Senior scientists and government officials are meeting in Japan to agree a critical report on the impact of global warming.
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.
This will be the second of a trilogy of reports on the causes, effects and solutions to climate change, from a body made up of some of the leading researchers in the world.
Last September in Stockholm, they produced a summary on the physical science of climate change, arguing that it was real, and humans were the "dominant cause".
Now in Yokohama, the second IPCC working group will set out the impact that rising temperatures will have on humans, animals and ecosystems over the next century.
The scientists and government officials will agree on the exact wording of the final summary over the next few days, with publication coming early next Monday, UK time.
The summary is a short, dense document that sums up the findings of 30 underlying chapters, each made up of detailed assessments of relevant research that has been published since 2007.
A leaked draft of the summary, seen by the BBC, points to a range of negative effects that will, in some instances, be "irreversible".
Millions of people living in coastal areas in Asia will be affected by flooding, and displaced due to land loss.
The draft says that crop yields around the world will decline by up to 2% per decade for the rest of the century.
If the world warms by 4C towards the end of this century, this will pose a "significant risk to food security even with adaptation".
The summary says that in the near term, at levels of warming that scientists say we are already committed to, there is a very high risk to Arctic sea ice and coral reefs.
They warn that the oceans will become more acidic as they warm, and species will move towards the poles to escape the heat.
The researchers say that in this report they have been able to call on a broader range of observations. Instead of just adding up all the effects, saying that together they suggest an influence of climate change, they have been able to look at individual events.
"We've reached the stage where we can go impact by impact, and say is there an influence of climate change?" Dr Chris Field, co-chair of Working Group II told BBC News.
"We don't see it with every one but we do see it with a lot. It's a real difference. Before it was a very general concept, now it is much more specific."
But some researchers are decidedly unhappy with the draft report.
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.
"But if you ask people in Syria whether they are more concerned with chemical weapons or climate change, I think they would pick chemical weapons - that is just silliness."
The assertions that the summary for policymakers is too alarmist has been countered by Dr Arthur Petersen, the chief scientist at the Netherlands Environmental Assessment Agency, who is representing his government in Yokohama.
He said that this working group had to anticipate all the challenges that might arise from a warming world.
"Working group I (the physical sciences) doesn't want to sound alarmist. In working group II, they don't want to chance not having spotted a particular risk so they have a bias in the other direction," he said.
"In this report, they are more honest and open that they have a risk orientation because they do focus more on the risks than the opportunities."
The report is shaping up to be more nuanced, with far more emphasis on adaptation than the last one in 2007.
According to many familiar with the text, it is about managing the risk rather than waiting to see if things get worse.
"We are going to frame the issue of climate change as more of a distributional issue," said Dr Petersen.
"It's not doom and gloom but an additional stress on countries that are already severely stressed."
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