From 2023 to 2024, the global average concentration of CO2 rose by 3.5 parts per million - the largest increase since modern measurements started in 1957, according to the report, published ahead of next month's United Nations Climate Change Conference in Brazil.
The burning of fossil fuels and an increase in wildfires, particularly in South America, drove the rise in CO2 levels over the last year, it said, stressing more needs to be done to reduce emissions.
"The heat trapped by CO2 and other greenhouse gases is turbo-charging our climate and leading to more extreme weather," WMO Deputy Secretary-General Ko Barrett said.
Concentrations of other important greenhouse gases, methane and nitrous oxide, also rose to record levels, increasing by 16 per cent and 25 per cent respectively against pre-industrial levels, while CO2 rose by 52 per cent.
"This gas (CO2) accumulates in the atmosphere. It has a very long lifetime ... every single molecule which is emitted in the atmosphere will have a global impact," Oksana Tarasova, WMO senior scientific officer, told a briefing in Geneva.
About 50 per cent of carbon emissions are soaked up by forests, land, and oceans, however, the ability of these so-called carbon sinks to absorb the gases is lessening, Tarasova said.
"We rely on natural systems to help us offset our impacts, and those systems are so stressed that they start reducing their help," Tarasova said.
Trees in the Amazon, for example, became stressed from rising temperatures and low rainfall during the periodic warming in the Eastern Pacific Ocean known as El Nino in 2023, and the onset of drought which continued into 2024, Tarasova stated.
"If the tree is under stress, if it doesn't have water and has a very high temperature...it does not photosynthesize," she said.
