A three-million-year record of ice and air
In the 18 March issue of Nature, two companion papers reconstruct three million years of the climate record, one tracing atmospheric carbon dioxide and methane, the other tracing ocean heat content. The pair shows how stable the ice-age world was, and how far the present departure from it has already gone.
The climate record has a problem of depth. Instrumental measurements go back a couple of centuries, ice cores push that to roughly a million years for the atmosphere, and beyond that the record grows patchy, assembled from indirect evidence, sediments, isotopes, fossils, each with its own uncertainties. Two papers in the 18 March issue of Nature reach further, reconstructing three million years of climate history from two different angles: one tracing atmospheric carbon dioxide and methane, the other tracing the heat content of the oceans. Together they provide one of the longest, most coherent views yet of the world before the ice ages took their modern form.
The first paper reports that, over the past three million years, atmospheric carbon dioxide and methane were broadly stable, fluctuating within a range that looks narrow from the vantage of today. The second reconstructs global ocean heat content over the same span, tracing how much heat the oceans stored through the glacial cycles that dominated the period. The two records fit together: the atmosphere set the boundary conditions, and the oceans, the planet’s dominant heat reservoir, responded on the slow rhythms of ice expansion and retreat. The picture that emerges is of a system in equilibrium, cycling between ice ages and warm intervals within a relatively narrow climatic envelope.
The word that recurs in the papers’ framing is “broadly stable,” and it is worth understanding precisely. Three million years of glacial cycles were not quiet; the world swung between deep glacials and interglacials, and the swings were large by any human standard. But measured against the atmosphere’s composition, the range was bounded, the carbon dioxide concentration oscillating within a band that the modern record has far exceeded. The stability was the stability of a system with natural limits, and the limits were set by processes, weathering, ocean uptake, biological cycling, that operated on timescales far longer than any human institution.
The modern departure is the point of the exercise, and the papers do not belabour it. Concentrations today have risen far above the three-million-year range in a few generations, a rate of change with no analogue in the record the papers reconstruct, and the oceans are absorbing heat at a pace the geological archive does not show. The past is not a prediction, and the authors are careful not to make it one, but the record gives the present a scale: the atmosphere’s current composition is off the chart that three million years drew, and the oceans are being asked to store heat at rates their three-million-year history never recorded.
There is a quieter wonder in the reconstruction itself. Carbon dioxide that was in the air when the first hominins were learning to walk, heat stored in oceans before the genus Homo existed, read today from chemical traces in sediments: the papers are acts of forensic patience, assembling a record that no instrument ever kept. The value of the work is not only the conclusion, important as it is, but the demonstration that such a record can exist at all. Three million years is deep time, and it is now, in these papers, legible.