IN THE full glare of Washington’s summer sunshine, Barack Obama unveiled what he called “a co-ordinated assault on a changing climate” on June 25th. He promised to deploy almost every green weapon at his disposal, from better insulation in public buildings to loan guarantees for clean energy. To engage the enemy as quickly as possible, he is relying solely on authority already granted to him by Congress. Yet most of the munitions in his atmospheric arsenal are less than fearsome—and Congress, which could provide reinforcements, prefers not to.

The centrepiece of Mr Obama’s “climate action plan” is a directive to the Environmental Protection Agency (EPA) to limit the amount of carbon dioxide that power plants may produce. The EPA was already working on such a rule for new plants; the president wants it to produce that by September 20th, and one curbing emissions from existing plants a year from now. Since power plants spew out almost 40% of America’s greenhouse gases, and are not yet subject to any restrictions, this order could in theory make a big dent in America’s contribution to global warming.

In addition, the president ordered the strengthening of fuel-economy standards for lorries and buses, on top of the increases for all vehicles adopted in his first term. He offered $8 billion in loan guarantees for the deployment of technologies that make fossil fuels less harmful to the climate, such as carbon capture. He promised to promote renewable power by encouraging the construction of wind farms and solar arrays on federal lands, by requiring government agencies to obtain more of their own power from such sources and by streamlining permits for a more efficient electricity grid.

Mr Obama said he would tighten energy-efficiency standards for federal buildings and try to get mortgage lenders to take more account of energy efficiency in home sales. There was talk of curbing leaks of natural gas, managing forests to trap more carbon and phasing out HFCs (chemicals used in air-conditioners and fridges that are especially potent greenhouse gases).

Some thought Mr Obama would approve Keystone XL, a pipeline for carrying Canadian oil to American refineries. Instead, he said the pipe could go ahead only if it “does not significantly exacerbate” carbon pollution—a high hurdle for oil from tar sands.

All this is supposed to help fulfil the president’s pledge to get America’s greenhouse-gas emissions to 17% below the level of 2005 by 2020. In fact, America’s emissions have been falling, thanks to the recession and widespread switching from coal- to gas-fired power stations. But not fast enough: they are only 7% below the level of 2005, and the administration estimates that they are poised to start rising again.

Green groups say that Mr Obama’s plan, if vigorously pursued, might propel America most of the way to its target. But the president is less agitated than they are about the need to reduce emissions from natural-gas drilling and transport, for example. He gave no indication of the scale of cuts he would like to see from existing power plants, the most important element of his plan. And however exacting the EPA’s rules are, drafting and implementing them will be a long and uncertain process.

The EPA’s authority over greenhouse gases stems from the Clean Air Act, which was first passed in 1970 to combat smog. Under its terms, the agency must propose standards for existing power plants, revise them after listening to comments and then wait for each state to come up with an implementation plan, which in turn will set compliance dates that may be several years in the future. Mr Obama is thus likely to be out of office before the new standards bring down emissions at all.

Moreover, even the relatively modest rules the agency has promulgated under the act on less pervasive pollutants than carbon dioxide have sparked endless lawsuits. This week, for example, the Supreme Court agreed to hear a challenge to restrictions on ozone that drifts over state boundaries. There is great uncertainty as to whether the relevant section of the act permits the EPA to adopt trading schemes and other market-based mechanisms to spur cuts, or only allows it to require particular technological fixes. And even if the rules survive in court, a future administration could reverse them, as George W. Bush’s underlings did in 2005 with a Clinton-era ruling that mercury was an especially dangerous pollutant to be regulated under a particular section of the act.

If you want less of it, tax it

Mr Obama himself admits that it would be far better if Congress adopted a more sweeping measure that, in effect, puts a uniform price on carbon from any source. That would allow cuts to be achieved more cheaply and efficiently. But all such proposals have run aground in Congress; the last big push, in 2009, cost many Democrats their seats in the next year’s mid-term elections.

Many House Republicans doubt that the planet is heating up at all. Others question the expense and effectiveness of Mr Obama’s plan. Almost all salivate at the prospect of excoriating Democrats at next year’s mid-terms for throttling the economy with green tape. No wonder, then, that the most impassioned part of Mr Obama’s speech was when he called on supporters to convince their neighbours that climate change is real, and to urge them to vote accordingly.