CO2 at Mauna Loa: the rise slowed in 2026 after two record years, but levels hit a new high of 432 ppm
NOAA Mauna Loa data show carbon dioxide rose 1.70 ppm in the year to September 2026, down from 3.3 ppm a year in 2023 and 2024, while May set a new monthly record of 432.34 ppm.
The story in focus
Carbon dioxide at NOAA's Mauna Loa Observatory averaged 426.07 parts per million in September 2026, according to the HalcyonWire CO2 tracker built on NOAA data. That is 1.70 ppm higher than a year earlier, a much smaller rise than the record jumps of 2023 and 2024. Levels still keep climbing: May 2026 set a new monthly high of 432.34 ppm.
- Mauna Loa CO2 averaged 426.07 ppm in September 2026, up 1.70 ppm from 424.37 ppm in September 2025.
- NOAA's annual growth rate hit records of 3.32 ppm in 2023 and 3.33 ppm in 2024, then eased to 2.23 ppm in 2025.
- The seasonally adjusted 12-month rise fell to 1.25 ppm in July 2026, the smallest since February 2023.
- May 2026 set a new monthly record of 432.34 ppm, beating the 430.51 ppm reached in May 2025.
What the latest data show
The monthly mean carbon dioxide concentration measured at Mauna Loa, Hawaii, was 426.07 ppm in September 2026, NOAA's Global Monitoring Laboratory reports. September is normally near the bottom of the yearly cycle, as Northern Hemisphere plants draw CO2 out of the air over the summer. After removing that seasonal swing, the level was 429.55 ppm, the highest seasonally adjusted reading in the record.
The peak of the yearly cycle came in May, at 432.34 ppm. That beat the previous monthly record of 430.51 ppm set in May 2025. Monthly readings first passed 400 ppm in May 2013, 420 ppm in April 2022 and 430 ppm in May 2025.
The rise has slowed this year
Compared with the same month a year earlier, CO2 was up 1.70 ppm in September 2026. A year before, the September-to-September rise was 2.34 ppm, and in late 2024 and early 2025 the 12-month rise ran between 3.4 and 3.9 ppm. On a seasonally adjusted basis, the 12-month increase fell to 1.48 ppm in April 2026 and 1.25 ppm in July 2026, the smallest since February 2023.
NOAA's own annual growth rate figures show the same turn. The laboratory puts the growth rate at 3.32 ppm in 2023 and 3.33 ppm in 2024, the two highest values since its Mauna Loa growth series began in 1959, followed by 2.23 ppm in 2025. The year-to-year growth rate varies with natural factors such as El Niño and La Niña cycles, which change how much carbon land plants and oceans absorb, so a slower year does not mean emissions have fallen.
The long-run picture
Over longer periods the trend is still accelerating. Comparing seasonally adjusted September readings, CO2 rose by an average of 0.90 ppm a year in the 1960s, 1.33 ppm in the 1970s, 1.53 ppm in the 1980s, 1.58 ppm in the 1990s, 1.99 ppm in the 2000s and 2.46 ppm in the 2010s. The average over the ten years to September 2026 was 2.47 ppm a year.
The annual mean concentration was 427.35 ppm in 2025, NOAA data show, up from 316 ppm in 1959. The average for January through September 2026 was 429.53 ppm, 1.82 ppm above the same months of 2025.
About the data
Monthly means and seasonally adjusted values come from NOAA Global Monitoring Laboratory measurements at Mauna Loa Observatory, which began in March 1958 under Charles David Keeling of Scripps Institution of Oceanography. Annual growth rates are NOAA's published figures. NOAA notes that the most recent months are preliminary and can be revised slightly.
The full monthly series since 1958 is available in the HalcyonWire CO2 tracker, which refreshes daily and offers a CSV download and an embeddable chart.
What is the current CO2 level at Mauna Loa?
NOAA's Mauna Loa Observatory recorded a monthly mean of 426.07 ppm of carbon dioxide in September 2026, 1.70 ppm higher than in September 2025. The highest monthly reading on record is 432.34 ppm, set in May 2026.
Sources & further reading
- Trends in Atmospheric Carbon Dioxide, Mauna Loa (NOAA Global Monitoring Laboratory) ↗gml.noaa.gov
- Annual mean growth rate of CO2 at Mauna Loa (NOAA GML) ↗gml.noaa.gov
- HalcyonWire CO2 tracker: Mauna Loa monthly readings since 1958 ↗halcyonwire.com
- Mauna Loa monthly CO2 series (CSV) ↗halcyonwire.com
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