julio 24, 2010


Analysis by John D. Cox Fri Jul 16, 2010 02:42 PM ET 10 Comments | Leave a Comment

The human impacts on climate come into sharper focus in a new National Research Council report that warns policymakers that carbon dioxide lives so long in the atmosphere "it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe."

The range of impacts -- temperatures, crop yields, precipitation, streamflow, wildfires -- have long been part of the global warming scenario, but the new analysis by leading climate scientists brings new consistency and clarity to model simulations by focusing on changes to expect from each additional degree of future temperature rise.

For each single degree Celsius (1.8 degree Fahrenheit) of additional warming, 5 to 10 percent less rain falls in the U.S. Southwest, the Mediterranean and southern Africa; 5 to 10 percent less streamflow occurs in some river basins, including the Arkansas and Rio Grande; and 5 to 15 percent crop-yield declines occur in corn in the United States and Africa, and wheat in India.

That's according to the panel, which was led by senior scientist Susan Solomon of the National Oceanic and Atmospheric Administration.

(The map, right, depicts the changes in runoff in rivers for each degree Celsius increase in rising temperatures over the globe.)

While rainfall declines in some areas, each degree Celsius of warming brings 3 to 10 percent more rain during heavy rainfall events.

In western North America, the area burned by wildfires doubles or even quadruples for each degree of warming up to 2 degrees C (3.6 F), when "fuel" begins to run out.

Warming of 3 degrees C (5.4 F) puts "many millions more people at risk of coastal flooding, and 4 degrees C (7.2 F) brings far warmer summers over almost all land areas.

Specific impacts of the various climate models wander over a wide range of uncertainty when simulations are based on different levels of carbon dioxide concentration, different time periods, or different emissions scenarios, Solomon noted.

But their results are "considerably more robust" and consistent when climate impacts are viewed "as a function of the warming level."

Beyond the specific impacts, the 242-page report describes carbon dioxide as the dominant and most unique greenhouse gas. Once released, it remains in the atmosphere for centuries, even thousands of years, and commits generations in the distant future to many years of a warming climate.

"Every pound that every one of us emits, every ton that we emit as a group, will be with us for a long time," she said. "Some of what we will be releasing now will be with us for generations."

Panelist David Lobell, a crop-impact specialist at Stanford University, said he was impressed by the degree of "lag time" built into the climate system and how long it takes for the full impact of heightened carbon dioxide concentrations to become measurable changes.

"In a sense, it's like we're paying for things with credit, running up a deficit, but we don't have the full consequences of that for quite some time," he said.

Taking this lag time into account, Solomon said that the changes described in the report are only "about half of the eventual impacts" for a given unit of heightened carbon dioxide concentration.

With CO2 concentrations at current levels of 390 parts per million, the report shows that the climate system already is committed to at least 1 degree C (1.8 F) of warming.

IMAGES: National Research Council

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