Climate scientists are warning the Paris Agreement's target to keep global warming to just 1.5C could be passed within the next five years.
A new assessment by the World Meteorological Organization (WMO) and the UK's Met Office says there's a 40 percent chance this year, or any of the next four, will breach that marker.
There's a 90 percent chance we'll have the hottest year on record between now and 2025, and the overall average is likely to be at least 1C above pre-industrial times - possibly up to 1.8C warmer.
The newly calculated chance of breaching 1.5C of warming is more than double what it was last year, the WMO said, saying it had improved its data collection - particularly in the fast-warming polar regions.
"These are more than just statistics," said WMO Secretary-General Petteri Taalas.
"Increasing temperatures mean more melting ice, higher sea levels, more heatwaves and other extreme weather, and greater impacts on food security, health, the environment and sustainable development."
The primary goal set in Paris in 2015 was to keep temperatures to 2C above pre-industrial levels, but nations are required to "pursue efforts to limit the temperature increase to 1.5C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change".
"This is nevertheless very bad news," said Joeri Rogelj, director of research at the Grantham Institute at Imperial College London.
"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."
The hottest year on record to date was 2016. Some months that year breached the 1.5C mark.
"To limit global warming to 1.5C... we need to hit the brakes on emissions now and stop global warming in the next 30 years or so," University of Oxford geosystem science professor Myles Allen.
"That hasn't changed, apart from the fact that five years have passed since Paris and we are still only talking about hitting the brakes."