Pacific Corals Protected by Natural Cooling?
WHOI climate scientist Kristopher Karnauskas examined this global satellite map of chlorophyll in surface waters. Higher chlorophyll is shown in green, yellow, and red; lower in aqua, blue, or purple. Chlorophyll indicates the growth of phytoplankton. To the left of South America, a line of green chlorophyll extends left (westward) toward the left edge of the map. (Photo: NASA)
For the Geo Quiz, we’re looking for a chain of coral islands in the Pacific Ocean.
The 16 tiny islands straddle the equator, and were inhabited by Micronesians for thousands of years before they were discovered by the west and renamed for a British sea captain.
They later became part of the British Empire, and played a strategic role in the battles between the US and Japan during World War II.
Today, the tiny islands are part of the republic of Kiribati.
And they may become the center of yet another battle – the effort to protect coral reefs from the effects of global warming.
The answer to our Geo Quiz is the Gilbert Islands.
Coral islands and reefs around the world face bleaching from temperature changes in global warming.
Some scientists think the corals of the Gilbert Islands might be spared some of the worst effects because of the Islands’ geography.
At the national aquarium in Baltimore, children ooh and aah at the colors of fish and coral waving and squirming in the Pacific Coral Reef tank in front of them. A fleshy colored Brent Whittaker, the aquarium’s senior director for biological programs, watches from a foot away. Whittaker says the colors in the corals actually come from symbiotic algae that live within them.
But one day, he says he came up to the tank and all the color was gone.
“The corals were bleaching,” Whittaker says. It turns out a construction crew doing renovation work was to blame. The lights were being left on all night long, which Whittaker says may have raised the temperature in the tank ever so slightly.
Corals are very sensitive to temperature. A rise of just one degree Celsius can mess up their relationship with the algae. The algae leave, Whittaker says, and if it goes on too long, “they basically starve to death without them.”
With the corals go the fish, the eels, the shrimp – all the things that live on reefs. And that’s important to people, because while coral reefs take up less than one percent of the surface area of the ocean, it’s estimated that a quarter of all marine life call coral reefs their home at some point in their life, including 25% of seafood consumed by humans.
What happened in the National Aquarium tank was an unintentional demonstration of a situation that’s playing out in real life in reefs all around the world’s oceans.
“Globally, the numbers of corals have reduced substantially,” says the Smithsonian’s Mary Hagerdorn. “Everyone has a different number, but it’s going down, it’s not going up.”
That’s because like the lights above the aquarium’s coral reef tank, global warming is heating up the world’s oceans.
The Gilbert Islands, part of the nation of Kiribati, are coral atolls, strips of land surrounding shallow lagoons. Seen from space, the central lagoons are aqua; deeper water appears black; and low-lying land, composed of broken coral skeleton, forms a bright, narrow border along the lagoons. (Image: NASA)
Scientists predict that in the South Pacific, in particular, the reefs are basically going to cook over the next century. Anne Cohen, of the Woods Hole Oceanographic Institution on Cape Cod says the area includes some of the most remote and pristine coral reefs in the world, but that climate models predict it will warm by three degrees Celsius or more by the end of the century.
“That is a huge rate of warming. Much more than we know corals can actually survive,” Cohen says.
But the rate won’t be the same everywhere. And the variation was driven home when Cohen and her colleague Kris Karnauskas began looking at satellite photos of the Pacific.
They saw some funny little dots, zoomed in on them, and found what they call a “signature” of cold water in an otherwise warm part of the ocean. A handful of coral islands near the equator—the Gilbert Islands, in the nation of Kiribati–were cooler and livelier than their neighbors. Karnauskas says it turns out there’s an ocean-long deep-water cooling current full of nutrients that wells up in the region, like air rushing over a mountain, and fuels the islands.
The WHOI researchers ran some numbers with supercomputers and found that as the climate changes, that current will strengthen. It’ll be like an air conditioning unit, slowing the rate of warming by about .7 degrees Celsius.
“That doesn’t sound like a lot,” Karnauskas says, “and this effect may not spare the corals the inevitable warming for the region as a whole. But that the rate of warming will be slower in these key pockets of coral life may offer them a better chance in the long run for adaptation and survival.”
Anne Cohen puts it a bit more directly. “It’s not looking good for anybody,” she says. “But it’s looking marginally better for a small subset of islands that are geographically well placed.”
Cohen says the Gilbert Islands should get special protections from short term threats like sewage dumping, runoff and fishing so the corals there at least have a chance.
George Stanley, a paleobiologist at the University of Montana, says corals have been around for hundreds of millions of years and have survived mass extinction events. He says they might yet survive climate change too, but that recovery of ecosystems can takes 3-10 million years.
“That’s an incredible amount of time from our standpoint,” Stanley says.
And just a bit too long for humans to wait.
TOP (Present): At present at the equatorial Pacific Gilbert Islands, east-to-west trade winds produce a surface current along the equator. The Equatorial Undercurrent (EUC) flows in the opposite direction at 100 to 200 meters depth. When it hits a barrier (such as an island), the cooler, higher-nutrient water in the EUC flows up toward the surface. The waters near the island exhibit both cooler temperatures (shown as blue) and higher productivity of chlorophyll-containing marine phytoplankton, both of which diminish to the west.BOTTOM (Future): In the future, global climate models predict a sea surface temperature rise of nearly 3 degrees C (5.4 degrees F) by the end of the century. According to Karnauskas’s and Cohen’s fine-scale model, equatorial trade winds will weaken, causing a weakening of the surface current. In turn, the frictional drag on the EUC will lessen, and the EUC will strengthen, carrying more cool, nutrient-enriched water to the surface around the Gilbert Islands. The result will be enhanced productivity close to the islands, and slower warming during the coming century than neighboring islands not in the EUC’s path. The slower warming may allow corals to adapt and survive, making the Gilberts a refuge for coral reef ecosystems. (Illustrations: Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)
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