Web edition: October 5, 2010
Fresh water evaporates from the oceans, rains out over land and then runs back into the seas. A new study finds evidence that global warming has been speeding up this hydrological cycle recently, a change that could lead to more violent storms. It could also alter where precipitation falls — drying temperate areas, those places where most people now live.
Among the new study’s more dramatic calculations: River runoff into the seas has been increasing by some 540 cubic kilometers per year, or about 1.5 percent annually over the period analyzed (1994 to 2006). While that may not sound like much, “over 20 or 30 years it would really add up,” notes study author James S. Famiglietti, a hydrologist at the University of California, Irvine.
Global annual precipitation also appears to be on the rise, but at only half the increase seen in river runoff. If prolonged, this differential would suggest that major terrestrial stores of water — such as ground aquifers and glaciers — are drying up (a trend that other studies have been chronicling). This would also be expected to eventually raise sea levels and generally dry temperate regions that depend on rivers to slake their thirsts.
Famiglietti and his colleagues reported their findings October 4, early online in the Proceedings of the National Academy of Sciences.
Changes were predicted
Scientists have been predicting climate change would intensify the global water cycle, Famiglietti says — “increasing the amount of precipitation and evaporation, globally, and stream flows from the continents.”
Computer models indicated that if this happened, there should also be “a redistribution of precipitation,” he adds. Driven by changes in atmospheric circulation patterns, rains and snowfall would increasingly ignore temperate regions in favor of zones nearer to the poles and tropics. Another feature of an intensifying water cycle: Storm intensity would also tend to increase.
With climate warming, long-frozen stores of water — glaciers and permafrost — could be gushing into the seas from regions largely out of humanity’s view. So fitting their contribution into the global tally would offer important clues to changes in water cycling, Famiglietti explains. But until now, calculating this has proven difficult.
In theory, one of the easiest ways to gauge ice and permafrost melting would be to tally stream flows around the world. Except that stream monitoring programs — never ubiquitous — have been diminishing in recent decades. So a large share of the water flowing over land escapes any accounting.
To estimate global stream flows “we had to get creative,” Famiglietti explains, and “took a balancing-the-checkbook approach.” They subtracted automated withdrawals (evaporation) and large direct deposits (precipitation figures) from the account balance (sea level values) to figure what the collective value of all cash deposits (river flow) into the account had been.
To do this, they mined satellite data on sea level values — a gauge of the source water — and data on sea-surface and cloud temperatures. Those temps provided a gauge of ocean evaporation and eventual rainout.
Identifying a planet-wide increase in stream flows “was a surprise,” Famiglietti acknowledges; “we never expected to see a trend.” His group now reports that the driver of this intensifying cycle, evaporation from the oceans — a symptom of global warming — appears to be increasing by some 768 cubic kilometers per year. That these trends fit together so well with what would be expected in a warming world “really underscores for me that the [projected] acceleration of water cycle may already be underway,” he concludes.
What his team can’t tease out of the data they analyzed is the proportion of river flows coming from particular sources: rainwater runoff, snowmelt, ice sheets in Greenland and Antarctica, groundwater mining or the melting glaciers residing anywhere from the poles to atop tropical mountain peaks.
T.H. Syed, J.S. Famiglietti, et al. Satellite-based global-ocean mass balance estimates of interannual variability and emerging trends in continental freshwater discharge. Proceedings of the National Academy of Sciences (in press). doi: 10.1073/pnas.1003292107