As Temperatures Rise, Corals Fall:The Effect of Climate Change on Reefs
With increased levels of carbon in the atmosphere, the temperature of the sea and its elevation are expected to rise. For corals and coral reefs, the impacts may well be devastating. And, should reef-building corals decline, the protection that they offer to Hawai`i’s coasts would be correspondingly diminished or lost altogether.
The threat of climate change to coral reefs has long been discussed, but one of the most thorough studies was released only in early July. That report, by Ove Hoegh-Guldberg of the University of Sydney School of Biological Sciences, contains some of the direst predictions yet for the fate of coral reefs in a warming environment.
As sea surface temperatures rise, Hoegh-Guldberg predicts a rise also in the phenomenon known as coral bleaching. This is not new; a link between high temperatures and coral bleaching has been observed since the 1980s. But what Hoegh-Guldberg does is predict future bleaching events based on past observations and future expected sea surface temperatures drawn from four different climate-change models. All the models predict much the same thing: coral bleaching events will increase in frequency and severity over the next century. While he does not predict that corals will become extinct, he does anticipate that — barring any action to reduce the expected rise in atmospheric carbon levels — by the middle of the 21st century coral bleaching will be an annual event in most parts of the world where coral reefs are found.
This near-constant stress on corals, Hoegh-Guldberg says, “is expected to severely degrade reefs and is likely to cause a phase shift away from coral dominated communities by approximately 2050.” In addition, he writes, “coral reefs are also threatened by changes in the alkalinity of seawater and by rising sea levels that are also associated with global climate change.
“These changes, combined with the increasing stress on reefs from human related activity, suggest that coral reefs may become dysfunctional within the near future.”
A dysfunctional reef would no longer be able to serve as a nursery and habitat for many species of commercially valuable fish. It would not be able to serve as a first line of coastal defense against storm surges and hurricanes. Revenue associated with diving, snorkeling, fishing, and other reef-centered activities would plummet. Beach losses would surely increase, along with associated economic activity. Property damage from storms and high waves may be predicted to rise, as would insurance rates for owners of coastal land.
Heat Waves
The effect of warming seas on coral will be different from place to place, Hoegh-Guldberg predicts. One of the first areas to be hit will be Australia’s Great Barrier Reef, which could experience annual severe bleaching events by the year 2030, starting at the southern end. The last areas to be affected, under Hoegh-Guldberg’s model, would be the Central Pacific, including Hawai`i, where severe bleaching on an annual basis may not begin until the middle of the 21st century.
That predicted delay does not mean Hawai`i has nothing to worry about. If global warming scenarios are correct, and Hoegh-Guldberg’s predictions of annual bleaching events moving slowly northward up the Pacific are borne out, Hawai`i will be in a position of doing little but wait for the inexorable arrival of the heat wave.
The economic impacts of a loss of coral reefs in Hawai`i would be almost impossible to measure, but by any standard, it would be huge. According to 1990 survey figures gathered by the Department of Business, Economic Development, and Tourism, ocean recreational activities contributed some $560 million a year to the state’s economy. Much, if not most, of that would be lost if coral reefs die off. With much of Hawai`i’s attraction to tourists based on ocean recreation opportunities, a decline in visitor arrivals would be expected as well, with proportional fall-off in hotel occupancy rates, car rentals, restaurant revenues, and the like. Families who derive income from these sectors would experience catastrophic loss, and homeowners’ insurance rates would probably rise far in excess of anything seen in the wake of Hurricane `Iniki.
Offsetting Gains?
In the few weeks since Hoegh-Guldberg’s article has been published, some scientists have said his predictions are too bleak and do not take into account the possibility that global warming could open up new habitat for corals. This, they say, could offset losses due to coral bleaching.
Hoegh-Guldberg addresses this possibility. “Changes in sea level will mean that reef ecosystems at the depth limit of coral growth will experience light conditions that will no longer sustain growth,” he writes. “Consequently, coral communities at these depths would be expected to disappear from these regions. As sea levels rise, however, new spaces for growth will become available at the upper regions of coral growth.”
But new growth is unlikely to outstrip coral losses. “If growth rates are reduced by thermal and other stresses, then sea level change would be expected more and more to exceed coral growth even under moderate global climate change scenarios. This effect alone means additional challenges for coral reefs in the future.”
In addition to thermal changes, changes in the chemistry of near-shore ocean water are likely to occur with global warming. Increased levels of carbon dioxide dissolved in the water will almost certainly diminish the saturation levels of aragonite (or calcium carbonate). This, in turn, “is expected to decrease the calcification rate of corals and other organism by 14 to 30 percent by 2050,” Hoegh-Guldberg notes. Noting that the net rate of calcification is the outcome of a balance between coral growth and erosion, he goes on to say, “a reduction in the calcification rate of this size is likely to tip the balance in favor of the net disappearance of coral reef calcium carbonate in many locations… [A] decrease in the rate of calcification of as little as 5 percent could lead to a net loss of calcium carbonate…”
“The implications of a net loss of calcium carbonate from the reef systems protecting coastlines are enormous. Not only are millions of human dwellings at risk, but substantial proportions of other vital coastal habitats… The effect of reduced coastal protection due to weakened or rapidly eroding coral reefs could add substantially to the potential costs associated with warming tropical seas, especially as these areas in turn support fisheries and crucial nursery areas for up to 90 percent of all commercial fisheries.”
Corals will probably not become extinct, but, as Hoegh-Guldberg observes, that is not the issue: “[C]orals as species have survived greater changes over geological time and are unlikely to be forced into extinction by the projected changes to sea temperatures. The projected increases in sea temperature will, however, cause teh condition of coral reefs to be severely compromised over the next several hundred years at least. While a mere blink in geological time, this length of time is highly significant to the 100 million people who are estimated to depend directly on coral reefs for their livelihood. Arguments based on geological time do not compensate current and future generations for the trillion dollar costs of losing healthy tropical marine communities for the next several centuries.”
Other Impacts
Coral reefs have been likened to the proverbial canary in the coal mine, giving early, visible warning of dangerous ocean trends. But they are not the only biological systems that stand to be harmed by shifts in the Earth’s climate.
One of the potentially most serious disturbances in ocean ecology was reported in May in Science magazine (“Long-Term Discrepancy Between Food Supply and Demand in the Deep Eastern North Pacific”).
A seven-year-long study of food supply in the deep North Pacific “revealed a long-term deficit in food supply,” write Kenneth L. Smith Jr., of Scripps Institution of Oceanography, and Ronald S. Kaufamnn, of the University of San Diego Marine and Environmental Studies Program. They speculate that the decrease in food reaching ocean depths results from the documented increase in sea surface temperatures and an accompanying decrease in the volume of plankton in the eastern North Pacific. “Continuation of this trend,” they write, “could profoundly impact geochemical cycling as well as the structure and dynamics of deep-sea communities.”
For organisms living on the deep ocean floor, food rains down in the form of particulate organic carbon from disintegrating plants, animals, and fecal matter. If the food deficit continues, “it is going to change the configuration of the deep-sea communities,” Smith told The New York Times.
— Emma Yuen
Volume 10, Number 6 December 1999