Levels of greenhouse gases surged to a record level in 2023, locking in future temperature rises for years to come, UN experts have warned.

The World Meteorological Organisation (WMO) said carbon dioxide (CO2) is accumulating in the atmosphere faster than any time experienced during human existence, having risen by more than 10% in just two decades.

In its annual Greenhouse Gas Bulletin, released on Monday, the WMO said globally-averaged surface concentrations of the greenhouse gases CO2, methane and nitrous oxide reached 151%, 265% and 125% of pre-industrial levels respectively in 2023.

The increase was driven by large vegetation fires and a possible reduction in carbon absorption from forests in 2023 combined with stubbornly high fossil fuel emissions from human activities, the experts added.

The bulletin – which reports on greenhouse gas concentrations rather than on emission levels – is published ahead of the annual UN climate conference to inform negotiations, with Cop29 taking place in Azerbaijan next month.

WMO secretary-general Celeste Saulo said: “Another year. Another record. This should set alarm bells ringing among decision makers.”

She added that the world is “clearly off track” to meet the UN Paris Agreement goal of limiting global warming to well below 2C and aiming for 1.5C above pre-industrial levels.

“These are more than just statistics,” she said. “Every part per million and every fraction of a degree temperature increase has a real impact on our lives and our planet.”

The 2023 increase of CO2 in the atmosphere was higher than that of 2022, although lower than that of the three years before that, the meteorologists said.

But as long as emissions continue, greenhouse gases will continue accumulating in the atmosphere leading to global temperature rise.

Given the extremely long life of CO2 in the atmosphere, the temperature level already observed will persist for several decades even if emissions are rapidly reduced to net zero.

In the last 20 years, CO2 levels have increased by 11.4% above that recorded in 2004 by the WMO’s Global Atmosphere Watch network of monitoring stations.

The WMO said the last time Earth experienced a comparable concentration of CO2 was 3-5 million years ago, when the temperature was 2-3C warmer and sea level was 10-20 meters higher than now.

Analysis of data shows that just under half of CO2 emissions remain in the atmosphere, the organisation said.

Just over one quarter is absorbed by the ocean and just under 30% by land ecosystems – although there is considerable year-to-year variability in this because of naturally occurring phenomena like El Nino and La Nina.