UN Reports Record Greenhouse Gas Concentrations in 2025

Record concentrations of carbon dioxide, methane and nitrous oxide accumulated in Earth's atmosphere in 2025, the World Meteorological Organization (WMO) reported on Tuesday, October 27, warning that the resulting heat retention will intensify climate risks for decades or centuries. The findings highlight the persistent effects of fossil fuel consumption and arrive ahead of the United Nations climate conference in Antalya, Türkiye.

The WMO's annual Greenhouse Gas Bulletin recorded average atmospheric carbon dioxide concentrations of 426.8 parts per million (ppm), equivalent to 153% of preindustrial levels measured before 1750. Methane reached 1,948 parts per billion, or 267% of its preindustrial concentration, while nitrous oxide climbed to 339.1 parts per billion, equivalent to 126% of its preindustrial level.

According to the WMO, carbon dioxide accounts for approximately 66% of the warming effect produced by long-lived greenhouse gases. Methane contributes about 16%, while the remaining warming effect comes from other gases, including nitrous oxide.

Ko Barrett, WMO Deputy Secretary-General, said the accumulating gases were increasing the amount of heat retained in the atmosphere.

"Another year, another new record," Barrett said. "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 also warned that carbon dioxide's long atmospheric lifetime means its effects will extend well beyond the period in which emissions occur.

"The past 11 years have been the warmest on record and this trend is set to continue. Carbon dioxide (CO2) has a very long life-time and so will affect the global climate not just for years but for decades and even centuries."

In remarks reported by the UN, Barrett described how additional atmospheric carbon dioxide contributes to escalating weather extremes.

"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," Barrett told journalists in Geneva.

The latest measurements show that atmospheric carbon dioxide increased by 2.8 ppm between 2024 and 2025. Although that rise was smaller than the previous year's exceptional increase, the WMO said it remained above the average annual growth rate recorded over the past decade.

Oksana Tarasova, a senior scientific officer at the WMO, attributed the slower increase partly to improving natural carbon absorption and lower fire-related emissions following the strong 2023–2024 El Niño event.

"However, a 2.8 ppm annual increase is still a substantial addition of carbon to the atmosphere from fossil fuel burning."

Tarasova warned that fossil fuel consumption and another powerful El Niño could push concentrations higher in 2026 by weakening the ability of land ecosystems to absorb carbon dioxide.

"We can expect to see a further increase in concentrations in 2026 because of fossil fuel burning and the development of a powerful El Nino which will impact land carbon sinks, as it did in 2023/2024," Tarasova said.

The WMO reported that the previous El Niño contributed to drought, drier vegetation and more wildfires, reducing the biosphere's effectiveness as a carbon sink. The agency also noted that oceans absorb a substantial share of annual carbon dioxide emissions, although concerns are growing that their capacity to absorb additional carbon could weaken.

Historical measurements underline the acceleration. According to the WMO, average atmospheric carbon dioxide concentrations stood at 377.5 ppm when its Greenhouse Gas Bulletin was first published in 2004. The UN agency also reported that the warming effect of greenhouse gases increased by 56% between 1990 and 2025, with carbon dioxide accounting for approximately 81% of that increase.

Beyond the three main greenhouse gases, the WMO highlighted sulfur hexafluoride (SF6) and molecular hydrogen (H2) as substances requiring closer monitoring during the transition to cleaner energy systems.

SF6 is widely used in electrical switchgear and power distribution equipment because of its insulating properties. According to the WMO, its atmospheric lifetime exceeds 1,000 years, and its global warming potential over 100 years is approximately 24,300 times that of carbon dioxide. Its atmospheric concentration increased steadily over recent decades, reaching a record annual rise in 2025.

The agency also reported that atmospheric hydrogen concentrations increased by 5.3% between 2010 and 2024. Although hydrogen is not itself a greenhouse gas, leakage from production facilities, pipelines, storage tanks and fuel cells can alter atmospheric chemistry and indirectly contribute to warming.

The WMO said monitoring these substances is important because leakage and long atmospheric persistence could reduce the climate benefits expected from electrification and other energy-transition measures. Its annual bulletin provides scientific evidence for international climate negotiations.

The findings come ahead of the COP31 United Nations climate conference scheduled for November 9–20 in Antalya, Türkiye, where participating countries are expected to address the implementation of existing climate commitments.