Refuge from Global Warming May Create Seed Bank for the Future
A pair of scientists who were working to determine how climate change would affect central equatorial Pacific reefs have discovered that some Pacific islands within two degrees north and south of the equator may become isolated climate change refuges for corals and fish.
“The finding that there may be refuges in the tropics where local circulation features buffer the trend of rising sea surface temperature has important implications for the survival of coral reef systems,” said David Garrison, program director in the National Science Foundation’s (NSF) Division of Ocean Sciences, which funded the research.
Here’s how it could happen, according to the study by Woods Hole Oceanographic Institution (WHOI) scientists Kristopher Karnauskas and Anne Cohen, published today in the journal Nature Climate Change:
At the equator, trade winds push a surface current from east to west.
About 100 to 200 meters below, a swift countercurrent develops, flowing in the opposite direction.
This, the Equatorial Undercurrent (EUC), is cooler and rich in nutrients. When it hits an island, like a rock in a river, water is deflected upward on an island’s western flank.
This upwelling process brings cooler water and nutrients to the sunlit surface, creating localized areas where tiny marine plants and corals flourish.
Karnauskas, a climate scientist, was working with Cohen on how climate change would affect central equatorial Pacific reefs. One day he changed the map view on his screen in order to view the entire tropical Pacific at once. What he saw immediately surprised him.
“I’ve been studying the tropical Pacific Ocean for most of my career, and I had never noticed that,” he said. “It jumped out at me immediately, and I thought, ‘there’s probably a story there.'”
The “it” was the global ocean chlorophyll levels as depicted on colour-enhanced satellite maps. They stand out as bright green o red spots, for example, near the Galapagos Islands in the Eastern Pacific, but then fade away like a comet trail as you move your gaze west. However, when Karnauskas changed his map view, he found that the chlorophyll levels jumped up again exactly at the Gilbert Islands on the equator.
Together, Karnauskas and Cohen started investigating how the EUC would affect the equatorial islands’ reef ecosystems, starting with global climate models that simulate the effects of a warming world. But even the best global model will have trouble predicting conditions in small areas such as the islands that scatter the ocean along the equator. So Karnauskas and Cohen combined global models with a fine-scale regional model to focus on the much smaller areas, and used the high-performance computer cluster at WHOI called “Scylla.”
“Global models predict significant temperature increases 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.
“To predict what the coral reef will experience in global climate change, we have to use high-resolution models, not global models.”
The model predicts that as air temperatures rise and equatorial trade winds weaken, the Pacific surface current will also weaken by 15 percent by the end of the century. As a result, the then-weaker surface current will impose less friction and drag on the EUC, strengthening the deeper current 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 0.7 C (1.25 F) per century,” Karnauskas said.
What possible benefit could the survival of a few small coral atolls be?
These coral atolls are some of them only 4 square kilometres in area. It doesn’t seem like much at all. But Karnauskas’ and Cohen’s results say that waters on the western sides of the islands will warm more slowly than at islands 2 degrees, or 138 miles, north and south of the equator that are not in the path of the EUC. This gives the Gilbert Islands a great advantage over neighbouring reef systems in the coming decades.
“While the mitigating effect of a strengthened Equatorial Undercurrent will not spare corals the perhaps-inevitable warming expected for this region, the warming rate will be slower around these equatorial islands,” Karnauskas said.
“This may allow corals and their symbiotic algae a better chance to adapt and survive.”
What this really means is that with the slower increase in warming the atolls may have enough time to adapt to the new environments, and may have enough time to produce larvae of corals and other reef species that could re-colonize damaged reefs.
“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 effect on their own future,” Karnauskas said, “which I think is a beautiful concept.”
Source: National Science Foundation
Image Source: NOAA