The researchers estimate that the proposed pipeline, which would carry oil from tar sands in western Canada to refineries on the Texas Gulf Coast, would increase world greenhouse gas emissions by as much as 121 million tons of carbon dioxide a year.
The department said this year that at most, the pipeline would increase world carbon dioxide emissions by 30 million tons.
Such emissions have been on the mind of President Barack Obama, who has said his administration would allow the pipeline to be built "only if this project does not significantly exacerbate the problem of carbon pollution."
The new estimates, from scientists at the Stockholm Environment Institute, were published Sunday by the journal Nature Climate Change. Peter Erickson, lead author, said his work implies that the pipeline could basically wipe out reductions from some potential pollution-cutting policies under discussion.
The State Department declined to comment on the research by Erickson and co-author Michael Lazarus.
Lower prices may sound good, but there's no free lunch, said Wesleyan University environmental economist Gary Yohe, who praised the work.
"Lower fuel prices are bad if they don't include all of the social costs," Yohe wrote in an email. "Consumers are happy, but the planet is not necessarily."
An increase of 121 million tons of carbon dioxide is dwarfed by the 36 billion tons of carbon dioxide the world pumped into the air in 2013. That's why University of Sussex economist Richard Tol dismissed the calculated Keystone effect as merely a drop in the bucket. If somebody is concerned about climate change, he wrote in an email, the pipeline "should be the furthest from your mind."
Ken Caldeira of the Carnegie Institution of Washington agreed the amount is small, but said the concern is more about the idea of boosting emissions than the degree of change.
Independent energy economist Judith Dwarkin in Calgary, Alberta, Canada, dismissed the study, faulting the idea that added oil production will lower the price and boost demand. Usually, she said, it's consumption that spurs price and then oil production.
Nature Climate Change: http://www.nature.com/nclimate
Seth Borenstein can be followed at http://twitter.com/borenbears