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Western Fire on the Rise

Friday, October 29, 2010

Author: Laura Marshall

It’s become a regular thing to turn on the news during the hot months of the year, and see images of wind-whipped flames devouring a California subdivision or a forested mountainside. In the last decade, fires in the West have burned much larger areas than previously. This phenomenon appears to be linked to the record warmth of the past decade, as well as with forest management choices.

Consider the Santa Catalina Mountains outside Tucson, where in 2002 and 2003 the Bullock Fire and then the Aspen Fire burned nearly the entire mountain range, including the Summerhaven community. A few years later the Rodeo-Chedeski Fire in northern Arizona became the largest fire in the state’s history. These happened during some of the hottest years on record, globally.

Fire is a natural occurrence in the forests of the West, and by looking at the rings of fire-scarred trees and logs, dendrochronologists can tell how often fires burned through a forest. These frequencies of past wildfires are known as fire regimes; they vary primarily by type of forest. - Fire regimes can range from a fire on average every five years up to hundreds of years between fires. Tree-ring based studies indicate that the natural, historical fire regime for the Santa Catalinas, largely Ponderosa pine forest, is one where a slow-burning surface fire every ten years or so clears out undergrowth and leaves the large trees alive and well.

Due to fire suppression activities during the 20th century, normal surface fires didn’t happen for multiple decades, leaving many forests with a much higher load of fuel than would have ever existed naturally.  More recently, efforts to prevent huge, hard-to-control fires have led to the use of small “controlled burn” fires as a management tool, but the effects of all-out 20th century fire suppression linger. Dead wood and many small trees that natural fires would have removed, instead built up in the forest over the years. Thus when a fire starts, from lightning or a careless human, there is often abundant fuel to stoke it.

The Sangre de Cristo Mountains in New Mexico, a year after a thinning treatment and a controlled burn. Such forest management practices can reduce the potential for large forest fires in the Southwest. Photo credit: L. Marshall.

But fuel loads alone are not to blame. A 2006 peer-reviewed study by Westerling et al. in the journal Science shows a link between warming temperatures, earlier spring snowmelt, and a clear rise in the frequency of large wildfires. Snow melting sooner means the ground is much drier earlier in the summer. Higher summer temperatures dry out the forest,  enabling it to burn more easily. Both of these climate factors show strong correlation with the increased incidence of large wildfires in recent years. As the world is expected to continue to warm, larger and more extreme wildfires may become the new norm. Large, climate-driven fires look to be a major challenge for forest managers, residents, and outdoors enthusiasts over the coming century.


  1. Westerling, A.L., H.G. Hildago, D.R. Cayan, and T.W. Swetnam. (2006) Warming and Earlier Spring Increase Western US Forest Wildfire Activity.  Science 313, 940-943.

The Southwest Monsoon Under Climate Change: What the Models Tell (and Don’t Tell) Us

Friday, October 15, 2010

 Now that we (hopefully) have a better understanding of how climate models make long-term, generalized projections, can we use them to model our monsoon to figure out how it might be different under global climate change?  The answer is yes and no, and that has to do with scale. To understand that, we first need to consider how the monsoon works.

Every spring and summer, a large subtropical high-pressure system called the Bermuda High (which is, by the way, responsible for gorgeous weather in–you got it–Bermuda!) spreads out across the southern United States. This large high-pressure system starts to circulate mid- to upper-level moisture around it, and into our region.

Figure 1: The upper-level Bermuda High extending to the west in July, when our monsoon begins. The red colors are higher pressures; the center of the Bermuda High is marked by an 'H'. This image is from the National Weather Service. 

At the same time, it gets pretty hot in southern Arizona and New Mexico. This hot air rises, and as it does, it draws in moist air from the Gulf of California and Gulf of Mexico. Once a sufficient amount of moisture is present, thunderstorms can start.

Figure 2: The major features of the North American monsoon. The center of the high pressure system is again shown with an “H”. Moisture from the Gulf of Mexico is largely at upper- and mid-levels of the atmosphere, whereas moisture moving up the Gulf of California is a lower levels of the atmosphere. This image is from the UA College of Agriculture and Life Sciences: Climate Science Application Program.

These are all relatively large-scale events, but anyone who has lived here long enough knows that individual monsoon thunderstorms usually pop up over the mountains during the day and grow from there – a much smaller-scale process. This is because mountains heat up faster relative to the surrounding air, and in so doing they draw air from lower elevations up their sides. As the air rises, it cools and forms convective clouds; these may grow into thunderstorms if the conditions are just right. Representing this process accurately in an atmospheric model requires a relatively fine spatial scale, on the order of several kilometers. (Real-time monsoon forecasts at the University of Arizona are produced with a spatial scale of 1.8 km!) Such a fine scale is sufficient to resolve individual thunderstorms within a weather model simulation—and has resulted in much improved monsoon forecasts for southern Arizona. But a single thunderstorm is too small for a global climate model to pick up, as global models typically cover the entire state of Arizona with just four grid points – four points to represent all of Arizona’s interesting weather!

Figure 3: Monsoon precipitation as predicted by the models run for the IPCC 4th Assessment Report. The black line is observations, and the colored lines are the various model runs. This figure shows precipitation amount by month (1 is January, 2 is February, etc). Note that the precipitation amount and timing is not well captured by the models. This figure is from Lin, et al. 2008, J Climate.

To improve monsoon projections for the future, one way is to "downscale" (back to the issue of scale!), or create finer spatial information from a global climate model.  When downscaling is performed using a regional atmospheric model, the monsoon can be reasonably represented. Such work is currently underway at the University of Arizona and elsewhere and the preliminary results have been quite promising.

So what's the bottom line?

Ok, what will happen with global warming?  If it gets hotter, will the whole monsoon system become stronger? Will it get drier because of other atmospheric changes? Or will something else happen? 

Well, the 2009 monsoon was a bust. We were all very excited about it, and it started strong, but by mid-July, an El Niño event in the Pacific changed the dynamics of the atmosphere enough that the large-scale features responsible and necessary for our monsoon didn't quite work out. If El Niño strengthens or becomes more frequent in the future, as some models predict, we could begin to see our monsoon weaken significantly. On the other hand, as the jet stream shifts poleward (as recent research suggests), we could see the monsoon subtropical high pressure system develop sooner in the year, making the monsoon longer and/or stronger. Or there might be another aspect of our climate system that is changing that will affect the monsoon in yet another way.

This is pretty indecisive, isn’t it? Well, that's why this is an area of active research. Climate modelers will keep working on this, atmospheric scientists will work on downscaling the models to be even more accurate, paleoclimatologists will keep reconstructing past climate of the monsoon to see how it varied when conditions were warmer, and soon we'll have a better idea of what to expect. In the mean time, stay tuned. At the very least, the weather will continue to be...interesting...

Sarah Truebe is a PhD candidate in the Geosciences Department at the University of Arizona studying paleoclimatology and the North American monsoon. She would like to thank Jonathan Overpeck and Christopher Castro for helpful comments and suggestions on this posting. 

Climate Models Versus Weather Models: Different Approaches for Different Needs

Thursday, October 7, 2010

If you live in the Southwest, no doubt you know a little bit about our weather. It's interesting here. You likely know we have a monsoon during the summer, with awesome thunderstorms and flash floods, and our wettest winters and biggest floods happen when there's an El Niño event. We've also been hearing a lot about global climate change - and there's already a lot of evidence that it's happening here now. One big question is: how will our monsoon change in the future?

We have a few ways to look at this issue. We could look to the past hundred years or so, when we have temperature and rain gauge data, and try to make projections. We could reconstruct past climate during warm times to understand what might happen when temperatures are warmer in the future; we can do this using tree rings, cave formations, or lake sediments, for example. Or, we can use computer-based models to project what future climate might be like.

Now, I know that a lot of people don't trust models. Have you heard "scientists can't predict the weather in a few days; why should I trust them about fifty years out?" However...climate models are different than weather models. Each has uncertainty associated with it, but the uncertainties are different. 

Weather models are used to forecast day-to-day changes in weather, or rather to predict what will happen at a specific place and point in time in the near future, typically no more than five to seven days out.  Model-based weather forecasts generally less reliable beyond a week, because the atmosphere is an inherently chaotic system. Small changes in observed conditions, which are fed to the model regularly, can produce completely different weather predictions a week into the future because the atmosphere is so dynamic.

In contrast, climate models aren't trying to predict what is going to happen at a specific place and point in time. So they can’t produce a forecast for, say, March 15, 2077, or even tomorrow! Instead, climate models are used to determine how the average conditions will change - as in, will it be on average warmer or cooler, wetter or drier, in Tucson over the next 50 years? And this is information we need if we're going to plan for things like water shortages, or more frequent fires, or any of the other impacts on the Southwest that might result from the local effects of global climate change. To manage reservoir operations in Lake Mead, for example, the U.S. Bureau of Reclamation doesn't care about the exact conditions on March 15, 2077. They care whether over a 5 to 10 year period there will be enough water to meet the demands of Colorado River water users.

Using a suite - or "ensemble" of climate models can help sharpen the image somewhat. One model might project a 2.8°F change in 50 years (note: this is arbitrary). Another might say 3.1°F. By using 2, 4, 6, 10, or 15 of these models – all of which use slightly different approaches to represent atmospheric processes, we can begin to sharpen the focus of that predicted temperature change, precipitation change, or whatever variable is being looked at. Alternatively, an ensemble might consist of the same model run multiple times from a different starting point (say, using 1990 conditions, or 2010 conditions) – this will yield slightly different but realistic simulations of the future. For example, forecasts made by NOAA that indicate wetter or drier precipitation amounts, or hotter or cooler than normal temperatures for the next season, are largely based on an ensemble of future possibilities from a single model.

Ok, so that’s how climate models work. In my next post I’ll discuss how we can use a global climate model to project the North American monsoon into the future. Check back in a few days!