Is the US Congress ready to take action to limit global climate change? No. But a look at two key bills shows how much progress has been made since this time last year — and how many hurdles remain.

The legislation getting the most attention is a bill to mandate reductions in greenhouse-gas emissions, introduced by senators Joseph Lieberman (Independent, Connecticut) and John Warner (Republican, Virginia). They describe the measure as reducing emissions to as much as 10% below 2005 levels by 2020, and 63% by 2050.

On 1 November, a Senate subcommittee that Lieberman chairs approved the bill — just the first step in a tortuous legislative process, but the first time that any congressional panel has approved a measure that would require cuts in greenhouse-gas emissions. Moreover, the bill has gained support from middle-of-the-road members of both parties who have stayed away from previous climate measures, and a key swing senator from a coal state, Max Baucus (Democrat, Montana), voted for it in subcommittee.

That's a big difference from last year, when Lieberman backed a somewhat weaker bill he had introduced with John McCain (Republican, Arizona). With the Republicans in control back then, climate bills were not scheduled for committee votes — and they would not have passed in any event. But the Lieberman–Warner bill is not exactly sailing through Congress now; it squeezed by with a 4–3 vote in the panel most worried about climate change. Of even greater concern, the bill faced opposition not only from conservative Republicans, but also from the most liberal member of the panel, Bernie Sanders (Independent, Vermont). Echoing several environmental groups, Sanders argues that all emission credits in a cap-and-trade system should be auctioned off, whereas the bill provides some credits to corporations for free.

In one sense, the attacks from the left are a good sign. Proponents of action used to be happy to see almost any measure that acknowledged the need to cut emissions; nothing was going to pass anyway, so just getting officials on-the-record could be counted as progress. But now the details matter, and Lieberman and Warner are going to have to negotiate with both the right and the left. From the centre, they will also have to contend with a more restrained alternative offered by senators Jeff Bingaman (Democrat, New Mexico) and Arlen Specter (Republican, Pennsylvania). Cobbling together the votes for Senate passage has to be seen as a long-shot goal that is not likely to be met in this Congress.

If a bill ever did pass the Senate, it would probably face even greater obstacles in the House of Representatives, a more ideological body with a keener tendency to protect local economic interests. House Speaker Nancy Pelosi (Democrat, California) surprised everyone earlier this year by announcing that climate legislation would be a priority in the House and by setting up a new committee on the issue. But since then, climate bills seem to have slipped gradually from the House agenda, replaced, in part, by a related energy measure.

Different versions of the energy bill passed the House and Senate this summer, and both are largely a hodge-podge of less controversial ideas. But each version included at least one lightning rod — in the Senate, tighter mileage standards for cars, and in the House, renewable-energy requirements for utilities. If the House and Senate can't agree on those two items — and negotiations have been moving slowly — then it's hard to imagine action on a comprehensive climate measure, which would necessarily include much more controversial provisions. Indeed, some environmental groups view the bill as an unwelcome distraction, eating up the time of officials and staff who could be working on broader legislation. Worse still, they fear that if the energy bill passes, the transport and utility industries will claim that they already have been forced to make sacrifices and should therefore be treated gingerly in a larger climate bill.

'If' is a key word; the energy negotiations are clearly no cakewalk. The Senate approved the tougher mileage standards without a roll-call vote — in which all senators go on-the-record with their stance — so it's not clear how much latent opposition to the idea remains in the Senate, which had always defeated the standards by a sizeable margin in the past. The House, which has also defeated mileage standards in past years, didn't even take up the issue because its Democratic leadership asked proponents to hold off. Either the leadership didn't want the House on record defeating the standards, or it didn't want to put Democrats in the uncomfortable position of choosing between their environmental backers and their labour backers, who generally oppose the standards. Either explanation signals rough sledding for the proposal. The future of the utility provision is equally problematic.

Many observers think that an energy bill will finally emerge, in part because Pelosi has so vocally promised to produce one. The political fall-out from such a bill could be one factor that determines how much stomach Congress has for a fight over a measure that targets climate change. If Democrats are rewarded by the public and the media for taking long-debated steps to reduce fossil-fuel consumption, then that may encourage more high-profile action on the climate. It might also make the issue a more central part of the US presidential campaign.

But high-profile action is not necessarily speedy action. Complicated and controversial legislation usually takes several Congresses to pass. The first clean-air law was enacted in 1955, but no measure that had any real impact passed until the 1970s, and a real crackdown on many pollutants didn't begin until the 1990 version of the Clean Air Act. Partly, this is because no one can foresee all the intellectual and political issues that a bill raises until the detailed writing of it begins — and sometimes problems can't be foreseen until it is implemented. Congress has wasted years on an ideological debate on the reality of climate change; now at least it's getting down to the nitty-gritty. The biggest hurdle of all facing climate change legislation may be the steepness of the learning curve; Congress won't be able to make up all that lost time.