Climate Change Chokes Oceans

Rising temperatures have caused oxygen-starved swaths of ocean to expand over the last half-century -- a disturbing trend that, if it continues, could wreak havoc on global fisheries.

Led by University of Kiel oceanographer Lothar Stramma, researchers analyzed 50 years of ocean oxygen data. The findings, published today in Science, dovetail with predictions made by earlier climate models.

Two mechanisms are responsible for the drop. As water warms, it holds less oxygen. More significantly, Earth's oceans are oxygenated in large part by cold waters that sink at high latitudes, then ride deep-sea currents to the equator. But water becomes buoyant as it warms: it no longer sinks so readily into this cycle.

"The surface warmer getting warmer means it's harder for oxygen to mix down, to the deeper parts, and that's the dominant effect," said study co-author and National Oceanic and Atmospheric Administration researcher Greg Johnson.

Stramma said that data is still too patchy to estimate the total spread of low-oxygen zones, but it appears that existing zones are growing vertically -- reaching deeper into ocean depths, and closer to the surface.

These patterns are especially pronounced in the equatorial Pacific and tropical Atlantic: in the tropical North Atlantic, low-oxygen areas expanded from a thickness of 370 meters in 1960 to 690 meters in 2006.

"Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies," wrote the researchers.

I asked Francisco Chavez, a Monterey Bay Aquarium Research Institute scientist now devising fisheries management strategies suitable for a changing climate, what these consequences could look like.

According to Chavez, destructive Humboldt squid invading the coast of California are fleeing from spreading low-oxygen zones. Commercial fishing for bottom-dwellers, such as hake and rockfish, could be decimated.

When Chavez told me that, I remarked that -- in the larger scheme of things -- it didn't seem so terrible: fishing would take a hit, but hardly a fatal one.

"Well, there won't be an eruption in the ocean that's going to kill everyone," he said. "But if this phenomenon would spread everywhere, then the whole ocean would die. At the current rate of expansion, that's not something that would happen in 100 or 200 years -- it would be a long process."

Continued Chavez, "Do I think that will happen? No, I don't think so. We don't understand enough about how the climate and ocean interaction with each other yet. There's been a lot of discussion in the scientific literature about tipping points -- will we get to a place where we have dramatic and rapid change, where we can't get out of whatever we've done? Right now it seems that we could, if we curbed emissions and did some sort of geoengineering. But it's like predicting the stock market."

Of course, given the state of the American economy, that's not a comforting analogy.