Heat records could outnumber cold records by a factor of 15 (plus or minus 8) later this century, according to the study published in the Proceedings of the National Academy of Sciences.
“More and more frequently, climate change will affect Americans with record-setting heat,” said Gerald Meehl, of the National Center for Atmospheric Research and study lead author. Record-setting cold won’t altogether disappear, but will become rare, he said.
The growing disparity between record warmth and record cold will be pushed along by a steady rise in the average temperature over the Lower 48. The temperature in the Lower 48 is projected to warm about 5.4 degrees (3 Celsius) by 2065, assuming emissions of heat-trapping gases from human activity continue at the current pace.
“An increase in average temperatures of a few degrees may not seem like much,” Meehl said, “but it correlates with a noticeable increase in days that are hotter than any in the record, and nights that will remain warmer than we’ve ever experienced in the past.”
The amount of warming will determine just how disproportionate the record heat is. If the climate warms 7.2 degrees (4 Celsius), 38 times as many record highs as record lows would be expected, Meehl said. But if warming is limited to 3.6 degrees (2 Celsius), record highs would outpace lows by a much lower factor of about 5.5 (range of 3 to 8).
“Every degree of warming makes a substantial amount of difference,” Meehl said.
Meehl’s latest work, with co-authors Claudia Tebaldi and Dennis Adams-Smith, builds on a study he published (with colleagues) in 2009. It found that, since 2000, about twice as many record highs as record lows had occurred in the continental U.S.
If the climate was not changing, roughly an equal number of record highs and lows would have been expected.
The 2009 study also projected the number of record highs and lows into the future. But it found the underlying model was predicting too many record highs compared to reality. So the new study investigated the reasons the earlier model erred to make a more reliable forecast, more consistent with observations.
Since Meehl’s 2009 study, record highs have continued to greatly outnumber record lows but with considerable year-to-year fluctuation. For example, the ratio was about 5:1 in 2012, the warmest year on record in the U.S., but about 1:1 in 2013 and 2014, when the polar vortex was disrupted and frigid air gripped large portions of the nation. This year, the nation has witnessed five to six times as many record highs as record lows.
A disproportionate number of record highs has also been seen in individual cities. In Washington, for example, record highs have outpaced lows by an 8 to 1 ratio since 2000, and more than 16 to 1 since 2010.
(The increase in record highs in Washington may, in part, be due to urbanization around the observing site. Meehl said in an interview that, for his study, he relied on data sets for which methods were developed to correct for urbanization.)
In other parts of the world, the ratio of record highs to record lows has also become increasingly skewed. In Australia, warm records outnumbered cold records by more than 12 to 1 from 2000 to 2014, according to a study published last year.
“These changes pose adaptation challenges to both human and natural systems,” said Tebaldi, also at the National Center for Atmospheric Research. “Only a substantial mitigation of greenhouse gas emissions may stop this increase, or at least slow down its pace.”