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.
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"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.
Unprecedented back-to-back coral bleaching in the summers of 2015-16 and 2016-17 triggered the deaths of about half the corals on the Great Barrier Reef alone.
Of the World Heritage-listed natural coral reef properties, 15 were exposed to repeated severe heat stress between 1985-2013, while 25 of the 29 suffered bleaching in the subsequent three years, according to the assessment published last year.
The Turnbull government's handling of the Great Barrier Reef's health has lately been controversial following a decision to direct $444 million of reef research funding in the budget for the year ending this month to the little known privately funded Reef Foundation.
Keeping global warming to under 1.5 degrees, though, will be a difficult task given the roughly 1-degree increase so far and the timelag effect of the carbon pollution already emitted. Dr Heron's report notes the low-emissions scenario modelled implies pollution peaks during the 2010-2020 decade.
Even a low-carbon track, however, won't be enough to spare the Phoenix Islands Protected Area in Kiribati, which will be among the four sites likely to suffer severe coral bleach twice a decade by 2038, the report said. The other three are protected sites off Cocos Islands, the Galapagos Islands and Panama.
Dr Heron, who is a researcher at James Cook University and supported by the Australian Marine Conservation Society, will release his report at this week's World Heritage Committee meeting in Bahrain.
He is also planning to unveil a new Climate Vulnerability Index assessment tool to highlight the climate action needed to save all of the 1000-plus culture and natural World Heritage-listed sites world.
"There's not really the capacity within the World Heritage process to address [climate change]," said Dr Heron. "These are the best of the best places to protect."
Climate change impacts "have the potential to overwhelm any efforts that we put into local management", he said.
Separately, Hilde Heiner, President of the Marshall Islands, will on Thursday announce the first online political leaders summit to drum up support for lifting the emissions goals pledged at the 2015 Paris climate accord.
The summit is due to be held after the expected release in October of the Intergovernmental Panel on Climate Change's special report on the impacts of a temperature increase of 1.5 degrees.
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