The battle to keep global heating from passing the agreed 1.5-degree threshold has been lost, United Nations (UN) environment chiefs have conceded.

In their most explicit forecast yet, they say the target being exceeded is “unavoidable” and some of the effects of this will be “irreversible”.

They say the emphasis must now switch from trying to avoid hitting 1.5 degrees to limiting the extent and duration of the overshoot. New policies and technologies will be needed to halt the rise and begin a rapid reversal to 1.5 degrees.

“The rules of the game are now changing,” said climate scientist Debra Roberts, one of the authors of the Limiting Overshoot report for the UN Environment Programme (UNEP).

“We now have to reconfigure our policies for an exceedance of 1.5 degrees. None of our plans are prepared for that.”

The goal of limiting average global temperature rise to 1.5 degrees above pre-industrial levels – before fossil fuel use accelerated – was set in the 2015 Paris Agreement, signed by almost all the world’s countries.

The year 2024 was the first the threshold was exceeded every month but until a long-term exceedance was recorded, the UN held that the agreement was not breached.

The limiting overshoot report says that exceedance is inevitable in the next few years, “pushing climate risks and impacts to increasingly dangerous heights”.

The overshoot is expected to last several decades, with the “most optimistic scenario” – if behaviours change immediately and dramatically – marking a temperature rise kept to 1.8 degrees. A reversal to 1.6 degrees could then be possible by the end of the century.

However, the report’s international team of authors warn most scenarios predict a rise of more than 2 degrees and a return to or below 1.5 degrees is uncertain.

“Even if a return to below 1.5 degrees is secured, many nations and communities will face irreversible losses and permanently changed conditions,” they say.

Worsening extreme weather, ecosystem destruction and loss of small islands and low-lying coastal cities are anticipated.

“Severe damage to human health, food security, water supplies, nature, cities, infrastructure and economies is expected.”

Irreversible impacts include glacier loss, with more catastrophes such as the recent landslides in Nepal.

Permanent sea-level rise will also occur and disruption is expected to the Atlantic meridional overturning circulation, the currents that moderate weather in many countries including Ireland.

“There are no good outcomes if we remain above 1.5 degrees,” said UNEP executive director Inger Andersen.

She referred to the deadly heatwaves that struck Europe and large parts of Asia and North America this summer.

“Extreme heatwaves are already proving that climate impacts will strike faster, hit harder, and last longer.”

Peter Thorne, director of the Icarus Climate Research Centre at Maynooth University, said the science in the report was not radically new. But it was expressed in a more “unfiltered” way than usual. And he said this was because the report was not subject to line-by-line approval by UN member states, some of whom had an interest in retaining fossil fuels.

“There wasn’t really anything [in previous scientific analysis] that kept us at 1.5 degrees,” he said. “We’re still warming at 0.2-0.3 degrees centigrade per decade; we’re still year-on-year – albeit more slowly – increasing global emissions.”

Thorne said that “even with the most optimistic scenarios of what might happen in terms of emissions, we’re passing 1.5 degrees”.

He added: “The question is how far past and for how long might we stay above that. It’s not looking good.”

UN secretary general António Guterres appealed to governments to step up climate action. “We must make the overshoot above 1.5 degrees as small and short as possible. That demands an overshoot of ambition.”