U.N. 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 two million years. The climate time-bomb is ticking."

In a recorded address, Guterres described the sixth "synthesis report" from the U.N.'s Intergovernmental Panel on Climate Change (IPCC) as "a survival guide for humanity" and urged developed countries to commit to reaching net zero emissions 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.

U.N. Secretary-General Antonio Guterres attends a joint news briefing with Ukraine's President Volodymyr Zelenskiy, amid Russia's attack on Ukraine, in Kyiv on March 8, 2023. (Alina Yarysh/REUTERS)

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 C 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 3.2 C by century's end, and temperatures could still rise by at least 2.2 C even if existing pledges are met.

Average temperatures are already 1.1 C 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.

A view shows the partially dry Lake Montbel at the foot of the Pyrenees Mountains as France faces records winter dry spell, raising fears of another summer of droughts and water restrictions. (Sarah Meyssonnier/REUTERS)

Observers said the major areas of contention included the language around finance and the projected impacts of climate change, as well as the issue of "equity" and climate justice for poorer countries.

Some governments also wanted to give more prominence to their own favoured climate solutions, including solar power or carbon capture.

The IPCC says the world needs to accelerate the transition to green energy and transform agriculture and eating habits if it has any chance of making the necessary cuts in emissions.

It also warned of more extreme weather, rapidly rising sea levels, melting Arctic ice and the growing likelihood of catastrophic and irreversible "tipping points." They also said nearly half the world's population was already vulnerable to climate impacts.

"In short, our world needs climate action on all fronts — everything, everywhere, all at once," said Guterres.

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

Thumbnail image: A firefighter hoses down flames from the Mosquito fire as it burns in California on Sept. 7, 2022. (Fred Greaves/REUTERS)