The world's oceans cover more than 70% of our planet's surface and the rich web of life they support is the result of hundreds of millions of years of evolution. Nomadic peoples were collecting shellfish and harvesting fish long before the dawn of settled agriculture. Great human civilizations, from the Egyptians to the Polynesians relied on the sea for commerce and transport, and now, at the end of the Twentieth Century, our fate is as tied to the oceans as ever. We still rely on fish for a significant portion of our daily protein needs, and more than $500 billion of the world's economy is tied to ocean-based industries such as coastal tourism and shipping. Perhaps most important, this vast mass of water acts to help regulate the global climate and to ensure that a constant flow of vital nutrients is cycled throughout the biosphere.

But all is not well in the sea. Increased pressures from overfishing, habitat destruction, pollution and the introduction of invasive alien species have combined in recent decades to threaten the diversity of life in estuaries, coastal waters and oceans. Now a new threat, global warming, is making itself felt, and its impacts could be devastating for life in the sea.

There can be no doubt that our world is getting warmer. 1998 was the hottest year since accurate records began in the 1840s, and ten of the hottest years have occurred during the last 15 years. By examining growth rings from trees and ice cores drilled in Antarctica, scientists have determined that the past decade was the warmest in more than four centuries, and that the current rate of warming is probably unprecedented in at least 10,000 years. In 1992, the more than 2500 scientists comprising the Intergovernmental Panel on Climate Change concluded that the warming is caused at least in part by emissions of greenhouse gases from fossil fuel use. As the world warms, the outlook for marine wildlife looks bleak unless we can turn down the heat by reducing concentrations of the main greenhouse gas, carbon dioxide, in the atmosphere.

Sea-level Rise

One of the most talked about problems associated with climate change is sea-level rise. Rising oceans could flood many millions of hectares of valuable coastal ecosystems world-wide, from the shorebird havens of northern Europe's Waddensea and South America's Suriname, to the highly populated river deltas of China, Vietnam and Bangladesh, where millions of people live and depend on coastal fisheries. Sea-level rise threatens whole nations on the low-lying coral atolls of the Pacific and Indian Oceans and is already flooding valuable wetlands in America's Chesapeake Bay and Louisiana's Mississipi Delta.

Ocean Circulation

The havoc global warming could wreak on ocean life may be much greater than we have previously imagined. The water in the oceans circulates slowly around the world, driven by a complex system of currents. Wind and the rotation of the Earth are important in determining the flow of surface currents and local areas of upwelling and downwelling, but the true driving force of deep water movement is thermohaline circulation. Sometimes called the ocean conveyer belt, this mechanism is responsible for bringing the oxygen that sustains life to the deepest reaches of the sea, and in moving warmer waters from the tropics towards the poles. Movement of this conveyer belt depends on sinking of cold water in certain polar regions, thereby triggering the global thermohaline circulation. Global warming could alter this. Because freshwater is less dense than seawater, increased precipitation, melting of polar glaciers and ice caps could block the system by reducing the amount of cold water that sinks downwards. The consequences of shutting down the conveyer belt would be devastating for marine life.

Polar Impacts

Signs of global warming are being observed throughout the ocean realm. The most obvious changes are in the regions near the poles, where the warming has already been, and will continue to be, greatest. The ice-edge is retreating in the Antarctic, and scientists worry about irreversible changes, as happened when a huge portion of the Larsen ice shelf collapsed in 1995 and large areas of two more ice sheets shattered into the sea in 1999. Arctic ice masses appear to be thinning noticeably and seasonal ice is forming later and melting earlier. Scientific data is backed up by the first-hand observations of native Yupik and other indigenous communities in the Arctic. They report that changes in ice conditions are making it dangerous to hunt in the Spring, and that species dependent on sea ice, such as walrus, seals and polar bears are already suffering. Global warming may threaten a whole way of life for these subsistence communities. Other alarming signs of warming in 1997 and 1998 in the Arctic were never-before seen blooms of marine algae so massive that they were easily visible from space. In the Antarctic, declines in Adélie penguin and crabeater seal populations have been linked to altered snow and ice conditions, and warming has also been linked to reduced numbers of krill.

Coral Reefs

In the tropics, global warming may be precipitating a new crisis for coral reefs. Marine biologists note that 1997 and 1998 saw the worst episode of coral bleaching on record. Reports of bleaching due to high sea temperatures came in from all over the world, with more than 30 nations reporting losses in their reefs. Almost all species were affected, and the Indian Ocean was particularly badly hit, with more than 90% of coral dying in some areas. While currently, there is no direct scientific evidence to suggest global warming is threathening corals, the high temperatures in 1997 and 1998 were attributed to an unusually severe El Nino event, and signal the types of changes we can expect as temperatures continue to climb. New evidence suggests that even the increased levels of carbon dioxide in the atmosphere may be harmful for reefs as it weakens corals' calcium carbonate skeletons and makes them more susceptible to storm damage and other erosion. Mobile-gear fishing is clearly a bigger threat, particularly in the case of deep sea corals. Coral reefs are home to an extraordinarily rich array of marine life and provide vital resources to the tourism and fishing industries.

Seabirds and invertebrates

Global warming is expected to decrease ocean productivity. We may be seeing this effect already: zooplankton in the California Current have decreased more than 70% since the 1950s, and may explain dramatic declines in seabirds like sooty shearwaters and Cassin's auklets in recent years. Shearwaters and other seabirds off Alaska including common murres died from starvation by the tens of thousands in the exceptionally warm years of 1997 and 1998. Other species have been starting to change their range in response to warming. Studies by scientists at Monterey Bay indicate that species like anemones and crabs from the intertidal rock pools of the California coast may be shifting their distribution in response to warming. Southern species have increased in abundance and northern species have declined over the past 60 years as waters have warmed. Similar changes have been recorded for the English Channel.

Increasing Threats to Salmon

Among the most alarming recent changes has been the crash in populations of some north Pacific salmon populations in the unusually warm years of 1997 and 1998. Canadian scientists have shown that sockeye salmon are extremely sensitive to changes in water temperature, particularly in the winter. Other species, including steelhead, chum, and coho are similarly sensitive to water temperature. The warmer it gets, the more food salmon need to survive. Eventually they reach a temperature threshold and simply starve to death. Scientists predict that around the middle of the next century there may be no marine habitat cool enough for salmon in the surface waters of the Pacific. They will either have to migrate halfway up the Bering Sea, or learn to live deeper in the water column. If neither happens, they may simply go extinct.

The startling changes already beginning to affect marine life may turn out to be merely the tip of iceberg. Global warming is predicted to worsen rapidly, with average annual temperatures expected to increase by about 3 degrees C by the middle of the next century. Changes of this speed and magnitude could set off a chain reaction in marine ecosystems with truly appalling consequences for life in the sea and for human communities that depend on it. However, if we act now to reduce carbon pollution from the dirtiest power stations and from vehicle exhausts, we stand a good chance of slowing the warming and helping to save a healthy ocean for future generations.

Source:

WWF (1999) Global Warming: The Oceans in Peril.World Wildlife Fund (WWF) and Marine Conservation Biology Institute (MCBI)