Just two years ago, a U.N. panel that synthesizes the work of hundreds of climatologists around the world called the evidence for global warming 'unequivocal.' But as representatives of about 200 nations converge in Copenhagen on Monday to begin negotiating a new international climate accord, they do so against a background of renewed attacks on the basic science of climate change. The debate, set off by the circulation of several thousand files and e-mail messages stolen from one of the world's foremost climate research institutes, has led some who oppose limits on greenhouse gas emissions, and at least one influential country, Saudi Arabia, to question the scientific basis for the Copenhagen talks. The uproar has threatened to complicate a multiyear diplomatic effort already ensnared in difficult political, technical and financial disputes that have caused leaders to abandon hopes of negotiating a binding international climate treaty this year. Scientists and policy makers in the United States and abroad have said that nothing so far disclosed has undercut decades of peer-reviewed science. Yet on dozens of Web sites and blogs, skeptics and foes of greenhouse gas restrictions take daily aim at the scientific arguments for human-driven climate change. The stolen material was seized upon for the questions it raised about whether some data had been manipulated. At the same time, polls in the United States and Britain suggest that the number of people who doubt that global warming is dangerous or caused by humans has grown in recent years. If scientific predictions about global warming's effects are correct, inaction will lead at best to rising social, economic and environmental disruption, at worst to a calamity far more severe. If the forecasts are wrong, nations could divert hundreds of billions of dollars to curb the emissions of greenhouse gases while struggling to recover from a global recession. Yet the case for human-driven warming, many scientists say, is far clearer now than a decade ago, when the skeptics included many people who now are convinced that climate change is a real and serious threat. The Copenhagen conference itself is a reflection of the increasing acceptance of the scientific arguments: The negotiations leading to the talks were conducted by high-ranking officials of the world's governments rather than the scientists and environment ministers who largely shaped the 1997 Kyoto Protocol. 