Natural Ocean Thermostat Helps Protect Pacific Ocean Coral Reefs
Region northeast of Australia has world's highest coral biodiversity
Natural processes may prevent oceans from warming beyond a certain threshold, thereby helping to protect some of the world's largest coral reefs from the impacts of climate change, new research finds.
The research, by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and the Australian Institute of Marine Science, uncovered evidence that an ocean "thermostat" appears to be helping regulate sea-surface temperatures in a biologically-diverse region of the western Pacific Ocean.
The research will be published online on February 9, 2008, in the journal Geophysical Research Letters. It was funded by the National Science Foundation (NSF), NCAR's primary sponsor, with support from the U.S. Department of Energy.
"The research may help explain why this ocean region is the center of biological diversity in coral reefs worldwide," says Phil Taylor, director of NSF's Biological Oceanography program. "This thermostat may be fostering higher species diversity there."
The research team, led by NCAR scientist Joan Kleypas, looked at the Western Pacific Warm Pool, a region northeast of Australia where naturally warm sea-surface temperatures have risen little in recent decades.
As a result, the reefs in that region appear to have suffered relatively few episodes of coral bleaching, a phenomenon that has damaged reefs in other areas where temperature increases have been more pronounced.
The study lends support to a much-debated theory that a natural ocean thermostat prevents sea-surface temperatures from exceeding about 88 degrees Fahrenheit (31 degrees Celsius) in open oceans. If so, this would act to protect reefs that have evolved in naturally warm waters that will not warm much further, as opposed to reefs that live in slightly cooler waters that now face more significant warming.
"Climate change is damaging many corals, but it appears to be bypassing certain reefs that support some of the greatest diversity of life on the planet," Kleypas says. "In essence, reefs that are already in hot water may be more protected from warming than reefs that are not. This is some rare hopeful news for these important ecosystems."
Coral reefs face a multitude of threats, including overfishing, coastal development, pollution, and changes to ocean chemistry caused by rising levels of carbon dioxide in the atmosphere.
Global warming presents a threat because unusually warm ocean temperatures may lead to episodes of coral bleaching, in which the corals turn white after expelling colorful microscopic algae on which they feed. Unless cooler temperatures return in a few days, allowing algae to return, the bleached coral collapses and dies.
Bleaching can occur naturally, but it has become increasingly widespread in recent decades. This is largely because sea-surface temperatures in tropical waters where corals live have increased about 0.5 to 0.7 degrees Fahrenheit (0.3 to 0.4 degrees Celsius) over the last two to three decades, with temperatures occasionally spiking higher.
However, between 1980 and 2005, only four episodes of bleaching have been reported for reefs in the Western Pacific Warm Pool. This is a lower rate than any other reef region, even though the western Pacific reefs appear to be especially sensitive to temperature changes.
Sea-surface temperatures in the warm pool naturally average about 85 degrees Fahrenheit (29 degrees Celsius), which is close to the proposed thermostat limit. They have warmed up by only by about half as much as in cooler areas of the oceans.
Researchers have speculated about several processes that could regulate ocean temperatures. As surface waters warm, more water evaporates, which can increase cloud cover and winds that cool the surface. In some areas, warming alters ocean currents in ways that bring in cooler waters. In addition, the very process of evaporation removes heat.
"Warming waters are just one part of the picture, but they are an important part," says Kleypas. "As we evaluate how and where to protect reefs, we need to determine whether the ocean thermostat offers some protection against coral bleaching."
"Computer models of Earth's climate show that sea-surface temperatures will rise substantially this century," says NCAR scientist Gokhan Danabasoglu, a co-author of the study. "Unfortunately, these simulations show the Western Pacific Warm Pool warming at similar rates 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."
To conduct the research, the authors analyzed 1950-2006 sea-surface temperatures in tropical waters that are home to corals, relying on measurements taken by ships, buoys, and satellites.
They also used the NCAR-based Community Climate System Model (CCSM) 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.
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