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The average temperature of the planet for the next several thousand years will be determined this century—by those of us living today, according to a new National Research Council report which lays out the impact of every degree of warming on outcomes ranging from sea-level rise to reduced crop yields.
"Because carbon dioxide is so long-lived in the atmosphere, it could effectively lock Earth and future generations into warming not just for decades and centuries, but literally for thousands of years," atmospheric scientist Susan Solomon of the National Oceanic and Atmospheric Administration, who chaired the report, said at a July 16 press briefing held to release it. She compared CO2 to cheesecake: "If I knew that every pound of cheesecake that I ate would give me a pound that could never be lost, I think I would eat a lot less cheesecake."
According to the report, for every degree Celsius of warming, impacts include:
- A 5 to 15 percent lower yield for some crops, including corn in Africa and the U.S., and wheat in India
- A 3 to 10 percent increase in heavy rainfall globally
- A 5 to 10 percent drop in rainfall in southwestern North America, southern Africa and the Mediterranean, among other precipitation changes
- A 5 to 10 percent change (increases in some regions, decreases in others) in stream flow in many river basins globally
- A 15 to 25 percent decrease in the extent of Arctic Ocean sea ice
The report's authors were charged with evaluating a range of "greenhouse gas–stabilization targets and describe the types and scale of impacts likely associated" without any judgment on whether such targets are "technically feasible" or which is "most appropriate." In essence, the scientists evaluated the impacts associated with a given final level of carbon dioxide in the atmosphere, but did so through the lens of temperature change.
This represents a shift in the usual analysis of climate change, particularly in international negotiations, which tend to focus on how much concentrations of greenhouse gases in the atmosphere will rise by a particular date. "Many impacts respond directly to changes in global temperature, regardless of the sensitivity of the planet to human emissions of CO2 and other greenhouse gases," says geoscientist Katharine Hayhoe of Texas Tech University in Lubbock, a co-author of the report, excluding effects such as ocean acidification and CO2 as a fertilizer for plants. "Those impacts don't 'care' about what the CO2 concentration is."
It also eliminates much of the uncertainty surrounding potentially ill effects; whereas various mathematical models may disagree about when and at what concentrations Arctic Ocean sea ice disappears, they all agree that at roughly 3 degrees C of warming, the far north will be ice-free. "It's amazing how consistent they become," Solomon says. "At what point do you get to three to four degrees of warming, which is roughly the time when Arctic sea ice is mostly gone."
Adds economist Gary Yohe of Wesleyan University, another co-author: "We will commit to an ice-free Arctic sometime this century. We won't know definitively until 2090, but essentially there's nothing we can do about it at that point in time and it changes the climate system dramatically."
Already, the planet's average temperature has warmed by 0.7 degree C, which is "very likely" (greater than 90 percent certain) to be a result of the rising concentrations of greenhouse gases in the atmosphere, according to the U.N. Intergovernmental Panel on Climate Change. That's about half what can ultimately be expected from the roughly 390 parts per million of CO2 already in the atmosphere—the highest level the planet has experienced in at least 800,000 years.
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Source: Scientific American
