WASHINGTON—Dozens of leading scientists in the fields of climate change, public health and ecology have told the State Department that the findings of its draft assessment of the impacts of the Keystone XL pipeline are "without merit in many critical areas."

In unusually blunt advice, they urged the Obama administration to reject the pipeline as not being in the U.S. national interest.

"How is importing the world's highest carbon content crude consistent with national policy goals?" they asked. "Now is the time to make a serious commitment to place the United States on a lower carbon trajectory. If not now, when? If not here, where?"

Many of the 29 signatories, like retired NASA scientist James Hansen and Penn State professor of meteorology Michael Mann, are already well known for their opposition to the pipeline. Fourteen of the signatories were among a group of 18 leading climate scientists who wrote President Obama in January urging him not to approve the pipeline.

Several have played prominent roles in the decades-long work of the UN's Intergovernmental Panel on Climate Change, or IPCC, which has produced the overarching scientific consensus on the risks of climate change due to human pollution, mostly from burning fossil fuels.

This letter is significant not only because the group of luminaries is large and their arguments elaborate, but because they wrote specifically to discredit the justification for the pipeline laid out in the State Department's March 1 draft environmental impact statement, or EIS. The controversial draft EIS said the pipeline wouldn't have much effect on the environment or climate. It was widely seen as opening the door to the project, which would carry tar sands oil from Alberta to refineries on the Gulf Coast.

On April 29, the 29 scientists filed formal comments on the EIS, and declared that "the XL pipeline project poses a number of environmental and human health threats, in addition to exacerbating climate change."

This letter, coming from a group with strong credentials in science, health and environmental policy circles, is one of the more striking criticisms of the EIS to emerge from the bulging docket of more than a million public comments, which are being posted online by the State Department while it considers whether to adjust its findings.

The EIS is a critical step in the U.S. approval process for the Keystone, a decision that could take several more months. The draft statement's central finding—that the project would have no significant impact on global warming because expansion of the tar sands would continue unabated regardless of the pipeline's fate—has been denounced by environmental groups.

"Despite the claims of the EIS that the environmental impacts are minor these assertions are not supported by the science," the scientists wrote.

"Our scientific judgment is that the actual and potential environmental damage are sufficiently severe to reject the Keystone XL pipeline proposal in order to protect the climate human health and the multiple ecosystems this project threatens."

Their letter, submitted on behalf of the group by William R. Moomaw, a professor of international environmental policy at Tufts University and a leading IPCC author, disputed assertions by the pipeline's advocates that the United States must rely on Canadian crude oil for its energy security.

"The assertion that the additional oil will be needed by the US seems unlikely in light of the growing oil production within the US and declining demand," they wrote.

They ripped into the argument by pipeline proponents that Keystone, and the increased tar sands production it would support, was just a drop in the bucket of worldwide carbon emissions. This was suggested in the draft EIS, despite the report's conclusion that the carbon footprint of tar sands crude was higher than other oil, and that the products carried by the Keystone would add a billion tons of carbon dioxide to the atmosphere during the pipeline's lifetime.

"The assertion that the tar sands will not significantly increase global warming is simply not supported by the facts," they wrote.

"While this might be said of any single project this is simply false over the lifetime of the pipeline as even the calculations in the EIS demonstrate," they said.

They said the EIS had underestimated carbon emissions, ignored the problem of petroleum coke as a highly polluting byproduct, disregarded the damage to boreal forest ecosystems, inadequately presented the risk of spills and the difficulty of cleaning them up, and downplayed the exposure of people to toxic substances, including carcinogens.

And they said that by approving the Keystone, the United States would be locking in decades of environmental damage at just the moment when scientists say the need for action is most urgent.

"The building of this dedicated inflexible infrastructure assures the continued use of this destructively produced high emissions oil far into the future," they said. "This at a time when the United States must reduce its emissions, not increase them."