The Pew Center on Global Climate Change report, "Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems," will be released Friday at the annual meeting of the American Association for the Advancement of Science. The report, written by Kleypas with Robert Buddemeier (Kansas Geological Survey) and Richard Aronson (Dauphin Island Sea Lab), analyzes the likely impacts of climate change over the next century on coral reef ecosystems around the world. Kleypas and other scientists will discuss the threats to coral reefs at an AAAS session on February 14.
Scientists are finding that human emissions of carbon dioxide and other greenhouse gases, a primary cause of climate change, endanger reefs in two important ways. First, higher water temperatures are promoting coral "bleaching"--episodes in which corals and other reef-building species are weakened or killed after losing vital algae that lives within their tissues. Although coral species can recover from bleaching to some degree, repeated bleaching events are likely to eliminate sensitive organisms and reduce biodiversity. Elevated water temperatures are also thought to be a factor in the recent increase in coral diseases in the Caribbean.
Second, as carbon dioxide builds up in the atmosphere, more of it is dissolved into the ocean, which increases ocean acidity. This lowers concentrations of the carbonate ion, a building block of calcium carbonate that corals and other organisms use to grow their skeletons and build up the reefs. Carbon dioxide levels in the atmosphere, which were about 280 parts per million by volume (ppmv) in 1880, increased to 367 ppmv by 2000 and are expected to reach from 463 to 623 ppmv by 2050. An increase to 560 ppmv would cause an estimated 30% reduction in the carbonate ion concentration in the upper ocean and affect both skeletal growth rates and the structural growth of reefs.
"Coral reef ecosystems are going to be significantly impacted by climate change," Kleypas says. "They're already being degraded by both climate change and by direct impacts such as overfishing and habitat loss, and the combination of these stresses can be devastating."
The Pew report notes that coral reefs are among the most biologically diverse ecosystems on Earth, providing vital habitat to numerous species as well as economic benefits to society in the form of fishing and tourism. One recent estimate concluded the world's reefs provide annual net economic benefits of $30 billion. But more than 10% of the reefs have been destroyed by human activity, and an additional 16% were extensively damaged in 1997–98 alone by coral bleaching during a major El Niño event.
Kleypas emphasizes that it is impossible to predict the future of reefs with any precision because, in the millions of years that modern coral reefs have flourished, the atmosphere has rarely, if ever, contained the high levels of carbon dioxide that are expected to accumulate in coming decades. However, recent losses in reefs show that human-related impacts are exceeding the adaptive ability of coral reef organisms. The severity of what has come to be known as the "coral reef crisis" is expected to worsen with future climate changes.
"We're really outside of any normal envelope that coral reefs have existed in," Kleypas says. "We haven't had carbon dioxide levels this high for thousands of years, maybe millions of years, and given the sensitivity of coral reefs to high temperature and ocean acidity, we can't expect the ecosystem to stay the way it is."
The report notes that local and some regional stresses, including overfishing and development, can be managed more easily than global climate change. The establishment of coral reef refuges and marine protected areas would enable coral reefs to better adapt to the impacts of climate change.
"Reducing the direct impacts will go a long way toward slowing down the degradation, but if we don't also cut back on carbon dioxide emissions, we will probably lose some vital species," Kleypas says.
The report is the tenth in a series of Pew Center reports examining the potential impacts of climate change on the U.S. environment. The National Science Foundation, NCAR's primary sponsor, funds Kleypas's research.
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