Washington - President George W. Bush could be forcing President-elect Barack Obama to act almost immediately to curb global warming, after years of the Bush administration fighting attempts to crack down on greenhouse gas emissions.

    Or, depending on which interpretation prevails, Bush could be giving his successor much-needed breathing room on a volatile issue.

    In its final weeks, the Bush administration has moved to close what it calls "back doors" to regulating carbon dioxide and other greenhouse gases. It barred the Environmental Protection Agency from considering the effects of global warming on protected species. And it excluded carbon dioxide from a list of pollutants the EPA regulates under the Clean Air Act.

    Environmentalists call the moves a last-minute attempt to block speedy, executive action by the president's successor on climate change, an issue that Obama calls a top concern. But they say it could backfire, by prompting lawsuits and fueling fights over coal-fired power plants that the new administration would need to resolve quickly.

    Obama "now has to clean up a mess," said David Bookbinder, chief climate counsel for the Sierra Club, which has challenged the EPA over the Clean Air Act decision and plans to sue to block it. "They're forcing him to act sooner than he otherwise might have."

    Yet energy-industry lobbyists predict the challenges will fail. They say the Bush administration's actions give Obama time and political cover to take a more deliberative approach to emissions regulation and avoid overly broad, overly swift rules that could slow construction projects for schools and businesses, not just power plants.

    "I'm quite confident that the Obama administration will have no interest in coming in and immediately reversing" the decisions, said Jeffrey Holmstead, a former EPA clean air administrator who now represents energy industry clients at the lobbying firm Bracewell and Giuliani in Washington.

    Underlying the debate is the issue of how the federal government should reduce America's emissions of the gases scientists blame for global warming, including carbon dioxide. Congress has long debated, but never approved, a so-called cap-and-trade system to limit carbon emissions.

    Frustrated, environmental groups have looked for other ways to fight global warming. They have pressed to list the polar bear, which has seen its habitat dwindle as arctic ice melts, as a threatened species. The Interior Department consented this summer, but later declared that any protection for the bears under the Endangered Species Act didn't extend to regulating greenhouse gases.

    Environmental groups also sued to force the EPA to regulate carbon dioxide as a pollutant under the Clean Air Act. The Supreme Court ruled the EPA had the power to do that, but administration officials have declined to exercise it.

    EPA Administrator Steven Johnson issued a memo in late December - as part of a review for a proposed coal-fired power plant expansion in Utah - that excludes carbon dioxide from the list of pollutants the government must regulate under the Clean Air Act when approving construction projects.

    Environmentalists called the memo a gift to the coal industry and utilities.

    "This is a desperate attempt to interfere with the Obama administration's ability to deal with greenhouse gases from power plants," said John Walke, a former EPA attorney who is now clean air director for the Natural Resources Defense Council.

    Industry lobbyists say the memo leaves the door open for Obama to regulate carbon dioxide eventually through the EPA - and that it gives him time to solve a wider problem. A broad rule, they say, risks lumping school expansions, office construction and even some home building into the same regulatory process a power plant would face.

    The memo allows Obama's team time to solve those issues, Holmstead maintains, so "they don't sweep in hundreds of thousands of small building projects around the country."

    Obama vows to push aggressively for a cap-and-trade bill as president. Under this method of trading, overall air quality goals are set by the government, and individual facilities such as power plants are given allowances for what they can emit. Facilities that pollute less than they are permitted can trade a share of their allowance to others that pollute more.

    And the president-elect's top energy adviser promised during the campaign that Obama would move to regulate carbon under the Clean Air Act within 18 months of taking office.

    Now, environmentalists say, Bush has put pressure on Obama to act sooner - or risk watching states approve new power plants without regard to carbon emissions. Energy companies have taken quick notice of the EPA memo: Duke Energy recently cited it in a court filing supporting its bid to build a new coal-fired plant in Indiana.