Later this year, the State Department will decide whether to approve construction of a 1,700-mile oil pipeline from Canada to the Texas Gulf Coast called Keystone XL. The underground 36-inch pipeline, built by TransCanada, would link the tar sands fields of northern Alberta to Texas refineries and begin operating in 2013. The department should say no.

State is involved because the pipeline would cross an international boundary. Secretary Hillary Rodham Clinton first said she was “inclined” to support it, but has lately sounded more neutral. An environmental assessment carried out by her department last year was sharply criticized by the Environmental Protection Agency for understating the project’s many risks. The department has since undertaken another environmental review that will soon be released for public comment. It needs to be thorough and impartial.

Advocates of the Keystone XL, which include the Canadian government, the oil industry and its allies in Congress, argue that a steady supply of oil from a friendly neighbor is the answer to rising oil prices and turmoil in the Middle East. But the Energy Department says the pipeline would have a minimal effect on prices, and there is already sufficient pipeline capacity to double United States imports from Canada.

The environmental risks, for both countries, are enormous. The first step in the process is to strip-mine huge chunks of Alberta’s boreal forest. The oil, a tar-like substance called bitumen, is then extracted with steam or hot water, which in turn is produced by burning natural gas. The E.P.A. estimates that the greenhouse gas emissions from tar sands oil — even without counting the destruction of forests that sequester carbon — are 82 percent greater than those produced by conventional crude oil.