Climate change is partly responsible for a rapid change in the ecosystem along the continental shelf waters of the Northwest Atlantic, which has so far been blamed on the collapse of the cod stocks, according to a U.S. oceanographer.

"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 at Cornell University.

He makes the argument for the role of climate change in the area's changing ecosystem in a study published Friday in the journal Science.

"Predicting the fate of these ecosystems will be one of oceanography's grand challenges for the 21st century," he said.

Previous studies of the region have shown the loss of cod had a cascading effect that saw smaller predators including snow crab and herring increase in population.

But Greene's analysis of ocean currents suggests climate change might have helped increase the amount of food at the bottom of the chain: free floating phytoplankton and the zooplankton.

Climate change caused by global warming has made water in the Northwest Atlantic less salty, he said. 

Increased precipitation and melting polar ice have sent more fresh water into the ocean, which in turn has been driven into the northwest coastal waters by shifting Arctic wind patterns.

The introduction of a cooler current has interfered with the process in which the warmer, less salty summer water on the surface of the ocean cools in the fall and sinks as its density changes to mix with the cooler waters below.

Since those surface waters are already being cooled by the new fresh water currents, they sink gradually rather than rapidly.

Without the fall deepening of the surface layer of water, phytoplankton populations receive continued access to the sunlight needed for growth, and they have stayed abundant. In turn, zooplankton, which eats the phytoplankton, have increased during the fall and into the early winter.

Greene suggests the rise of these tiny life forms and the subsequent rise in predators normally considered food for the cod, like snow crabs and shrimp, would have happened even had the cod population not collapsed.

"We suggest that, with or without the collapse of cod, a bottom-up, climate-driven regime shift would have taken place in the Northwest Atlantic during the 1990s," Greene said.

The cold Arctic waters have also made the water off Canada's East Coast inhospitable to the slow-growing cod, Greene told the Boston Globe.