Warming temperatures are threatening coral reefs in the northern hemisphere, including many in U.S. waters, scientists are warning.

Unusually warm ocean temperatures are putting coral at risk across the equatorial and north Pacific and western Atlantic oceans, the National Oceanic and Atmospheric Administration says.

The rise in temperatures can lead to coral bleaching, a phenomenon that sees coral release the symbiotic algae that lives within it, turning the coral pale or white.

That eliminates a major source of food and leaves the coral more vulnerable to disease, NOAA scientists say, and in extreme cases can cause entire sections of reefs to die, destroying a habitat important to fish and shellfish.

"The bleaching that started in June 2014 has been really bad for corals in the western Pacific," says Mark Eakin, coordinator of the NOAA Coral Reef Watch. "We are worried that bleaching will spread to the western Atlantic and again into Hawaii."

If bleaching occurs again this year, it would mark the first time it has occurred in consecutive years in the Hawaiian archipelago.

Reef loss from coral bleaching has a long-term affect, as reefs normally take decades to recover, scientists point out. Dead reefs degrade as erosion destroys the structures the corals have painstakingly built up.

The result is less protection for shorelines and fewer habitats for marine life, they say.

In addition to Hawaii, Florida also experienced warmer temperatures, which hit coral nurseries in the Florida Keys in which researchers have attempted to grow threatened species of coral for harvesting and transplanting onto local reef systems.

More bleaching this year could be disastrous for corals still attempting to recover from last year's warming stress, Eakin says.

"Many healthy, resilient coral reefs can withstand bleaching as long as they have time to recover," Eakin says. "However, when you have repeated bleaching on a reef within a short period of time, it's very hard for the corals to recover and survive."

The problem is compounded by other stresses, including overfishing and pollution, he adds.

The warmer oceans in the Northern Hemisphere are being caused by both El Niño weather patterns and ongoing climate change, Eakin says.

Although bleaching can be the result of light and nutrient levels as well as ocean temperatures, only very warm temperatures can cause the widespread bleaching scientists have detected beginning in 2014.

Coral reefs play a critical role in marine ecosystems; although they are found on just 10 percent of the world's ocean floor, they provide homes and habitats for 25 percent of the globe's known marine species.