Levels of carbon dioxide in the atmosphere soared by a record amount in 2024 to hit another high, UN data shows, deepening the climate crisis that is already taking lives and livelihoods across the world.
Scientists are also worried that the natural land and ocean "sinks" that remove CO from the air are weakening as a result of global heating, which could form a vicious circle and drive temperatures up even faster.
The global average concentration of the gas surged by 3.5 parts per million to 424ppm in 2024, the largest increase since modern measurements started in 1957, according to the report by the World Meteorological Organization.
Several factors contributed to the leap in CO, including another year of unrelenting fossil fuel burning despite a pledge by the world's countries in 2023 to "transition away" from coal, oil and gas. Another factor was an upsurge in wildfires in conditions made hotter and drier by global heating. Wildfire emissions in the Americas reached historic levels in 2024, which was the hottest year yet recorded.
However, scientists are concerned about a third factor: the possibility that the planet's carbon sinks are beginning to fail. About half of all CO emissions every year are taken back out of the atmosphere by being dissolved in the ocean or being sucked up by growing trees and plants. But the oceans are getting hotter and can therefore absorb less CO while on land hotter and drier conditions and more wildfires mean less plant growth.
Carbon sinks were already known to be less effective in El Niño years such as 2023 and 2024, when changes in Pacific winds and ocean currents lead to higher global air temperatures. But human-caused global heating has already raised the average world temperatures by about 1.3C and researchers, who recorded an "unprecedented failure of the land sink in 2023" and 2024, fear this may be weakening the sinks.
Related: Trees and land absorbed almost no CO2 last year. Is nature's carbon sink failing?
Dr Oksana Tarasova, a WMO senior scientific officer, said: "There is concern that terrestrial and ocean CO sinks are becoming less effective, which will increase the amount of CO that stays in the atmosphere, thereby accelerating global warming. Sustained and strengthened greenhouse gas monitoring is critical to understanding these loops."
Rising CO emissions not only impact the global climate today, but will do so for hundreds of years because of the gas's long lifetime in the atmosphere, the WMO said.
Ko Barrett, the WMO deputy secretary general, said: "The heat trapped by CO and other greenhouse gases is turbo-charging our climate and leading to more extreme weather. Reducing emissions is therefore essential not just for our climate but also for our economic security and community wellbeing."
Atmospheric concentrations of methane and nitrous oxide - the second and third most important greenhouse gases related to human activities - also rose to record levels in 2024.
About 40% of methane emissions come from natural sources. But scientists are concerned that global heating is leading to more methane production in wetlands, another potential feedback loop. The rest comes from fossil fuel exploitation; livestock such as cattle; rotting waste in landfills; and rice paddies. Human-caused nitrous oxide emissions include those from overuse of fertiliser by farmers and some industrial processes.
The WMO released its annual greenhouse gas bulletin a monthbefore Cop30, the UN climate change summit in Belém, Brazil, where representatives from the world's countries will attempt to ramp up climate action. The data comes from a network of 500 monitoring stations around the world.