Global temperatures could reach 1.5 Celsius above pre-industrial levels in the next five years, a forecast by the Met Office has warned.
The world's first comprehensive climate treaty, the Paris Agreement, commits countries to holding global temperatures to "well below" 2C above pre-industrial levels and to strive to curb increases to 1.5C.
But the new long-range forecast suggests that the world could already, temporarily, hit the 1.5C threshold in the next few years.
Average temperatures around the world are likely to be more than 1C higher than those seen in the pre-industrial era, measured as between 1850 and 1900, and could reach 1.5C higher during the period 2018 and 2022.
There is also a small - around 10% - chance that one of the next five years could see global temperatures soar to more than 1.5C above 19th century levels, the Met Office said.
It is the first time such high values have been highlighted in the Met Office's decade-long predictions, which are updated every year.
The new highs could come if there is a strong natural El Nino weather pattern in the Pacific, which pushes up world temperatures, which would combine with global warming caused by human activity such as burning fossil fuels.
The latest warning of rising temperatures comes after three years of record heat, with 2016 globally the hottest year on record, and 2017 the warmest without the added impact of an El Nino.
This year is not expected to see temperatures exceed 1.5C over pre-industrial levels.
Professor Stephen Belcher, chief scientist at the Met Office, said: "Given that we've seen global average temperatures around 1C above pre-industrial levels over the last three years, it is now possible that continued warming from greenhouse gases along with natural variability could combine so we temporarily exceed 1.5C in the next five years."
The Paris Agreement's more stringent 1.5C limit was introduced into the global deal because some countries including low-lying island states say it is necessary to curb temperature rises for their very survival.
It relates to the global climate reaching such a level over a long-term average period, rather than hitting a temporary high, the scientists said.
Professor Adam Scaife, head of long range prediction at the Met Office, said: "These predictions show that 1.5C events are now looming over the horizon, but the global pattern of heat would be different to more sustained exceeding of the Paris 1.5C threshold.
"Early, temporary excursions above this level are likely to coincide with a large El Nino event in the Pacific."
But he added that continued greenhouse gas emissions leading to further warming would mean a greater chance of seeing years with temperatures of 1.5C or more above pre-industrial levels in future years.
Meeting either of the temperature limits in the Paris Agreement, which all countries in the world are currently signed up to, would require emissions to fall to net zero by the second half of the 21st century.