Keeping global temperature increases to the lower end of the Paris climate accord would make a dramatic difference to the severity of coral bleaching by mid-century, according to research to be presented to the UN's World Heritage Committee.

The study, conducted by scientists including Australians, found only four of the 29 World Heritage reefs are projected to experience severe bleaching twice a decade from heat stress by the second half of this century if warming can be kept to 1.5 degree, compared with pre-industrial levels.

That projection, in an update report to the First Global Scientific Assessment of the World Heritage reefs,  compares with an assessment last year that projected 25 of those 29 sites – including the Great Barrier Reef – would suffer severe bleaching twice a decade by 2040 if global greenhouse gas emissions were allowed to increase under business-as-usual conditions.

"Coral reefs are one of the more sensitive ecosystems and so the impacts [of climate change] are unveiled earliest," said Scott Heron, lead author of the report, and an oceanographer with Coral Reef Watch, run by the US National Oceanic and Atmospheric Administration. "Action on greenhouse gas emissions ... is critical at this stage."

Corals suffering sustained heat stress typically expel algae known as zooxanthellae, losing their main source of energy and colour.