Water habits on land cause oceans to swell.

Sea levels around the world are currently creeping upward by a few centimeters per decade, a trend most scientists attribute to warning global temperatures that melt glacial ice and cause ocean water to expand. But don't blame warmth for everything. A team of geoscientists reports that as much as a third of the observed sea level rise stems from human activities, such as cutting down forests and pumping groundwater out of aquifers.

"The direct [human] contribution to sea level rise is much larger than previously believed, so much larger that it competes with other sources of sea-level rise," argues Dork L. Sahagian of Ohio State University in Columbus. He and his colleagues describe their research in the Jan. 6 NATURE.

Extraction of groundwater raises the ocean level because it takes water stored underground and transfers it to Earth's surface. Although some returns to the aquifer, much of the water -- especially that used for irrigation -- eventually reaches the ocean by flowing into rivers or by evaporating into the atmosphere and then raining on the seas, Sahagian says. In the same way, lake water used for irrigation in arid regions adds to ocean levels because it greatly increases evaporation, permanently transporting water away from continental interiors.

To get a rough idea of how much water comes from such activities, Sahagian and his colleagues studied withdrawal figures for large aquifers in the central United States, the southwestern United States, central California, North Africa, and the Arabian Peninsula. Judging from the rate of removal since the 1930s, the scientists calculate that water from these five aquifers adds about 0.17 cm each decade to ocean levels. That equals about one tenth of the total sea level rise observed around the world.

The team also examined the Aral and Caspian Seas, two large, inland lakes whose levels have fallen markedly this century. Indeed, the Aral Sea has lost half its area since 1960. The scientists calculate that using ground and surface waters from the Aral and Caspian areas adds 0.21 cm per decade to sea levels.

In an unanticipated discovery, the team found that deforestation also pumps a significant amount of water into the ocean. Either through fire or decomposition, much of the water in leveled trees escapes to the atmosphere. There, it rains down into rivers or the oceans. Deforestation in the tropics alone contributes roughly 0.14 cm per year to sea-level rise, calculate Sahagian and his colleagues.

Combining all these sources, they conclude that human actions raise sea levels about 0.5 cm each decade, roughly one-third the observed rate of increase. Because the study included only five aquifers and left out other potential sources, Sahagian considers the figure an underestimate.

Others are not so sure. Mark F. Meier, a glaciologist with the University of Colorado at Boulder, questions some of the numbers and assumptions that Sahagian's group used in their calculations. He suggests that they overestimated the effects of deforestation and groundwater withdrawal. Nonetheless, he agrees that such activities do contribute significantly to rising sea levels.

According to Meier, Antarctica and Greenland represent the biggest problems for scientists seeking to understand sea-level rise. Researchers do not know how much these regions add to ocean levels through melting. In fact, some think the ice cover on Antarctica could be growing, which would slow the rise of ocean waters.

If greenhouse-gas pollution warms the Earth, it will raise sea levels by expanding ocean water and melting mountain glaciers. However, climate experts remain unsure how Antarctica and Greenland will respond to higher temperatures. Although the warmth will hasten melting along their margins, snowfall in the interior could increase, thereby removing water from the oceans.