The finding does not change our understanding of the warming power of carbon dioxide. In fact, it shows that human CO2 emissions have interrupted a long cooling period that would ultimately have delivered the next ice age.

But others have doubts. Mann argues that Esper's tree-ring measurements come from high latitudes and reflect only summer temperatures. "The implications of this study are vastly overstated by the authors," he says.

“It may seem impossible to imagine that a technologically advanced society could choose, in essence, to destroy itself, but that is what we are now in the process of doing.”