Some coral reefs could be protected from the impacts of climate change by an "ocean thermostat", a study says.
At higher temperatures coral lose their algae, becoming "bleached"
Researchers suggest that natural processes appear to be regulating sea surface temperatures in a region of the western Pacific Ocean.
Reefs in the area had only suffered relatively few episodes of bleaching because the naturally warm waters had remained stable, they observed.
The findings appear in the Geophysical Research Letters online journal.
The study, carried out by scientists from the US National Center for Atmospheric Research (Ncar) and the Australian Institute of Marine Science (Aims), appears to support a theory that natural processes prevent ocean sea surface temperatures exceeding 31C (88F).
A limit on the amount that water can warm, the team argues, will help protect reefs that have evolved in warm waters.
And reefs found in cooler waters will experience a larger degree of warming, it says.
"Global warming is damaging many corals," lead author Joan Kleypas explains, "but it appears to be bypassing certain reefs that support some of the greatest diversity of life on the planet.
"In essence, reefs that are already in hot water may be more protected from warming than reefs that are not; this is rare hopeful news for these important ecosystems."
Feeling the heat
Although reefs are exposed to a number of threats, such as overfishing, pollution and acidification, climate change is of particular concern to marine ecologists.
Unusually warm temperatures can cause "bleaching", which is when coral turns white after expelling colourful microscopic organisms that provides the community with its nutrients.
If the water temperature does not return to normal within a few days or weeks, the bleached coral collapses and dies.
The researchers say that a region, called the Western Pacific Warm Pool, to the north-east of Australia has only experience four episodes of bleaching between 1980 and 2005.
Sea surface temperatures in the area average 29C (84F), which is near the limit of the so-called thermostat.
It is suggested that as surface waters warm, more water evaporates, and this can lead to an increase in cooling cloud cover and winds.
"This year is the International Year of the Reef, and we need to go beyond the dire predictions for coral reefs and find ways to conserve them," says Dr Kleypas.
But her Ncar colleague, Gokham Danabasoglu, warns that projections do not paint an optimistic picture.
"Computer models of Earth's climate show that sea surface temperatures will rise substantially this century," he says.
"Unfortunately, these future simulations show the Western Pacific Warm Pool warming at a similar rate as the surrounding areas, instead of being constrained by a thermostat.
"We don't know if the models are simply not capturing the processes that cause the thermostat, or if global warming is happening so rapidly that it will overwhelm the thermostat."