Q & A: Jim McClelland, marine scientist

Focus on Science: The amount of freshwater in the North Atlantic and Arctic Oceans has been increasing. Why is that a big deal?

Jim McClelland: Changes in salinity of the oceans could alter ocean circulation patterns, and ultimately, climate. If you add more freshwater, you change the density of the ocean surface water such that it’s less likely to sink. Some models predict this could rapidly shut down the North Atlantic Deep Water formation, which is a driving factor behind the ocean’s “conveyor belt” current responsible for distributing heat around the world.

What could happen then?

If the North Atlantic Deep Water formation is shut down, the strength of the Gulf Stream—which brings warm water to the North Atlantic—could decrease. We could see a cooling of Northern Europe within this century.

You recently published a paper in Science that shows the origin of all this freshwater.

My colleagues from the Marine Biological Laboratory and I looked at freshwater coming from river discharge, precipitation, and melting sea ice and glaciers over a 50 year period, from 1950-2000. We found that the amount of excess freshwater coming into the North Atlantic and Arctic Oceans matches up with the amount of excess freshwater that’s being stored there. Normally, oceanographers would think these changes in freshwater storage are associated with changes in ocean circulation and mixing patterns, but this shows that rivers, ice melt and precipitation have a large influence.

Are the increases in freshwater related to global warming?

We’re observing changes that people have been modeling for a while, particularly models that show increased net precipitation in response to global warming. We’re not talking about something that might happen 100 years down the road. We’re not projecting out into the future about the freshwater increase. We’re seeing it right now. And this isn’t just a theoretical debate. Theory is meeting reality and that’s a major, exciting aspect to our study. We may need to prepare ourselves for the potential economic ramifications of ocean circulation change.

Jim McClelland is an assistant professor of marine science at the University of Texas Marine Science Institute in Port Aransas.