LOS ANGELES -- With President Obama's re-election, environmentalists are re-energized.

In his first term, the president disappointed just about everyone in the movement with (among other things) his lackluster support for the cap-and-trade bill passed by the House in 2009. Senate Democrats had a filibuster-proof majority at the time, so only a few senators from the party's conservative Blue Dog caucus or those representing fossil fuel-rich states would have needed some LBJ-style arm-twisting (or Clinton-style quid-pro-quos) to get the bill through.

The Senate version of the bill never even came up for a vote, and the Democrats have since lost their numerical advantage. Legislative action on reducing carbon emissions, therefore, is widely viewed as having a polar bear's chance in Phoenix.

Nonetheless, policy analysts say, there's much a president can do on his or her own to bring down the U.S. contribution to greenhouse gases. The executive branch is, of course, tasked with enforcing laws and regulations already on the books, creating new regulations and updating old ones. It has the power, technically, to exercise authority over every major source of greenhouse gases, from power plants (34 percent of U.S. emissions) to adipic-acid manufacturing (less than half a percent).

Producing electricity is the No. 1 source of greenhouse gas emissions in the United States. As natural gas' cost has fallen due to the fracking boom, it has replaced coal at many power plants. But coal is still the source of one-third of our electricity, and it contributes the majority of emissions from power plants.

The Supreme Court has ruled that the Clean Air Act gives the EPA the authority to regulate CO2 emissions, making the agency the lever by which the second Obama administration can force the greatest reduction in greenhouse gases. The traditional approach would be to require individual power plants to limit CO2 emissions -- much as the agency has done to reduce the amount of toxic chemicals, such as mercury, that come from burning coal. Carbon "scrubbers," though, are expensive, and requiring them would likely face legal challenges.

Obama's Department of Energy could also bring down emissions from power plants; it has the job of setting efficiency requirements on appliances and equipment. And EPA could squeeze coal plants further by forcing reductions of mercury and sulfur dioxide emissions, since those chemicals are closely correlated with CO2 emissions. Additionally, new regulations on coal ash disposal would make producing the stuff more costly for utilities, and thereby encourage alternate fuels.

Electricity from burning natural gas accounts for at least 5 percent of greenhouse gas emissions. Natural gas is mostly methane, a potent (though relatively short-lived) greenhouse gas, and when it's burned it produces CO2 (though in about half the concentration as coal).

Much of the natural gas industry's emissions occur before the gas is even burned in power plants, and there lies another opportunity for Obama to reduce greenhouse gases. Fracking operations (for oil as well as gas) routinely burn off methane at the source, so penalizing that practice would reduce CO2.

About 55 percent of U.S. homes get their heat from natural gas, accounting for about 7 percent of our emissions, and this combustion source represents another opportunity for reductions. As with electric appliances, the Department of Energy has issued standards for furnaces and water heaters; tightening them could lower emissions while offsetting increased energy costs that may result from conversion to renewable sources.

Transportation accounts for the most emissions after those associated with energy utilities -- about 29 percent of the total. Obama's National Highway Traffic Safety Administration issued fuel economy standards for light-duty vehicles (aka cars) in May 2010 that will reduce the amount of oil used in the United States by more than what we buy from Saudi Arabia. It was such a huge leap, in fact, that it's unlikely he'll be able to press for more fuel-economy improvements in automakers' consumer fleets during his second term. But the EPA has additional authority to effect reductions in cars' tailpipe emissions regardless of their mpg. And the efficiency requirements enacted in 2010 didn't cover vehicles like delivery trucks and 18-wheelers; given the strong buy-in from industry on the requirements for cars, upping the larger vehicles' fuel economy should prove doable if the administration chooses to prod the industry in this direction.

The sad fact is that without Congress' help, the president can't get the atmospheric concentration of greenhouse gases to a level that won't cause major problems in the lifetime of today's first-graders.

Paul Tullis, has written about science for the New York Times Magazine, Businessweek, Scientific American Mind and others.