Record Highs Far Outpace Record Lows, Thanks to Emissions

(CN) – Scorching, record-setting heat will be 15 times more common than record-low temperatures if the current pace of greenhouse gas emissions continues. And if emissions increase, that ratio could be much higher, according to findings published Monday in the journal Proceedings of the National Academy of Sciences.

The 15-to-1 ratio of record highs to record lows is based on average temperatures across the United States increasing by a bit more than 5.4 degrees Fahrenheit above recent years – the projected amount of warming expected to occur due to the current pace of emissions.

“An increase in average temperatures of a few degrees may not seem like much, 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,” said lead author Gerald Meehl.

elephantMeehl, a scientist at the National Center for Atmospheric Research, led a 2009 study that found the ratio of record daily high temperatures to record low temperatures has increased since the 1970s, as average temperatures in the United States rose overall. Computer models at the time predicted that the ratio would continue to increase during this century, however, the team noticed the models were overstating the ratio in recent years compared to observations.

After analyzing why the models didn’t match observations and studying the issue further, Meehl and his team were able to produce a more accurate projection of future record-setting daily highs across the United States. Their projections were based on average temperature increases across the lower 48 states, rather than a specific scenario of future emissions.

The team’s new models predict that U.S. temperatures will increase on average just over 5.4 degrees Fahrenheit by 2065, assuming the current trend of increasing greenhouse gas emissions continues. Such a rate of warming may lead to a 15-to-1 ratio of record highs to record lows, although the team found the ratio could ultimately be anywhere from 7 to 1 up to 22 to 1.

If temperatures rise more than 7.2 degrees Fahrenheit, the ratio could be about 38 to 1, which could happen if sufficient efforts to limit the production of greenhouse gases aren’t made, according to the study.

“Every degree of warming makes a substantial amount of difference, with the ratio of record highs to record lows becoming much greater,” Meehl said. “Even with much warmer temperatures on average, we will still have winter and we will still get record cold temperatures, but the numbers of those will be really small compared to record high maximums.”

While there is significant year-to-year variation, the ratio of record highs to record lows averaged about 2 to 1 during the first decade of this century.

“These changes pose adaptation challenges to both human and natural systems,” said study co-author Claudia Tebaldi. “Only a substantial mitigation of greenhouse gas emissions may stop this increase, or at least slow down its pace.”

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