Climate change brews ocean trouble
Scientists tie global warming to increased upwelling of deep ocean water, which can create crippling aquatic dead zones.
from the March 8, 2007 edition
Page 2 of 3
In 1990, Bakun published a paper in the journal Science that suggested a way global warming could accelerate upwelling, with uncertain consequences. He posited that by increasing the temperature difference between the already warmer land and the coastal ocean during the day, and reducing the rate at which that difference narrows at night, global warming could lead to stronger winds – accelerating the upwelling.
But until last month, this effect appeared largely in the output of computer simulations of global warming – and in measurements taken over too short a period to be of much use in testing the idea. On Feb. 2, however, the broad theme in Bakun's hypothesis got a boost from a team led by Helen McGregor, a marine scientist currently at the University of Wollongong in Australia, which also published its results in Science.
Using chemical stand-ins as thermometers, her team analyzed two columns of sediment pulled from the ocean bottom off Morocco – the site of one major coastal upwelling zone. They reconstructed sea-surface temperatures there during the past 2,500 years – and found the pattern Bakun had theorized.
During the Medieval Warm Period (AD 1000-1270), water temperatures were cooler, indicating more intense upwelling. During the Little Ice Age (about AD 1300-1850), water temperatures were warmer, pointing to a slowdown in upwelling.
As for today: "Over the last 50 to 100 years we see a strong trend to global warming, and we see a corresponding trend to intensified upwelling," Dr. McGregor writes in an e-mail. Looking back across the 2,500-year span, sea-surface cooling off Morocco during the 20th century is unprecedented – consistent with increased upwelling, the team says.
At this point, the story would seem to be bright for fish: faster upwelling, more food, more fish. But researchers looking at upwelling regions off the west coast of southern Africa and the Pacific Northwest are finding potentially disturbing signs – the emergence of oxygen-starved "hypoxic" zones, and more recently, a delay in the onset of upwelling that risks reducing the number of juvenile organisms that survive to adulthood.
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