According to a report by the World Wildlife Fund and Marine Conservation Biology Institute, rising global temperatures affect ocean ecosystems far more than previously acknowledged. From the tropics to the poles, widespread changes in marine life are occurring in step with rising water temperatures.

By Amy Mathews-Amos & Ewann A Berntson

September 1999

A report by the World Wildlife Fund (WWF) and Marine Conservation Biology Institute (MCBI) finds rising global temperatures affecting ocean ecosystems far more than previously acknowledged. From the tropics to the poles, widespread changes in marine life are occurring in step with rising water temperatures.

Especially disturbing is evidence that Pacific salmon may no longer find suitable habitat in the Pacific Ocean. Other effects of warming climate are appearing across the marine food chain, from plankton, penguins and polar bears to fisheries on which humans depend.

Effects on Birds, Fish and Mammals

The report, Turning Up the Heat: How Global Warming Threatens Life in the Sea, is based on a comprehensive review of the latest scientific literature.

Key findings include the ominous possibility that warming could eliminate much, if not all, marine habitat for Pacific sockeye salmon, and probably other salmons as well. Sockeyes are extremely temperature-sensitive: their metabolism increases in warmer water, requiring larger amounts of food. To avoid incurring large energy losses, the salmon must either move into deeper water or migrate northward into the Bering Sea, farther from the freshwater rivers where they spawn.

Drastic declines in western Alaska's Pacific salmon populations in 1997 and 1998 appear related to exceptionally high sea temperatures. Warm water caused a rare bloom of phytoplankton typical of waters closer to the equator. Sharply reduced size of returning salmon and dramatically decreased numbers suggest large-scale starvation.

Reef fish and intertidal invertebrates provide compelling evidence that fish and other species are shifting toward the poles in response to warming. Studies of rocky reef fishes off the California coast show the proportion of northern species declining, and southern species increasing.

Scientists have also linked population decline in seabirds to the warmer water. Sooty shearwaters off the California coast declined 90% in the late 1980s and early 1990s. Cassin's auklets have declined by half. In both cases, the cause seems to be decreases in plankton which forms the base of the birds' food chain. In Alaska, the severe decline in shearwaters from 1997 to 1998 was clearly due to starvation, as their crustacean prey was dramatically reduced in the unusually warm waters. Common murres also died by tens of thousands.

Coral reefs, the most biologically diverse and beautiful marine ecosystems, are also at extreme risk. The upper heat tolerance for many reef corals is just a few degrees above normal temperatures. Beyond that, they expel the colourful food-producing zooxanthellae algae in a process called bleaching. If it is too warm for too long, bleached corals die. High water temperatures in 1997 and 1998 sparked unprecedented bleaching in all major tropical regions. Large numbers of corals are turning completely white and dying, with over 90% mortality in parts of the Indian Ocean.

Polar Problems

Meanwhile, polar regions suffer worse biological impacts than lower latitudes. Sea ice is diminishing in both the Arctic and Antarctic, depriving wildlife of hunting and breeding grounds. Sea ice is habitat for the algae at the base of the polar food web. As ice shrinks, so does the food available at higher levels on the web, from zoo-plankton to seabirds.

In the Antarctic, air temperatures have been increasing since the 1950s, causing significant stress for species that depend on sea ice. Two closely-related species demonstrate the result: chinstrap penguins in the western Antarctic Peninsula have increased in numbers since the 1950s, while Adelie penguins have declined. Both eat the same prey, but Adelies winter on sea ice, while chinstraps prefer open water. Populations of crabeater seals, which also require pack ice, are falling, while southern fur, southern elephant, and other open-water seals are extending their ranges further south.

Researchers in western Hudson Bay have documented decreased weight in adult polar bears and a decline in the birthrate since the early 1980s. They suspect the cause is earlier spring breakup of sea ice. The bears use ice-floes as platforms from which to catch their prey, seals. During the summer months, bears are shore-bound and rely heavily on fat reserves to survive. The entire population must fast for at least four months after the ice has broken up, while pregnant females in Hudson Bay must fast for eight months.

Alaska's Bering Sea has exhibited many ecological changes over the past decade. In addition to major, long-term declines in Steller sea lion and northern fur seal populations, other mammal and bird species appear inreasingly stressed. Small forage fish, such as herring, capelin and larval fishes, have been declining for the past five years. Species that were previously known from more southern climes have appeared in Alaska including Pacific white-sided dolphins, albacore and yellow-fin tuna, and ocean sunfish, and herring spawned earlier than ever before.

Seals and sea lions suffered considerable declines during El Nino years. For example, studies during the 1983-84 El Nino showed females had to dive deeper to find food and were away from their pups longer, causing a drop in milk production and pregnancy rates. Young seals and sea lions had reduced growth rates and higher mortality as a result.

Evidence Warrants Action Now

Ocean temperatures have been rising steadily for as long as 60 years, increasing as much as 2-3% in some places. With such widespread changes in marine life already occurring, the implications for even more dramatic changes in the near future are grave.

'We have ample evidence that current global temperatures are significantly higher than any time in a thousand years,' says WWF's Adam Markham. 'Carbon pollution from the burning of coal and oil is projected to boost temperatures at an accelerating rate in the coming decades. The longer we wait to turn down the heat the fewer our options will be.' - Third World Network Features

About the writers: Amy Mathews-Amos and Ewann A Berntson work with the Marine Conservation Biology Institute. The above article first appeared in Earth Island Journal (Fall 1999). Read a copy of the report at <>