The World Meteorological Organization (WMO) has published Greenhouse Gas Bulletin No. 22, which reports global average concentrations for 2025. The headline numbers are 426.8 parts per million for carbon dioxide, 1,948 parts per billion for methane and 339.1 parts per billion for nitrous oxide. WMO says all three reached unprecedented highs in 2025. This article sets out what the bulletin page reports and how to read it.
What did the WMO bulletin report?
WMO’s Bulletin No. 22 gives these 2025 global averages, with their stated uncertainties:
- Carbon dioxide (CO2): 426.8 ± 0.2 ppm, which is 53% above pre-industrial levels
- Methane (CH4): 1,948 ± 2 ppb, which is 167% above pre-industrial levels
- Nitrous oxide (N2O): 339.1 ± 0.1 ppb, which is 26% above pre-industrial levels
WMO measures the increases against pre-industrial levels, meaning the years before 1750. The bulletin page says the concentrations of all three gases reached unprecedented highs in 2025. The detailed report and the press release are linked separately from that page, and we rely here on the bulletin page itself.
How does 2025 compare with 2024?
The page compares the 2024 to 2025 increases with earlier years rather than giving them in ppm or ppb. For carbon dioxide, the rise was lower than the 2023 to 2024 rise, which WMO calls anomalously high, and slightly higher than the average annual growth rate for 2016 to 2025. For methane, the increase was lower than in 2023 to 2024 and lower than the 2016 to 2025 average. For nitrous oxide, the increase matched 2023 to 2024 and was close to the 2016 to 2025 average. A slower rise is still a rise, because concentrations went up for all three gases.
What was the record jump in the previous bulletin?
WMO’s news release on the previous bulletin said global average carbon dioxide rose by 3.5 ppm from 2023 to 2024, the largest annual increase since modern measurements began in 1957. That release also said the annual growth rate averaged 0.8 ppm per year in the 1960s and 2.4 ppm per year in 2011 to 2020, and described growth rates as having tripled since the 1960s. It named continued human emissions, wildfires and reduced uptake by natural sinks such as land ecosystems and the ocean as drivers. Our entry on the carbon sink explains what a sink is.
Why do these three gases matter?
WMO describes methane as the second most important long-lived greenhouse gas linked to human activity and nitrous oxide as the third. Carbon dioxide is the largest. Each traps heat in the atmosphere, a process described in our entry on the greenhouse effect. Our article on methane from energy covers one major source of the second gas, and the entry on climate change puts the gases in a wider context. In the previous bulletin’s release, WMO Deputy Secretary-General Ko Barrett said the heat trapped by carbon dioxide and other greenhouse gases is turbo-charging the climate and leading to more extreme weather.
How to read the numbers
The percentages let readers back out the pre-industrial starting points. By our own arithmetic, a gas that is 53% higher than before means today’s value is 1.53 times the old one, so 426.8 divided by 1.53 gives about 279 ppm for carbon dioxide. For methane, 167% above means 2.67 times, so 1,948 divided by 2.67 gives about 730 ppb. For nitrous oxide, 26% above means 1.26 times, so 339.1 divided by 1.26 gives about 269 ppb. These implied baselines are our calculations from rounded percentages and are not stated on the WMO page.
The units also differ. One ppm means one molecule of the gas per million molecules of air, and one ppb means one per billion. That is why methane is quoted in ppb even though its percentage rise is the largest of the three. A general definition like this is not quoted from WMO.
Where do the numbers come from?
The WMO page presents global averages, which come from a worldwide monitoring network. This article does not describe the network in detail because the bulletin page does not. WMO’s earlier news release said that sustaining and expanding greenhouse gas monitoring is critical to support efforts to cut emissions, a point made by Oksana Tarasova, the bulletin’s coordinator. Our article on global CO2 emissions covers the separate question of how much carbon dioxide humans emit each year, and our piece on Copernicus climate data for 2025 covers temperature.
Why do the three gases get separate percentages?
Each gas started from a different pre-industrial level and has risen by a different proportion, so the percentages are not directly comparable as measures of warming. A small percentage rise in carbon dioxide still represents a very large amount of gas because the baseline is so much larger. This is a general point about the units and is not made on the WMO page, which reports the concentrations and percentages without ranking their climate effects.
Glossary
- ppm: parts per million, one molecule per million molecules of air
- ppb: parts per billion, one molecule per billion molecules of air
- Pre-industrial: the period before 1750, WMO’s reference point
- Sink: a natural system such as a forest or the ocean that absorbs carbon dioxide
- Long-lived greenhouse gas: a gas that stays in the atmosphere long enough to mix globally
The ppm, ppb and long-lived gas definitions are general explanations and are not quoted from WMO.
What is not covered on the bulletin page?
The page gives no absolute annual increase in ppm or ppb for 2025 and says nothing about carbon sinks. We have not filled those gaps. Readers who need the annual increase should open the full bulletin that WMO links.
Key facts at a glance
- Reference year: 2025
- Carbon dioxide: 426.8 ppm, 53% above pre-industrial
- Methane: 1,948 ppb, 167% above pre-industrial
- Nitrous oxide: 339.1 ppb, 26% above pre-industrial
- All three gases reached unprecedented highs
Frequently asked questions
What was the global carbon dioxide level in 2025?
WMO reports 426.8 ± 0.2 ppm.
Did the rise slow in 2025?
For carbon dioxide and methane, WMO says the increase was lower than in 2023 to 2024. Concentrations still reached new highs.
Which gas has risen most in percentage terms?
Methane, at 167% above pre-industrial levels.
What does pre-industrial mean here?
The years before 1750.