How to Read NOAA's Mauna Loa CO2 Record — Monthly Means, Annual Increase and Preliminary Values

As of 13 November 2026, 00:35 KST (retrieved by this page at that time); latest listed on the agency page at retrieval: the Mauna Loa monthly mean CO2 file of NOAA's Global Monitoring Laboratory, reached from the Trends in CO2 data page. Its last row is September 2026, the file header gives File Creation: Fri Oct 9 13:08:07 2026, and the trends page shows Last updated: Oct 09, 2026. Values for the last year are preliminary. Primary source: https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_mm_mlo.txt^[1][4]

Comparison basis: values are dry-air mole fractions in parts per million (ppm), printed to two decimals and used as printed; differences subtract the displayed values. The monthly mean has a strong seasonal cycle, so the month-to-month change in the first row is not a trend; the year-ago comparison or the de-seasonalized column is the like-for-like reading. NOAA states each monthly uncertainty in its own column; this page does not combine uncertainties into a confidence interval for the differences, and values are not rounded further.^[1][4]

In three lines

1. CO2 at Mauna Loa averaged 426.07 ppm in September 2026 (preliminary), 1.70 ppm above September 2025.
2. The 2025 annual mean was 427.35 ppm, 2.74 ppm above 2024.
3. The increase from January 1 to December 31, 2025 was 2.23 ppm, against 3.33 ppm in 2024, each with a stated uncertainty of 0.11 ppm.^[1][2][3]

Monthly means of background air. The monthly file says: "Monthly mean CO2 constructed from daily mean values." The trends page explains that daily means use only hours with background conditions, and that the de-seasonalized column removes the average seasonal cycle, estimated as a moving average of seven adjacent seasonal cycles. The record combines Scripps Institution of Oceanography data from March 1958 to April 1974 with NOAA data afterwards.^[1][4]

A site change in 2022-2023. After the Mauna Loa eruption, measurements were suspended as of Nov. 29, 2022, and observations from December 2022 to July 4, 2023 come from the Maunakea Observatories, about 21 miles north. The change log records that Mauna Loa data replaced the Maunakea data retroactively to July 5, 2023. Readers comparing those months should note the different site.^[1][4][7]

Mauna Loa is one site, not the globe. The trends page says the Mauna Loa data, taken at 3400 m in the northern subtropics, may not equal the global surface average. For growth rates the growth rate page adds: "The annual growth rate measured at Mauna is not the same as the global growth rate, but it is quite similar." and "One standard deviation of the annual differences MLO minus global is 0.26 ppm/year."^[4][5]

Two annual measures. The growth rate file defines its series as: "Annual CO2 mole fraction increase (ppm) from Jan 1 through Dec 31." The growth rate page says NOAA finalizes the previous year's figure in March, using the average of the most recent November to February months corrected for the seasonal cycle, and adds: "The estimated uncertainty in the Mauna Loa annual mean growth rate is 0.11 ppm/yr."^[3][5]

2.74 ppm or 2.23 ppm for 2025. The annual means give 427.35 for 2025 and 424.61 for 2024, a difference of 2.74 ppm; the growth rate file gives 2.23 ppm for 2025. They measure different things: the first compares two calendar-year averages, the second the change from the start to the end of 2025 as defined on the growth rate page. For 2024 the pair is 3.53 and 3.33. Neither figure is wrong; the definition must be named when quoting a yearly increase.^[2][3][5]

2024 and 2023 at the top. By this page's ranking of the growth rate file, 2024 (3.33 ppm) and 2023 (3.32 ppm) are the two largest annual increases since 1959, ahead of 2016 (3.03). The two top values differ by 0.01 ppm against a stated uncertainty of 0.11 ppm for each year, so their order is not firm.^[3][5]

Preliminary recent values. The trends page says the last year of data are still preliminary, pending recalibrations of reference gases and other quality control checks, and the data page says: "These values are subject to change depending on quality control checks of the measured data, but any revisions are expected to be small." The growth rate file notes that earlier years may also change and: "Usually these changes are minor." This page read only the current files, so it cannot say whether any value shown here has already been revised.^[1][4][6]

A record month in May. The highest monthly mean in the file is May 2026 at 432.34 ppm, by this page's search of all rows; September 2026 is 6.27 ppm lower in the monthly mean, while in the de-seasonalized column it is above May 2026 (429.55 against 429.05) and is the highest value in the file, 0.07 ppm above August 2026, a gap smaller than the stated monthly uncertainties of 0.23 and 0.18 ppm.^[1]

Dates kept apart. Data: through September 2026 for monthly values, through 2025 for annual values. File creation: Fri Oct 9 13:08:07 2026 (monthly file) and Fri Oct 9 13:08:13 2026 (annual mean and growth rate files). Trends page: Last updated: Oct 09, 2026. Scale: the change log entry for March 2021 says NOAA's Mauna Loa CO2 data starting in 1974 are now on the X2019 scale; the trends page note gives Feb. 10, 2021 as the start of the WMO X2019 scale for the data shown.^{[1][2][3][4][7]}

Next release. No October 2026 value was in the file at retrieval. The change log's January 2009 entry says: "We have also changed the monthly web update from the 3rd of each month to the 10th of each month to give us a few more days to properly evaluate the measurements for the just-completed month." The pages give no current schedule, so the next update date is not established.^[4][7]