Poring over 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.
Like homicide detectives, climate scientists are continually sifting through evidence looking for what they also call “fingerprints.” Over the years, they’ve teased out the human signal from Earthly noise in annual and decade-spanning temperature records, marine chemistry, rapid Arctic change and more.
What they discovered is an uneven pace of seasonal change in the atmosphere above the Northern and Southern Hemispheres’ temperate zones. While warming is famously 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.
It’s “one of the things that is most noticeable in everyday life.”
The study, published Thursday in the journal Science, also calls attention to a persistent disconnect between findings that attribute warming to humanity, and how the same research has been characterized in testimony before the U.S. Congress.
Santer has previously referenced congressional hearings in peer-reviewed journal articles, dedicating a May 2017 article in Nature Scientific Reports to fact-checking claims made by then-U.S. Environmental Protection Administrator Scott Pruitt in a written supplement to his confirmation hearings.
“To me when incorrect claims are elevated to the level of formal congressional testimony and are part of the Congressional Record, then it is important to address them,” Santer said.
Santer’s newest paper comes during a busy week for climate politics, as several Republicans back a resolution opposing carbon taxes, another Republican congressman preps a long-shot carbon-tax bill, and several research groups led by Columbia University’s Center on Global Energy Policy publish studies of new scenarios analyzing U.S. pricing of carbon dioxide emissions.
Climate models are famously imperfect. The authors indicate where simulated warming has been known to outrun actual temperatures, the focus of much attention in recent years. They walk briefly through several possible explanations, and dismiss concern among some scientific critics that models overestimate how fast the world will warm.
“The claim that overestimation of warming is solely due to a large error in climate model sensitivity has been tested elsewhere and is not credible,” the authors write.
In the end, five of six satellite data sets show that the warming signal has risen above the natural noise, according to the research. Changes miles above the ground are part of the same puzzle visible from your kitchen window.
“There are a lot of observations that the seasonal cycle is changing, and it is also one of the things that is most noticeable in everyday life with trees flowering earlier,” said Friederike Otto, an associate professor at the University of Oxford’s Climate Research Programme.
“But so far this has been tricky to disentangle formally and with high statistical significance from natural variability.”
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.”