To track global warming, watch the water flow

Say "climate change" and people tend to think global warming. But we also should think about water, specifically, the cycle of precipitation, evaporation, and river flow that is a key climate component. A little decline here, a little boost there, can have direct effects on how we live our lives.

In the Arabian Sea, for example, fishermen now enjoy richer fishing thanks to declining snow cover in Southeast Asia and the Himalayas. The links work this way: Less snow means more summer heating of the land, intensifying air pressure differences between land and sea, which in turn drive the seasonal monsoon winds. Stronger winds stir the Arabian Sea more vigorously, bringing more nutrients into its higher, sunlit levels. Microscopic plants and animals (fish food) flourish. Fisheries burgeon.

Arctic inhabitants aren't so fortunate. An intensified water cycle is increasing moisture in the American and Eurasian northlands. Rivers fed by stronger precipitation are pouring more freshwater into the Arctic Ocean. It caps the upward flow of fish food. This is bad for fisheries.

Changes in the water cycle itself may be subtle and often poorly understood. Yet their effects can sometimes be dramatic. Joaquim Goes at the Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine, and several colleagues studied satellite images of the western Arabian Sea and found sea-color changes due to seasonal blooms of phytoplankton (microscopic plants). In fact, the blooms have increased more than 350 percent in seven years, the research team reported in Science last month.

These robust phytoplankton blooms can enhance fisheries, Dr. Goes says. But too much phytoplankton can deplete the water's oxygen supply. That can kill fish and encourage bacteria that release nitrous oxide, a gas with 310 times the heat-trapping power of carbon dioxide.

Arctic water-cycle changes also have global implications. Peili Wu, Richard Wood, and Peter Stott at Britain's Hadley Centre for Climate Prediction and Research studied these changes by comparing computer simulations with actual river-flow data. Their conclusion, published in Geophysical Research Letters in January, indicated that we are seeing the beginning of an intensified water cycle. Dr. Wu calls the team's findings "evidence that changes in the global water cycle predicted to follow global warming are already happening."

That cycle is expected to remain in balance. Increased Arctic precipitation is balanced by decreased precipitation in the tropics. There may be large-scale shifts in ocean circulations and movement of water vapor through the atmosphere. That may involve a net movement of water from the Southern to the Northern Hemisphere.

One particular concern is the effect of more freshwater from the Arctic on the North Atlantic. This could alter the large-scale currents, including the Gulf Stream. "It is clear that further and more rapid warming will increase the vulnerability of this [North Atlantic] circulation system, possibly leading to a permanent circulation change in the climate system," warned Thomas Stocker and Christoph Raible of the University of Bern, Switzerland, in a comment on the Hadley study published last month in Nature.

Public concern about water has concentrated on maintaining clean-water supplies for human use. That's too self-centered. The way water moves around our planet has fundamental environmental implications. We never will understand where climate change is headed without taking account of precipitation, evaporation, and river flow.