It's becoming more likely that a key global temperature limit will be reached in one of the next five years.
A major study says by 2025 there's a 40% chance of at least one year being 1.5C hotter than the pre-industrial level.
That's the lower of two temperature limits set by the Paris Agreement on climate change.
The conclusion comes in a report published by the World Meteorological Organization (WMO).
The analysis is based on modelling by the UK Met Office and climate researchers in 10 countries including the US and China.
In the last decade, it was estimated that the chance of any one year reaching the 1.5C threshold was only 20%.
This new assessment puts that risk at 40%.
Leon Hermanson, a senior Met Office scientist, told BBC News that comparing projected temperatures with those of 1850-1900 shows a clear rise.
"What it means is that we're approaching 1.5C - we're not there yet but we're getting close," he said.
"Time is running out for the strong action which we need now."
The researchers point out that even if one of the next five years is 1.5C above the pre-industrial level, it'll be a temporary situation.
Natural variability will mean the following few years may be slightly cooler and it could be another decade or two or more before the 1.5C limit is crossed permanently.
The Paris Agreement established the goal of keeping the increase in the global average temperature to no more than 2C and to try not to surpass 1.5C - and that's understood to mean over a long period rather than a single year.
According to Dr Joeri Rogelj, director of research at the Grantham Institute, Imperial College London, "the 1.5C in the Met Office announcement should not be confused with the 1.5C limit in the Paris Agreement".
"The Paris targets refer to global warming - that is, the temperature increase of our planet once we smooth out year-to-year variations," he explained.
"A single year hitting 1.5C therefore doesn't mean the Paris limits are breached, but is nevertheless very bad news.
"It tells us once again that climate action to date is wholly insufficient and emissions need to be reduced urgently to zero to halt global warming."