Sinking solution for carbon dioxide.

With nations scrambling to find ways of reducing carbon dioxide pollution, the oceans might seem an ideal place to dump this troublesome greenhouse gas. Earth's oceans already hold 65 times the amount of carbon dioxide present in the atmosphere, and they have a vast potential for absorbing even more. So why not pump power plant exhaust into the deep sea?

Aside from the obvious environmental concerns, the sheer cost of such pumping has dampened enthusiasm for ocean disposal. But a new report may buoy hopes for this technical fix. Studies of ocean disposal over the last 15 years have suggested that once removed from exhaust gases, carbon dioxide must be pumped extremely deep -- more than a kilometer below the sea surface--to prevent it from rising back up to the atmosphere. One way around such an expensive method would be to inject the gas into sinking ocean currents, such as those flowing out of the Mediterranean Sea. The problem is that scientists know of relatively few currents of this type, and they lie far from most power stations.

Peter M. Haugan and Helge Drange of the Nansen Environmental and Remote Sensing Center in Solheimsvik, Norway, argue in the May 28 NATURE that deep disposal may not be necessary. Using thermodynamic calculations, they show that when carbon dioxide is pumped only 200 to 400 meters down, the dissolved gas increases the density of surrounding seawater, causing it to sink. These density currents could then carry the gas into the deep sea, where it would eventually disperse.

Although the shallow-pumping idea sounds promising on paper, James C. Orr of Princeton University points out several potential complications. It may still prove quite expensive to separate carbon dioxide from exhaust gases, compress it and pipe it to even shallow depths in the sea, he notes in a commentary accompanying the research report.

Then there are the environmental concerns. The concentrated streams of carbon dioxide-rich water would be extremely acidic and therefore deadly to organisms. The dissolved carbon dioxide could also cause seawater to lose a substantial amount of its dissolved oxygen -- another life-threatening effect. Even if future studies show that the environmental consequences will be negligible, Orr cautions against viewing ocean dumping as a complete fix for the greenhouse problem. "It could only hope to be one part of a larger strategy," he writes, "simply because many fossil-fuel-fired power plants are far from the nearest coastline."