The devastating heat wave that hit Asia in 2016 and the unprecedented warmth of ocean waters off of Alaska that year had something in common: neither would have been possible without the excess carbon dioxide that humans have pumped into the atmosphere over the past century, according to new research.
That year was the warmest on record globally, and that extreme also would have been impossible without climate change, the report said.
The findings marked an ominous first for the American Meteorological Society's annual report on the role of climate change in extreme weather events, which was released Wednesday. While five previous editions included research showing that climate change made dozens of heat waves, droughts and storms more likely or more severe, none had determined that the events could not have occurred under "natural" conditions.
"The conversation needs to change," Jeff Rosenfeld, editor-in-chief of the Bulletin of the American Meteorological Society, said at a press conference Wednesday. "These are not just new odds. These are new weather extremes that are made possible by a new climate."
The report included 27 peer-reviewed studies that examined extreme weather events around the globe in 2016. All but six found that climate change had played a role. El Niño, the periodic warming of Pacific waters, also contributed to extreme weather that year.
Overall, the studies found that human-caused warming had increased the risk of heat waves, heavy precipitation, frost, drought, marine heat, wildfires and coral bleaching across five continents, while making cold weather less likely in China. Among the events for which no climate link was found was a winter storm in the eastern U.S., a drought in Brazil and marine heat in the eastern Pacific.
The report, which was compiled and edited by scientists affiliated with the National Oceanic and Atmospheric Administration, several universities and the British government, says we are witnessing what climate models have long predicted: that eventually, greenhouse gas emissions from human activities would warm the planet enough to push extreme weather beyond the bounds of natural variability. The models suggested these events would come first in areas that are warming the fastest, including northern latitudes.
"It is striking how quickly we are now starting to see such results," the editors wrote in the introduction.
One of the studies, by researchers at the University of Alaska, found that water temperatures in the Bering Sea last year were unprecedented, and that while natural variability contributed to the extreme, the heat "cannot be explained without anthropogenic climate warming."
Another, by researchers in Japan, determined that the record-breaking heat wave that hit much of southern Asia last year "would never have happened without the anthropogenic warming."
At the news conference, Andrew King, of the University of Melbourne in Australia and an author of two of the reports, said the studies are conservative with their conclusions. "So for scientists to say that an event would be virtually impossible without climate change," he said, the odds of that conclusion being wrong are "very short."
Finding Human Fingerprints in the Storms
The annual report includes studies that examine what role, if any, human-caused warming played in extreme weather.
The studies use a statistical approach, examining the chances that the event would occur given the current climate and atmospheric conditions. The researchers then use models to remove human-caused changes to see the likelihood under "natural" conditions before comparing the two.
In the case of 2016's record global temperature, "when they tried to simulate this event without human-caused climate change, they couldn't do it," said Stephanie Herring, a climate scientist at NOAA and the lead editor of the report.
This year, several studies examined the impacts of extreme weather on species and ecosystems. One paper found that warmer ocean waters, heated in part by human-caused warming, contributed to coral bleaching and a drop in fish and seabird numbers in parts of the Pacific Ocean.
2017: Hurricane Harvey's Extreme Rainfall
The American Meteorological Society report looked back to 2016 and did not examine the extreme events of this year, such as the record-breaking hurricane season, California's devastating wildfires or the new low for Arctic sea ice.
In a separate study published Wednesday in the journal Environmental Research Letters, scientists affiliated with the group World Weather Attribution examined the record rainfall that Hurricane Harvey brought to Texas in August 2017. One station near Houston recorded more than 51 inches of rain. The scientists determined that human-caused climate change made Harvey's extreme rainfall three times more likely and 15 percent more intense.