A low-key case filed in a San Francisco court last August promises to be just the first ripple. The suit, now with the Friends of the Earth, Greenpeace and the cities of Boulder, Colo., and Oakland, Calif., as plaintiffs, seeks to force two government agencies to assess the total impact on climate of the projects they finance. Rather than treaties and regulations, litigation may soon be the weapon of choice for those concerned about human-induced global warming.

In the San Francisco case, the plaintiffs charge that in the past decade, the Overseas Private Investment Corporation (OPIC) and the Export-Import Bank of the United States (ExIm) have provided $32 billion in loans, insurance and loan guarantees for oil pipelines, oil drilling and other fossil-fuel endeavors that will ultimately result in the emission of 32 billion tons of carbon dioxide over the life of the projects. (All human activity currently emits about 24 billion tons of CO2 a year.) In contrast, the agencies provided only $1.3 billion for renewable-energy projects during the same period. (A spokesperson for OPIC states in the agencys defense that OPIC-supported efforts are not major contributors to global greenhouse gas emissions or climate change.)

The lawsuit does not attempt to cancel ongoing projects but asks only that OPIC and ExIm determine the cumulative impact on the climate of every future project. Such a review, asserts Jon Sohn of Friends of the Earth, is required by the National Environmental Policy Act.

The plaintiffs are confronted with many hurdles. To begin with, they will have to demonstrate that they face harm from global warming and, in particular, from the agencies actions. The cities contend that their water supplies are in jeopardy. Boulder depends on runoff from mountain snow, but the snowpack at lower elevations has evaporated. Oakland fears that rising seas will salinate its underground aquifer. Other litigants include a coral-reef scientist who finds that his object of study is vanishing, and a couple who fear that their island home will be washed away.

Scientific uncertainties over such claims can be partly overcome by aggregating harm done over a large span of space and time, contends David Grossman, a recent graduate of Yale Law School and now a law clerk in Anchorage. In a paper to be published in the Columbia Journal of Environmental Law, Grossman argues that tort litigation over global warmingin which communities or states seek damages from oil companies, electric utilities and automobile manufacturersis entirely feasible. The main problem is causationthat is, proving that the defendant caused harm to the plaintiff. Statistics can help, he says, as when a towns residents can attribute an enhanced frequency of cancer to a nearby pesticide plant. Thus, a homeowner will probably not be able to show that the hurricane that destroyed his house was spawned by global warming, but the state of Florida may well prove that increased damage to coastal property over several years has a lot to do with climate change.

In truth, sea-level rise and greater frequency of storms are higher-order results of global warming, in that they would require several links in a causal chain to be proved. An easier case to make, notes Donald Goldberg of the Center for International Environmental Law in Washington, D.C., will simply be warming. In Alaska, for example, average temperatures have risen by about two degrees Celsius since 1970. Two coastal villages, Kivalina and Shishmaref, have suffered from erosion that Gunter Weller of the University of AlaskaFairbanks attributes to three factors, all directly deriving from warming. Permafrost has thawed, causing houses to slide off suddenly muddy cliffs; sea ice has thinned, creating expanses of open water that rise up in ever higher storm surges; and glaciers are melting, leading local sea levels to climb (albeit very slightly). The townships must be relocated (at an estimated cost of more than $100 million), so they should stand a good chance of a court upholding a claim that they suffered damages because of global warming.

A plaintiffs next task would be to show that the defendants are meaningfully responsible. The issue will be vigorously fought, Grossman predicts. Environmentalists can estimate the quantity of greenhouse gases for which, say, a large oil producer is responsible. But calculating the fraction of warming is a far more contentious task, points out climatologist Stephen H. Schneider of Stanford University, because of the inherent uncertainty and variability of climate models. Even so, Goldberg holds that U.S. courts can solve the problem of apportioning blame: It may take a few cases, but ultimately the courts will figure out a formula for assigning responsibility.

Shifting the cost of global warming to those who are disproportionately the perpetrators, Grossman argues, could make fossil fuels more expensive and thus force corporations to pay more attention to renewable energy. Environmental groups have been frustrated by the Bush administrations rejection of the Kyoto treaty and what Sohn describes as its tendency to deny, deflect blame and delay when it comes to issues involving global warming. So dont be surprised if See you in court becomes the environmentalists new rallying cry.

Madhusree Mukerjee is based in Darien, Ill.