Global concentrations of carbon dioxide (CO2), methane and nitrous oxide reached record levels in 2025, intensifying the risk of extreme weather and threatening economies and societies, the World Meteorological Organization (WMO) has warned.
According to the agency’s latest Greenhouse Gas Bulletin, the average global concentration of CO2 reached 426.8 parts per million (ppm) last year, equivalent to 153 percent of the pre-industrial level recorded in 1750.
“Every additional part per million matters because it traps additional heat. In effect, we are supercharging the ocean and the atmosphere. That means that heat waves, tropical cyclones, and rainstorms are intensifying faster than our historical baselines prepared us for,” WMO Deputy Secretary-General Ko Barrett told journalists in Geneva.
Methane and nitrous oxide, the second and third most significant long-lived greenhouse gases generated by human activities, also recorded unprecedented concentrations in 2025.
“Another year, another new record. Greenhouse gases are trapping an unprecedented amount of heat in the atmosphere, providing energy for more extreme weather and associated risks to our economies and societies,” Barrett said.
She noted that the past 11 years had been the warmest on record and that the trend was expected to continue. As CO2 remains in the atmosphere for a long time, its effects on the global climate will persist for decades and even centuries.
When the WMO first published the bulletin in 2004, the annual average CO2 concentration recorded by its Global Atmosphere Watch monitoring network stood at 377.5 ppm, compared with nearly 427 ppm today.
El Niño accelerates the rise
The rate of CO2 accumulation has tripled since the 1960s, rising from an annual average of 0.8 ppm to 2.4 ppm per year during the decade from 2011 to 2020.
WMO senior scientific officer Oksana Tarasova said the exceptional 3.5 ppm increase recorded in 2023-2024 coincided with a strong El Niño event, which weakened the biosphere’s capacity to absorb carbon because of increased drought, drier vegetation and more wildfires.
Although the annual rise slowed to 2.8 ppm between 2024 and 2025, Tarasova described it as a substantial addition of carbon to the atmosphere from fossil fuel burning.
She warned that CO2 concentrations could rise further in 2026 because of continued fossil fuel use and the development of a powerful El Niño, which could weaken land-based carbon sinks, as happened in 2023-2024.
Greenhouse gases drive greater warming
The WMO bulletin measures the accumulation of greenhouse gases in the atmosphere rather than annual emissions. Around half of the CO2 emitted each year remains in the atmosphere, while the remainder is absorbed by land ecosystems and oceans.
The US National Oceanic and Atmospheric Administration’s Annual Greenhouse Gas Index shows that the warming effect of greenhouse gases on the climate increased by 56 percent between 1990 and 2025. CO2 accounted for about 81 percent of that increase, known as radiative forcing.
Energy transition brings new concerns
As countries shift towards cleaner energy systems, the WMO has called for close monitoring of two less abundant gases associated with electricity production and distribution: molecular hydrogen (H2) and sulphur hexafluoride (SF6).
SF6, widely used to suppress electrical sparks in power distribution systems, has seen its use increase steadily over the past three decades alongside global electrification.
The gas remains in the atmosphere for more than 1,000 years and has a 100-year global warming potential approximately 24,300 times that of CO2. The increase in atmospheric SF6 in 2025 was the highest on record, according to the bulletin.
Meanwhile, atmospheric hydrogen concentrations rose by 5.3 percent between 2010 and 2024.
Hydrogen is increasingly used for energy storage and in sectors that are difficult to decarbonise, including steelmaking and shipping. It also supports renewable energy systems, long-duration storage and transport.
However, hydrogen leakage from production facilities, pipelines, storage tanks and fuel cells is emerging as a concern.
Although hydrogen is not itself a greenhouse gas, it affects atmospheric chemistry and can contribute indirectly to global warming, the WMO said.
The agency warned that hydrogen leakage and the long atmospheric lifetime of SF6 could undermine the climate benefits of the global energy transition.
It stressed that continuous monitoring of the atmosphere is essential to identify, understand and prevent unintended consequences of changes in energy systems.
The WMO publishes the bulletin to provide authoritative scientific evidence for negotiators attending the annual UN climate conference.
The 31st Conference of the Parties (COP31), scheduled to take place in Antalya, Türkiye, from November 9 to 20, aims to translate climate commitments into concrete action.