The current state of most of the world’s coral reefs is so calamitous that it’s difficult to over-dramatize the situation.

Reefs have seen massive declines around the globe, and while there is much debate about which particular threat is most responsible, most scientists agree humans are to blame.

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Overfishing, pollution, climate change pose major risks.

Overfishing throws reef systems out of balance, pollution fertilizes the spread of smothering algae, sediment from land clearing washes into the ocean gumming up reefs, and all of this can be exacerbated or even eclipsed by climate change. Indeed, coral reefs are considered one of the most obvious climate change canaries, because even slight temperature increases can send them into a dangerous or even fatal tailspin, and carbon dioxide itself poses a direct threat by altering seawater chemistry in a way that handicaps corals.

And by most accounts, the situation is getting worse. That’s of serious concern, because reefs provide far more than pretty scenery as they protect coasts from storm damage, are the focus of many tourism economies, and support commercially and recreationally important fish populations, among other benefits.

The National Oceanic and Atmospheric Administration has the unwieldy task of summarizing every few years, most recently in 2008, the state of the widespread reefs under U.S. jurisdiction, a task that includes gauging the degree to which those human threats are harming the reefs. It’s not a pretty summary, to say the least.

Mostly Dismal Numbers

It’s impossible to make any blanket statements about reefs because, scattered as they are in all tropical seas save the Indian Ocean, they experience widely varying conditions and are often monitored inconsistently. Nonetheless, if you are a coral, the Pacific Ocean is undoubtedly a safer place to be than the Atlantic. In some of the more remote regions such as the Northwest Hawaiian Islands and Guam, reefs may be more than 70 percent covered in live coral. But coral health tends to decline with increasing proximity to humans, which leads to coral cover in the main Hawaiian Islands at about 20 percent.

This all compares to the Atlantic and Caribbean, where it’s difficult to find a reef with more than 10 percent cover. In southeast Florida, it’s generally less than 2.5 percent, a fact you’re not likely to spot in tourism campaigns for Miami and the Florida Keys.

To give some historical perspective, the population of Acropora corals, once common if more fragile than some other species groups, has declined more than 90 percent at numerous sites since the 1970s, and two Acropora species are now officially considered threatened.

Such numbers give some indication of the starting point against which future climate change threats will be measured. Most agree that if business as usual warming scenarios play out, reefs’ fate will fall somewhere between serious damage and alteration to total annihilation. And even the most remote coral outposts are still subject to damage from climate change impacts.

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Calamitous declines of reefs are reported worldwide.

The Threats

The most immediate concern is that as atmospheric temperatures increase, the oceans warm, at times causing coral bleaching. This occurs when the individual coral animals that make up a reef kick out of their cells, by mechanisms still not quite clear, the symbiotic algae that provide most of their food. Water temperatures even one or two degrees Celsius above their normal ceiling can cause bleaching, and while corals can recover from this after reacquiring algae, in many cases they do not. Bleaching often increases corals’ susceptibility to disease, which can also cause death.

The most recent known major bleaching event in U.S. waters devastated many Caribbean reefs in 2005. “It confirmed that our worst nightmare was coming true,” says Jenny Waddell, chief editor of the NOAA reefs report. “When you see a loss of live coral cover of 50 to 90 percent in a nine-month period, it increases the urgency with which people are working.”

As if all that weren’t enough to deal with, over the past decade it has become clear that the increased carbon dioxide in the atmosphere largely responsible for global warming also has an insidious and potentially devastating direct impact on reefs. Carbon dioxide is a weak acid and in seawater it reacts with the carbonate that corals use to build reefs, converting it into a form the corals can’t use, thus decreasing the amount of building material available. Eventually, the carbonate concentration can go so low that corals simply can’t build reefs at all. Ocean acidity has already increased measurably in past decades, meaning reefs may have already been weakened.

As dire as the situation is for coral reefs, the writing on the wall may not yet be entirely indellible. A recent paper in the journal Coral Reefs, where debates among the notoriously argumentative reef researchers often play out, suggested that the evidence may not yet be strong enough to support the theory that reefs could be completely wiped out this century as a result of climate change. Though the authors recognized the potential for major damage, they are also concerned that talking about threat to corals as if there is absolutely no hope may curtail needed efforts to protect reefs. Another respected researcher rebutted the points, making the case that the situation is as dire as, if not worse than, the messages that have been getting out to the public.

Even sparring researchers agree there is still hope for reefs if greenhouse gas emissions can be substantially curtailed. But modeling suggests there’s not much time, particularly given that the greenhouse gas already pumped into the atmosphere would cause significant additional warming even if all emissions ceased today – which, for those not following international climate negotiations, is not terribly likely.

Playing the Odds

Researchers can use modified climate models to gauge the threats to reefs under various future scenarios. An issue of key concern is when reefs in a given region will be exposed to such frequent bleaching events that they won’t be able to recover sufficiently in between. Work led by Simon Donner, an ecologist and climate modeler at the University of British Columbia in Vancouver, suggests that in most regions that point could be reached within just a few decades or sooner.

Many variables will determine what actually happens, however, not only in the way climate change will affect temperatures in a given region, but also how individual reefs, which have varying susceptibilities to bleaching, will respond to warmer waters. For reasons not yet clear, some corals, for instance those found in a small lagoon in American Samoa, bleach frequently, even once a year, but can bounce back each time. Just why is not yet understood, but studying such resiliency has become a major priority in reef research.

A much more typical result of coral bleaching is slow recovery with significant losses of coral, as happened with the 2005 Caribbean event. The Donner group has shown that this bleaching was about 1,000 times more likely to be the result of the warming effect of human greenhouse gas emissions. “That was really like a once in 500 or 1,000 years event in a pre-industrial climate,” says Donner, “It might happen but it would be extremely rare.” In today’s climate, the modeling suggests such events could occur ever 100 years, or even once per decade.

For the short term, U.S. agencies and managers are looking for ways to improve the chances of sufficient recovery after a bleaching by reducing human impacts, which appear to increase a reef’s susceptibility to damage, much as AIDS increases humans’ susceptibility to other threats.

The idea, still in its infancy, would be to reduce fishing, activities that put silt or pollutants in the water around reefs, and tourism activities in the months leading up to and during bleaching events.

NOAA already predicts the onset of bleachings, giving managers time to implement emergency plans, though such actions clearly can encounter opposition from those worried about short-term economic impacts.

Along with The Nature Conservancy, NOAA now runs training sessions with managers to teach potential bleaching responses.

“There is value in this idea of reducing the other immediate stresses, but there is only so much we can do,” says Donner, because research suggests that most corals at best have a survival window of just a couple of degrees Celsius. Once ocean temperatures rise beyond that, bleaching and acidification may well destroy reefs as we know them.

For this reason, many coral scientists, as evidenced through various papers and declarations, are strong and outspoken proponents of major reductions in greenhouse gas emissions as the only option for long-term survival of coral reefs resembling to any extent even the degraded reefs we now enjoy.

Mark Schrope is a freelance science writer living in Melbourne, FL.