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On the eve of the release of a landmark climate change report Friday, U.S. officials said Thursday that the nation is making progress in cutting its heat-trapping greenhouse gas emissions but still has "more work to do."

Average U.S. emissions were lower from 2009 through 2011 than in any three-year period since 1994-1996 and are on the way to meeting President Obama's pledge to cut them 17% below 2005 levels by 2020, according to a State Department report to the United Nations.

"The United States has already made significant progress, including doubling generation of electricity from wind and solar power and establishing historic new fuel efficiency standards," the report says. Still, it makes the case for additional steps such as cutting emissions from U.S. power plants, saying total U.S. emissions — based on measures in place as of September 2012 — will be 4.6% below 2005 levels in 2020. That falls far short of Obama's pledge.

The report comes as the U.N.-created Intergovernmental Panel on Climate Change prepares to release a draft of its latest assessment of the science on climate change and its impacts Friday in Stockholm. Based on research by hundreds of the world's top climate scientists, the panel is expected to say with heightened certainty that humans are responsible for the planet's rising temperatures and that surface temperatures are not the only indicators of climate change.

The panel releases reports every few years that synthesize the latest in peer-reviewed research, and its fifth assessment — to be released in several parts over the coming year — is the first since 2007.

This assessment is likely to paint a dire portrait of climate change. Yet some scientists say it actually underestimates the problem. Kevin Schaefer of the National Snow and Ice Data Center in Boulder, Colo., says it doesn't, for example, include the significant and increasing greenhouse gas emissions from thawing permafrost, which is perennially frozen ground in Alaska and other Arctic regions.

The State Department's report, updated every four years as part of the U.N.'s monitoring of global climate change efforts, agrees that the challenges ahead are formidable. "Climate change represents one of the greatest challenges of our time," it says.

The report says total U.S. greenhouse gas emissions rose 8% from 1990 through 2011 but began falling dramatically in 2005, partly because of the economic downturn and fuel switching from coal to natural gas. It expects more progress through increased energy efficiency, greater use of solar and wind power and Obama's requirement that new cars and light trucks nearly double their fuel efficiency by 2025.

Yet if the United States is going to meet Obama's pledge, it has to take bolder steps, says Frances Beinecke, president of the Natural Resources Defense Council, an environmental group. She says it needs to, among other things, adopt the Environmental Protection Agency's plan to cut carbon emissions from power plants.

The EPA released an updated proposal last week that would, for the first time, set limits on emissions from new coal and natural gas plants. It aims to finalize the rule and propose a new limit for existing plants next year. Yet the coal industry and some Republicans on Capitol Hill object to these standards as job-killing regulations.

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