Not only are a reef's coral bleached during a marine heatwave, the new research shows, but also the skeletons of the coral - which are animals - decay within weeks.

This bleaching and skeletal decay risks the lives of the many other sea creatures call the coral framework home.

Researchers suggest that "severe heatwave-induced mortality events" should therefore be considered a separate phenomenon to coral bleaching, and one which causes more damage.

The study published in journal Current Biology on Friday says a temperature increase of half-a-degree Celsius in the ocean changes the extent of mortality in coral during bleaching.

Report co-author and UNSW associate professor Tracy Ainsworth said the research confirms what many scientists have known for a while: that marine heatwaves are becoming more severe and are having greater impacts.

"It confirms that we are on a trajectory where heatwave events and heat stress is becoming so severe it's beyond the capacity of the ecosystem to withstand it," she told AAP.

"We are just going so far beyond what's normal. These systems can't keep up."

The report showed that a severe marine heatwave will not only trigger bleaching events, but can lead to heat-induced death of the coral itself.

"The water temperatures are so warm that the coral animal doesn't bleach - in terms of a loss of its symbiosis - the animal dies and its underlying skeleton is all that remains," she said.

The skeleton is immediately overgrown by algae and bacteria, a process that was devastating not just for the animal tissue, but also for the skeleton that is left behind which is rapidly eroded and weakened, the study found.

The research was conducted by a team from four Australian universities and the US National Oceanographic and Atmospheric Administration.

Australian Associated Press