Some Florida Keys coral reefs are losing their color weeks earlier than normal this summer because of record-high water temperatures, meaning they are under stress and their health is potentially endangered, federal scientists said.

The corals should be vibrant and colorful this time of year, but are swiftly going white, said Katey Lesneski, research and monitoring coordinator for Mission: Iconic Reefs, which the National Oceanic and Atmospheric Administration launched to protect Florida coral reefs.

“The corals are pale, it looks like the color’s draining out,” said Lesneski, who has spent several days on the reefs over the last two weeks. “And some individuals are stark white. And we still have more to come.”

Scientists with NOAA this week raised their coral bleaching warning system to Alert Level 2 for the Keys, their highest heat stress level out of five. That level is reached when the average water surface temperature is about 1.8 degrees Fahrenheit (1 degree Celsius) above the normal maximum for eight straight weeks.

Surface temperatures around the Keys have been averaging about 91 degrees (33 Celsius), well above the normal mid-July average of 85 degrees (29.5 Celsius), said Jacqueline De La Cour, operations manager for NOAA’s Coral Reef Watch program. Previous Alert Level 2s were reached in August, she said.

Coral reefs are made up of tiny organisms that link together. The reefs get their color from the algae that live inside them and are the corals’ food. When temperatures get too high, the coral expels the algae, making the reefs appear white or bleached. That doesn’t mean they are dead, but the corals can starve and are more susceptible to disease.

Andrew Bruckner, research coordinator at the Florida Keys National Marine Sanctuary, said some coral reefs began showing the first signs of bleaching two weeks ago. Then in the last few days, some reefs lost all their color. That had never been recorded before Aug. 1. The peak for bleaching typically happens in late August or September.

“We are at least a month ahead of time, if not two months,” Bruckner said. “We’re not yet at the point where we are seeing any mortality … from bleaching. It is still a minor number that are completely white, certain species, but it is much sooner than we expected.”

Still, forecasting what will happen the rest of the summer is hard, De La Cour and Bruckner said. While water temperatures could continue to spike — which could be devastating — a tropical storm or hurricane could churn the water and cool it down. Dusty air from the Sahara Desert moving across the Atlantic and settling over Florida could dampen the sun’s rays, lowering temperatures.