Photograph by Randy Olson, Nat Geo Image Collection
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Lightning cracks in a cloud-filled sky as rain falls in the distance.

Photograph by Randy Olson, Nat Geo Image Collection

2018 was the U.S.'s third-wettest year on record—here's why

The eastern U.S. got a ton of rain, while the west stayed dry. A warming climate probably contributed.

On Wednesday, NASA and NOAA announced that 2018 was the fourth hottest year on record. But the impacts of a warming planet extend beyond just warming air; the feverish state of the planet is also changing when, where, and how intensely rain and snow fall. And 2018, the reports say, was the third-wettest year since 1895, when steady record-keeping began.

Overall, the U.S. got 4.68 inches more precipitation in 2018 than the 20th-century average, but that rain and snow was not anywhere close to evenly distributed across the country. In the eastern half, several states, like North Carolina and Virginia, blew past their previous precipitation records, while most of the western U.S. remained mired in drought.

It’s too early to know exactly how a warming climate influenced the individual storms that hovered over much of the eastern U.S. this year. But, says Karin Gleason, a meteorologist at NOAA who was involved in preparing the annual report, the pattern fits with what we might expect from a hotter world.

“In a warming climate, one of the things we anticipate seeing is precipitation extremes at both ends,” she says—meaning the wet places are likely to get wetter and the dry places drier.

The extreme extremes

The 2018 pattern toward extremes was global. Places like Austria, which are generally not rain- or snow-limited, saw huge extremes: last January, the country piled up nearly double their normal precipitation for the month.

“We’ve seen a marked upward trend in short-duration extreme events,” says Ken Kunkel, a climate scientist at North Carolina State University who was not involved in preparing the reports. “It’s an obvious trend that’s been going on for the last few decades and continues to go up.”

The ripping winds of Cyclone Mekunu, for example, dumped 12.9 inches of rain on Salalah, Oman, in just 36 hours—more than double the rain it usually gets in a whole year. And Hurricane Florence doused North Carolina in several feet of rain in the few days it hovered over the state, far more rain than the rivers, soils, and communities could absorb.

And a single tropical storm dropped just under 50 inches of rain on Kauai in 24 hours, breaking the U.S. record for most rain in a single day by over six inches (the record was previously held by a 1974 storm in Texas).

Why is this happening?

There is growing scientific evidence that a warming planet fuels stronger, wetter storms.

In the southeast and eastern U.S., the trend toward stronger storm events is primarily driven by strong warming in the oceans that fringe their shores. The Gulf of Mexico and the western part of the Atlantic Ocean have seen temperature increases of up to four degrees Fahrenheit over their long-term average in recent years. That number bounces up and down as phenomena like El Niño’s cycle through, but the long-term pattern trends steadily upward.

Warm oceans evaporate more water into the air. And warm air holds more water than cooler air—think of a sauna, where the water vapor can hang so thick it’s hard to breathe. So the toasty oceans feed more vapor into the atmosphere. Then, when storm systems come along to sweep that moisture-rich air onto the continent, out rains the water.

“The higher daytime temperatures we’re seeing are increasing the atmosphere’s ability to hold more moisture and also to wring it out, so to speak,” says Gleason.

Another factor? Warmer, moisture-laden air acts like a “blanket” over the land, keeping heat trapped near the ground. This year, many of the states that had their wettest-ever years also hit records for their minimum temperatures—meaning that the coldest temperatures were less cold than in the past. Usually, those coldest temperatures come during the wee hours of the morning, when land has cooled down all night and the sun hasn’t yet come up.

But last year, for many states, that tail end of nighttime was warmer and cloudier than ever. Those two factors are related, explains Gleason.

Think of it like this: “In the winter, when it’s really cool and crisp out, everything cools down really quickly because there’s nothing to hold that heat in,” she says. But if there’s more cloud cover, the heat sticks around—leading to warmer temperatures overall.

Air temperatures are projected to warm up even further in the coming years, and many scientists are anticipating that with a warmer planet, the extreme precipitation events will only get more extreme.

“This pattern—of drought in the west and wetness in the east—is likely to stay,” says Kunkel.