The landmark report by the Intergovernmental Panel on Climate Change is expected to state with more confidence than its previous four assessments that global warming is mostly man-made.
It's also going to provide updated observations and projections of the changes happening in the climate system, from the melting of Arctic sea ice to the warming and acidification of oceans.
Thomas Stocker, co-chair of the working group that wrote the report, said it has undergone multiple stages of review, with more than 50,000 comments considered by the authors. The final version is scheduled to be adopted at the end of an IPCC conference this week in Stockholm.
"I know of no other document that has undergone this scrutiny," Stocker said as the meeting opened. "It stands out as a reliable and indispensable source of knowledge about climate change."
He said millions of measurements on land, at sea, in the air and from space underpinned what he called an "unprecedented and unbiased view of the state of the climate system."
The IPCC's work to improve the world's understanding of climate change won it the Nobel Peace Prize in 2007 together with climate campaigner Al Gore. But a series of errors embarrassed the authors of its previous climate assessment, which was completed that same year.
Among the most prominent was an incorrect statement that the glaciers in the Himalayas were melting faster than others and that they would disappear by 2035 - hundreds of years earlier than other information suggests.
An independent review of the U.N. climate panel in 2010 found that overall it has done a good job but needs more openness and regular changes in leadership. It also called for stronger enforcement of its reviews of research and adoption of a conflict of interest policy.
IPCC chairman Rajendra Pachauri told The Associated Press on Monday that he hopes the review has helped enhance the panel's credibility.
"Our processes were found to be very strong. Very good. Very effective," he said. "But you know we have been in existence for 20-odd years and therefore it was time for us to get a second opinion on how we could improve ourselves. And I'm sure we've done very well in implementing the recommendations."
The report being completed in Stockholm deals with the physical science aspects of the climate system and is the first of a four-part assessment that covers several aspects of global warming.
Earlier Monday, Pachauri told delegates in Stockholm that the latest report marks "a new milestone in the understanding of climate change."
He said the fact that 60 percent of the authors were new to the IPCC process "show the inclusivity and openness" of the U.N.-sponsored panel "and the emphasis we place on new knowledge and expertise and fresh perspectives and approaches."