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.
