Atmospheric CO₂.
Explore the monthly Mauna Loa atmospheric record alongside its seasonal context. This is a separate dataset from the EPA water observations.
What does this finding mean?
The latest published monthly mean is 426.07 ppm for 2026-09. That is 1.70 ppm higher than the same month one year earlier. CO₂ is a greenhouse gas. The monthly record has a seasonal cycle, so a month-to-month fall does not by itself indicate a reversal of the longer-term change.
This is atmospheric climate context, not a local AQI score, indoor-air result or an explanation of conditions at a particular lake.
Loading monthly observations…
| Month | Mean (ppm) | Seasonally adjusted (ppm) | Contributing days | Source status |
|---|---|---|---|---|
| 2026-09 | 426.07 | 429.55 | 18 | Published source value |
| 2026-08 | 427.55 | 429.48 | 17 | Published source value |
| 2026-07 | 429.13 | 428.74 | 21 | Published source value |
| 2026-06 | 431.43 | 429 | 19 | Published source value |
| 2026-05 | 432.34 | 429.05 | 19 | Published source value |
| 2026-04 | 431.12 | 428.6 | 23 | Published source value |
| 2026-03 | 430.15 | 428.69 | 26 | Published source value |
| 2026-02 | 429.35 | 428.39 | 17 | Published source value |
| 2026-01 | 428.62 | 428.32 | 25 | Published source value |
| 2025-12 | 427.49 | 428.22 | 24 | Published source value |
| 2025-11 | 426.46 | 428.4 | 20 | Published source value |
| 2025-10 | 424.87 | 428.12 | 19 | Published source value |
How to read this record
Parts per million describes the dry-air mole fraction: roughly how many CO₂ molecules occur per million molecules of dry air. The seasonally adjusted series helps separate the recurring seasonal cycle from longer-term change. These observations are not CO₂ emissions, and a concentration difference is not an emissions total.
Coverage and source notes
The series starts in March 1958. NOAA notes that the late-2022 eruption interrupted Mauna Loa observations; measurements from December 2022 to July 4, 2023 came from Maunakea. Missing-month interpolation and unavailable historical precision metadata are preserved in source-status fields rather than treated as new measurements.
Attribution and freshness
NOAA Global Monitoring Laboratory and Scripps Institution of Oceanography; reference attribution: Xin Lan and Ralph Keeling. Research background: Keeling and colleagues, Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii (1976); Thoning, Tans and Komhyr, Atmospheric carbon dioxide at Mauna Loa Observatory, part 2 (1989).
Source retrieved 2026-10-19T02:22:14+00:00. Latest published month: 2026-09. Checked daily; the source’s observation cadence is monthly. Revisions may occur. No NOAA endorsement is implied.