Just two years ago, a United Nations panel that synthesizes the work of hundreds of climatologists around the world called the evidence for global warming “unequivocal.”

A globe hung at a conference center in Copenhagen where climate change is to be discussed.

Negotiations by high-ranking government officials prepared the way for the United Nations conference in Copenhagen.

But as representatives of about 200 nations converge in Copenhagen on Monday to begin talks on 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...