Broadly stable atmospheric CO2 and CH4 levels over the past 3 million years
Article Date: 2025-10-15
Article URL: https://www.nature.com/articles/s41586-025-10032-y
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Summary
The study compiles and analyses palaeo-atmospheric records (ice cores, blue-ice exposures and multiple proxy archives) to reconstruct atmospheric carbon dioxide (CO2) and methane (CH4) across the last ~3 million years. The central finding is that, despite major climatic shifts from the Pliocene into the Pleistocene and through many glacial–interglacial cycles, mean atmospheric CO2 and CH4 concentrations remained broadly within a long-term envelope until the recent anthropogenic rise. The paper synthesises high-resolution ice-core chronologies, boron- and alkenone-based proxy data, and advances in ice dating (AICC2023 and related methods) to quantify variability, identify uncertainties, and discuss implications for drivers of past warm climates (for example, Pliocene warmth) and for climate sensitivity estimates.
Methods: integration of ice-core gas measurements, blue-ice sampling, boron-isotope and other marine proxies, updated chronologies and careful treatment of gas-ice processes and diffusion effects. The authors compare proxy stacks with model expectations to tease apart the roles of greenhouse gases versus ocean/ice-sheet and orbital forcings.
Key Points
- Reconstruction across ~3 million years shows CO2 and CH4 largely remained within a persistent range prior to the modern anthropogenic increase.
- High-resolution ice-core records and blue-ice samples plus multiple marine proxies were combined to extend the atmospheric record beyond the usual ice-core limits.
- The findings imply that Pliocene warmth and some other past warm intervals cannot be explained solely by sustained, large increases in CO2 or CH4 relative to the long-term envelope; other factors (ocean heat content, albedo, gateway changes, regional feedbacks) were important.
- Authors highlight advances in chronology (AICC2023), gas extraction and isotopic methods that reduce some prior uncertainties but also note remaining proxy limitations and resolution gaps.
- Results provide a stronger constraint for climate models and estimates of Earth-system sensitivity by showing the greenhouse-gas boundary conditions for much of the Plio-Pleistocene.
Context and relevance
This paper sits at the intersection of palaeoclimate reconstruction and climate model evaluation. By extending and harmonising greenhouse-gas records back to ~3 Ma, it tightens the boundary conditions models use to test how temperature, ice sheets and sea level respond to radiative forcing. That has direct relevance to understanding long-term climate sensitivity and to interpreting why some past warm intervals were warmer than expected from greenhouse gases alone.
It also matters for policy and communication: the long-term stability of natural CO2 and CH4 prior to human influence emphasises how exceptional and rapid the modern rise is, providing context for the scale and speed of anthropogenic change.
Why should I read this?
Because if you care about how the Earth pulled off past warm episodes without runaway greenhouse-gas jumps, this paper is your short-cut. The authors have done the tedious cross-dataset work — merging ice-core, blue-ice and marine proxies — so you don’t have to. It clarifies what greenhouse gases did (and didn’t) do over millions of years, which helps make sense of Pliocene warmth, ice-sheet behaviour and what models should be matching. Also, it’s a handy reality check: natural CO2/CH4 stayed mostly stable until humans came along.