Two degrees Celsius. It has become a familiar number to anyone who follows news about global warming and climate change.
Climate negotiators in Paris and others have repeatedly stated that limiting global temperature rise to two degrees Celsius, or about 3.5 degrees Fahrenheit, is a reasonable target for mitigating the worst effects of climate change. It was even written into the formal COP21 agreement reached over the weekend.
But there’s a problem with this number, according to one prominent scientist: It doesn’t make sense.
“The number was picked out of a hat,” says James Hansen, a former NASA scientist and now a professor at Columbia University “Simply because people thought that's probably the best we can hope for — because we've already got almost one degree of warming and we know there's more in the pipeline. And it’s very difficult to move off of our present largest energy source, which is fossil fuels.”
“We know it’s not a sensible target, because if we look at the Earth's his