Before: cold kills 20 people, heat kills 1 person. Cold kills 20x more, for a total of 21 deaths.

After: cold kills 15 people, heat kills 15 people. Cold kills the same number of people as heat, for a total of 30 deaths.

"However, if—as the data seem to show—extreme cold is relatively unimportant, then a few degrees of warming will not yield a large reduction in cold-related mortality. Moreover, if extreme heat is important, then the same few additional degrees might cause a substantial increase in heat-related mortality."

The study warns that global warming won't appreciably reduce deaths due to cold, but might substantially increase deaths due to heat. Again, the current 20-fold factor between deaths due to cold and heat says nothing about future mortality.