Even before Hurricane Harvey made landfall in Texas on August 25, veteran weathercaster Dan Satterfield was calling the storm’s predicted rainfall totals “astounding and astonishing.”

“I’ve never seen consistent model output forecasting rainfall amounts of over 30 inches before,” Satterfield, chief meteorologist with WBOC-TV in Salisbury, Md., wrote online for an American Geophysical Union site.

The weather models proved correct: Astronomical, record-breaking amounts of rainfall dumped on Texas, causing catastrophic flooding in Houston and killing at least eight people.

“It’s hard, if not impossible, to compare this to any other storm,” Eric Fisher, chief meteorologist at WBZ-TV in Boston, told the Washington Post. “It may stand alone when all is said and done in terms of rainfall.”

If Hurricane Harvey is unprecedented, does that mean it was caused by climate change? Not quite: After all, it’s normal for hurricanes to form during hurricane season.

But climate change could have played a role in worsening the storm. Here’s why.

Climate change is putting more moisture in the atmosphere

As Kevin Trenberth, a scientist at the National Center for Atmospheric Research, explains in this video, the rise in Earth’s temperature now serves as the background for weather.

“The environment in which all events occur these days is different, so in fact every storm is different, every event is actually different. It has to be,” Trenberth says. “The air, being warmer, means it can also hold more moisture. There’s more moisture in the air, especially coming off of the oceans.”

That extra moisture matters, adds climate scientist Katharine Hayhoe of Texas Tech: “So when that storm comes along, as it always does, there’s more water vapor sitting up there for that storm to pick up and dump on us.”

Hot oceans can make hurricanes more intense

Warm water is fuel for hurricanes. In fact, Harvey had weakened to a tropical depression as it crossed the Yucatan Peninsula days before it made landfall in Texas. But then it hit the unusually hot waters of the Gulf of Mexico and quickly reorganized into a powerful hurricane.

As scientist James Elsner of Florida State University explained to YCC in 2016, “With warmer oceans caused by global warming, we can expect the strongest storms to get stronger.”

Higher sea levels make storm surge worse

When a hurricane makes landfall, its winds push seawater ashore, creating a dangerous storm surge. Adding sea-level rise to storm surge can make that flooding even worse than it was in the past.

In places like Miami, flooded streets and houses now flood routinely, even in the absence of a hurricane.

“The water is here. It’s not that I’m talking about some sci-fi movie here. No. I live it. I see it, it’s tangible,” South Beach resident Valerie Navarrete told YCC in February 2017. She said her garage floods about once every other month, and she often has to wear rain boots just to get to her car. As sea levels continue to rise, low-lying coastal areas will grow even more vulnerable to hurricanes.

Climate change ‘systemically caused’ Superstorm Sandy

For all of these reasons – extra moisture in the atmosphere, hotter oceans, and higher sea levels – linguist George Lakoff of Berkeley argued in 2012 that it was accurate to say that climate change “systematically caused” Superstorm Sandy. That storm slammed the northeast U.S. in 2012:

Yes, global warming systemically caused Hurricane Sandy – and the Midwest droughts and the fires in Colorado and Texas, as well as other extreme weather disasters around the world. Let’s say it out loud, it was causation, systemic causation.

Systemic causation is familiar. Smoking is a systemic cause of lung cancer. HIV is a systemic cause of AIDS. Working in coal mines is a systemic cause of black lung disease. Driving while drunk is a systemic cause of auto accidents. Sex without contraception is a systemic cause of unwanted pregnancies.

Climate scientists are sure to study Harvey closely for many years to come. But climate scientist Michael Mann at Penn State University has already offered an initial assessment in The Guardian: “We cannot say climate change ’caused’ Hurricane Harvey (that is an ill-posed question), we can say is that it exacerbated several characteristics of the storm in a way that greatly increased the risk of damage and loss of life,” he wrote. “Climate change worsened the impact of Hurricane Harvey.”

Want to know more?

Explore this curated list of books on hurricanes and climate change.