What do we know about the relationship between climate change and Hurricane Harvey?

Hurricane Harvey has dumped more water on the United States than any other weather event in history, and its costs are expected to be huge.

The unprecedented storm has inevitably posed the question: What impact has climate change had on producing Hurricane Harvey? We took a look at what scientists had to say.

Keep in mind: None of the experts we talked to said climate change caused Hurricane Harvey. Instead, some posited that climate change exacerbated the effects of the storm. The degrees of exacerbation vary, though, sometimes significantly.

A draft report on climate science conducted by 13 federal agencies as part of the National Climate Assessment said models showed the number of very intense storms have been rising as a result of a warmer world. But the trend has yet to rise above normal variation.

The report also said that scientists are better able to attribute weather events to climate change than they used to be, but linking individual events to climate change is more complicated. The scientists we spoke to about Hurricane Harvey expressed a similar challenge.

Below, we’ll outline some general concepts of climate change and then address whether they influenced Hurricane Harvey.

In a warmer world, there’s more water vapor that storms can sweep up and dump on us.

"While it is not yet possible to determine exactly how much of the rainfall associated with Harvey was due to climate change versus how much would have occurred naturally, nearly every scientific study agrees that, as the world warms, on average the amount of rainfall associated with hurricanes will increase," said Katharine Hayhoe, a climate scientist at Texas Tech University in Lubbock.

Heavy downpours are on the rise in the United States, according to a 2014 report by 13 federal agencies. But we found differing estimates for how climate change might have affected rainfall from Harvey.

According to Chris Landsea, science and operations officer at the National Hurricane Center, the amount of increased precipitation in Harvey is not significant. Landsea expects a 10 percent surge in rainfall by the end of the century due to climate change, which he predicts would only have only increased rainfall by an inch or two in this case.

Kevin Trenberth, a scientist at the National Center for Atmospheric Research, said he could justify a 5 to 15 percent increase in rainfall during Harvey from climate change effects, which then increase with natural variability.

"So the storm is a bit more intense, bigger and longer lasting than it otherwise would be," Trenberth wrote to PolitiFact.

As Earth’s temperature warms, land-based ice melts and ocean water expands, causing sea levels to rise. This in turn increases the risks that the sea will rise with the atmospheric pressures of a storm, causing more waves and flooding.

If sea level rise continues at the current pattern, Landsea expects every single hurricane to have a 2-foot higher storm surge by the end of the century.

Trenberth attributed half a foot of the flooding in this case to climate change.

In addition to rising sea levels, the storm surge in Harvey was also tied to erosion, subsidence, saltwater intrusion and the impact of past hurricanes.

Warm oceans may also increase the intensity of hurricanes. Tropical storms use the ocean to fuel their growth like a battery, so warmer temperatures make storms stronger and intensify them more quickly. Over 90 percent of the excess heat in the atmosphere caused by climate change is going to the ocean.

However, it’s not so simple to connect climate change to the intensity of Harvey, or any other hurricane for that matter. There are other atmospheric factors at play, such as changes in air temperature at the top of the hurricane, that can offset warmer ocean temperatures.

Climate science researchers want to answer this question, but they haven’t reached a verdict just yet. What some researchers have shown is that the frequency of hurricanes is decreasing, while the intensity increases. That would mean fewer storms, but the storms would be more intense with greater potential for destruction.

Some scientists attribute abnormally high Gulf of Mexico temperatures to the storm’s rapid intensification before it hit the ground, but they also can’t wholly link the temperatures to climate change. The Gulf has been ripe enough for hurricanes for three decades, but it still doesn’t produce them every year.

Almost every scientist we spoke with agreed that the most remarkable feature of Harvey is how long it lingered atop Houston, which was crucial in creating the staggering rain totals. But that is also the part of the storm with the least conclusive evidence linking it to climate change.

Several studies found that hurricane-steering winds have weakened and shifted northward in recent decades over North America, increasing the likelihood of slow-movers like Harvey in the future. A recent paper by Penn State professor Michael Mann showed how blocking patterns in general become more common in a warmer world.

"These studies require painstaking, detailed analyses that take years to complete so there is nothing we as scientists can say right now about the extent to which human-induced change was involved in determining Harvey’s path, or even whether it was at all," Hayhoe said.

Kerry Emanuel, a professor of atmospheric science at Massachusetts Institute of Technology, found that steering currents had weakened significantly since 2010, but "the suddenness of the decline weighs against an explanation in terms of anthropogenic climate change."

Scientists may disagree on the degree to which anthropogenic (or human-caused) climate change intensified Harvey, but almost all concurred that Houston’s lack of preparation for it magnified its ramifications.

Urbanization turned prairies and forests into concrete, reducing the land’s capacity to absorb rainfall, and lax zoning codes gave way to development more prone to cave to the flooding. Paired with an explosion in the city’s population, the damages snowballed.

Many of the scientists we spoke with nonetheless stressed that in addition to infrastructure, lawmakers must focus on addressing climate change.

"We’re doing a huge disservice if we put off the reality that we are changing the climate and the sea levels if we wait until we do fancy analyses (on storms like Harvey) years later. It’s time to face up to the fact that some of these are going to become the new norm," said Harold Wanless, chair of the geological sciences department at the University of Miami.