The latest prediction for the risk of coral reef bleaching.

The latest prediction for the risk of coral reef bleaching.

NOAA

Looming coral reef disaster? Scientists divided

Marine biologists are keeping a watchful eye on coral reefs stressed by rising temperatures in the western Pacific—and debating the signals. "The signs are still there that we may see the third global-scale bleaching event in 2015," says C. Mark Eakin, a coral reef ecologist in charge of the Coral Reef Watch, a service of the U.S. National Oceanic and Atmospheric Administration (NOAA), in College Park, Maryland. Others say the prognostication is premature. “I just don’t think we know at this stage," says David Wachenfeld, director of reef recovery for the Great Barrier Reef Marine Park Authority in Townsville, Australia.

The debate centers on how much to trust a computer model and how to assess the variability of local weather. There is a wild card as well: the episodic climatic event known as El Niño, which dramatically warms Pacific waters but affects weather worldwide. One point all reef scientists agree on is that rising seawater temperatures due to climate change make the survival of coral reefs increasingly precarious.

Corals harbor colorful symbiotic algae called zooxanthellae, which use photosynthesis to produce nutrients for themselves and their hosts. When the water gets too hot the corals expel the zooxanthellae and turn white, or bleach. If the water cools soon enough, the algae return. But prolonged bleaching can be lethal.

Previous extensive bleaching events occurred in 1998 when what was arguably the biggest El Niño on record heated up ocean waters, and in 2010, which was also an El Niño year. There may be a mild El Niño now getting under way. "A weak El Niño probably would not have triggered mass coral bleaching and mortality in the late 1990s but is much more likely to do so today after 25 years of ocean warming. This is why there is considerable concern among key scientific groups about the growing risks posed by even a mild El Niño event on the world's coral reefs," says Ove Hoegh-Guldberg, director of the Global Change Institute at the University of Queensland, St. Lucia, in Australia.

NOAA keeps a close eye on sea surface temperatures. Scientists with its Coral Reef Watch use satellite data to plot sea surface temperatures on online maps. Colors indicate four levels of concern for corals: watch, warning, alert level 1, and alert level 2. The office also forecasts seawater temperatures several months in advance.

Modeling late last year predicted the two highest stress levels occurring in January for many areas throughout the western Pacific, the Indian Ocean, and Australia's Great Barrier Reef.              

The NOAA model is a long-term climate model that doesn’t take into account short-term weather events. But with the most recent data on sea surface temperatures, Coral Reef Watch's latest modeling now shows just "watch" for the Great Barrier Reef through January with "alert level 1" arriving toward the end of February. Eakin says that higher temperatures that late in the austral summer probably won't produce severe bleaching on the reef. In addition, the Great Barrier Reef area got some welcome relief in late December from rain, winds that churned up cooler water, and shade from clouds. 

Eakin remains concerned about significant and widespread bleaching elsewhere. Waters are warming up around the Pacific islands of Kiribati, Nauru, and the Solomons. “My big concerns are the reefs on these South Pacific Islands, the Indian Ocean, and perhaps Southeast Asia later this year," he says.

Given the uncertainties, Wachenfeld and other reef scientists want to see how conditions evolve in the coming weeks. Others note bigger threats, such as chronic pollution and destructive fishing. Reducing the impact of these insults will make reefs more resistant to bleaching, says Terry Hughes, a coral reef scientist at James Cook University, Townsville. And, voicing a common sentiment, he says it is already known what needs to be done to limit bleaching: "The sooner we put a limit on carbon emissions the better."

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