The new findings examine the impact of the U.S. energy and transportation sectors as well as sources such as methane releases from landfills.
The study gives a pessimistic view of the future even though carbon emissions have fallen in recent years because of the economic downturn and increased use of natural gas to produce electricity.
While the Obama administration has taken several steps to curb greenhouse gas emissions, such as imposing the first carbon limits on vehicles and new power plants, the analysis suggests that non-carbon emissions from the U.S. natural gas boom and from chemicals used as refrigerants are on the rise.
The U.S. target is to cut greenhouse gas emissions 17 percent by 2020 compared with 2005 levels.
Energy-related carbon dioxide emissions have fallen 8.7 percent compared with 2005 levels and are projected to stay near that level through 2035. But greenhouse gas emissions from other sources are expected to increase 18 percent by 2020 compared with the 2005 baseline and 36 percent by 2035.
Imposing greenhouse gas emission limits on existing power plants -- a policy the White House is considering -- could halve the gap between the current trajectory and the country's 2020 climate target. Phasing out hydrofluorocarbons, used in cooling equipment from soda machines to many car air conditioners, would make up 23 percent of the gap, according to the report, while stricter federal rules for natural-gas methane emissions and energy efficiency standards would make up 11 percent and 8 percent, respectively, of the difference.
"The U.S. is not yet on track to hit its 17 percent target, but we have the tools to get there," said Nicholas Bianco, a senior associate at World Resources Institute and the report's lead author.
Michael Levi, a senior fellow for energy and the environment at the Council on Foreign Relations, praised the report as "the first serious attempt to show what it would take to slash emissions over the next two decades without new legislation."
Facing stiff congressional opposition, President Barack Obama has made clear that he plans to undertake more ambitious action on climate change in his second term by using existing regulatory authority.
Durwood Zaelke, president of the Institute for Governance and Sustainable Development, noted that the car sector accounts for roughly half of U.S. HFC use, "making this the biggest opportunity for getting rid of this super greenhouse gas."
"The last time we changed the coolant in our cars, it only took three years to change the fleet in the U.S. and most of the world," he added.
Without setting these and other climate polices in motion, the WRI analysts warn, the United States will find itself falling short of the pledge it made in 2009 as part of U.N. climate negotiations. While the commitment is more modest than many scientists and other world leaders have called for, the United States' ability to meet it could influence whether more than 190 nations can broker a new climate pact over the next three years that would take effect in 2020.
Neil Morisetti, Britain's climate and energy security envoy, said the United States and other industrialized nations need to fulfill their climate pledges both to build trust among negotiators and to ensure that any global warming agreement delivers results.
"It is important, having made that commitment, that you deliver against it," Morisetti said of the current U.S. climate pledge. He added that when it comes to any future treaty, "it's important not only that we sign bits of paper, but we have a plan to get there. It is that action by national governments that is the wind beneath the sails."
Jake Schmidt, international climate policy director for the Natural Resources Defense Council, an advocacy group, said that the rest of the world "will be looking to see what the U.S. does in the next few months," given the signal that Obama has sent about tackling global warming.
"It will show the U.S. can follow through, even after the climate bill demise" of 2010, Schmidt added.
Still, Levi warned, the report "also emphasizes how unlikely we are to achieve deep emissions cuts without meaningful congressional action, particularly beyond 2020."
Ultimately, Levi said, the critical climate question is how the United States and the rest of the world will cut greenhouse gas emissions through 2030 and 2050, since that will have a much bigger impact on future warming.
"Steps between now and 2020 should be evaluated primarily based on how they set the U.S. up for the longer term, not on the exact number of tons that get cut in the next eight years," he said.
Evaluating how much methane is leaking from an increasing number of natural-gas operations and then limiting such emissions will be a challenge, Bianco said. The Environmental Protection Agency has tightened air emissions standards for new equipment in the oil and gas sector, but it could require other equipment changes and new inspection measures.
Large oil and gas facilities are the second-biggest source of greenhouse gas emissions after utilities, the EPA reported Tuesday, although they are a far smaller contributor.
In a statement, Dan Whitten, a spokesman for the trade group America's Natural Gas Alliance, said that the report "demonstrates the enormous role natural gas must play in order for the U.S. to meet any emission reduction targets," noting that the U.S. Energy Information Administration "has credited the switch to gas in the power sector as a main factor in the U.S. reaching 1992 emissions levels as of last spring."
The sector is taking voluntary steps to curb leaks and is complying with new federal air rules "that further cut emissions," Whitten added.
When asked about the study, State Department spokeswoman Molly Lynn Westrate replied in an e-mail, "We are aware of and are reviewing the WRI report."
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