Climate Change Affecting Marine Mammals?
22 July 1998 - 10:05am
WASHINGTON - A sudden 2-degree rise in ocean water temperature 20 years ago may be causing a decline in fish, birds, seaweed and some mammals along the West Coast, researchers say in a study published today.
John A. McGowan of the Scripps Institution of Oceanography, co-author of the study, said the warming trend may signal a climate change deeper than just a temporary El Nino effect.
"It's clear the entire system has warmed up," said McGowan, co-author of the study in the journal Science.
The warming trend was spotted in an analysis of coastal temperature measurements taken daily at 17 sites from La Jolla, Calif., to Charleston, Ore. Temperature readings at some of the sites started in 1916 and are some of the most extensive ocean thermal readings in the world, McGowan said.
The rise in sea surface temperature, said McGowan, "is not just in California. It is the whole eastern half of the northern Pacific and the Gulf of Alaska."
The warmer waters are linked to declines in some temperate species and to the wholesale migration into northern waters of fish and animals that normally live in the tropics, researchers say.
Sea surface temperatures suddenly jumped up in 1977 and the average has remained about 2 degrees warmer than the previous averages, said McGowan.
"The whole temperature range has shifted upward so the lows are not as low as they used to be and the highs are higher," he said.
The change occurred within just 10 months of 1977, and McGowan noted, "we didn't know before that such changes could occur that rapidly."
Among the biological changes linked to the warmer water since 1977, McGowan cited these examples:
* A 70 percent decline in zooplankton, the tiny animals that feed on microscopic plants and that are a major link in the natural food chain.
* A 90 percent decline in an important sea bird called the sooty shearwater. "It was once here by the millions and now they are uncommon," said McGowan.
* Warm water fishes and other animals have migrated northward and are now common in places that they once shunned. The short-beaked dolphin, a sea mammal that prefers warm water, had a California population of about 15,000 in 1970, according to the National Marine Fisheries Service. In 1991, the agency estimated the dolphin population at 226,000 and scientists now say the numbers could be as high as 400,000.
* Near-shore species like abalone, sea urchins and kelp plants "have been devastated," said McGowan. A survey has shown a severe reduction in young kelp plants, called "spikes."
* A surge of warm weather in the 1980s is linked to the wholesale death of young among fur seals, sea lions and related animals in Alaska. Those animal populations have yet to recover.
* Warm surface waters are blocking the upwelling of nutrient-rich cold waters, said McGowan. As a result, the warmer surface waters lack some of the chemicals that supports plankton, which is at the base of the food chain.
* Fish populations have declined about 5 percent per year since 1986.
Karin Forney, a biologist with the National Marine Fisheries Service, said the McGowan study "is a solid paper" and "an important study."
But she cautioned that it is impossible to say with absolute certainty that declines in the population of fish or other sea animals is caused by a single, basic factor, such as temperature.
"There is no doubt that there have been biological changes" said Forney, "but it is not certain what is causing it. There is some evidence that is due to climate change."
McGowan said there is not enough data to say for sure that the warming of the coastal waters is caused by global warming, a planetwide increase in temperature thought to be caused by increased carbon dioxide in the atmosphere.
"It is consistent with global warming, but we can't attribute it to that," he said. "If global warming occurs, it would probably look something like this."