Ecosystems along the continental shelf waters of the Northwest Atlantic Ocean, from the Labrador Sea south of Greenland all the way to North Carolina, are experiencing large, rapid changes, reports a Cornell oceanographer in the Feb. 23 issue of Science.
While some scientists have pointed to the decline of cod from overfishing as the main reason for the shifting ecosystems, the article emphasizes that climate changes are also playing a big role.
"It is becoming increasingly clear that Northwest Atlantic shelf ecosystems are being tested by climate forcing from the bottom up and overfishing from the top down," said Charles Greene, director of the Ocean Resources and Ecosystems Program in Cornell's Department of Earth and Atmospheric Sciences. "Predicting the fate of these ecosystems will be one of oceanography's grand challenges for the 21st century."
Most scientists believe the planet is being warmed by greenhouse gases emitted in the burning of fossil fuels, and by changing land surfaces. Early signs of this warming have appeared in the Arctic: Since the late 1980s, scientists have noticed that pulses of fresh water from increased precipitation and melting of ice on land and sea in the Arctic have flowed into the North Atlantic Ocean and made the water less salty.
At the same time, climate-driven shifts in Arctic wind patterns have redirected ocean currents. The combination of these processes has led to a freshening of seawater along most of the Northwest Atlantic shelf.
In the past, during summer months, a wind-mixed layer of warmer, less salty water (which is less dense and lighter) floated on the ocean surface. When the air temperature cooled during autumn, temperature and density differences lessened between the surface mixed layer and the cooler, saltier waters below. Similar to the flow of heating and cooling wax in a lava lamp, as the density differences became smaller, mixing between the layers typically increased and the sur
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