In just a few days, policymakers will descend on Copenhagen for an international climate summit that has been pored over by the press since its announcement two years ago. Initially billed as the event that would herald a new era for international climate policy, it's now clear that Copenhagen will not be the birthplace of a post-2012 climate treaty.

But even if the summit's ambitions have been scaled back, Copenhagen is still as much 'crunch time' for climate change as it ever was. The level of commitment shown by nations in the coming two weeks will set the tone of talks aimed at reaching a legal agreement in 2010. For that reason alone, the announcement that the world's two largest greenhouse gas emitters — the United States and China — will arrive in the Danish capital with carbon reduction targets is to be welcomed, even if the level of their proposed cuts is less than needed.

While these moves are being hailed as major political strides by world leaders, the scientific community is growing increasingly frustrated with the sluggish progress on the policy front. In this issue of Nature Reports Climate Change, scientists warn that whatever approach is taken to mitigating climate change — be it putting in place a target for emissions reductions (see page 147) or setting a limit on the rate of warming (page 141) — failure to act soon will come with substantial risks, one of which is hitting 4 °C of global warming within the next half-century (page 143). Most vocal of all is climatologist James Hansen, director of NASA's Goddard Institute for Space Studies in New York, who says (page 139) that politicians are deceiving themselves and the public on the ability of their proposed plans to solve the climate problem.

That greater urgency is needed is news to no one. And shouting it from the rooftops may backfire, causing 'climate fatigue' (Science 326, 926-928; 2009). Yet there are several reasons to reiterate this message now. In the last few weeks, what appears to be a deliberate attempt to cast doubt on the science of climate change — and on the need for action — has sent shockwaves through the scientific community. The content of illegally obtained e-mails belonging to scientists from a leading British climate research institute is, in some cases, professionally embarrassing, and it could yet lead to charges of misconduct.

Though none of the content undermines the evidence of the role of humans in planetary change, the political fallout of this controversy is already being felt in the US Senate, where Republican Senator James Inhofe is ordering an investigation. Scientists must communicate clearly in the coming weeks to minimize the spill-over of this storm onto the international stage. Lacking a firm deadline for agreeing a climate treaty, negotiators already have one reason to postpone key decisions into next year. With further setbacks, the UN climate negotiations risk becoming the next Doha round of trade talks, stuck forever in limbo over deep divides between rich and poor.