Isolated Pacific Could Become Safe Haven In A Warming World
May 1, 2012

Isolated Pacific Could Become Safe Haven In A Warming World

Jason Farmer for

Based on current models of climate change, scientists predict that ocean temperatures will continue to rise in the equatorial Pacific, potentially devastating coral reef ecosystems. However, a recent study by Woods Hole Oceanographic Institution scientists Kristopher Karnauskas and Anne Cohen suggests that climate change may also cause ocean currents to operate in ways that could attenuate the warming near a number of islands located right on the equator. These isolated Pacific islands could then become safe havens for corals and fish.

Trade winds (winds blowing from the northeast and southeast) push the surface current from east to west at the equator. A swift countercurrent flowing in the opposite direction called the Equatorial Undercurrent (EUC) develops about 100 to 200 meters below the surface. This current is cooler than the surface current and rich in nutrients. When the Equatorial Undercurrent runs into an island, water is deflected upward on the western flank and around the island. This process of ℠upwelling´ brings the cooler water and important nutrients to the surface and creates areas where marine plants and corals can thrive.

Karnauskas and Cohen have been studying the effects of ocean warming trends on central equatorial Pacific reefs using global-scale climate models. These models predict that ocean temperatures will likely rise by 5.4oF in the central tropical Pacific.

The warming of the waters often precipitates a process known as bleaching to occur in which coral lose the algae that live inside them. This symbiotic algae is a vital source of nutrition to the coral. In the last three decades, bleaching has been a major cause of the loss of coral reefs.

Karnauskas and Cohen then began to investigate how the EUC would affect the equatorial islands' reef ecosystems.

"Global models predict significant temperature increase in the central tropical Pacific over the next few decades, but in truth conditions can be highly variable across and around a coral reef island," Cohen said in a recent statement. "To predict what the coral reef will experience under global climate change, we have to use high-resolution models, not global models."

The researchers combined global models with a fine-scale regional model to focus on much smaller areas around islands scattered along the equator. This combination of models yielded some surprising results.

Their model predicts that as air temperatures rise and equatorial trade winds weaken, the Pacific surface current will weaken by 15 percent by the end of the century. The weaker surface current will cause less friction, and therefore drag, on the EUC. As a result, this deeper and cooler current will strengthen by 14 percent.

"Our model suggests that the amount of upwelling will actually increase by about 50 percent around these islands and reduce the rate of warming waters around them by about 0.7oC (1.25oF) per century," Karnauskas said.

"While the mitigating effect of a strengthened Equatorial Undercurrent will not spare the corals the perhaps-inevitable warming expected for this region, the warming rate will be slower around these equatorial islands, which may allow corals and their symbiotic algae a better chance to adapt and survive," Karnauskas said.

These coral reefs might then be a source of novel corals and other species that could potentially re-colonize reefs damaged by global warming.

"The globe is warming, but there are things going on underfoot that will slow that warming for certain parts of certain coral reef islands," said Cohen.

"These little islands in the middle of the ocean can counteract global trends and have a big impact on their own future, which I think is a beautiful concept," Karnauskas said.

The study was published April 29 in the journal Nature Climate Change.


Image Caption: Corals exposed to warm that is too warm often lose the symbiotic algae that live in their tissues and provide them nutrition. They turn white, a phenomenon known as bleaching and a major cause of coral mortality, said WHOI coral scientist Anne Cohen. WHOI climate scientist Kristopher Karnauskas teamed with Cohen to study how global warming may affect reefs in the equatorial Pacific, using a fine-scale model to predict future water conditions around the Gilbert Islands. (Photo by Jessie Kneeland, Woods Hole Oceanographic Institution)