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Global warming determined to be "unequivocal"
Published March 20, 2007
A University of Arizona geosciences professor was among the world’s leading scientists to issue a recent climate change report that asserted for the first time that global warming is “very likely” driven by human activity.
Jonathan Overpeck, director of the UA’s Institute for the Study of Planet Earth and a coordinating lead author of the United Nation’s latest Intergovernmental Panel on Climate Change (IPCC) report, said the document represents an international scientific consensus on climate change.
“The most striking thing to me is that we now have presented a much clearer picture of climate change and its causes, both past and future,” Overpeck said. “The word we used for the evidence of climate change and global warming is now ‘unequivocal.’ That is a very strong statement.”
The assessment was released in Paris after 113 governments unanimously agreed to the language in the report.
In the last IPCC report, issued in 2001, scientists concluded that industrial emissions “likely” caused a rise in temperatures over the last century. That warming is manifested in observed increasing air, deep ocean, and sea surface temperatures; melting snow, ice, and permafrost; and rising sea levels, said Overpeck, who also is a Climate Assessment for the Southwest investigator.
“All of these observations and others mentioned in the report are consistent and give us a much firmer basis for asserting that climate change is indeed real and that warming has been significant,” he said. “I think everyone is pretty comfortable now in saying that we see the climate change and that you cannot get the kind of climate change we’re seeing without human-generated greenhouse gases.”
Scientists have observed heat-trapping greenhouse gases in the atmosphere, particularly carbon dioxide and methane, at levels that far surpass those seen in the last 650,000 years, Overpeck said. Unless steps are taken to curb these gases, droughts likely will become more frequent. Hurricanes are projected to intensify, boosting the potential for destruction. Some areas, like the Maldives in the Indian Ocean and Tuvalu, a nation of islands and atolls in the Pacific Ocean, could disappear if sea levels rise just three feet. Much more sea level rise will likely be unstoppable over coming centuries if global warming continues unabated.
In the western and southwestern United States and in northern Mexico, climate models agree that winter precipitation will decrease sharply in this century, Overpeck said. The model projections also align with what has actually been happening in the region over the last several years.
One reason for the drying out is that in the winter, the jet stream and the average position of storms will enter the western United States in a more northerly position, bypassing the Southwest, Overpeck said. On top of that, he said, the West has seen a steady downward trend in late spring snowpack because of warmer temperatures and earlier snow melt. Snowpack acts as the region’s natural water reservoir and is especially crucial in the dry period that follows winter. A decline in snowpack and streamflow would cut into water supply resources. And with warmer-thanaverage temperatures continuing into summer, demand for water would spike further still.
The climate models are less certain when it comes to the future of the monsoon, the region’s primary source of summer precipitation, and the El Niño Southern Oscillation (ENSO), which is linked to variability in winter precipitation, Overpeck said.
While the region is expected to dry out, it paradoxically is likely to see larger, more destructive flooding as hurricanes, also known as tropical cyclones or typhoons, intensify in all of the oceans.
The largest floods in the Southwest tend to occur when a remnant tropical storm in the fall or late summer hits a frontal storm from the north or northwest,providing enough energy to wring out the moisture in the remnant tropical storm, Overpeck explained.
Overall, he said, the Southwest should brace for a number of far-reaching climate changes as the planet warms.
“You take all of these things together and you can clearly see in the report a strong case that the western U.S. and particularly the Southwest—Southern California into Texas—will probably be one of the hardest and soonest hit parts of the country,” he said.
Climate Change Projections http://www.geo.arizona.edu/dgesl/research/regional/projected_US_climate_...
Climate change and Southwest Hydrology http://www.swhydro.arizona.edu/archive/V6_N1/
UA Global Climate Change Lecture Series podcasts http://podcasting.arizona.edu/globalclimatechange.html
Jonathan Overpeck ISPE webpage http://www.environment.arizona.edu/jonathan-overpeck
UA News release on IPCC report http://uanews.org/cgi-bin/WebObjects/UANews.woa/5/wa/MainStoryDetails?Ar...