NATURE has her own way of dealing with excess carbon dioxide. When human activities spew CO 2 into the atmosphere, plants absorb more of it than usual, leading to profuse growth. The ocean, too, swallows more than it otherwise would. Many scientists fret that these so-called carbon sinks risk getting clogged up. Some even suggest that this has already started happening.

Ashley Ballantyne, a geologist at the University of Colorado, and colleagues are less gloomy. In a paper published recently in Nature they show that over the past 50 years Earth’s absorption of CO 2 has nearly doubled. Yet they see no evidence of a slowdown in the rate at which this takes place. If the climate models suggest otherwise, the researchers argue, then the modellers must have got their sums wrong.

One reason, points out Jean-Baptiste Salée, an oceanographer with the British Antarctic Survey, might be that little is known about how exactly the CO 2 is absorbed by the ocean, which quaffs more than half of the man-made stuff. There has been much speculation about this, but little hard evidence. Theory points to three main mechanisms: mixing the ocean's surface layers (up to a few hundred metres) by wind; mixing of deeper layers by ocean currents; and eddies, swirls created when warm ocean currents meet cold ones, blending large swathes of the ocean 10-100km across. It had been assumed that most of the CO 2 is captured by the surface layers, which would then be stirred by wind, distributing the carbon dioxide over a larger patch, and pulled down by ocean currents, freeing the surface layers to absorb yet more.