A weather arm of the United Nations on Wednesday warned that within the next five years, the world will likely surpass a critically important warming threshold, reinforcing the urgent need to slash planet-heating greenhouse gas emissions.

In a landmark report timed to coincide with World Environment Day, the World Meteorological Organization said there is now an 80% chance that global average temperatures will exceed 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above pre-industrial levels for at least one year between 2024 and 2028.

That prediction marks a stark change from 2015 when the WMO considered the prospect of temporarily overshooting 1.5 degrees Celsius close to zero.

The 1.5-degree Celsius limit is the aspirational target of the landmark 2015 Paris Agreement, an international treaty on climate change.

Scientists say that exceeding this temperature threshold over the long term will lead to increasingly frequent and catastrophic extreme weather events.

Even at current levels of global warming, there are already devastating climate change impacts. These include record-breaking heatwaves, extreme rainfall events and droughts, accelerating sea level rise and ocean heating and dramatic reductions in ice sheets and sea ice.

“We are playing Russian roulette with our planet,” U.N. Secretary-General António Guterres said in a speech on Wednesday. “We need an exit ramp off the highway to climate hell. And the truth is we have control of the wheel.”

Guterres said that the battle to limit long-term temperatures to 1.5 degrees Celsius will be won or lost in the 2020s under the watch of today’s world leaders.

Speaking at the American Museum of Natural History in New York City, Guterres called for much more ambitious action to tackle the climate crisis ahead of the G7 summit in Italy from June 13 to June 15.

“All depends on the decisions those leaders take — or fail to take — especially in the next 18 months. It’s climate crunch time,” he added.

Extreme heat is fueled by the climate crisis, which is chiefly driven by the burning of fossil fuels.

The WMO said in its report that global average temperatures for each year between 2024 and 2028 were expected to be between 1.1 degrees Celsius and 1.9 degrees Celsius higher than the 1850 to 1900 baseline.

The agency said there is a roughly 50/50 chance that global average temperatures over the entire five-year period will exceed 1.5 degrees Celsius compared with the pre-industrial era — up from a 32% likelihood from last year’s report assessing the period from 2023 to 2027.

At least one of the years through 2028 will likely set a new temperature record, the report said, which would beat 2023, currently the hottest year on record.

“Behind these statistics lies the bleak reality that we are way off track to meet the goals set in the Paris Agreement,” WMO Deputy Secretary-General Ko Barrett said in a statement.

“We must urgently do more to cut greenhouse gas emissions, or we will pay an increasingly heavy price in terms of trillions of dollars in economic costs, millions of lives affected by more extreme weather and extensive damage to the environment and biodiversity,” Barrett said.

The world surpassed 1.5 degrees Celsius across an entire year for the first time on record between February 2023 and January 2024, although scientists have stressed that monthly and annual breaches of the threshold limit do not mean that the world has failed to adhere to the Paris Agreement’s long-term goals.

The WMO said it is “sounding the alarm” that the world is likely to exceed the 1.5 degrees Celsius level on a temporary basis with increasing frequency.