WMO’s 2025 greenhouse gas record
A global record built from local air samples
WMO reports record greenhouse gas levels in 2025; the global figures measure accumulated gases, not one year’s emissions. The bulletin released October 27 puts the global surface average for carbon dioxide at 426.8 parts per million. Methane reached 1,948 parts per billion and nitrous oxide 339.1 parts per billion. These are observations for 2025, assembled and analyzed for the 2026 publication—not a completed account of the current year.
The three gases did not climb at identical speeds. Carbon dioxide rose more slowly than during the exceptional previous interval, methane’s increase was below its recent decadal average, and nitrous oxide continued upward at roughly its recent pace. Yet all three finished at record annual concentrations. A slower increase and a higher total can coexist; that distinction is the central fact behind the apparently contradictory headlines.
For American readers, this international assessment also has a domestic foundation. NOAA supplies observations to the global system, while the Japan Meteorological Agency hosts the World Data Centre for Greenhouse Gases. The resulting record connects remote sampling locations, laboratories and common measurement standards. Understanding that chain makes the numbers more useful than treating each annual record as an isolated warning.
Read the units before comparing the gases
Carbon dioxide’s 426.8 ppm means roughly 427 molecules in every million molecules of dry air. Methane and nitrous oxide are reported in parts per billion, a unit one thousand times smaller. Converting the methane value gives 1.948 ppm. That arithmetic describes abundance only: it does not mean methane contributes a correspondingly tiny fraction of warming, because different molecules interact with radiation differently and persist for different periods. The preindustrial comparisons require equal care. WMO places carbon dioxide 53 percent above its baseline before 1750, methane 167 percent above it and nitrous oxide 26 percent above it. A concentration equal to 153 percent of an earlier level is an increase of 53 percent, not 153 percent. The distinction matters when comparing summaries of the same release.
Emissions inventories answer another question and use another unit: how much gas entered the atmosphere over a period, generally expressed as mass. Carbon-dioxide-equivalent emissions multiply that mass by a warming-potential factor for a stated time horizon. Neither a ton of CO2 equivalent nor a percentage change in emissions can be substituted directly for the parts-per-million reading on an atmospheric chart.
Why two credible global averages can differ
NOAA’s fall 2026 assessment reports a 2025 marine-background carbon dioxide average of 425.62 ppm, whereas WMO’s surface figure is 426.8 ppm. Choosing whichever number suits an argument would miss the documented reason for the difference. NOAA emphasizes well-mixed air in the marine boundary layer. The WDCGG analysis also admits inland stations after evaluating their representativeness and their connection to the WMO calibration scales.
WDCGG explicitly says this station selection tends to make its carbon dioxide average somewhat higher while leaving the broad trend and year-to-year variability similar. The two estimates therefore describe closely related, but not identical, sampling frameworks. They should retain their agency, method and observation year when quoted.
The published uncertainty also belongs to the particular estimate, not to every possible definition of a global average. WDCGG lists 68-percent confidence intervals. Such an interval quantifies uncertainty under the analysis used; it does not erase the difference between a marine-background series and one incorporating screened inland observations. Comparing methods first prevents a legitimate distinction from becoming a false claim that the measurements disagree about the direction of change.
The annual increase has more than one definition
NOAA’s global growth-rate page gives 2.06 ppm for 2025. That number measures the change between seasonally adjusted estimates around the beginning and end of the year. WMO’s 2.8 ppm increase instead compares successive annual averages. Averages across two full years and endpoints within one year need not produce the same result, especially when growth changes substantially over the intervening months.
A household comparing its average bank balance across years would obtain a different figure from the change between January 1 and December 31. The atmospheric calculation is more elaborate, but the time-window problem is similar. A headline should state which comparison it uses rather than splice together values from different series.
NOAA also publishes monthly marine averages. Its October 9 update listed 426.10 ppm for July 2026 against 424.09 ppm a year earlier. Those observations extend beyond the bulletin’s year, but a single month cannot replace a full-year mean. Seasonal adjustment and the distinction between preliminary and finalized data remain part of the comparison; a newer date does not automatically make a differently defined number a better substitute.
The land and ocean change the yearly balance
Atmospheric accumulation is what remains after emissions and removals interact. WMO says approximately half of emitted carbon dioxide stays in the atmosphere, with land ecosystems and the ocean taking up the rest. That broad division is not a fixed annual allowance. Changes in vegetation, drought, fire and ocean conditions alter the balance from one year to the next.
WMO attributes the smaller 2025 increase partly to recovering land uptake and reduced fire-related carbon emissions after the strong 2023–24 El Niño. The previous event had brought conditions that weakened the biosphere’s ability to absorb carbon. This explanation makes the slower growth significant without turning it into evidence that fossil-fuel emissions have disappeared.
An emissions reduction and a stronger natural sink can both slow atmospheric growth, but they carry different implications for the next year. The first concerns a change in the amount released; the second concerns how much the environment removes. An assessment of progress therefore needs inventories and observations together. The atmosphere records the net result, while additional evidence is needed to separate the mechanisms producing it.
A flask is a time-stamped sample, not a souvenir
The international network is partly a shipping operation. NOAA’s cooperative sampling effort began at Niwot Ridge, Colorado, in 1967 and now includes fixed sites, baseline observatories and commercial ships. Air collected roughly weekly is returned for analysis. A bottle taken at a remote coastline can consequently be measured on laboratory instruments alongside samples from another continent.
NOAA’s behind-the-scenes documentation describes collectors filling flasks, commonly in pairs, and returning them to Boulder. Each arriving sampling event receives a unique identifier connecting the air to its collection location, time and subsequent measurements. The visual sameness of the containers makes that record essential: a precise result assigned to the wrong site would be scientifically misleading.
The laboratory’s published educational account describes evacuation and leak checks before flasks are recirculated. It also explains that different samples follow different measurement paths rather than every container being tested for every possible gas. These routine controls supply context for the global figures. The headline average depends on the integrity of individual samples long before an analyst combines them into a worldwide estimate.
A common scale lets countries combine their observations
NOAA serves as the WMO Global Atmosphere Watch central calibration laboratory for carbon dioxide, methane, nitrous oxide, sulfur hexafluoride and carbon monoxide. Its responsibility includes maintaining and distributing the reference scales used to express amounts of these gases in air. Instruments in different laboratories can then relate their results to a common system rather than relying on incompatible local benchmarks. The standards are physical gas mixtures, not simply numbers entered into a spreadsheet. NOAA calibrates reference cylinders, and its scale documentation identifies the primary standards and the years associated with successive scales. Recalibration can lead to small retrospective adjustments in a data series. Keeping the scale and version with a result is therefore part of preserving comparability across decades.
Inside the measurement laboratory, NOAA describes routing sample air and standard air through the analysis system. Tubing must be evacuated, connections sealed and leaks repaired so that room air does not contaminate the sample. Software tracks which container is being measured, but technicians still check the output. This is why quality assurance includes both the instrument’s response and the physical handling of the air reaching it.
Why a mountain station is not the whole planet
Mauna Loa’s high, remote setting has made it an emblem of atmospheric monitoring. NOAA places the observatory about 11,135 feet above sea level, above the region’s strong marine temperature inversion. That location helps researchers observe air with limited immediate influence from nearby vegetation and human activity. Nevertheless, a mountain station and a worldwide surface average remain different products.
The observatory’s operational history also matters. After the 2022 eruption severed road access, temporary measurements on nearby Mauna Kea helped preserve the record. NOAA’s May 18, 2026 account says some Mauna Loa measurements resumed in 2023 using solar generation and batteries, and a temporary road reopened March 26, 2026. At the time of that May report, 62 of 91 daily measurement programs were active. The current facility notice still says public access is closed. Road access, measurement continuity and complete restoration are distinct milestones.
In its measurement-method description, NOAA explains why carbon dioxide is expressed relative to dry air. Changing moisture can dilute a wet-air ratio without removing carbon dioxide. Dry-air mole fractions avoid confusing that dilution with a real change in abundance. The method is especially important when comparing humid marine locations with colder or higher stations: the same label must describe the same physical quantity.
How scattered observations become a global curve
NOAA’s marine reference is deliberately selective. It favors remote, near-sea-level sites where incoming marine air represents a large atmospheric volume. Its methodological description excludes elevated locations such as Mauna Loa and locations strongly influenced by nearby sources or sinks. Exclusion from this particular calculation does not make a station unimportant; it reflects the question the reference is designed to answer. The method first smooths each station’s measurements through time, reducing short-term weather-related variability while retaining the longer evolution. The synchronized records are then combined across latitude. Measurements with more consistent sampling and clearer signals receive greater weight than interpolated portions. Finally, a latitude-weighted calculation yields the global marine mean.
This is why a global average cannot be reproduced by adding the latest readings from a handful of famous observatories and dividing by their number. Geographic coverage, timing and selection all matter. The network is distributed unevenly across a planet whose land, oceans and populations are unevenly arranged. The analysis must explicitly handle those features instead of allowing the places with the most convenient instruments to dominate the result.
Aircraft and balloons add the missing vertical dimension
Air at the surface is only part of the atmospheric picture. NOAA’s aircraft program collects flask samples during descents from altitudes reaching about 26,000 feet, while other instruments sample air more continuously. These profiles help reveal differences with height that a surface network alone cannot resolve and provide comparisons for satellite estimates through the atmospheric column.
The practical value is not that every flight reading enters the bulletin’s surface average. Vertical profiles support a different layer of evidence: how gases are distributed and transported, and how well models represent that distribution. A concentration observed over a city, for example, needs an understanding of air movement before it can become an estimate of emissions from that city.
NOAA’s AirCore provides another approach. A long, narrow coil of tubing collects and preserves a sequence of air as a balloon-borne instrument descends. The recovered air can then be pushed through an analyzer. The narrow tubing limits internal mixing, allowing the sample sequence to retain information about height. Flying paired systems supplies a cross-check. Such complementary measurements strengthen the larger observing system without pretending that surface, aircraft and satellite products are interchangeable.
The warming index answers a different question again
NOAA’s Annual Greenhouse Gas Index translates measured abundances of long-lived gases into an estimate of their effective radiative forcing—the added influence on Earth’s energy balance. For 2025, the index was 1.56 with 1990 set to one, meaning that influence was 56 percent greater than in the reference year. Carbon dioxide accounted for about 81 percent of the increase over that interval.
That is not a statement that global temperature rose 56 percent. It is also not a forecast of the temperature in any particular American city. Temperature outcomes depend on other influences and on the response of the climate system. The index isolates a defined part of the changing energy balance so that its progression can be followed consistently.
For comparisons among gases, abundance, persistence and heat-trapping behavior must be considered together. EPA’s warming-potential metric instead compares the effect of a mass of gas over a chosen period with the same mass of carbon dioxide. The atmospheric bulletin, the forcing index and an emissions inventory are therefore complementary tools. Each is informative when its unit, baseline and purpose remain attached.
What to watch after the record
The next useful evidence will arrive on different schedules. Monthly atmospheric estimates offer a more current view but can change as late samples and quality checks are incorporated. NOAA explains that its annual growth estimate is finalized in the following fall partly because some samples arrive late. An annual synthesis trades speed for a more complete and harmonized record. For a reader following climate diplomacy or an American company’s emissions claims, a practical comparison begins with three questions: what was measured, where was it measured and over what interval? A corporate inventory reports activity within an accounting boundary. A remote-air network samples the accumulated result of many sources and removals. Connecting those two scales requires additional analysis; neither can stand in for the other.
The latest bulletin establishes a clear outcome for 2025: the three major long-lived greenhouse gases reached higher annual concentrations even as their growth rates differed. Its deeper value is the durable international measurement system behind that finding. Carefully identified samples, shared scales and published methods allow future claims of improvement to be tested against something more substantial than a promise or a single dramatic chart.
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