Spatial patterns of temperature differences from normal for April 1998. (University of Indiana)

As the Earth has warmed over the last three decades, it’s the warmest temperatures that have warmed the most, a new study finds.

This warming of the hottest temperatures since 1984 (examine the red bars in the first image below) reverses the long-term trend  which shows cold temperature extremes warming at the fastest clip (examine the blue bars in the second image below).

To put it another way, the long-term trend shows global warming concentrated on easing the severity of the most extreme winter cold, while the short term trend shows warming focused on intensifying some of the most oppressive heat.

Linear trends of the 5th to the 95th percentile of each month’s temperature anomalies for 1984-2013. (Robeson/AGU)
Linear trends of the 5th to the 95th percentile of each month’s temperature anomalies for 1984-2013 for the Northern Hemisphere. (Robeson-AGU, adapted by CWG)
Linear trends of the 5th to the 95th percentile of each month’s temperature anomalies for 1881-2013. (Robeson/AGU)
Linear trends of the 5th to the 95th percentile of each month’s temperature anomalies for 1881-2013 in the Northern Hemisphere. (Robeson-AGU, adapted by AGU)

To discover these trends in temperature extremes, Scott Robeson, a climate researcher at the University of Indiana, and two colleagues, analyzed surface temperature data from 1881 to 2013 from the United Kingdom Hadley Centre.

Specifically, they focused their analysis on the cold and warm “tails” of the temperature distribution, or the 5th and 95th percentile.

Overall, the researchers found temperatures at the tails rising more than the average temperatures.

“Average temperatures don’t tell us everything we need to know about climate change,” said Robeson, lead author of the study published by the American Geophysical Union. “Arguably, these cold extremes and warm extremes are the most important factors for human society.”

An increase in high temperature extremes is significant since hot weather worsens air quality, increases heat-related illness and death, and can exacerbate drought, for example. Alternatively, a warming of the coldest temperature can be beneficial, by reducing cold-related health problems and easing heating costs.

Over the last 16 years, during which the rate of climate warming slowed – the so-called “global warming hiatus” –  both cold and warm extremes continued warming. But there’s one surprising exception: during winter in the Northern Hemisphere, both warm and cold temperature extremes actually cooled.

“There really hasn’t been a pause in global warming,” Robeson said. “There’s been a pause in Northern Hemisphere winter warming.”

In other words, an unexpected cooling of Northern Hemisphere winters has put the brakes on global warming.

Robeson and his co-authors suggest the winter cooling over the Northern Hemisphere is consistent with the hypothesis that the warming Arctic is causing the jet stream to become more extreme, transplanting unusually cold weather into the mid-latitudes.

You can read more about this hypothesis here: There’s growing evidence that global warming is driving crazy winters

This hypothesis is controversial, but the Robeson et al. study provides a bit of supporting evidence – albeit over a short time period.