Hurricane Harvey – one of the big names of the 2017 Atlantic hurricane season – unleashed catastrophic flooding in southeast Texas at the end of August. Now, after months of reviewing the data, scientists say the storm was exacerbated by climate change.

According to a peer-reviewed report by World Weather Attribution (WWA), an international coalition of scientists, human-caused climate change made Harvey’s devastating rainfall three times more likely to occur and fifteen percent more intense. Using historical rainfall data and high-resolution climate models to compare conditions in a pre-warming world to those at the time of the storm, the WWA team was able to separate the climate signal from natural variability. They found that the deluge caused by Harvey would have been a 1-in-2400-year event in the absence of global warming, but is now a 1-in-800-year event and becoming more likely.

Heavy rainfall events, in general, are becoming more frequent in many different places, because as the atmosphere warms it can hold more moisture. In fact, it can hold four percent more moisture for every 1°F of warming. This means there is more water vapor available in the air that can fall as precipitation.

After rapidly intensifying in the Gulf of Mexico, Hurricane Harvey made landfall as a category-4 storm in the Texas Coastal Bend region on August 25. It then stalled over the area for several days, unleashing massive amounts of rainfall. Cedar Bayou, outside of Houston, reported a staggering 51.88 inches of rain, setting a new record for the continental US. The storm claimed the lives of 80 people and more than 120,000 residents across the area had to be rescued from their homes. The economic impacts of the deluge are still being tallied, but it is expected to be one of the most expensive in US history.

The WWA study only analyzed the impact of climate change on Harvey’s rainfall, not its role in the storm’s formation or strength.  Those connections remain an active area of research.

Climate change made Hurricane Harvey’s rainfall worse. Credit: Climate Central. (World Weather Attribution is led by Climate Central, a non-profit research group.)