The IPCC Synthesis Report will summarise the causes and impacts of - and solutions to - rising temperatures.
It will be the bedrock of talks on a new global climate deal.
But there are concerns that political battles could neuter the final summary.
Over the past 13 months, the IPCC has released three major reports on the physical science, the impacts and the potential methods of dealing with climate change.
On Sunday they will release the Synthesis Report. This new study is meant to take the most important elements of all three and blend them into something new. It is not meant to be a cut-and-paste exercise.
"The new thing is there is going to be a stand-alone document that will be the most important for policymakers for the next few years," Prof Arthur Petersen, a member of the Dutch government's team at the Copenhagen meeting, told BBC News.
"It will be the document for the Paris summit."
The UN hopes to deliver a new global treaty on climate change at a meeting in the French capital at the end of 2015. The IPCC Synthesis Report will, in the eyes of many, play a critical role in that.
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The reality is we are facing a bleak future in absence of any ambition on mitigation and that needs to be addressed”
Sanjay Vashist
Climate Action Network
"It is the last word that science will have in this process," said Alden Meyer from the Union of Concerned Scientists, who is also attending the Copenhagen meeting as an observer.
"So it is important that they get it right and make it as relevant to policymakers as possible."
'Warming unequivocal'
A draft of the report, seen by the BBC, underlines once again the near certainty of scientists about warming and man's role in it.
Continued emissions of greenhouse gases, it warns, increase "the likelihood of severe, pervasive and irreversible impacts for people and ecosystems".
The authors reiterate that each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850.