The report, examining research on two dozen weather events, was compiled and edited by scientists from the National Oceanic and Atmospheric Administration, as well as the Met Office Hadley Centre for Climate Prediction and Research.
Climate scientists have long predicted that the progression of climate change would bring about changes in both the severity and frequency of extreme weather events around the world. In recent years, a new area of climate science has begun to emerge, one that seeks to figure out the likelihood that any given event was influenced by climate change.
“We do this for the same reasons we’ve always tried to understand our weather,” said Stephanie Herring, a NOAA climate scientist and lead editor of the new report, at a Thursday press conference. “We know if we can better understand why these events are happening, we can better predict and prepare for them in the future.”
The new report includes one study focusing on the 2015 Alaskan wildfire season — which burned the second-largest number of acres since records started being kept in 1940 — finds the severity was likely influenced by climate change. The study concludes that “2015’s fuel conditions reached a level that is 34 percent to 60 percent more likely to occur in today’s [human-changed] climate than in the past.”
Another study investigated the intense cyclones that occurred in the western North Pacific in 2015, and found that human-caused, or anthropogenic, climate change substantially increased the odds of such a phenomenon. And another found that the extreme heat wave that central Europe experienced in 2015 was also likely influenced by global warming — it concluded that anthropogenic climate change could account for two thirds of the warming.
On the other hand, scientists did not detect a climate change influence for some events — for instance, the extreme cold experienced by the eastern U.S. in February of 2015. Two separate studies suggested that climate change did not play a major role in this event, although factors related to climate change — such as reductions in Arctic sea ice — may have played a small role in influencing the behavior of the jet stream and thus “could contribute in the near future to enlarge the probability of such extreme cold spells in the region.”
It’s important to note that the results of these studies are “necessarily probabilistic and not deterministic,” the editors of the report have noted. In other words, no study can claim that any given event was definitively caused by climate change — rather, the studies generally calculate a probability that climate change played a role. Experts have consistently pointed out that individual weather events are typically the product of multiple complex factors.
The year 2015 was a particularly complex year because of the added influence of a severe El Niño event occurring at the time. El Niño events can change climate and weather patterns around the world in ways that may resemble the influence of climate change — for instance, warmer and wetter conditions in some places or increased likelihood of drought in others. Parsing out the influence of El Niño versus human-caused climate change was one of the major challenges of analyzing 2015’s weather events.
“This climate ‘double jeopardy’ presented a worthy scientific challenge,” said Jeff Rosenfeld, editor-in-chief of the Bulletin of the American Meteorological Society, at the press conference. “Just as importantly, it served as an opportunity to showcase the growing value of climate attribution studies.”
In recent years, researchers have become increasingly confident about their ability to separate the effects of natural influences, like El Niño, from human ones. And their studies are helping both scientists and policymakers better predict the likelihood that such extreme events will occur in different regions in the future — and how severe they might be.
Scientists “are now able to ask questions that the public is asking, like, ‘Is this the new normal?,’ ” Rosenfeld said.