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March 2008

Ocean’s natural thermostat may protect some coral reefs

But reefs still face a multitude of threats

Coral reefs are in hot water, both literally and figuratively: it’s estimated that 27% of the planet’s reefs are now dead, half of these due to global warming.

As Earth gets hotter, the oceans warm. When the temperature of seawater gets high enough, corals expel the colorful microscopic algae that provide them with nutrition. In a process known as coral bleaching, they turn white. Unless cooler temperatures return in a few days or weeks, the corals often collapse and die.

Beleaguered reefs recently got a rare bit of good news, however. Natural processes in the ocean may prevent waters from warming beyond a certain point, helping protect some reefs from the impacts of climate change, according to new research led by Joanie Kleypas (SERE/ISSE), in collaboration with Gokhan Danabasoglu (ESSL/CGD) and colleagues at the Australian Institute of Marine Science.

The researchers found evidence that a natural ocean thermostat appears to be regulating sea-surface temperatures in a biologically diverse region of the western Pacific. Their study, published in Geophysical Research Letters in February, lends support to the much-debated theory that such a thermostat prevents sea-surface temperatures from exceeding about 88°F (31°C) in open oceans. If so, this thermostat would protect reefs that have evolved in naturally warm waters that will not warm much further under climate change (as opposed to reefs that live in slightly cooler waters that are primed for more significant warming).

“In essence, reefs that are already in hot water may be more protected from warming than reefs that are not,” Joanie says. “This is some rare hopeful news for these important ecosystems.”

A close look at bleaching

Often described as the rainforests of the sea, coral reefs are rich with biodiversity, sheltering thousands of marine plants and animals. They also protect coastlines from erosion, create habitat for mangrove forests and sea grass beds, add to local economies, and attract tourists.


Before-and-after photos of a coral reef show the dramatic effects of bleaching. (Photos courtesy Ray Berkelmans, Australian Institute of Marine Science.)

Although reefs around the world are stressed by pollution, development, and overfishing, rising water temperatures that cause coral bleaching are perhaps their greatest threat. Bleaching can occur naturally, but it has become increasingly widespread over the last several decades as sea-surface temperatures in tropical waters where corals live have increased about 0.5–0.7°F (0.3–0.4°C), occasionally spiking higher.

For the new study, Joanie and her colleagues looked at a region of the ocean northeast of Australia known as the Western Pacific Warm Pool. The pool has some of the warmest ocean temperatures in the world, averaging about 84°F (29°C). Its waters have warmed up about half as much as cooler areas of the oceans, and its reefs appear to have suffered relatively few episodes of coral bleaching. Between 1980 and 2005, only four episodes of bleaching were reported—showing a lower rate of bleaching than any other reef region.

“Global warming is damaging many corals, but it appears to be bypassing certain reefs that support some of the greatest diversity of life on the planet,” Joanie says.

To study the correlation between temperatures and bleaching, the researchers analyzed sea-surface temperatures from the period 1950–2006 in tropical waters that are home to corals, relying on several existing sea-surface temperature data sets compiled from measurements taken by ships, buoys, and satellites. They also used the NCAR-based Community Climate System Model to study computer simulations of past and future sea-surface temperatures. The team compared the actual and simulated temperatures to a database of coral bleaching reports, mostly taken from 1980 to 2005.

Inside the thermostat

Scientists speculate that several processes could regulate ocean temperatures. As the ocean’s surface waters warm, more water evaporates, which removes heat and can increase cloud cover and winds that in turn cool the surface. And in some areas, warming alters ocean currents in ways that bring in cooler waters.

Scientists are uncertain, however, whether global warming will alter the thermostat, raising the upper limit for sea-surface temperatures. Computer model simulations tend to capture the slow rate of warming in the western Pacific Ocean over the last few decades, but they show the warm pool heating rapidly this century in line with other ocean areas.

“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,” Gokhan says. “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.”

pacific warm pool

The Western Pacific Warm Pool contains some of the warmest ocean waters in the world. Water temperatures in the warm pool have risen less than elsewhere in the tropics, which may explain why reefs there have experienced less coral bleaching. (Illustration by Steve Deyo.)

Year of the Reef

The year 2008 has been named International Year of the Reef as part of a campaign to raise awareness about the importance of coral reefs and threats to their sustainability.

In addition to reefs’ contributions to biodiversity, coastline protection, and healthy marine ecosystems, it is estimated that the net economic benefit of the world’s reefs amounts to about $30 billion.

Reefs are highly sensitive to environmental perturbations. Slight changes in incoming sunlight, water temperature, salinity, and acidity affect coral’s ability to thrive. Human impacts, both indirect (greenhouse gas emissions) and direct (sewage or sediment runoff, for example, or blast-fishing with dynamite) threaten the long-term survival of many coral colonies. Furthermore, research led by Joanie in 2006 showed that carbon dioxide emissions are increasing the ocean’s acidity (see On the Web). This lowers concentrations of carbonate ion, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and create coral reef structures.

“We need to go beyond the dire predictions for coral reefs and find ways to conserve them,” Joanie says.

On the Web

NCAR Field Guide: Climate’s Impact on Coral and Reef Systems
Report Warns about Carbon Dioxide Threats to Marine Life

In this issue...

Tim Killeen to head geosciences for NSF

Project BudBurst blooms

Upcoming conferences

Ocean’s natural thermostat may protect some coral reefs

Let it melt

Delphi Question

Short Takes

Just One Look

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