Some coral reefs could be protected from the impacts of climate change by an "ocean thermostat", a study says.
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.
Whereas reefs found in cooler waters will experience a larger degree of warming.
"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 evapourates, 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."
Ocean "Thermostat" May Be Secret Weapon Against Warming
Brian Handwerk
for National Geographic News 8 Feb 08;
A natural but mysterious "ocean thermostat" may be limiting seawater warming in at least one Pacific Ocean locale.
The phenomenon may help protect some of the world's largest and most ecologically diverse coral reefs from the effects of climate change, a new study says.
"There appear to be natural negative feedbacks that keep water temperatures in check—at least in this part of the planet," said study co-author Joan Kleypas from the National Center for Atmospheric Research in Boulder, Colorado.
Kleypas and colleagues focused on the Western Pacific Warm Pool, a region in the open ocean northeast of Australia.
Water temperatures at the pool, which on average has been about the size of Australia, have risen little in recent decades, even as the rest of Earth's oceans have heated up.
"In the 20th century, warming has been less in that region, and coral bleaching has been less in that region," Kleypas said.
Coral bleaching occurs when warming waters cause corals to expel the colorful algae that sustain them. The corals turn a ghostly white and die in a few days unless temperatures cool down and the algae return.
"The model shows that there is a reason that the water is warming less—it's not just a fluke," Kleypas continued.
"The models and observations are showing the same thing, [which] points to some sort of mechanism or feedback process that is keeping temperatures in check."
Mysterious Process
Scientists have proposed several ways that the world's oceans might be able to regulate sea-surface temperatures.
One theory suggests that warming waters cause more evaporation, which in turn creates cooling cloud cover and prevalent winds.
Warmer waters could also alter the flow of ocean currents and initiate an influx of cooler water in certain areas.
The controversial thermostat theory holds that an undefined process will naturally prevent sea-surface temperatures from rising above 88 degrees Fahrenheit (31 degrees Celsius) in the open oceans.
This is good news for corals in warm waters, such as those that live in the Western Pacific Warm Pool, where the average temperature is about 85 degrees Fahrenheit (29 degrees Celsius).
That's because stable water temperatures—rather than any specific temperature—are what reefs need to survive, said coral expert Nancy Knowlton of the Smithsonian Institution's National Museum of Natural History.
"In general, if the temperature gets above a degree Centigrade [two degrees Fahrenheit] over the normal seasonal maximum, coral reefs are often in trouble," she said.
(Knowlton, who was unaffiliated with the new study, is a National Geographic Committee for Research and Exploration grantee. National Geographic News is owned by the National Geographic Society.)
Warm-water corals will experience less drastic changes than their cold-water counterparts if an ocean thermostat is in effect.
(Related news: "Corals May Have Defense Against Global Warming" [October 4, 2007].)
The new research, which will appear tomorrow in the online edition of the journal Geophysical Research Letters, lends support to the thermostat theory.
The team's analysis of historical data combined with computer simulations show that the pool has warmed up only half as much as colder ocean regions.
With little idea of how the effect operates, it is unknown if such a thermostat is unique to the Western Pacific Warm Pool.
Such a process may be more widespread and either remain undetected or be overwhelmed by a larger and more complex mix of other climate drivers in waters nearer continental landmasses.
The effect's apparent beneficial impact is likely to draw much more attention to the Western Pacific Warm Pool and its reefs.
"It means that these kinds of places might be high-priority places to protect," the Smithsonian's Knowlton said.
Ancient Regulator
Study author Kleypas and colleagues also say that the pool's thermostat process may have been at work for a long time.
Evidence from the paleontological record suggests that the pool's water temperatures were not much warmer in the past—even during the Cretaceous period, which had higher-than-present levels of the greenhouse gas carbon dioxide (CO2).
But those ancient climates were more stable, and the team's computer models suggest that the projected rates of carbon entering the atmosphere—and subsequent warming—could overwhelm the regulating effect.
"The bad news is that we did look at some existing runs where [our modeled increase in atmospheric CO2] was pretty strong, and we did not see an effective thermostat," Kleypas said.
While the research raises more questions than it answers, Kleypas said, it provides a hopeful path to pursue.
"It's about solutions to deal with global warming," she said.
"It gives us some hope that these feedbacks exist, and we should be paying attention."