A federal judge has blocked a coal project in the wilds of Colorado because federal agencies failed to consider the future global-warming damages from burning fossil fuels.

U.S. District Court Judge R. Brooke Jackson's decision halts exploration proposed by Arch Coal that would have bulldozed six miles of roads on 1,700 untrammeled acres of public land.

When the agencies touted the supposed economic benefits of expanded coal mining in the Sunset Roadless Area, Jackson ruled, they should also have considered any global-warming costs.

The decision was a significant judicial endorsement of a policy tool known as the "social cost of carbon," which economists and climate scientist use to put a price in today's dollars on the damages from drought, flood, storm, fire, disease and so forth caused by future global warming due to our emissions from burning fossil fuels.

"It is arbitrary to offer detailed projections of a project's upside while omitting a feasible projection of the project's costs," Jackson decreed.

The Obama administration has increasingly used the social-cost figure to help decide whether restraints on carbon dioxide emissions are worthwhile. Industry groups and their allies in Congress have sought to limit its use. Environmentalists call it a useful device for making clear that there are high costs as well as benefits to burning fossil fuels.

"This decision means that these agencies can't bury their heads in the sand when confronting the very real impacts of climate change," said Ted Zukoski, an attorney with Earthjustice, which represented conservationists who sued to block the mining expansion.

If the judge's reasoning holds up in other challenges to agency decisions, it could undermine the rationales for much bigger projects, such as the proposed Keystone XL tar sands pipeline. In its environmental review of the KXL, the State Department ignored repeated requests by the Environmental Protection Agency to estimate the social cost of carbon from burning the unusually dirty fuel the pipeline would deliver from Canada to the Gulf Coast.

In the Colorado case, the judge wrote, "by deciding not to quantify the costs at all, the agencies effectively zeroed out the cost."

That violated a key precept of the National Environmental Policy Act, the judge said, which requires a "hard look" at all the environmental costs of government decisions.

That this can be difficult or contentious does not allow agencies "completely to ignore a tool in which an interagency group of experts invested time and expertise," he wrote. "Common sense tells me that quantifying the effect of greenhouse gases in dollar terms is difficult at best. The critical importance of the subject, however, tells me that a 'hard look' has to include a 'hard look' at whether this tool, however imprecise it might be, would contribute to a more informed assessment of the impacts than if it were simply ignored."

Correction: An earlier version of this article misstated the surname of the federal judge who blocked the Arch Coal project. He is R. Brooke Jackson, not Johnson.