Top climate scientists will launch a study this week of how hard it would be to limit global warming to 1.5C, although many of them fear it might be too late to reach that level.
Where are temperatures at now?
The world's average surface temperatures reached 1C above pre-industrial times in a record-hot 2015. They will rise by 3C or more by 2100 if current trends continue, many projections show.
Yet 1.5C is the target?
A 195-nation climate summit in Paris in December asked the UN's Intergovernmental Panel on Climate Change for a report in 2018 on limiting warming to just 1.5C. The IPCC began a three-day meeting in Nairobi today to consider how to do that.
How likely is it?
"Do we know how? No. It is definitely a moon shot," Christiana Figueres, the UN's climate chief, said at a conference in London today. Paris set a goal of limiting average surface temperatures to "well below" 2C while "pursuing efforts" for 1.5C. Experts say the IPCC will comply with the Paris request, with misgivings. "I don't seek how they can say 'No'," David Victor, a professor of international relations at the University of California, said.
"But I don't see how they say 'Yes' with a straight face."
What do scientists think?
Documents prepared for the Nairobi meeting say scientific literature about 1.5C is thin. Many scientists have barely focused on the 1.5C goal, reckoning it would require unrealistically deep cuts in emissions.
What would need to happen?
Some IPCC studies suggest 1.5C will be feasible if the world develops low-cost technologies later this century to extract greenhouse gases from the atmosphere.
What would be the benefits?
Limiting warming to 1.5C rather than 2C would limit, for instance, sea level rise, the melt of Arctic sea ice, damage to coral reefs and the acidification of the oceans, according to IPCC studies.
Who is pushing for 1.5C?
Many poor nations, fearing melting ice that will raise sea levels and swamp their coasts, campaign for "1.5 to stay alive". Myles Allen, a professor at Oxford University, said: "My concern is that the 2018 report may have lots of information about how hard it will be to achieve 1.5C, and relatively little about the benefits". He noted that countries pushing hardest for the 1.5C limit, including small, low-lying island states such as the Marshall Islands or the Maldives, wanted to stress the advantages.