The amount of fresh water entering the Arctic Ocean from the rivers that feed it is increasing, UK scientists report.
Writing in the journal Geophysical Research Letters, they say the increase is caused in part by human activities and is an early sign of climate change.
The rise in fresh water entering the Arctic Ocean could change the global distribution of water, the team says.
It could also affect the balance of the climate system itself and even possibly alter the behaviour of the Gulf Stream.
The team is from the Hadley Centre for Climate Prediction and Research, part of the UK Met Office.
Knock-on effects
The global hydrological cycle is the exchange of water between the land, the oceans and the atmosphere. The rate of the exchange is expected to increase as the Earth warms.
Part of the process is likely to mean more precipitation (hail, rain, sleet and snow) at higher latitudes, and so more water flowing down the rivers.
If the global water distribution changes, this could have important social and economic consequences. An altered hydrological cycle might conceivably have a profound cooling effect on north-west Europe as well.
The American Geophysical Union, publisher of the journal, says: "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."
The Hadley researchers compared data published in 2002 from observations of Siberian river flows with model simulations, to see whether they could identify a human influence on the increase in fresh water.
Making allowances
They point out that higher emissions of greenhouse gases, caused by human activities, are expected to intensify the hydrological cycle in the Arctic, with higher precipitation there balanced by a reduction in the tropics.
They tested the model with four simulations which took into account both human inputs and natural factors, including solar variability and volcanic eruptions.
The results showed a steady increase in river discharges, especially since the 1960s, with the annual rate of increase since 1965 8.73 cubic kilometres, far greater than the long-term trend.
The simulations excluded human impacts in one instance and natural impacts in another, and included all factors in a third.
The team concluded that if there had been no human inputs, the hydrological cycle would have shown no trend at all in the 20th Century.
Over the past four decades, they say, human activity played the major role in the increased flows, and it is likely that the upward trend is part of the early stages of an intensified hydrological cycle.
Dr Peili Wu, a team member, told the BBC: "It looks clear to us that this is an early signal of human-induced climate change. If only natural factors were involved, you wouldn't get these results.
"It is possible the increase in fresh water entering the Arctic Ocean could contribute to an alteration in the thermohaline circulation, because it is diluting the saltiness of the seawater and reducing its density."
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