Major Report: Some Extreme Weather Can Only Be Blamed on Humans

Wildfires are still raging across southern California, marking the end of a destructive year of extreme weather events around the world. In the U.S. alone historic floods hit Missouri and Arkansas in May, drought parched the Dakotas and Montana from spring through fall and autumn hurricanes ravaged the U.S. Gulf Coast, Florida and the Caribbean.

Scientists have long predicted such extreme events (pdf) would become more frequent or intense, and sometimes both, due to human-influenced climate change. And as extreme as this year seems, it turns out last year’s events were already a landmark of sorts. This week the Bulletin of the American Meteorological Society published an assessment of the connection between climate change and extreme events in 2016, the society’s sixth annual report on the topic. The report selects a handful of extreme events from the previous year and disentangles anthropogenic climate change’s effects from natural variability (meaning what we would expect to happen without human influence). For the first time in the report’s history, scientists said that they have found that several of the events could not have occurred if the planet was not heating up.

“Climate change was a necessary condition for some of these events in 2016, in order for them to happen,” Bulletin Editor in Chief Jeff Rosenfeld said in a presentation at the American Geophysical Union (AGU) conference in New Orleans this week. “These are new weather extremes made possible by a new climate. They were impossible in the old climate.”

According to the new report, three human-caused extreme events in 2016 were: the overall global temperature increase; record heat in Asia, with crises like the dangerous heat wave that hit Thailand in April 2016; and finally, marine hot spots in the Gulf of Alaska, Bering Sea and off the coast of northern Australia. Areas of the Bering Sea became part of a mysterious mass of warm ocean water dubbed “the Blob”—a phenomenon that cannot “be explained without anthropogenic climate warming,” according to a press release from the Bulletin.

Of course, many other extreme weather events struck the world in 2016, and scientists found climate change played a role in most of them—even though they did not find it was an absolutely necessary condition for them to occur. The report lists examples such as unusually warm temperatures in the Arctic in November and December 2016 and abnormally dry air on the U.S. west coast, which helped drive wildfires that year. It further notes southern Africa’s flash droughts and record-breaking rainfall in places such as Wuhan, China. The report adds that climate change also intensified heat waves around the planet and strengthened El Niño, with the latter resulting in problems such as food shortages in southern Africa.

The report also identified three events apparently not connected to global warming, including “Winter Storm Jonas,” a massive blizzard that hit the mid-Atlantic states in 2016. But researchers say this does not necessarily rule out the possible influence of human-caused climate change in such events; some scientists say they simply may not have the tools to detect a human factor.

It will be awhile before experts can make such definitive statements about 2017. “To do analyses of [these] connections requires a lot of detailed analysis, trying to separate the natural factors [from human influence],” says Donald Wuebbles, a professor of atmospheric science at the University of Illinois at Urbana–Champaign. Scientists have already identified some climate change trends sweeping across the U.S. over the past decades, however. They have found heat waves are becoming more frequent and intense whereas the number of cold waves has dropped. Extreme rainfall events are rising in frequency and strength; the risk of floods is climbing in the U.S. Northeast and Midwest. More large wildfires are hitting places such as western Alaska and Atlantic hurricanes are intensifying. These findings came from this year’s Climate Science Special Report, written by the U.S. Global Change Research Program, and Wuebbles presented them this week at the AGU conference.

Scientists do have some results for at least one big event this year, though: Hurricane Harvey, which swamped the Houston area in August. Two new studies found climate change likely strengthened Harvey’s rainfall. “Warm water in the oceans helps drive a large hurricane like that. These storms are picking up huge amounts of water as they would not have done 40 years ago,” Wuebbles says. “There are studies really showing the human connection to this big storm that was devastating.”

All this means anthropogenic climate change’s effects on extreme weather are becoming increasingly obvious to scientists—even for some individual events. “Climate change is influencing weather in almost all aspects,” Wuebbles says. He cites the National Oceanic and Atmospheric Administration records (pdf) of U.S. climate disasters that do $1 billion or more [adjusted] in damages and costs. “In the 1980s we were getting two extreme events [that cost $1 billion], now we’re getting 10,” he notes. “This year there will probably be 17 or 18. We’re seeing a lot more of these billion-dollar events, and finding some ties to climate change in almost all of them.” The billion-dollar disaster list already cites 15 extreme events for 2017—and the year has yet to close.

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