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How has the Mauna Loa CO2 record (Keeling Curve) changed year-by-year since 1958?
Executive summary
The Keeling Curve measured at Mauna Loa has shown an unambiguous, accelerating rise in atmospheric CO₂ since systematic measurements began in 1958, with year‑to‑year increases small in the early decades and larger in recent years; the record includes a seasonal “sawtooth” cycle superimposed on this long‑term trend. Recent sources document higher annual growth rates and record one‑year jumps around 2023–2025, and datasets from NOAA, Scripps, and related summaries are the primary, publicly available references for year‑by‑year values [1] [2] [3].
1. How the record is described when you look closely — steady climb, seasonal teeth, faster recent steps
The Mauna Loa record is portrayed consistently across the provided analyses as a continuous upward trajectory with seasonal oscillations: levels rise and fall within each year due to Northern Hemisphere biospheric uptake and release, but the annual baseline climbs. Early decades saw smaller average annual increments—analysts cite roughly ~0.8 ppm/yr in the 1960s rising to ~1.6 ppm/yr in the 1980s, and substantially larger mean increases in the 2010s–2020s [1]. Recent summaries explicitly note record annual increases and heightened year‑to‑year variability tied to anthropogenic emissions and natural climate variability such as El Niño [4]. These characterizations align across NOAA and Scripps descriptions that present monthly, annual and growth‑rate products for clear, reproducible year‑by‑year analysis [5] [3].
2. Specific recent numbers and notable record years — agreement and caveats in the sources
Multiple recent summaries report higher absolute concentrations (over 420 ppm in 2024–2025) and cite exceptionally large single‑year rises around 2023–2025, with one analysis naming a 3.75 ppm increase in 2024 as the largest on record [1] [2]. NOAA and Scripps product notes confirm that 2024–2025 values were among the highest recorded and that some year‑to‑year growth estimates remain preliminary until quality control and cross‑instrument calibration are completed [2] [5]. The data providers also emphasize the availability of monthly, weekly and annual series so researchers can verify year‑by‑year changes themselves, while flagging that short‑term anomalies can reflect both emission trends and natural climate drivers [3] [5].
3. What different sources emphasize — human signal, natural variability, or data mechanics?
The analytic set divides emphasis among drivers, milestones, and data access. Several summaries stress the human fingerprint—burning fossil fuels and land‑use change—as the primary cause of the long‑term rise and of the continued acceleration in recent decades [1] [6]. Others highlight natural amplifiers like El Niño that can increase interannual growth and cause record jumps in specific years [4]. A third theme is data management and transparency: NOAA and Scripps note ongoing maintenance, data formats and potential interruptions or delays in public posting, which affects how quickly year‑by‑year updates appear in publicly curated tables [5] [7]. Each emphasis is factual but serves different user needs—policy framing, scientific attribution, and data reproducibility.
4. Reconciling differing tone and numbers — why summaries sometimes disagree
Differences among the provided analyses arise from choice of metrics (monthly vs annual means), the sample end date, and preliminary versus finalized values. Some statements present rounded averages by decade (e.g., 0.8 ppm/yr in the 1960s, 2.6 ppm/yr in the last decade) while others cite exact monthly figures for individual months in 2024–2025 [1] [2]. Reports written earlier in 2025 use data then available and mark recent years as provisional; later summaries include adjusted, final annual growth rates once quality control is applied [2] [5]. These procedural distinctions explain apparent contradictions: they do not dispute the core factual trend of accelerating CO₂ concentrations, only differences in rounding, endpoints, and whether an annual change is flagged provisional [3] [2].
5. What is missing from these summaries and what to watch next
The documents reliably show year‑by‑year increases but often omit a consolidated table listing each calendar year’s final Mauna Loa annual mean and growth increment from 1958 to the present; users must pull monthly or annual files from NOAA or Scripps and compute the series themselves [3] [5]. Policy discussions sometimes foreground milestone headlines—e.g., exceeding 400 ppm in 2013 or record single‑year jumps—without linking to longer‑term cumulative emissions or multi‑site global columns that place Mauna Loa in the broader atmospheric context [6]. For the most authoritative, up‑to‑date year‑by‑year numbers, download the combined Scripps/NOAA annual mean file and check the providers’ notes on provisional values and recalibrations, then monitor updates through the coming months for finalized 2024–2025 adjustments [3] [2].