Last Thursday, forecasters were saying that what was then called Tropical Storm Michael “does not pose a threat to land.” Just days later on Tuesday, as they further tracked its path, Hurricane Michael made landfall as a Category 4 storm and the most intense hurricane to hit Florida since 1851.

The rapid increase in Michael’s force came because of a phenomenon that meteorologists call “rapid intensification,” which the National Hurricane Center defines as an increase in sustained wind speeds of at least 35 miles per hour in one day. The most likely cause for such sudden changes? It’s large bodies of warm water, scientists say.

Michael gained sudden force Tuesday night as it churned over the Gulf of Mexico. “It is most likely that the very warm water in the Gulf .. is likely contributing to the intensity and the intensification that we have seen,” Jim Kossin, an atmospheric scientist with the U.S. National Oceanic and Atmospheric Administration told Reuters.

Rapid intensification poses new dangers to communities in the path of tropical storms and hurricanes because it makes early forecasts less reliable and because it leaves less time to evacuate or prepare for severe damage. But warmer water in the Gulf and other large bodies of water is here to stay because of climate change.

In 2016, Hurricane Matthew, which caused the deaths of nearly 600 people in Haiti, quickly grew to a Category 5 storm with sustained winds up 165 miles an hour because of rapid intensification. Forecasters failed to predict its sudden intensity. Michael’s winds were recorded at 155 miles an hour on Wednesday afternoon.

Corene Matyas, a climatologist at the University of Florida in Gainesville, told NBC that instruments measuring water temperature found warm water 300 feet below the surface level. “Storms are known to do this, but normally we see this happening when it’s away from land,” Matyas said. “What’s unusual is that it’s happening so close to land.”

Hurricane Michael made landfall two days after an alarming report from the United Nations that said a rise in global temperatures above 1.5 degrees Celsius—which could come as early as 2030—could cause catastrophic damage unless “rapid and unprecedented” changes in energy use is made before then.