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Continued Bering Sea warming could cause major ecosystem change

Observations for almost three decades show a warming in the climate of the Bering Sea. If such a shift continues for another decade, it will have a major effect on the ecosystem as some species seek colder waters, say researchers for the National Ocean and Atmospheric Administration (NOAA).

The findings of James E. Overland and Phyllis J. Stabeno, both oceanographers at NOAA’s Pacific Marine Environmental Laboratory in Seattle, Wash. are published in today’s issue of EOS, the newspaper of the American Geophysical Union (Editor’s note: available to AGU members).

The Bering Sea provides 47 percent (by weight) of the U.S. fishery production, and is home to 80 percent of the U.S. seabird population, 95 percent of northern fur seals and major populations of the Steller sea lions, walrus, and whales, said Stabeno.

A warming can also affect the production of zooplankton, which is a major food source for many marine species.

Observation data of surface air temperature shows a transition to warmer summer for virtually all years after 1976. Researchers also see an earlier arrival of spring since 1996, although several other springs (1989, 1990, and 1993) were also warm.

Overland and Stabeno note that the change in the climate after 1976 was affected by the Pacific Decadal Oscillation (PDO) that resulted warmer sea temperatures along the U.S. west coast, and, in the early 1990s, the Arctic Oscillation (AO) that weakened the storms in the northern North Pacific.

However, while the two climate modes have shown variability, the main characteristic of the past four years in the southeastern Bering Sea has been a persistent lack of sea ice, warm ocean temperatures, and warm air temperature anomalies in late winter through summer.

When the researchers looked at sea ice concentration, they found a decrease in area and number of days of coverage. Looking at one spot around 57-degree North latitude, researchers saw the first shift in 1977 with a decrease in the mean number of days from 130 in 1971-1976 to 67 in 1977-1989 in which there was more than five percent ice cover after January 1. After 2000 there was almost an absence of sea ice in that area.

Sea ice is related to the spring phytoplankton bloom. In years when there is sea ice after mid-March, there is an early phytoplankton bloom. During the years with no ice cover after mid-March, the bloom does not occur until May or June.

Annual fisheries surveys also show a decline in the addition of young fish to some stocks, especially those species that prefer cold waters, such as Greenland turbot, arrowtooth flounder, and rock sole. However, Walleye Pollock, which prefers warmer waters, has increased by 400 percent since 1978, and is characterized by a large, rather stable, population over the last decade. Pacific Walrus is another species showing northward movement due to lack of sea ice and warmer temperatures.

These indicators, such as the sea ice cover and the change in the types of fish that are in the Bering Sea, should be watched closely for the next five years to confirm or reject our hypothesis of the northward movement of the cold water curtain, said Overland.

The Commerce Department’s NOAA is dedicated to enhancing economic security and national safety through research to better understand atmospheric and climate variability and to manage wisely our nation’s coastal and marine resources.

Contact: James Overland
(206) 526-6795

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