Global warming boosting tropical reefs?
Rising sea temperature expands range of Caribbean coral.
November 4, 2003
BETSY MASON - Nature Magazine
Little is known about the reaction of tropical species to global warming
© Alamy
Global warming might not be all bad. Some corals are flourishing, heard this week's annual meeting of the Geological Society of America in Seattle, Washington.
At least one Caribbean genus, Acropora, seems to have capitalized on warmer sea temperatures to expand its range northwards, as it did in the past. So say William Precht of PBS&J, an environmental engineering company based in Miami, Florida, and Richard Aronson of Dauphin Island Sea Lab in Alabama.
In 1998, live samples of the coral were found near Fort Lauderdale, Florida, and in 2002 in the northern Gulf of Mexico off the coast of Texas. Similar northward expansions have been spotted in Pacific corals. "The silver lining in the cloud is that there are these bright spots where some corals aren't going to go extinct and might even expand," Precht told the meeting.
Global warming's impact on plants and animals at middle and high latitudes is well documented. Here temperature gains have been greatest and there have been northward range shifts and changes in the timing of migrations and blooms.
Much less is known about the reaction of tropical species. And for corals, most of the news has been bad. Warmer waters are thought to boost diseases and cause bleaching - the release of algae that are crucial to the coral's survival.
We might not want to put Acropora in the global warming winners' column just yet, counters geologist Dennis Hubbard of Oberlin College in Ohio. The population is likely to swell as Earth warms, but only if tropical sea temperatures stay below the coral's upper temperature limit of 32 °C.
Most models predict that temperatures will not breach this threshold, but that's just our best guess, Hubbard cautions. What's more, today's corals must contend with other human impacts such as pollution - potentially making them more vulnerable to temperatures near their upper limit. "We have thrown a man-made wildcard into the system that the coral reefs haven't had to deal with before," says Hubbard.
Fossil record
Precht and Aronson found their first evidence while repairing a coral reef off the coast near Fort Lauderdale that had been severely damaged by a US nuclear submarine. They noticed that the collision had exposed a fossil reef. The ancient coral was Acropora, whose current range ends 50 kilometres south of the crash site, near Miami.
Using radiocarbon dating, the researchers found that the coral was 7,000 years old. Between 9,000 and 4,000 years ago, sea temperatures in the Atlantic were 2-4 °C warmer than today. "Thousands of years ago there was a luxuriant, ten-metre-thick reef far north of its present extent," says Precht. He has since found several other ancient reefs off the Florida coast, extending as far as Palm Beach, 150 km north of the coral's current limit.
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