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Hurricane Harvey and Climate Change

Hurricane Harvey has brought with it both a record amount of rain and questions about how much climate change can be blamed for the storm. Climate change did not cause Harvey, or any other storm, but it makes intense storms like Harvey more likely to occur, scientists say.

The most recent analysis of what’s known about the effect of climate change on hurricane activity comes from the June 28 draft of the U.S. Global Change Research Program’s Climate Science Special Report.

One of that report’s key findings said that human activities have “contributed to the observed increase in hurricane activity” in the North Atlantic Ocean since the 1970s. The Gulf of Mexico, where Harvey formed, is part of the North Atlantic Ocean.

The draft report echoes the findings of the U.N.’s Intergovernmental Panel on Climate Change’s 2013 assessment report, which found that scientists are “virtually certain” (99 percent to 100 percent confident) that there has been an “increase in the frequency and intensity of the strongest tropical cyclones since the 1970s” in the North Atlantic Ocean.

Still, the U.S. Global Change research program’s report says there’s uncertainty in how much humans have contributed to hurricane activity relative to natural causes. The studies that have looked at this question have come up with a “fairly broad” range of contributions for humans, but “virtually all studies identify a measurable, and generally substantial, [human] influence,” it adds.

Another key finding of the report concluded that climate models generally show that a “warmer world” leads to an increase in the intensity of hurricanes, as well as an increase in the frequency of “very intense” hurricanes. The report also says the Atlantic Ocean specifically is projected to see increases in hurricane precipitation.

These projections also fit with scientists’ theories about how a warmer world leads to greater moisture in the atmosphere, which leads to greater precipitation, which leads to more intense storms.

But there is some uncertainty: The projections don’t yet align with the observed data on hurricanes because the data are “not of a high enough quality” to detect human contribution to the trend, the report says.

What about Hurricane Harvey in particular?

As we’ve said, it’s inaccurate to say that climate change is the cause of Harvey, which dumped a record 50 inches of rain on southern Texas over four days.

But scientists have said that Harvey was likely worsened by climate change for a few reasons.

Michael Mann, a professor of atmospheric science at Penn State, pointed to at least three factors that likely affected Harvey — higher sea levels, warmer ocean waters and weak prevailing winds — in an article published in the Guardian on Aug. 28.

Sea level rise contributes to higher storm surges, explains Mann. Higher storm surges then lead to more coastal flooding, he says.

Warmer waters, on the other hand, lead to increased moisture in the atmosphere, which “creates the potential for much greater rainfalls and greater flooding,” Mann explains. “The combination of coastal flooding and heavy rainfall is responsible for the devastating flooding that Houston is experiencing.”

Though “more tenuous” than the other two factors, says Mann, Harvey also may have stalled near the coast because of weak prevailing winds that failed “to steer the storm off to sea.” This stalling then led to continued heavy rainfall on Texas that eventually topped 50 inches.

The weak prevailing winds are caused by a “greatly expanded subtropical high pressure system” currently over much of the U.S., which is “predicted in model simulations of human-caused climate change,” says Mann.

Scientists say other storms were likely made worse by climate change as well.

In a perspective paper published in Nature Climate Change in 2015, Kevin E. Trenberth, a senior scientist at the National Center for Atmospheric Research in Colorado, and others discussed how climate change likely affected Hurricane Sandy, which hit New York and New Jersey particularly hard in October 2012.

Trenberth et al., June 2015: Although perhaps only one-half to one-third of the [sea surface temperature] increase can be blamed on global warming from human activities, it is readily apparent that the storm surge and associated damage was considerably influenced by climate change. It is quite possible that the subways and tunnels [in and around New York City] might not have flooded without the warming-induced increases in sea level and in storm intensity and size, putting a potential price tag of human climate change on this storm in the tens of billions of dollars.

So is climate change all to blame for Harvey? No. But it did play a role in making the storm worse, as it has with other storms, scientists say.

Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation. For more information about extreme weather and climate change, please see our item “Precision in Climate Science.”