In the Feb. 5 editorial "Say yes to this pipeline," The Post conceded that Canada's tar sands oil is "nasty" and that we will all be better off the sooner we stop burning it.

Yet the editorial went on to endorse construction of the massive Keystone XL tar sands pipeline, which the oil industry has proposed building across America's Midwest.

This pipeline would not just feed further development of Canada's toxic tar sands - linked to cancer spikes, forest destruction, loss of wildlife and climate-disrupting pollution. It would also cut across America's most important agricultural aquifer. A single pipeline spill like the one we saw in Michigan last year could devastate our farming and drinking water.

The State Department needs to carefully consider all of these threats and decide whether it is really in our national interest to allow Canada's oil industry to build the pipeline.

The Post argued that the real answer is to reduce our demand for dirty fuels such as tar sands oil. Saying no to toxic foreign oil pipelines and investing instead in clean-energy infrastructure - like the president's proposal last week to kick-start high-speed rail - is the first step.

Michael Brune, San Francisco

The writer is executive director of the Sierra Club.