Water exchange between the ocean, atmosphere, and land is called the global hydrological cycle. As Earth's climate warms, the rate of this exchange is expected to increase. As part of this process, high- latitude precipitation and, consequently, river runoffs are also expected to increase. This could change the distribution of water on Earth's surface, with important social and economic consequences.

It could also alter the balance of the climate system itself, such as the Atlantic thermohaline circulation, a kind of conveyor belt. Cold water flows southward in the Atlantic at great depths to the tropics, where it warms, rises, and returns northward near the surface. This flow helps keep northern Europe at a temperate climate, whereas the same latitudes in North America are sparsely settled tundra or taiga.

Researchers Peili Wu, Richard Wood, and Peter Stott of the Hadley Centre compared observational data reported in Science in 2002 by Peterson and others with model simulations, produced by Hadley, part of the United Kingdom's Met Office.

Writing in the journal Geophysical Research Letters (21 January), they note that increased human-caused greenhouse gas emissions are expected to intensify the Arctic hydrologic cycle, that is, the cycle of water as it rains onto land and sea, runs off into rivers, and evaporates to continue the cycle. The increased Arctic precipitation is balanced by decreased precipitation in the tropics, they say.

Wu and his colleagues tested the model with four simulations that took into account both human inputs and natural factors, including solar variability and volcanic eruptions. The results showed a steady increase in Arctic river discharges, especially since the 1960s. The annual rate of increase since 1965 was 8.73 cubic kilometers [2.31 million gallons] per year, far greater than the long term trend.

Seeking to determine the source of the upward trend of recent decades, the researchers asked first whether it could be the early part of the predicted increase in the global hydrological cycle, caused by global warming. Their simulations excluded human impacts in one instance and natural impacts in another, and included all factors in a third. They concluded that had there been no human inputs, the hydrological cycle would have shown no trend at all in the 20th century.

Had there been only human inputs and no natural ones, Wu and colleagues say, the long term trend would be 50 percent higher than when all factors were considered. They conclude that over the past four decades, human activity played the major role in increased river flows into the Arctic.

The observed data conform well to the predictions of the Hadley climate model, they say, regarding human inputs. They say it is likely that the upward trend in river flow changes is part of the early stages of an intensified hydrologic cycle.

Title: "Human influences on increasing Arctic river discharges" Authors: Peili Wu, Richard Wood, Peter Stott, Hadley Centre for Climate Prediction and Research, Met Office, Exeter, United Kingdom.

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