The letter notes that the State Department’s environmental review concluded that the process of extracting oil from the Canadian oil sands produces about 17 percent more greenhouse gas pollution than the process used to extract conventional oil — but that the oil was likely to be produced, extracted and brought to market with or without construction of the pipeline.

It concluded that without construction of the pipeline, the oil would be brought to market by rail. However, that review also found that demand for the oil sands fuel would drop if oil prices fell below $65 a barrel, since moving oil by rail is more expensive than using a pipeline. The price of oil was $53.70 a barrel at midafternoon Tuesday.

Ms. Giles noted that over the long term, the price of oil would probably rise. But, she wrote, “Given the recent large declines in oil prices and the uncertainty of oil price projections, the additional low price scenario included in the Final S.E.I.S. should be given additional weight during decision making, due to the potential implications of lower oil prices on project impacts, especially greenhouse gas emissions.” Final S.E.I.S. is the State Department’s final supplemental environmental impact statement.

It is a small distinction, but one that shows the E.P.A. emphasizing quietly that, in some outcomes, the pipeline could contribute to additional greenhouse gas emissions.