UN Secretary General Antonio Guterres warned that the “climate time bomb is ticking” as he urged rich nations on Monday to slash emissions sooner after a new assessment from scientists said there was little time to lose in tackling climate change.

“The rate of temperature rise in the last half century is the highest in 2,000 years,” he said. “Concentrations of carbon dioxide are at their highest in at least 2 million years. The climate time-bomb is ticking.”

In a recorded address, Guterres described the sixth “synthesis report” from the UN’s Intergovernmental Panel on Climate Change (IPCC) as “a survival guide for humanity” and urged developed countries to commit to net zero by the earlier date of around 2040.

The synthesis report summarized findings from three expert assessments published between 2021 and 2022 that looked at the physical science, impacts, and mitigation of climate change. The summary report is designed to provide clarity for policymakers as they consider further action to slash emissions.

“We have the tools to stave off and reduce the risks of the worst impacts of the climate crisis, but we must take advantage of this moment to act now,” said U.S. climate envoy John Kerry.

The 37-page report was distilled from thousands of pages of previous assessments after a week of deliberations in Interlaken, Switzerland.

The document will also serve as a guide for a global climate change “stocktake” set to take place this year, in which countries will assess progress. Under the 2015 Paris Agreement, nations are also expected to update climate pledges by 2025.

According to the IPCC, emissions must be halved by the mid-2030s if the world is to have any chance of limiting temperature rise to 1.5 degrees Celsius (2.7 Fahrenheit) above pre-industrial levels – a key target enshrined in the Paris accord.

“If we act now, we can still secure a liveable, sustainable future for all,” said IPCC Chair Hoesung Lee.

On current trajectories, the planet is on track to warm by 3.2C by century’s end, and temperatures could still rise by at least 2.2C even if existing pledges are met.

Average temperatures are already 1.1C higher than 1850-1900 levels, driving more extreme weather events worldwide.

“In the words of very senior colleagues in the IPCC, we’re up the proverbial creek – that’s really the key message from the report,” said synthesis report co-author Frank Jotzo of Australian National University.

(Reporting by David Stanway in Singapore; editing by Bernadette Baum and Hugh Lawson)