Using groundwater for crop irrigation or industrial purposes adds more planet-warming carbon dioxide to the atmosphere than volcanoes do, a new study suggests.

As water soaks through soil, it picks up carbon dioxide that's generated when organic matter in the soil decomposes. On average, groundwater holds from 10 to 100 times as much carbon dioxide as the water in lakes and rivers, says Gwen L. Macpherson, a hydrogeologist at the University of Kansas in Lawrence. When groundwater is pumped to Earth's surface, carbon dioxide escapes to the atmosphere, where it acts as a greenhouse gas.

In recent years, people have been pumping about 740 cubic kilometers of water from the ground each year, Macpherson said last week in Denver at a meeting of the Geological Society of America. Chemical analyses of groundwater suggest that this water use adds about 300 million metric tons of carbon dioxide to the atmosphere annually.

The volume of groundwater-generated carbon dioxide is, at most, a small percentage of the emissions produced by burning fossil fuels, says Macpherson. Nevertheless, her analysis suggests that the amount of carbon dioxide fizzing from groundwater is about three times that spewed skyward by volcanoes, a major natural source of atmospheric carbon dioxide.

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