Bill McKibben: Jim, more than a dozen nations have set new high-temperature records this year, and we’ve seen the all-time marks set for Asia (Pakistan at 129 degrees Fahrenheit) and Southeast Asia. Given that the global temperature has “only” gone up about a degree, can you explain how this kind of heat is possible?

James Hansen: Sure. What we see happening with new record temperatures, both warm and cold, is in good agreement with what we predicted in the 1980s when I testified to Congress about the expected effect of global warming. I used colored dice then to emphasize that global warming would cause the climate dice to be “loaded.” Record local daily high temperatures now occur more than twice as often as record daily cold temperatures. The predominance of new record highs over record lows will continue to increase over the next few decades, so the perceptive person should recognize that climate is changing.

Yes, global average warming is “only” about a degree, but that is actually a lot. During the last major ice age, when New York, Minneapolis, and Seattle were under an ice sheet a mile thick, global average temperature was about 5 degrees colder than it is now. The last time Earth was 2 degrees warmer so much ice melted that sea level was about twenty-five meters (eighty feet) higher than it is today.

We scientists create a communications problem by speaking about average global warming in degrees Celsius. Global warming in degrees Fahrenheit is almost twice as large (exact factor is 1.8) and warming is about twice as much over land (where people live!) than over ocean. Also certain regions and times experience bigger changes than others. (So far the United States has been lucky, with smaller average warming than most land areas. There is no reason to think that luck will continue.)

But remember that weather variability, which can be 10 to 20 degrees from day to day, will always be greater than average warming. And weather variability will become even greater in the future, as I explain in the book, if we don’t slow down greenhouse gas emissions. If we let warming continue to the point of rapid ice sheet collapse, all hell will break loose. That’s the reason for “Storms” in the book title.