A Few More Hot Days Does Not Equal a Lot More Heat Waves

Whatever the combination of human and natural causes, the world has warmed by about 0.85 degrees Celsius (1.5 degrees Fahrenheit) over the last century or so.

During this time, there have been no increases in tornadoes, hurricanes, floods or droughts associated with this mild increase in world temperature. But much has been made about a dramatic increase in extreme heat and heat waves.

We should expect overall warming to contribute to some increase in the hottest temperatures, but the reporting often implies a dramatic increase in extreme heat. For instance a much-cited study asserts that 75 percent of heat extremes (defined to mean high temperatures that happen on average once every 1,000 days) are caused by the 0.85 degrees of global warming. Does that mean there would be 75 percent fewer extremely hot days if we hadn’t had the 0.85 degrees of global warming? No.

There are many more days when temperatures rise to within one degree of a day defined as “extremely hot” than there are days that actually meet the definition. So, adding a degree across the board will increase the number of days that can be considered “extremely hot” by hundreds of percent. This doesn’t mean we have more heat waves; it just means every day is a little hotter.

We can use a set of temperatures from the National Climatic Data Center to illustrate the misleading assertion of dramatic increases in extreme heat. The daily high temperature at Reagan National Airport for the roughly 5,600 days between Jan. 1, 2000, and April 30, 2015, had three days with temperatures exceeding 39 degrees C (a little over 102 degrees F). Using 39C as our threshold for extreme heat and adding 0.85 degrees to all the daily high temperatures (matching the amount of warming since the pre-industrial age) we would find 10 additional days that exceed the threshold for extreme heat.

So the number of days that can be considered extremely hot increases by more than 300 percent. But does that mean extreme heat waves tripled or more? No. All that has happened is each day is 0.85 degrees C warmer, and the 10 days in our record with temperatures between 38.15 degrees and 39 degrees are now above the threshold.

The same game could be played at the cold end of the spectrum. A temperature increase of 0.85 degrees C means 75 percent fewer “killer” cold days. Who wouldn’t favor that?