Coral cover on the reef has already shrunk by as much as half since 1985, with outbreaks of the prickly starfish blamed for about 42 per cent of the loss, or more than four times the impact of coral bleaching during that period.

Scientists from the Australian Institute of Marine Science studied how rising temperatures affect starfish larvae and found that a 2-degree increase improved their likely survival rate by 240 per cent, or more than tripling it.

"Warmer sea temperatures were found in this study to enhance [starfish] survival along with other, cumulative pressures on the reef," Sven Uthicke, the report's lead author and AIMS scientist, said.

The development time of larvae to so-called settlement stage was shortened by 30 per cent, according to the study, published in Scientific Reports on Thursday.

The study assumed that sufficient nutrients would be available to feed the increased number of survivors. Nutrient sources can include increased run-off from land, a scenario in line with predictions that climate change will increase terrestrial flooding events.

"Recognising the role of synergistic effects of increased nutrient flows and sea-surface temperatures on [starfish] survival better enables scientists to understand the science behind outbreaks," Dr Uthicke said.

The reef has had four big outbreaks of the starfish – in 1966, 1979, 1994 and 2011-12 – starting in a region north of Cairns.

University of Queensland's Global Change Institute  director Ove Hoegh-Guldberg said temperature increases would tend to speed things up for most cold-blooded organisms – up to an optimal point.

In the case of the crown-of-thorns, with each starfish capable of producing millions of eggs each reproductive season, any change in the odds of survival would potentially have a big impact on numbers.

"Larval life is a very hazardous phase. If you can shorten the phase, you increase the percentage of larvae making it through to settlement," Professor Hoegh-Guldberg said. "You start to change the balance."

Reefs, including the Great Barrier Reef, are struggling in many regions as rising temperatures increase the likelihood of coral bleaching. Rising acidity, starfish infestation and cyclones are also taking their toll.

"Corals are just not growing back as fast after natural disturbances," he said.

With the impacts evident after increases of just 0.6 degrees in Coral Sea temperatures, it was "lunacy" to be projecting rises of as much as 4 degrees if greenhouse gas emissions continued on their present trajectory, Professor Hoegh-Guldberg said.

"Complex systems are proving much more sensitive to small shifts in temperatures and climate than we first thought."