Greenhouse gas levels in the atmosphere reached unprecedented highs in 2025, potentially further warming the planet and leading to more extreme climate events, the World Meteorological Organisation said in a report on Tuesday.
The global average concentration of atmospheric CO2 reached 426.8 parts per million in 2025, about 153% of the 1750 pre-industrial level, the report said. The level rose by 2.8 ppm from the previous year, a bigger annual increase than the average over the past decade.
Concentrations of other important greenhouse gases — methane and nitrous oxide — reached record highs in 2025, standing at 267% and 126% of pre-industrial levels, respectively. The report measures the amount of gas that accumulates in the atmosphere, not new annual emissions.
The WMO will share the findings with negotiators at the COP31 UN climate conference in Antalya, Turkey, in November, and warned that a powerful El Niño could push concentrations higher this year.
"Every additional part per million matters because it traps additional heat. In effect, we are supercharging the ocean and the atmosphere," WMO Deputy Secretary-General Ko Barrett said.
"That means that heatwaves, tropical cyclones, and rainstorms are intensifying faster than our historical baselines prepared us for."
This year, parts of the world including Europe saw scorching summer temperatures, as well as devastating floods in Asia and elsewhere.
El Niño likely to add to the problem
The WMO said it expected CO2 concentrations to rise further this year due to fossil fuel burning and the onset of a powerful El Niño, which will cause drier weather and make it harder for carbon sinks to absorb emissions.
The WMO also raised concern about sulfur hexafluoride, or SF6, a greenhouse gas widely used in electrical equipment, saying its increase in 2025 was a record high. The global warming potential of SF6 is more than 24,000 times that of CO2 and it can remain in the atmosphere for more than 1,000 years.
The agency also pointed to rising hydrogen concentrations, which increased 5.3% between 2010 and 2024. Although hydrogen is not a greenhouse gas, leaks from infrastructure used to produce, transport and store it can affect atmospheric chemistry and indirectly contribute to warming. Hydrogen produced using renewable electricity can be used as a low-carbon fuel for transport.
The WMO said both hydrogen leakage and the long persistence of SF6 could undermine some of the climate benefits of the global energy transition.
"The energy transition is essential and welcome, but as with every transition, it has to be done in a smart way and informed by environmental observations," Barrett said.