Climate scientists, who specialize in thinking about the Earth system as a whole, are often reticent to link any one weather event to global climate change. But they say that aspects of the case of Hurricane Harvey—and the recent history of tropical cyclones worldwide—suggest global warming is making a bad situation worse.

It may not be obvious why global warming has anything to do with hurricane strength. Climate change is caused by the release of greenhouse gases like carbon dioxide and methane into the atmosphere. These gases prevent some of the sun’s rays from bouncing back into space, trapping heat in the planetary system and raising air temperatures all over the world.

As the world warms, evaporation speeds up. So on avg there's more water vapour for a storm to sweep up & dump now, compared to 70 years ago. https://t.co/M4R9OFFZt9 — Katharine Hayhoe (@KHayhoe) August 24, 2017

This warmer air causes evaporation to happen faster, which can lead to more moisture in the atmosphere. But that phenomenon alone does not explain climate change’s effects on Harvey.

Storms like Harvey are helped by one of the consequences of climate change: As the air warms, some of that heat is absorbed by the ocean, which in turn raises the temperature of the sea’s upper layers.

Harvey benefited from unusually toasty waters in the Gulf of Mexico. As the storm roared toward Houston last week, sea-surface waters near Texas rose to between 2.7 and 7.2 degrees Fahrenheit above average. These waters were some of the hottest spots of ocean surface in the world. The tropical storm, feeding off this unusual warmth, was able to progress from a tropical depression to a category-four hurricane in roughly 48 hours.

“This is the main fuel for the storm,” says Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research. “Although these storms occur naturally, the storm is apt to be more intense, maybe a bit bigger, longer-lasting, and with much heavier rainfalls [because of that ocean heat].”

This also suggests an explanation for one of Harvey’s strangest and scariest behaviors. The storm intensified up until the moment of landfall, achieving category-four strength hours before it slammed into the Texas coast. This is not only rare for tropical cyclones in the western Gulf of Mexico: It may be unique. In the past 30 years of records, no storms west of Florida have intensified in the last 12 hours before landfall.

Why do storms normally weaken—and why didn’t Harvey? As mentioned above, hurricanes feed and grow on warm ocean surface waters. But as they grow, their strong winds often pick up seawater, churning the oceans and moving the warmest waters deep below the surface. The same winds also bring newer, colder water closer to the atmosphere, which usually serves to drain energy and weaken the storm.