Phasing out fossil fuel leases on public lands would be one of the most significant new policies the United States could adopt to help meet international climate goals, according to a new report.

In a world cutting its use of carbon fuels to bring warming under control, "at some point in the next two decades, there is potentially no need for federal fossil fuels," said the analysis, published on Tuesday by the Stockholm Environmental Institute.

Annual global emissions of carbon dioxide could drop by 100 million tons by 2030 if the Interior Department stopped issuing or renewing leases from federal lands and waters, it said. Thirty percent of the emissions reduction would occur overseas, as coal exports from federal lands dwindle.

The study is among the most detailed assessments yet published of the effectiveness of a hotly disputed policy proposal that is being closely studied by the Obama administration. The review will not be completed until after a new president takes office.

The study's conclusions were presented in a detailed paper and in summary form. It was written by Pete Erickson and Michael Lazarus of the institute's Seattle office.

About a quarter of U.S. fossil fuel energy comes from federal lands, including 40 percent of coal. These subsidized leases are facing new challenges from environmental advocates who say they unwisely lock in high-carbon infrastructure for decades to come.

After the international Paris climate agreement set the goal of rapidly phasing out worldwide emissions of greenhouse gases from fossil fuels, President Obama declared in his State of the Union address that he wanted to change the leasing system to better reflect its fiscal and planetary costs – including its carbon footprint.

The Interior Department then announced an environmental review to consider the options. Proposals have also been floated in Congress to raise royalties, and lawsuits are under way to force leasing agencies to take environmental impacts into account.

Teasing out the climate implications of a leasing phase-out is complicated for several reasons. After all, the U.S. energy boom in recent years has occurred mostly on private lands, not public lands. And any changes would affect coal, oil and natural gas in different ways.

Taking into account switching between various fuels, the Stockholm study found that restricting coal leases would cut annual emissions by 107 million tons, partially offset by 36 million tons of additional emissions from natural gas. Restricting oil leases would cut emissions by 54 million tons, offset by additional emissions of 23 million tons from other fuels. Cutting natural gas leasing would have only negligible net effects, it found.

Phasing out all fossil fuel leasing would add up to 100 million tons of emissions reductions in 2030, and probably more in later years, the study found.

By comparison, other policies would be much less effective. For example, across-the-board regulations on emissions from the whole oil and gas industry would cut emissions just 13 million tons, and methane leakage restrictions on oil and gas operations on federal lands just 5 million tons, the study said.

The Stockholm institute's work builds on recent projections of emission pathways toward the international goal of keeping warming below 2 degrees Celsius, such as one published in the journal Nature and another published by the International Energy Agency. In Paris, negotiators called for an even more ambitious goal, 1.5 degrees.

The study compared those approaches to the current U.S. Energy Department's energy outlook, which assumed no new restrictions beyond the Clean Power Plan, and saw energy production growing 11 percent by 2040.

"Between them, these studies suggest that to be consistent with a 2 degree Celsius goal, the U.S. would need to cut aggregate fossil fuel production by 40–44 percent from current levels by 2040," the Stockholm paper found.

One big unknown: how much a unilateral step by the federal government to phase out leasing would influence other nations. If the rest of the world barrels down the business-as-usual path, the U.S. change might have less effect, the authors said. If it inspires others to raise their ambitions, it might have even more impact than this study forecasts.