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Last weekend, Ellicott City's quaint Main Street, with red, white, and blue decorations draped from storefronts, became a churning brown river.

While several factors resulted in this Maryland town's second "1,000-year storm" occurring in just the last 22 months, one increasingly significant contribution to the flooding is the amount of water vapor accumulating in Earth's warming atmosphere.

The flooding in Maryland fits within the broader context of how human-caused climate change is altering our atmosphere, making these extreme events more commonplace in the future.

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NASA scientists say that average global temperatures have been on a "rapid warming trend" in the last four decades. This warming means that the air is capable of holding more water. Specifically, for every 1 degree Celsius (1.8 degrees Fahrenheit) of warming, the air can hold 7 percent more water.

Accordingly, this loads storm systems with more water.

"So when it rains, it really, really pours, in the future -- and now," Kate Marvel, a climate scientist at NASA's Goddard Institute for Space Studies, said in an interview.

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"This is super basic physics and chemistry," said Marvel of increased temperatures allowing the air to absorb more water.

Today, the consequences of this elementary science are pummeling the U.S.

"We’ve already observed an increase in heavy precipitation events," said Marvel.

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In 2017, the Environmental Protection Agency (EPA) released a report showing extreme, single day downpours have been on the rise since the late 1970s. The United States' 2014 National Climate Assessment -- guided by 13 federal agencies -- found that "Across most of the United States, the heaviest rainfall events have become heavier and more frequent."

"Increases in extreme precipitation are projected for all U.S. regions," the report added.

A map showing increases in the amount of precipitation falling in very heavy events.Credit: U.S. government/National climate assessment

That said, Marvel emphasizes it's inaccurate to say that any extreme downpour was the direct or single result of more water vapor being held in the warmer air.

Destructive weather will still happen -- but now it has an added kick -- human-induced global warming.

"Unlucky, bad things happen all the time," said Marvel. "Weather happens, but we know what climate change is doing."

Tropical cyclones, for example, have ravaged U.S. coastlines throughout history, and will continue to do so.

The first named storm of the 2018 Atlantic hurricane season, Alberto -- which is not a hurricane but is still carrying bounties of water -- has now causing flooding so severe in North Carolina that the National Weather Service tweeted "HEED ALL EVACUATION ORDERS IMMEDIATELY!" in areas around the Lake Tahoma dam.

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NASA also notes that the severity of downpours during tropical storms and hurricanes are on the rise.

Marvel uses disgraced U.S. cyclist Lance Armstrong, who admitted to doping, as an analogy. The talented Armstrong would still have probably won some races without enhancing his performance with prohibited drugs.

But, "we know that he was doping, and we know what doping does," Marvel said.

Similarly, "we know we’re doping the weather," said Marvel. "And we know what that does."

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