Poring through four decades of satellite data, climate scientists have concluded for the first time that humans are pushing seasonal temperatures out of balance – shifting what one researcher called the very “march of the seasons themselves.”

Ever-mindful of calculable uncertainty and climate deniers, the authors give “odds of roughly 5 in 1 million” of these changes occurring naturally, without human influence.

Climate scientists are continually sifting through evidence looking for what they call “fingerprints.” Over the years, they’ve teased out the human signal from Earthly noise in annual and decade-spanning temperature records, marine chemistryand rapid Arctic change.

What they discovered is an uneven pace of seasonal change in the atmosphere above the Northern and Southern Hemispheres’ temperate zones. While warming is global, summers in the troposphere are heating faster than winters, in a way physics would dictate if greenhouse gases were the culprit. The satellite data and computer models for seasonal temperature change used by the study agree with each other even more closely than they do when gauging average annual temperature.

Ben Santer, an atmospheric scientist at Lawrence Livermore National Laboratory and the study’s lead author, likens the temperature results to a wave washing up on a beach. For every year in the 38-year satellite record, the team captured the monthly temperature lows (troughs) and highs (crests). In the early years, the “waves” came in small. By the end of the data set under study, 2016, the waves crashed ashore with higher troughs – and much higher crests.

Santer sees the work as an uncomfortable reminder of the overall climate trend.

“The piling on of evidence is worrying me,” he said. “This is the kind of stuff you don’t want to be right about.”

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