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Volume 5, Number 3
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Global Warming
and the Bay

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Signs of Change

The first thing to happen under global warming will be rising temperatures. Worldwide projections now forecast an increase of 2.5°F to 10.4°F by the end of the century. According to oceanographer Ray Najjar from Penn State University, who studies climate effects on the Chesapeake region, the Bay has never experienced years as warm as those predicted since we started keeping records. This means we don't have a clear sense of how the estuary will respond.

Najjar even questions whether temperature increases will bring more or less precipitation to the area. The response to temperature increases may vary depending on the month of the year, he explains. This frustrates scientists, because they have a better grasp on how the Bay responds to precipitation extremes than to temperature changes. "We know that it's going to get warm, but we don't know what the effects of that warmth will be. We don't really know whether it's going to get wetter or drier, but if we did know, then we'd really be able to say how the Bay would respond," Najjar says. "So we know half the story."

Two Eurytemora affinis copopods.  Photograph by Adam Frederick

Kimmel has linked robust, healthy striped bass populations to wetter winters that bring an abundance of their major food source, the zooplankton Eurytemora affinis.

The second change global warming will bring is a rise in sea levels around the world, up to 50 cm by the year 2100. What will that mean for the Bay?

In the Chesapeake region, sea level rise could go twice as high (up to 1 meter by the end of the century). While the sea is rising worldwide, land in Bay country is subsiding (sinking), making the relative impact of sea level rise much greater, explains Thomas Cronin, a paleoclimatologist with the U.S. Geological Survey in Reston, Virginia. During the last ice age — when the great ice sheets pushed down to New York, New Jersey, and Pennsylvania — the ice was so heavy that it depressed the area underneath.

"It's like squeezing a rubber ball," says Cronin. "The area you squeeze goes down, but the area around it would go out." But when the ice began to melt, the area previously buried began to bounce back (rebound), while the land outside the ice sheet, like the Bay, began to sink. Low-lying regions like Smith Island and the Blackwater National Wildlife Refuge could eventually be underwater.

Sea level rise could also raise salinity in the Bay. Salt water will move in from the ocean and flow from the southern reaches of the Bay northward, similar to what happens in a drought year now, but on a sustained basis, explains Michael Kearney, a geographer at the University of Maryland College Park.

Evidence for the sea level-salinity hypothesis comes from recent modeling efforts. Using a historical data record for salinity over the past 50 years, mathematical modeler Ming Li from the UMCES Horn Point Lab is able to predict future salinity patterns. If the 1-meter rise in the level of the Bay comes to pass, Li's model predicts that the salinity in the southern part of the Bay could see an increase of up to 5 psu (practical salinity units) by the end of the century.

To further complicate matters, rainfall could also increase with global warming, and river flow from the tributaries may extend the reach of freshwater farther south. "This could cause a biological squeeze," says Victor Kennedy, a biologist at the UMCES Horn Point Laboratory and the first author of a 2002 report for the Pew Center on Global Climate Change on coastal and marine ecosystems. "You've got sea level increasing from one end and rainfall at the other end," he says. "If the estuary becomes constricted in this manner, it would have an effect on the organisms that cannot tolerate either higher or lower salinities."

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