“Climate change is adding to what’s going on naturally, and it’s that extra stress that causes things to break,” said Trenberth, a scientist at the National Center for Atmospheric Research in Boulder, Colo. “It takes the experience well outside anything that’s been experienced before. It crosses thresholds. As a result, things break, people die, and things burn.”

There is no single metric for measuring extreme weather globally and comparing 2018 with previous years. The American Meteorological Society puts out an annual report on extreme weather, but it just published the 2017 results and won’t issue its report on 2018 until late next year.

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But it was definitely a hot and perilous year. Perhaps most striking were the temperature extremes. It was not the hottest year on record in terms of overall global temperature — the three previous years were slightly warmer — but many places around the planet set high-temperature records.

Africa may have endured the hottest temperature ever reliably measured since record-keeping began: 124.3 degrees Fahrenheit in the Sahara Desert city of Ouargla, Algeria, on July 5.

That same day, temperatures may have reached 90 degrees F. on the coast of the Arctic Ocean in northern Siberia. And in the Middle East, the low temperature of the day in Quriyat, Oman, on June 28 was 109 degrees F.

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Among the many striking events of 2018 was the late intensification of Hurricane Michael in early October in the Gulf of Mexico. Normally, when a hurricane moves north in the gulf, it weakens somewhat. But Michael exploded from a tropical depression into a major hurricane with startling speed and then strengthened all the way to landfall, when it was nearly a Category 5 storm on the Saffir-Simpson hurricane wind-speed scale. After shredding Panama City and Tyndall Air Force Base and destroying the town of Mexico Beach, Fla., with a 16-foot storm surge, the storm remained a full-blown hurricane deep into Georgia.

Just weeks earlier, Hurricane Florence triggered record flooding on numerous rivers in the Carolinas after coming ashore as a broad, soggy Category 1 storm and slowing to a crawl. That storm track invited comparisons with Hurricane Harvey in Texas in 2017. Harvey stalled after landfall and dropped as much as 60 inches of rain in Southeast Texas, triggering catastrophic flooding that killed scores of people. Florence did not quite match Harvey in terms of rain or fatalities, but it still set many records for high water on Carolina rivers.

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Natural disasters cost the world $155 billion this year, and several of them struck the United States particularly hard. Michael and Florence, the California wildfires and a volcanic eruption in Hawaii are all on that list, according to the Zurich-based reinsurance company Swiss Re. But it doesn’t match what happened in 2017. That was the costliest weather year in U.S. history, with more than $300 billion in damage, Woods Hole Research Center senior scientist Jennifer Francis said in an essay published by the National Oceanic and Atmospheric Administration.

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“People are being affected directly by these events, and increasingly they're asking, ‘What's up with this? Is climate change playing a role?’ Scientists can now answer a confident ‘yes’ to that question, though the exact degree of influence is difficult to pin down,” she wrote.

The slow-moving or stalled hurricanes may be carrying a climate signal, in the same way that unusually long droughts — and the associated wildfires — may be telling us something about the changing planet. One striking feature in recent years has been the behavior of the jet stream in the Northern Hemisphere.

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The jet stream is caused by the sharp temperature gradient between the Arctic and the tropics. But as the Arctic has warmed — at a rate far faster than the rest of the planet — there is less of a gradient, and the jet stream has slowed down and meandered, Francis noted. This wavy pattern in the jet stream has the direct effect of locking down weather patterns for weeks or even months, extending droughts or periods of unusually heavy rain — something the nation’s capital can understand, having endured the wettest year in the city’s history.

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“The jet stream has always had periods where it got really wavy and caused temperature extremes. What we’re really talking about is that the proportion of time it gets into those high-amplitude waves appears to be increasing,” said Matthew Kelsch, a hydrometeorologist at the University Corporation for Atmospheric Research.

Kelsch has kept a chronological list of extreme weather events in 2018, beginning with the dizzying temperature swings in the northeastern United States — from minus-20 degrees Fahrenheit in Burlington, Vt., on Jan. 7 to a record-high temperature of 61 degrees just four days later. Then, on Jan. 13 and 14, Alaska had record warmth. It hit 63 degrees in Sitka.

Kelsch said the world is experiencing many more hydrological extremes — record rainfall, record drought. That makes perfect sense given the warmer atmosphere, which can hold more moisture. That doesn’t necessarily translate to more reliable rainfall, he noted: “Even though there’s a lot of moisture in the atmosphere, it may be holding it more rather than turning it into clouds and rain.”

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Trenberth believes that major reports from the U.N. Intergovernmental Panel on Climate Change and the U.S. National Climate Assessment have been too conservative in estimating the costs of changing climate.

“Climate change is here, and it’s already costing tens of billions of dollars a year. I think the climate costs in the future are greatly underestimated,” Trenberth said.