate variables. MS Thesis. Department of Agricultural and Resource Economics, University of Arizona.

Drought and the Environment

Sunday, September 14, 2008

Drought deeply affects the land, water, and people of the Southwest. It occurs when precipitation averages fall below the norm. A drought can persist for many years, punctuated by particularly severe dry stretches and sometimes a relatively rainy year. The cloudless skies associated with drought not only imply below-average rainfall, but also an increase in the amount of direct sunlight hitting the ground, which leads to higher evaporation rates.

The projected temperature increases and possible precipitation decreases for the Southwest will likely intensify the frequency of drought events and the severity of their impacts on the landscape. During the recent regional drought in the early 2000s, rainfall tallies were comparable to those during the regional drought in the 1950s. However, the effects on Arizona and New Mexico’s forests were much more severe in the early 2000s; researchers link this difference to warmer temperatures during the recent drought.1

During a drought, the combined effect of reduced rainfall and increased sunlight creates a number of environmental effects, including:

Soil moisture depletion

In the Southwest, the winter rains are most important for replenishing soil moisture and recharging groundwater aquifers. Decreased levels of precipitation during winter months, less cloud cover, increased sunlight, and warmer temperatures cause moisture to evaporate from the ground. The combination of increased soil aridity and associated plant mortality makes the soil more prone to wind erosion. Wind erosion can cause dust storms and increased sand deposition, which often kills even more vegetation.

Vegetation stress and die-off

Climate change will likely affect regional vegetation patterns in the Southwest, particularly along ecotones, or the boundaries between ecosystems.2 In northern New Mexico in the 1950s, the fastest recorded ecotone shift occurred bewteen semi-arid ponderosa pine forest and piñon-juniper woodland. In a study site spanning almost 2,400 hectares, the landscape shifted more than two kilometers in less than five years caused by the lack of precipitation during the early 1950s. Parts of this forest became fragmented and soil erosion increased. Some of this landscape has never shifted back.2

In 2002–03, Southwest lands reacted to severe drought when thousands of acres of trees began to die. Lack of water instigated a chain of events causing forest die-offs in ponderosa pine and piñon-juniper ecosystems across Arizona and New Mexico. As the soil became dry, so did the trees; more intense heat and direct sunlight caused faster evapotranspiration rates (when plants lose moisture to the atmosphere). Without water, the trees were unable to defend themselves against predators — in this case, bark beetles. Normally, the trees used sap to push beetles and other pests out of their bark. But under these dry conditions, the trees had to conserve water and could not produce the sap.

David Breshears and his colleagues investigated the extent of die-off from the particularly severe 2002–03 drought event and its related bark beetle outbreak (Figure 1).1 After 15 months of depleted soil water content, study sites in Arizona, Colorado, and New Mexico lost more than 90 percent of their piñon trees.1 These numbers indicate that this drought had more deeply-rooted impacts than the 1950s drought, even though the earlier drought was somewhat drier. Temperatures however, were significantly warmer during the drought in the early 2000s. Research from the National Data Climatic Center shows that one-fifth less land would have experienced severe drought during the recent drought if average temperatures had not increased since 1976.3 With temperatures projected to rise in the Southwest, the convergence of hotter temperatures and drier conditions could lead to more regional vegetation die-offs (Figure 2).


Drought impacts the frequency and the severity of forest fires by setting up ideal fire conditions. Warmer average temperatures during spring and summer also correlate to higher frequency wildfires and intensify the effects of drought.4 Large-scale forest die-offs create prime conditions for high intensity wildfires. The recent drought and related beetle outbreaks mostly affected tall, overstory trees, but as these trees died-off, other changes to the ecosystem occurred. Shorter trees, shrubs, grasses, and other vegetation beneath the overstory dried out too.

Historical tree-ring records show that the largest fires before the 1900s all correspond to years of severe drought, preceded by wet years.5 This cycle of wet and dry years appears to promote wildfires. Wet years encourage plant growth, while dry years cause plant mortality, thereby creating the fine fuels wildfires need to burn.

During 2002, drought conditions facilitated the burning of over 500,000 acres in northern Arizona, a wildfire known as the Rodeo-Chediski fire. These fires are extremely costly—the combined cost of wildfires that occurred between 2002 and 2004 was estimated at $196.8 million.

Degraded wildlife habitat

With drought bringing so many changes to the Southwest ecosystems, wildlife is sure to feel the impacts. Animals will face a reduction in available drinking water, habitat, and food (both vegetation and prey). Mortality rates could increase for the most vulnerable animal species, especially regional endangered species. This may cause increased competition between livestock and wild animals for grazing and water.

Lack of water, food, and habitat protection may also cause decreased reproductive success and survivorship. During the 2002 drought event, over 80 percent of the existing pronghorn population died, leaving an estimated 21 pronghorn in Arizona.6 Research shows that annual pronghorn reproduction is significantly tied to winter rainfall (Figure 3).

Lack of water, food, and habitat protection may also cause decreased reproductive success and survivorship. For example, the Arizona Game and Fish Department estimates the endangered Sonoran pronghorn population fell by 4,000 between 1987 and 2000. The pronghorns normally get enough water by eating forbs and other vegetation throughout the year. But when the forage dies due to drought, they congregate in areas near surface water, increasing competition for resources and the threat of predation.

Drought may also cause existing animal habitats to become patchy, isolating some populations. Other populations will be forced to migrate in search of new resources, increasing the potential for human-wildlife contact.


  • Breshears, D. et al. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences, Vol 102, 15144–15148 pp.
  • Allen, C., and D. Breshears. 1998. Drought induced shift of a forest woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences, Vol. 95, 14839–14842 pp.
  • Lenart, M. 2007. Southwestern drought regimes might worsen with climate change. In Lenart, M. (ed.) Global Warming in the Southwest: Projection, Observations, and Impacts. Climate Assessment of the Southwest, Tucson, AZ.
  • Westerling, A.L., H.G. Hidalgo, D.R. Cayan, and T.W. Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313:940–943 pp.
  • Swetnam, T.W., and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate, 11:3128–3147
  • Bright, J., and J. Hervert. 2005. Adult and fawn mortality of Sonoran pronghorn. Wildlife Society Bulletin, 33:43-50 pp.


  • Figure 1. Areas in Arizona and New Mexico with bark beetle-caused pinon and ponderosa pine mortality (2003). - Credit: U.S. Forest Service
  • Figure 2. The forest around Los Alamos before and after drought stress and a bark beetle outbreak.  Credit: Craig Allen, U.S. Geological Survey
  • Figure 3. Endangered Sonoran pronghorn fawn survival is linked to winter precipitation. Many fawns, as seen in the graph, survived during years of higher winter precipitation (when there was sufficient forage to last until the next rains), while fawn mortality was greatest during very dry years.Credit: James R Bright, Harvard University.


Sunday, September 14, 2008

In the Southwest, normal changes in seasonal climate make the landscape ripe for fires. Every winter, precipitation spurs plant growth, while the dry months of April, May, and June turn the vegetation into tinder. At the time in which the landscape is most primed for fire, convective monsoon storms generate lightning, providing the match.

Figure 1. The Rodeo-Chediski fire on June 21, 2002 before the fires converged.
| Enlarge This Figure |
Credit: Jesse Allen, NASA

Under normal conditions, the Southwest experiences recurring fires. Add in fire suppression by federal agencies, population growth that increases numbers of campfires and careless people, and human-caused climate change and the relationship between fire and climate becomes complicated.

In recent years, wildfires have charred increasing areas of western U.S. forests, burning homes and wildlands and regularly siphoning more than $1 billion per year from federal land-management agencies. With the general belief that future climate in the Southwest will become hotter and drier, recent research has focused on understanding:

  • Observed links between climate and fire
  • Climate change and wildfire in forests and deserts
  • The predicted character of future fires

Observed links between climate and fire

In recent years, the size and frequency of fires have increased. In some cases, either human-caused forest change or climate variability is primarily responsible for this increase. In other cases, both factors play important roles.

Figure 2. Relationship between Western U.S. forest wildfires and spring-summer temperature. The top graph shows the positive relationship between annual frequency of large (>400 hectare) wildfires (bars) and average spring and summer temperatures (line) in Western U.S. forests. Using the same x-axis, the bottom graph shows the first principal component of the center timing of streamflow in snowmelt dominated streams (pink = early, white = average, blue = late).
| Enlarge This Figure |
Credit: The American Association for the Advancement of Science

The frequency and size of fires in the Southwest is influenced by many variables. Climate variability, both short- and long-term, and natural- and human-caused, is one variable. For example, shifts in temperature can influence the timing of snowmelt, leading to drier conditions later in the year.  Oscillations between wet and dry periods, such as those related to phase changes of the El Niño/Southern Oscillation can also influence the amount and size of fires in a given year. Fluctuations in the vigor of monsoons can influence the number of lightening strikes. Other influences on fires are the incursion of invasive species, particularly in desert environments where exotic grasses often provide fine fuels that feed fires; land use changes, such as grazing and logging; and changes in fire management policy.

Although human actions and the climate both influence the timing and size of fires, analysis of historical fire records have helped identify relationships between climate and fire. Some of the more conclusive relationships are:

  • In western mountains during the last two to three centuries, years that had a high number of fires coincided with drought years.
  • The frequency of fires increases more during hotter springs and summers than during cooler springs and summers (Figure 2).
  • Wildfire activity at elevations between 5500 and 8500 feet has occurred predominantly during warm years and is associated with earlier spring snowmelt, which is driven by warmer temperatures.
  • In general, the size of fires in the Southwest is greatest during intense La Niña years in which winter and spring precipitation is low, while fewer acres burn during El Niño years characterized by exceptionally wet springs.

Climate change and wildfire in forests and deserts

Historically, wildfires have been a recurring disturbance in conifer forests, pinyon-juniper woodlands, chaparral shrublands, and grassland ecosystems of the Southwest. Lightning strikes frequently initiated surface fires in ponderosa pine forest, clearing out much of the surface brush and vegetation while killing only a few of the thick-barked mature ponderosa trees. These low intensity fires created a more open forest, preventing the build-up of vegetation that could fuel more severe wildfires. Meanwhile, fires were rare in the higher elevation spruce-fir forests, but were likely severe when they did occur.

Since about the mid-1970s, however, the total acreage of area burned and the apparent severity of wildfires in pine and mixed-conifer forest have increased, both from climate change and human activities. Management practices have made many forests more susceptible to severe wildfires; fire suppression and clear-cutting promote a high density of small trees, while grazing has reduced the grassy understory that formerly sustained surface fires.

Warming temperatures also have a clear link to the observed frequency of large western wildfires in recent times. An analysis of U.S. Forest Service and National Park Service data from 1970–2003 revealed that high spring and summer temperatures correlated with an unusually high number of large western wildfires. The researchers identified earlier snowmelt as a contributing factor to the increase.

Wildfires have also been occurring in desert ecosystems that are ill-adapted to flame. A major contributor to desert fires in the Southwest is invasive grass. Red brome, Mediterranean grass, and buffelgrass are now growing in southwestern deserts and are providing fine fuels for wildfires. Desert fires now expand more easily than in the past, especially after wet winters, which boost the growth of the grasses. Wildfires in desert ecosystems have also increased the fire risk in urban areas and housing developments where urban areas and desert meet.

Predicted character of future fires

Increases in future temperatures are nearly certain. Regardless of precipitation, temperature alone will likely amplify the frequency and total area burned by wildfires. If drought conditions become more common or if precipitation becomes more variable, these will likely combine with temperature change to exacerbate future fire risk. In addition, continued population growth will likely cause greater human-started fires since nearly half of the fires in the Southwest are started by humans. In 2002, for example, the Rodeo-Chediski fires in northern Arizona (Figure 1, above) were both started by humans and combined to burn nearly half a million acres, the largest fire on record in Arizona.

The frequency and size of fires will likely increase in the future because:

  • warmer annual temperatures will cause the snowpack to melt earlier in the spring, therefore increasing the length of the dry season between winter and monsoon rains—a longer dry season creates drier fuels as well as a longer period in which fires can occur
  • increases in summer temperatures will cause trees to be more moisture-stressed, making forests more susceptible to fire
  • fire suppression has created an over abundance of snags and small trees that have increased fuel supplies
  • growth in the human population will increase the number of wildfire starts


  1. Whitlock, C. 2004. Forests, fires and climate. Nature, 432:28-29.
  2. Consortium for Integrated Climate Research in Western Mountains (CIRMOUNT). 2006. Mapping New Terrain: Climate Change and America’s West. U.S. Department of Agriculture.
  3. Westerling, A.L., H.G. Hidalgo, D.R. Cayan, and T.W. Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313:940-943.
  4. Swetnam, T.W. and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climate variability in the American Southwest. Journal of Climate, 11:3128-3147.
  5. Grissino-Mayer, H.D. and T.W. Swetnam. 2000. Century-scale climate forcing of fire regimes in the American Southwest. The Holocene, 10(2):213-220.

Phenology: Changes in Ecological Lifecycles

Friday, September 12, 2008

Lilac flowers bloom with cues from the weather. Caribou give birth at the peak of plant abundance so that their newborns have plenty to eat. In the Southwest, as well as all other parts of the world, variations in the climate trigger life cycle events in plants and animals. Studying these events and their relation to climate is known as phenology. The information obtained is vital for understanding the impact climate change has on humans and ecosystems.

A Gila woodpecker feeding on the flowers of the giant saguaro cactus. The timing of blooming may shift in a changing climate.
Credit: ©Frank Leung,

Phenology includes the timing of flower blooms, agricultural crop stages, insect activity, and animal migration. All of these events are changing as a result of climate change and these changes impact humans. The date flowers bloom, for example, controls the timing of allergens and infectious diseases—impacting human health—and alters when tourists visit regions to enjoy wildflowers, which impacts economies. Variations in crop phases affect agriculture by influencing the timing of planting, harvesting, and pest activity.

Quantitative assessments of the impact of phenological changes on humans in the Southwest are scant primarily because phenology is a relatively recent scientific endeavor in the Southwest. However, increasing concern about climate change has amplified efforts in the following areas:

  • Documenting observed phenological changes
  • Projecting phenological changes from climate change
  • Establishing a national phenological network

Observed phenological changes

Phenology in the Southwest is relatively young and there are only a few observational records more than 20 years old. Nonetheless, records less than 20 years are sufficient to observe trends in phenological changes, and experts believe that changes in life cycle events in the Southwest will be similar to those documented in other parts of the world where longer records exist.

Two of the more important and well-documented effects of climate change on phenology are changes in the date of flowering and food-chain disruptions.

Changes in flower blooms

Studies indicate an advance in the date that flowers bloom in the West. Important conclusions include the following:

  • Shrub specimens collected in the Sonoran Desert of the southwestern U.S. and northwestern Mexico and biological models suggest that the spring bloom of shrubs may have advanced by 20 to 41 days between 1894 and 2004
  • A study published in 2001 concluded that the average date of bloom for lilacs in the western U.S. advanced by 7.5 days between 1957 and 1994, while the average bloom date of honeysuckle advanced by 10 days between 1968 and 1994.
  • A 20-year record of the timing of flower blooms for hundreds of plant species across 4,000 vertical feet in the Santa Catalina Mountains near Tucson, Arizona, suggests more than 15 percent of the surveyed species bloom at elevations as much as 1,000 feet higher than they did in the past.
  • The same 20-year record showed the average total number of species in bloom per year increased over the 20-year period by nearly three species per year at the highest elevations—this increase was associated with increasing summer temperatures.

Food chain disruption

Important life cycle events in plants and animals are often triggered by each other. When the timing of life cycle events changes in one species, it can disrupt symbiotic relationships and affect other species. For example, in the northeastern U.S., nectar-producing trees currently bloom 25 days earlier than in the past. As a result, honey bees have switched their source of nectar from the tulip poplar tree to black locust tree, impacting the pollination of tulip poplars and causing their numbers to crash. In the Arctic, the peak in plant abundance and caribou births no longer coincide, causing a 400 percent jump in offspring mortality.

Projected phenological changes

Future phenological changes will be localized, depending on the specific plant and animal species and the magnitude of climate change. Some species may profit, while others suffer. In general, flowers will likely bloom earlier and food-chain disruptions will likely be more frequent. Several changes are likely in the Southwest:

  • Because the date and abundance of flower blooms are highly correlated with winter snowpack, projected declines in snowpack will decrease flower abundance and advance the date of flowering.
  • Global warming may have a disproportionate effect on montane plant communities. Some mountain species may not be able to respond to changes in temperature by migrating north or south. In addition, an upward shift in altitudinal range of species to encounter cooler temperatures will decrease habitat area.
  • Earlier flower blooms could have substantial impacts on plant and animal communities in the Sonoran Desert, especially on shrubs and migratory hummingbirds.

In addition, climate change will cause plant species to move in response to changes in temperature and precipitation. This may be most evident on mountains, where changes in elevation help create specific habitat zones within small areas. In the Santa Catalina Mountains near Tucson, Arizona, for example, the habitat of many species has expanded upslope, and to a lesser extent downslope.

The main message is that different plants respond differently to climate and other changes.

National phenological network

The USA National Phenology Network (NPN) is headquartered in Tucson, Arizona. Its mission is to facilitate collection and dissemination of phenological data from the United States. NPN primarily supports scientific research concerning interactions among plants, animals, and the lower atmosphere, especially the long-term impacts of climate change.

NPN encourages involvement in phenological research and provides opportunities for interested people to contribute to science. Scholars, students of all grades, and citizens record the timing of life cycle events in key plant and animal species and submit their observations on-line. In this manner, a detailed database is growing. Currently, 800 people in the U.S. participate in NPN. Among them, amateur scientists in the Southwest have provided some of the more valuable and longer observational data.


  1. Bowers, J. E. 2007. Has climatic warming altered spring flowering date of Sonoran desert shrubs? The Southwestern Naturalist, 52(3):347-355.
  2. Cayan, D. R., et al. 2001. Changes in the onset of spring in the western United States. Bulletin of the American Meteorological Society, 82(3):399-415.
  3. Personal communication with Dave Bertelsen, August 4, 2008.
  4. Crimmins, T. H., M. A. Crimmins, D. Bertelsen and J. Balmat. 2008. Relationships between alpha diversity of plant species in bloom and climatic variables across an elevation gradient. International Journal of Biometeorology, 52:353-366.
  5. Personal communication with Jake Weltzin, July 21, 2008.
  6. Inouye, D. W., M. A. Morales and G. J. Dodge. 2002. Variation in timing and abundance of flowering by Delphinium barbeyi Huth (Ranunculaceae): The roles of snowpack, frost, and La Niña, in the context of climate change. Oecologia, 130:543–550.