Atmospheric scientists John Fasullo and Kevin Trenberth studied global humidity patterns to get at an elusive question: When atmospheric carbon dioxide (CO2) levels double—as is expected by late this century—how much warmer will it become?

Estimates of this temperature increase, called equilibrium climate sensitivity, hover around 5 degrees F (2.8 degrees Celsius) by about 2100. But predictions vary more than twofold, from 3 to 8 degrees F (1.7 to 4.4 degrees Celsius).

The difference matters because higher temperatures mean larger problems with sea level rise and extreme weather, as well as large-scale changes in ocean circulation—which could in turn mean big changes on the ground.

With a 3 degree F increase, for example, New York City would