I passed the building at 10pm on Thursday night and the delegate's coats were still hanging in the cloakroom. Some spent all night on the final night, going line by line through thousands of scientific papers, putting the finishing touches to the science report of the Intergovernmental Panel on Climate Change (IPCC).
Now it is released, what does it say about the state of the Earth's climate system?
There is no disputing that it is a grim picture: a world with more frequent heatwaves, an ice-free Arctic in the summer, devastating floods, and sea levels rising fast.
"Warming is unequivocal," IPCC vice-chairman Jean-Pascal van Ypersele tells me. "It is extremely clear that most of it is due to human activity. The good news is that we can now act to reduce emissions and protect climate for future generations."
But is that too optimistic? And can the world's governments and businesses act, when nothing in the IPCC's previous reports has made them take the issue of climate change seriously?
The UN's last report in 2007 suggested the Earth would warm between 2C and 4.3C on pre-industrial levels by 2100. This report updates that.
Even with drastic cuts to greenhouse gas emissions, it says temperatures will still almost certainly rise 1.5C. More likely, it says, we will see a rise of at least 2C, possibly as much as 4C.
The report says sea levels are rising faster than they previously thought, and by 2100 the oceans will be between 26cm and 97cm higher. That's bad news for millions of people living in low lying areas, which are fast becoming prone to flooding.
The report says it is very likely the Arctic sea will continue to shrink and become thinner in the years ahead. Last year, the ice melted to a record low - just 3.4 million square kilometres - just a third of what it was just 20 years ago.
This means the Arctic may become ice-free in summer months - if not by 2050, then by the 2100. If that happens, a vast volume of frozen methane currently trapped in Arctic permafrost could be released, adding to existing greenhouse gases.
The scientists I spoke to were united in their call for action - cuts to greenhouse gas emissions are the only way the worse effects of climate change are to be avoided.
But even with drastic cuts the scientists say some of the changes to the planet are irreversible. The best we can do is, prepare to live in a changed, and fast changing world.