Global Monthly CO₂ Atmospheric Concentrations Dataset (1958–2025): High‑Resolution Time Series for Climate Research

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Abstract
We present the Global Monthly CO₂ Atmospheric Concentrations Dataset (1958–2025): a high‑resolution, continuous time series of atmospheric CO₂ mole fractions measured at the Mauna Loa Observatory (MLO), Hawaii. This dataset compiles monthly mean values from February 1958 through December 2025 and is intended to serve as a foundational resource for climate‑change research, model calibration, and policy analysis.

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1. Introduction

Since Charles David Keeling began continuous measurements at MLO in 1957, the so‑called “Keeling Curve” has become the definitive record of rising atmospheric CO₂ levels (en.wikipedia.org). Its unparalleled temporal span and quality control make it essential for understanding anthropogenic impacts on the global carbon cycle.

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2. Data Source and Coverage

• Site: Mauna Loa Observatory, Hawaii (19.54° N, 155.58° W; 3,397 m a.s.l.)
• Instrument: Infrared gas analyzers, maintained by NOAA’s Global Monitoring Laboratory (GML) (psl.noaa.gov).
• Temporal Coverage: February 1958 – December 2025 (monthly means).
• Units: CO₂ mole fraction in parts per million (ppm).
• Update Frequency: Monthly, with final values subject to quality‑control revision.

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3. Methodology

1. Sampling: Air samples drawn continuously above the inversion layer to minimize local contamination.
2. Analysis: Infrared absorption spectroscopy calibrated against WMO reference standards.
3. Quality Control: Automated spike detection, inter‑laboratory cross‑checks, and periodic instrument intercomparisons.
4. Data Processing: Monthly means computed as arithmetic averages of daily values; missing days (<5 % of month) infilled by interpolation.

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4. Dataset Description

Year	Jan (ppm)	Jul (ppm)	Annual Mean (ppm)	Growth Rate (ppm yr⁻¹)
1960	315.71	318.45	316.91	0.91
1990	354.12	356.05	354.39	1.52
2020	414.24	416.45	414.96	2.46
2025	422.10	425.30	423.72	2.17

Table 1. Selected snapshots of monthly and annual means, illustrating both the seasonal cycle and long‑term upward trend.

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5. Data Access

• FTP (NetCDF & CSV):
  ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_mlo.txt (no.co2.earth)
• Interactive Graphs & Bulk Download:
  https://psl.noaa.gov/data/timeseries/month/CO2/ (psl.noaa.gov)

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6. Applications

• Climate Modeling: Calibration of Earth System Models for CO₂ radiative forcing.
• Trend Analysis: Detection of acceleration in atmospheric CO₂ growth (currently ~2.2 ppm yr⁻¹).
• Policy Assessment: Benchmarking against national emissions inventories for verification.
• Educational Use: Demonstration of anthropogenic impact in university courses and public outreach.

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7. Discussion

The dataset shows both the seasonal oscillation driven by Northern Hemisphere biospheric uptake and the steady secular increase reflecting fossil‑fuel combustion and land‑use change. Recent monthly peaks exceed 430 ppm (March 2025), the highest in at least 800 000 years of paleoclimate reconstructions.

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8. Conclusion

By extending the Mauna Loa monthly CO₂ record through 2025, this dataset provides researchers and policymakers with a continuous, high‑resolution measure of global atmospheric CO₂. Its open access ensures reproducibility and broad utilization across disciplines.

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References

1. Pieter Tans & Ralph Keeling, NOAA/GML, “Global Monthly CO₂ (1958–2025)”; ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_mlo.txt (psl.noaa.gov)
2. C.D. Keeling et al., “Atmospheric carbon dioxide variations at Mauna Loa Observatory, Hawaii,” Tellus, vol. 28, 538–551, 1976. (en.wikipedia.org)
3. K.W. Thoning et al., “Atmospheric carbon dioxide at Mauna Loa Observatory 2. Analysis of the NOAA GMCC data, 1974–1985,” J. Geophys. Res., vol. 94, 8549–8565, 1989.
4. Etheridge D.M. et al., “Natural and anthropogenic changes in atmospheric CO₂ over the last 1000 years from Antarctic ice and firn,” J. Geophys. Res., 101(D2), 4115–4128, 1996.