Since global warming hogs the environmental limelight, the fact that our oceans are also a mess often gets overlooked. But here's an eye-opener: According to a new study co-authored by the Virginia Institute of Marine Science's Robert Diaz, the number of "dead zones" in the ocean has roughly doubled every decade since the 1960s:

Many coastal areas of the world’s oceans are being starved of oxygen at an alarming rate, with vast stretches along the seafloor depleted of it to the point that they can barely sustain marine life, researchers are reporting.

The main culprit, scientists say, is nitrogen-rich nutrients from crop fertilizers that spill into coastal waters by way of rivers and streams.

A study to be published Friday in the journal Science says the number of these marine “dead zones” around the world has doubled about every 10 years since the 1960s. About 400 coastal areas now have periodically or perpetually oxygen-starved bottom waters, many of them growing in size and intensity. Combined, the zones are larger than Oregon....

The trend portends nothing good for many fisheries, said Dr. Diaz, a professor at the Virginia Institute of Marine Science at the College of William and Mary. “Dead zones,” he said, "tend to occur in areas that are historically prime fishing grounds."

The worst dead zones are in the Black Sea, the Gulf of Mexico, and especially the Baltic Sea, where the marine ecosystem is on the verge of total collapse. It's also worth noting that fertilizer runoff, while a major problem, isn't the sole culprit. Hypoxic conditions in Chesapeake Bay and Long Island Sound are also created partly by emissions from coal plants along the Ohio River, for instance. Phosphorous from human sewage and nitrogen-oxide emissions from car exhaust also play a role.

Still, fertilizer's the big one—and the good news is that the problem isn't totally unfixable. As Nancy Rabalais of the Louisiana Universities Marine Consortium told Mother Jones's Julia Whitty (whose 2006 article on the fate of the oceans is still the best single magazine piece on the subject): "There are modeling studies that show if you reduce nitrogen fertilizer applications by 12 to 14 percent, you can reach the target without losing crop production." It just takes better management techniques—for instance, farmers could plant winter crops like rye or wheat after the fall harvest to absorb fertilizer that would otherwise get washed down toward the ocean.

--Bradford Plumer