Oceans, which cover 71 percent of the Earth's surface, respond more slowly than land to global warming. This warming, however, may cause damage to Arctic and Antarctic ecosystems due to melting sea ice, bleaching and die-off of reef corals, disruption of ocean currents, and shifting predator-prey relationships favoring reduced biodiversity. Long-term effects could include massive extinctions due to changes in seawater chemistry.

Until quite recently, scientists and the general public considered the Earth's oceans to be impervious to anthropogenic degradation. Oceans cover 71 percent of the Earth's surface and account for a little less than half of its primary productivity, that is, the photosynthetic conversion of carbon dioxide (CO2) into the organic compounds that make up the bodies of living organisms.

The ocean is far from being a uniform habitat; however, with the exception of some near-shore environments, ecological niches cover wide areas and intergrade, meaning that species can readily adapt by shifting their ranges. In consequence, environmental pressures producing elevated extinction rates on land have a less dramatic effect in the open ocean.

Nonetheless, human activity has had an adverse effect on marine life, from phytoplankton to top marine predators such as sharks. While overfishing, pollution, and damming of rivers that serve as spawning grounds for marine fish have all taken their toll, these are only indirectly related to global warming.

Present effects attributable to elevated land temperatures include displacement of currents and upwelling zones and increased runoff in major river systems. Effects attributable to elevated sea surface temperatures include melting sea ice in the Arctic and Antarctic, reducing habitat for polar bears, penguins, and the many humbler species of plants and animals that thrive at the margins of the polar ice caps. In the tropics, higher sea surface temperatures alter coral metabolism, causing corals to bleach when they lose symbiotic algae (zooxanthellae) critical to their growth. Degradation of coral reefs profoundly affects the many organisms restricted to this habitat.

If atmospheric CO2 continues to increase, altered seawater chemistry will become a concern. An increase in dissolvedCO2 increases acidity, which in turn inhibits production of shells. Coelenterates such as corals, whose skeletons are made up of aragonite, are more susceptible than are mollusks that have calcite shells. Present-day stunting effects attributable to this cause have not yet been observed in nature, but such stunting is suspected in the geologic record, so coelenterates are being closely monitored.

Scientists recognize at least five major global massextinction events, of which the one at the Permian- Triassic boundary, 251 million years ago, was the most devastating. At that time, 95 percent of marine genera and 70 percent of land genera became extinct in three distinct pulses over a period of about eighty thousand years. At least two respected theories suggest that climate change caused these extinctions.

According to these theories, massive volcanic eruptions in Siberia started the catastrophe in motion. Each eruption caused cooling due to atmospheric volcanic dust and sulfur dioxide, followed by warming due to the longer-lived carbon-dioxide, augmented by heightened decomposition. Over the course of a million years, repeated eruptions,dwarfing anything humans have experienced in their brief tenure on Earth, eventually overwhelmed the Earth's capacity to self-correct. One theory postulates that the ocean depths became increasingly oxygen-depleted, favoring the growth of bacteria that produce hydrogen sulfide. High pressures and cold temperatures in the abyss allowed that gas to build up, only to be released in a gigantic "burp" of highly toxic fumes. Another theory points to the storage of large quantities of methane in the form of clathrates in deep-sea sediments. It suggests that this methane was abruptly released when warming raised the temperature of the deeper regions of the ocean by 5œ Celsius. In addition to being toxic and a powerful greenhouse gas (GHG), methane is explosive at concentrations as low as 5 percent.

Whatever the cause, the extinction, which devastated every group of plants and animals, was extremely abrupt by geological standards. Following the cataclysm, sedimentary rocks are nearly bare of fossils for the first ten million years of the Triassic period (Benton, 2003).

Unless the most carefully researched models are far off the mark, nothing resembling the devastating geochemical upheavals of the Permian-Triassic period looms in the foreseeable future, even if present levels of fossil fuel consumption persist. These models presuppose that volcanic activity will continue at levels typical of the Holocene and that no asteroids are headed in Earth's direction (Peters, 1992).

Possible changes due to increasing acidity are being closely monitored, but so far no notable effects on organisms have been observed in nature. Scientists working with a tiny marine snail that is crucial to Antarctic food chains have demonstrated, however, that there is ample cause for concern, because both higher temperatures and increasing acidity inhibit growth and shell production and can be expected to act synergistically.

Acidity alone is not expected to reach lethal levels for another fifty years, but temperatures are rising rapidly, and marine organisms in the Antarctic cannot adjust their ranges southward. A wide variety of fish and birds depend on this snail for continued survival.

In both the Arctic and the Antarctic, chilled surface seawater sinks, allowing nutrient-rich waters to well up from below and support high phytoplankton productivity. The polar seas teem with life. The lower surfaces of ice sheets also support dense growth of attached algae. Global warming near the North Pole causes the most productive zone to retreat northward and contract in extent. This restricts the number of herbivores and carnivores the system can support. Most polar animals are unable to extend their ranges into temperate seas, because their unique adaptations to frigid temperatures make them poor competitors and susceptible to disease in warmer climates. The situation in the Southern Hemisphere is even more acute, as species migrating southward encounter the continental margin.

The plight of polar bears has received considerable attention. These huge carnivores prey almost entirely on seals that they hunt on sea ice; they hibernate on land. The seals are declining in numbers and retreating farther from shore as the ice cap shrinks. Bears are starving and failing to reproduce. Whale populations that had begun to recover from overexploitation by the whaling industry are also declining again as a result of low food supplies. Antarctic penguins also face declining food supplies and an influx of predators, including sharks, which are extending their ranges southward.

Reef-building corals, and the numerous species that depend upon them, have a narrow temperature range for optimum growth. They are also vulnerable to changes in sea level due to either global warming or global cooling. During the last Pleistocene glaciation, the resulting drop in sea level exposed much of Australia's Great Barrier reef, restricting this unique ecosystem to isolated pockets. A rapid rise in sea level would damage existing reefs by reducing light levels below those needed by symbiotic algae. A 2œ Celsius rise in surface temperature is sufficient to cause bleaching in corals as the individual polyps eject symbiotic algae. Bleaching initially causes growth to cease and eventually kills the coral colony. In recent years, there have been massive die-offs of corals--80 percent in the Caribbean and 50 percent in the South Pacific--but it is uncertain how much of this is due directly to global warming. The die-off in the South Pacific was associated with a severe El Nino-Southern Oscillation (ENSO) event to which global warming may have contributed. Near-shore pollution also devastates coral reefs in populated areas.

References

1. Benton, Michael J., and Richard Twitchett. "How to Kill (Almost) All Life: The End-Permian Extinction Event." Trends in Ecology and Evolution 18, no. 7 (July, 2003): 358-365.

2. Peters, Robert L., and Thomas Lovejoy, eds. Global Warming and Biological Diversity. New Haven, Conn.: Yale University Press, 1992.

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