evidence: Our atmosphere and oceans can absorb only so much heat before climate change, intensified by various feedback loops, spins completely out of control. Some scientists and policymakers fear that we’re in danger of passing this point of no return if the global mean temperature rises by more than two degrees Celsius (maybe more, but also maybe less). The I.P.C.C.—the Intergovernmental Panel on Climate Change—tells us that, to limit the rise to less than two degrees, we not only need to reverse the trend of the past three decades. We need to approach zero net emissions, globally, in the next three decades. This is, to say the least, a tall order. It also assumes that you trust the I.P.C.C.’s calculations. New research, described last month in Scientific American, demonstrates that climate scientists, far from exaggerating the threat of climate change, have underestimated its pace and severity. To project the rise in the global mean temperature, scientists rely on complicated atmospheric modelling. They take a host of variables and run them through supercomputers to generate, say, ten thousand different simulations for the coming century, in order to make a "best" prediction of the rise in temperature. When a scientist predicts a rise of two degrees Celsius, she’s merely naming a number about which she’s very confident: the rise will be at least two degrees. The rise might, in fact, be far higher.