At the end of every year, U.S. meteorologists look back at what the nation's weather was like, and what they saw in 2015 was weird. The year was hot and beset with all manner of extreme weather events that did a lot of expensive damage.

December, in fact, was a fitting end.

"This is the first time in our 121-year period of record that a month has been both the wettest and the warmest month on record," says Jake Crouch, a meteorologist at the National Oceanic and Atmospheric Administration. The rest of the year was very wet and hot too, he says — the second-hottest period on record for the U.S.

The cause: a warming climate and a superstrong El Nino. El Nino is a weather phenomenon out of the Pacific Ocean that hits every few years and affects weather globally. It starts with a large body of unusually warm weather in the western Pacific that sloshes eastward; it changes wind and weather patterns as far away as the Atlantic and Indian oceans.

Together, climate and a very strong El Nino pushed the average temperature in the U.S. up over its 20th century average by 2.4 degrees Fahrenheit.

And even when the atmosphere is only that much warmer, it holds more moisture, leading to record snows in the Northeast last February and March, and record rain in the South and Midwest. NOAA'S Deke Arndt says he and other climate scientists expect more of the same.

"The fact is, we live in a warming world," Arndt says, "and a warming world is bringing more big heat events and more big rain events to the United States."

All this weather led to 10 extreme events that each did at least $1 billion in damage. These events included drought, flooding, severe rainstorms, big wildfires and winter storms. That's a wider variety of different types of $1 billion-plus weather events than usual.

Insurance companies are paying for most of the damage. Surprisingly, 2015 payouts were lower than the previous few years, though still high historically. That's mostly due to luck, says Mark Bove, a meteorologist at Munich Reinsurance America, a firm that insures insurance companies for their losses. No serious hurricanes hit the U.S. in 2015, he explains.

But that luck is not likely to last, Bove says.

Moreover, he is noticing a trend that has been going on for years and is likely to continue: "We seem to be seeing more extreme precipitation events," Bove says. "When it rains today, it seems to rain harder and heavier."

But even as the rain gets more intense, Bove says, people don't seem to be taking notice. "We tend not to build buildings to withstand the storms that we already see," he says, "let alone how they might change in the future."

That will mean higher costs in a future where weather becomes even less predictable.

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