Ever since the 2009 climate talks in Copenhagen, world leaders have agreed on 2 degrees Celsius (3.6 degrees F) as the maximum acceptable global warming above preindustrial levels to avert the worst impacts of climate change (today we're at about 0.8 degrees C). But a new study, led by climatologist James Hansen of Columbia University, argues that pollution plans aimed at that target would still result in "disastrous consequences," from rampant sea level rise to widespread extinction.
A major goal of climate scientists since Copenhagen has been to convert the 2 degree limit into something useful for policymakers, namely, a specific total amount of carbon we can "afford" to dump into the atmosphere, mostly from burning fossil fuels in power plants (this is known as a carbon budget). This fall, the UN's Intergovernmental Panel on Climate Change pegged the number at 1 trillion metric tons of carbon, or about twice what we've emitted since the late 19th Century; if greenhouse gas emissions continue as they have for the last few decades, we're on track to burn through the remaining budget by the mid-2040s, meaning immediately thereafter we'd have to cease emissions forever to meet the warming target.
The study, which was co-authored by Columbia economist Jeffrey Sachs and published today in the journal PLOS ONE, uses updated climate models to argue that the IPCC's carbon budget would in fact produce warming up to twice the international limit, and that even the 2-degree limit would likely yield catastrophic impacts well into the next century. In other words, the study says, two of the IPCC's fundamental figures are wrong.
"We should not use [2 degrees] as a target," Hansen said in a meeting with reporters on the Columbia campus in Manhattan. "It doesn't have any scientific basis."
A better target to avoid devastating climate impacts, Hansen said, would be 1 degree Celsius of warming (only slightly above what we've already experienced), although he readily admitted that such a goal is essentially unattainable. According to IPCC estimates, human activities have already committed us to that level of warming even if we suddenly stopped burning all fossil fuels today. A grim, but perhaps more realistic, vision of what the end of this century will hold comes from the the International Energy Agency, which predicts that temperatures could rise as much as 6 degrees Celsius by 2100 if greenhouse gas emissions continue unabated.