By Janet Raloff, Science News
There’s not much time to reach binding international agreements for limiting greenhouse gas emissions if climate negotiators are going to meet the goal of keeping global average temperatures from climbing no more than 2 degrees Celsius above those typical of pre-industrial times. Continuing with a business-as-usual approach to energy use into the foreseeable future could foster a 4 degree C warming, perhaps as early as the 2060s, a package of new papers concludes.
Such a dramatic temperature increase would be expected to trigger extensive, recurring droughts in some parts of the world, flood coastlines as sea levels rise and drastically alter the types of crops that can survive where lands remain arable, notes Mark New of the University of Oxford in England. New contributed to several of the 11 papers in the Philosophical Transactions of the Royal Society A. The journal issue, officially dated January 13, 2011, has been posted early online to coincide with the November 29 start in Cancun, Mexico of the United Nations Framework Convention on Climate Change.
Climate negotiators tend to focus on political goals, like setting timetables of 2020 or 2050 for when binding emissions limits should go into effect, notes Oxford’s Niel Bowerman. “But Mother Nature doesn’t care about what we emit in any particular year,” the atmospheric physicist says. “What matters is cumulative carbon emissions.”
His Oxford colleagues have calculated that to keep maximum global temperature increases below 2 degrees Celsius, humanity can spew greenhouse gases equivalent to no more than 1 trillion metric tons of carbon (or 3.67 million metric tons of carbon dioxide) by 2200. Already, he adds, regarding this carbon limit: “We’re just over halfway there.”
Also important, he and his colleagues argue in their new paper, is the maximum rate at which society emits that carbon. If it’s too high, warming may occur faster than society can accommodate. That peak emission rate will occur within the next decade or so, he says, so “there’s an urgent need to peak emissions quickly if we want to prevent dangerous rates of warming within our lifetime.”
Climate scientist Richard Betts of the United Kingdom’s Meteorological Office in Exeter led a team that attempted to gauge how soon a global 4 degree C warming might occur. They considered seven different “business-as-usual” scenarios of energy use.
“We don’t really know which [energy use and emissions] trajectory we’re on yet,” Betts says. But by using the upper value for likely emissions into the future—based on what industrial nations are emitting and what rapidly industrializing nations like China and India probably will soon be releasing annually—“you get to this projection showing us reaching 4 degrees in the 2060s,” Betts says.
There’s plenty of uncertainty surrounding such a projection, he acknowledges. “But if our models are a good judge,” he cautions, greenhouse gas emissions will have to peak within about five years or so if humanity hopes to dodge temperature increases exceeding 2 degrees C. When Betts’ group used less extreme versions of the business-as-usual emissions trajectories, the time to excessive warming lengthened.
“But most of these business-as-usual scenarios still gave you a 4 degree warming at some point in the next century,” he reports.
Unfortunately, climate models “don’t have a full grasp of reality,” notes Alice Bows, a climate scientist with the Sustainable Consumption Institute at the University of Manchester in England. The models can’t anticipate how people or cultures might react to climate threats to their economy. “So we started asking: ‘How realistic is it to think global emissions might stop growing in five or 10 years?’”
In their new paper, Bows and Manchester colleague Kevin Anderson decided to separately project likely greenhouse gas emissions scenarios for industrial and newly industrializing countries. As developing nations seek to increase their standards of living and market shares for new products, the researchers say, models should expect these nations’ near-term emissions to climb.
To slow the global growth of emissions, industrial powers may need to put a heavy brake on their own, Bows says. How much? “More than 6 percent per year—beginning immediately,” she says. And that’s to keep warming below 2 degrees Celsius.
She and Anderson also reran their analyses to look at what might occur if industrial countries did limit their reductions to what might be deemed “economically feasible.” This yielded at best a 3 percent drop in annual emissions and eventually, a 4 degree warming of the globe. So in the absence of at least these emissions reductions, she says, “we could be looking at much more than a 4 degree warming.”
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