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People in the Florida Panhandle recently had to dodge flash floods after two feet of rain fell in 26 hours. Torrential rains caused a Washington State hillside to collapse and bury a community earlier this year. Tumultuous rainstorms and floods overwhelmed Colorado last year, and sudden floods swept through Nashville in 2010, and Atlanta in 2009.
We’re seeing a pattern here.
In the National Climate Assessment, the experts reported huge increases since the mid-20th century in the amount of precipitation falling in very heavy rainstorms: up 71 percent in the Northeast, 37 percent in the Midwest and 27 percent in the Southeast. The effect was seen on a smaller scale west of the Mississippi River, too, even in parts of the country where the climate is drying out over all.
What led the researchers to expect this long before it actually happened?
Initially, the forecast was based on simple physics from the 19th century. As we pour carbon dioxide into the air, the lower atmosphere has to warm. As it does, it is able to hold more moisture, and as the surface of the ocean also warms, more moisture tends to evaporate from it.
In the United States, the increase in water vapor has been on the order of 3 percent or 4 percent since the 1970s (most of the human-caused global warming has occurred since then). That may not sound like a big jump, but the effect is enormous.
Two leading scientists, Kevin E. Trenberth at the National Center for Atmospheric Research and David R. Easterling at the National Oceanic and Atmospheric Administration, ran some calculations and agreed that the warming has, on average, put more than a trillion gallons of extra water into the air over the contiguous 48 states, probably closer to two trillion.
That extra water has to fall as rain or snow. But from the elementary physics, it was long unclear whether this would mean more rainy days over all, or more intense rains, or both.
It was the computer models of the climate that suggested, starting in the late 1980s, that the answer would be the latter, and so it has turned out. One way to think of it is that even with a lot of moisture in the air, conditions are not always right for rain, but when they are right, the skies have a lot more water to dump.
“It rains harder than it used to,” said Dr. Trenberth, who could not resist adding: “When it rains, it pours.”
Researchers sponsored by the Australian government were the first to really drill into the implications of the finding. In their 1995 overview paper, A. M. Fowler of the University of Auckland in New Zealand and K. J. Hennessy of Australia’s national research program warned that society needed to start thinking about the risks. They suggested toughening standards for the designs of levees and dams, and hardening roads and culverts against the possibility of more flash floods.
Society responded by ignoring them. For someone sitting in Pensacola, Fla., wondering why the roads were washed out the other day, that longstanding refusal to confront reality might be a good part of the answer.
The warming of the planet has slowed in recent years, but scientists think that is likely temporary. They expect it to get much, much warmer as this century progresses, and that can only mean that the rains will fall harder still.
So if you are still a little amazed at what these heavy downpours have been doing to communities around the country, the message from science is pretty blunt: Get used to it.
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