THEGREENGROK    Planetary Watch

Coral Reefs Head North

by Bill Chameides | February 24th, 2011
posted by Erica Rowell (Editor)

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Global warming is a coral reef migration.

As the globe warms, many temperature-sensitive species have to adapt or expire. Many in the scientific community have been particularly concerned about coral reefs. A new study in the journal Geophysical Research Letters by researchers in Japan suggests that the threat to coral reefs may not be that dire.

Coral Reefs: Rain Forests of the Ocean

Coral reefs are fundamental to the world’s oceans and marine life. Maybe you’ve heard the expression that rain forests are the lungs of the planet? Well, to marine scientists, coral reefs are the ocean’s rain forests — areas of incredibly rich biodiversity that serve as habitat and food for a wide array of marine life, on the order of nine million species. And so the fate of these reefs has major implications for fisheries and marine ecosystems. Roughly half a billion people depend on coral reefs for some part of their food, shelter or income.

The Global Warming Threat

Reefs face many threats — natural ones include biological infestation, diseases, storms, and excessive rainfall. Manmade hazards include over-fishing and coastal development. Into this mix we’ve added risks from global warming.

Rising temperatures can disrupt the symbiotic relationship between coral reefs and the algae that live on them, causing the reefs to suffer and even die. As temperatures rise, temperature-sensitive algae that sustain the coral are expelled or killed. Though corals can recover from such events, which are known as coral bleaching, they can also be seriously damaged or perish from them, depending on the temperature or duration of the event.

Such phenomena have been observed from the Caribbean to the Pacific and Indian Oceans, and have often been especially extreme following summers with unusually warm sea surface temperatures. For example, in 1998 when temperatures in the Indian Ocean spiked to levels not seen in half a century, bleaching led to the death of 50-90 percent of corals on many reefs. It’s estimated that 16 percent of the world’s corals died from that event. In the Caribbean in 2005 some 28 percent of observed coral was bleached.

Ocean acidification represents another hazard for coral reefs. All that extra carbon dioxide (CO2) added to the atmosphere is also finding its way into the ocean causing a tiny but potentially life-threatening change in ocean chemistry. The resulting drop in ocean alkalinity makes it harder for many marine organisms with calcareous shells and exoskeletons, including corals, to produce those hard exteriors. If they have a harder time surviving, eventually so do all the organisms dependent on these creatures.

The View From Japan

Researcher Hiroya Yamano of the National Institute for Environmental Studies in Tsukuba, Japan, and colleagues looked specifically at corals in temperate waters to gauge their response to global warming. They examined data on the distribution of nine coral species off the coast of Japan spanning an 80-year period during which minimum wintertime sea surface temperatures have systematically increased [pdf] (between about 1 to 3 degrees Fahrenheit over the last 100 years).

You might think the authors found corals in distress. You would be wrong, at least for the nine species they studied. Of those nine, five species showed little change — no evidence of extinction and no significant change in geographic distribution. The other four did show changes, but not the way one might have expected. These species actually expanded their range — they remained in their southerly locations despite the increase in temperatures, in addition to expanding northward. Interestingly, they also found that two of the four species that expanded their range in the temperate waters off Japan’s coast are species typically found in the tropical waters of the Indian and Pacific Oceans.

The good news in all this is that the ability of coral species and their communities to move northward suggests that more temperate regions “may serve as refugia for tropical corals … while corals in tropical areas suffer declines because of rising [sea surface temperatures] SSTs.” The future of coral reefs may not be as grim as many have worried.

But there is another side. With the expansion of the tropical corals northward into more temperate waters, other “endemic species … could suffer declines as a result of competition caused by the invasion of the tropical species.”

And that raises a larger question and, I think, a difficult conundrum: what is, and what can be done about, invasive species in the context of global warming? It is generally accepted in many circles that the migration of species (often propagated by humans) can damage and disrupt the ecological balance of the native species and should be controlled or even eliminated — these are the so-called invasive species, the undesired outsiders. But with climate change, species are often going to have to move to survive. When they arrive in a new region, are they climate refugees in need of protection and stewardship … or invaders that need to be sent packing?

filed under: climate change, coral reefs, faculty, global warming, oceans, Planetary Watch
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