Members of the Intergovernmental Panel on Climate Change (IPCC) will publish their first update in seven years on the scale of the threat.
Leaked documents speak of significant effects on economies, food supplies and security.
But some attendees say the summary, due out next Monday, is far too alarmist.
Prof Richard Tol is an economist at the University of Sussex, who has been the convening lead author of the chapter on economics.
He was involved in drafting the summary but has now asked for his name to be removed from the document.
“The message in the first draft was that through adaptation and clever development these were manageable risks, but it did require we get our act together,” he told BBC News.
“This has completely disappeared from the draft now, which is all about the impacts of climate change and the four horsemen of the apocalypse. This is a missed opportunity.”
Critics say that some aspects of the projected effects are “alarmist”, such as the impact on conflict and migration caused by climate change.
“You have a very silly statement in the draft summary that says that people who live in war-torn countries are more vulnerable to climate change, which is undoubtedly true,” said Prof Tol.