In the ancient ice sheets near Earth’s poles, tiny bubbles trapped in the compacted layers of snowfall provide a natural archive of air from ages past, but one that is not straightforward to read. Forty years ago, the glaciologist Robert Delmas and colleagues developed a technique for reliably measuring the amount of carbon dioxide in the bubbles1. Their work paved the way for modern measurements showing that atmospheric CO 2 levels have been linked to Earth’s temperature for hundreds of thousands of years.

Read the paper: Polar ice evidence that atmospheric CO2 20,000 yr BP was 50% of present

Towards the end of the nineteenth century, Swedish scientist Svante Arrhenius became interested in the causes of Earth’s ice ages. Through calculations, he estimated how changes in CO 2 levels might affect the atmosphere’s ability to absorb heat from the ground, rather than allowing it to escape into space. He suggested that the cooler temperatures during glacial periods resulted from a decrease in this absorption2.

This greenhouse theory was an alternative to the theory backed by James Croll, who proposed3 that glaciation cycles arose from changes in Earth’s orbit, amplified by natural feedbacks such as changes in snow cover. Another scientist, Milutin Milankovitch4, refined this idea by looking at fine variations in Earth’s movements and the resulting changes in insolation (incoming sunlight) at different latitudes. His orbital theory gained support from marine sediment records5, which exhibit periodicities similar to Earth’s orbital parameters. Thus, the orbital theory was promoted as a pacemaker of the ice age, with various feedback loops amplifying tiny changes in insolation.

In the 1960s, air inclusions in ice cores (Fig. 1) drilled from the depths of glaciers sparked interest as natural samples of the ancient atmosphere. To quote researchers at the time6: “if the CO 2 concentration were found to vary in time, one would have the means for testing the well-known greenhouse theory of climate change.” Attempts were made during the 1960s and 1970s to obtain a reliable atmospheric CO 2 record from gas extracted by melting ice samples (‘wet extraction’). But results showed unexplained high concentrations of CO 2 , sometimes ten times that of today’s atmosphere, and scattered over a bafflingly wide range of values.

Figure 1 | Ice record. This thin section of glacier ice, visualized using polarized light, contains ice crystals and trapped bubbles of ancient air.Credit: Vladimir Lipenkov

The answer to the puzzle came from studies of the chemistry of polar ice. At Grenoble in France, Delmas was researching the acid rain that was decimating northern European forests. Suspecting that the industrial sulfur emissions thought to be responsible might be recorded in Greenland ice, he and his students Michel Legrand and Jean-Marc Ascencio developed a technique7 for measuring ice acidity, adapted to be exceptionally sensitive to impurities and to avoid bias from ambient atmospheric CO 2 . They discovered that, even without volcanic or human activity, the ice was tinged with sulfuric acid arising from sulfur compounds emitted by plankton. If an ice sample contained carbonaceous dust, melting the ice would allow the background acid to react with the carbonate. This, they thought, might be producing extra CO 2 in analyses of ice-core bubbles and confounding the CO 2 measurements.

To test their idea, Delmas and colleagues set up a dry-extraction system that avoided melting the ice — instead, the ice was crushed under vacuum in a vessel at –40 °C. They analysed selected samples from two Antarctic ice cores (Dome C and D10), which contained very few carbonates. For ice from the past 10,000 years, they found CO 2 levels comparable to then-current atmospheric values (around 300 parts per million; p.p.m.), with good reproducibility. For ice deposited during the last glacial period, around 20,000 years ago, levels were much lower, around 190 p.p.m. (Fig. 2a), thus confirming Arrhenius’s prediction.

Figure 2 | Ice-core records past and present. a, Forty years ago, Delmas et al.1 found a way to measure ancient atmospheric carbon dioxide levels from bubbles trapped in ice. Their results show that a dip in CO 2 concentrations coincided with the temperature drop associated with the glacial period around 20,000 years ago. Top: red dots show the authors’ CO 2 measurements in parts per million (p.p.m.) and the corresponding depth of the ice; the dashed red line represents the CO 2 concentration in 1980, the time of the study. Bottom: blue line indicates oxygen isotope ratios18 (a proxy for temperature) expressed as the quantity δ18O. b, Similar methods have now produced CO 2 records for much longer periods of time from deep ice cores. Here, CO 2 results from Antarctic ice march in step with temperature cycles (temperature anomaly represents the difference between the actual temperature and the mean temperature of the past millennium) over the past 800,000 years, revealing how strongly this greenhouse gas is coupled to Earth’s temperature13.

At around the time that Delmas et al. carried out their work, a Swiss group, using a refined version of wet extraction to analyse ice cores, also suggested8 that CO 2 levels were depleted during the last glacial period. But the dry technique subsequently became widely adopted because of its greater reliability and accuracy. Delmas and co-workers’ paper thus paved the way for today’s more extensive comparisons of climate and CO 2 concentrations.

A milestone was reached in 1987 with dry analyses of an ice core more than 2 kilometres deep from the Vostok site in Antarctica. This provided a 160,000-year record9–11 of temperature and CO 2 , depicting a complete climate cycle in which CO 2 levels varied from 290 p.p.m. during the warm period to 190 p.p.m. in the cold period, and correlated with temperature. Over the following decades, the climate–CO 2 correlation was demonstrated over a period of 400,000 years from extended Vostok records12, and then as far back as 800,000 years13 (Fig. 2b) from ice drilled at another Antarctic site, EPICA Dome C.

So Delmas and colleagues’ pioneering work covering a few tens of thousands of years sparked research that now gives us an accurate record of atmospheric CO 2 levels undulating with glacial–interglacial climate cycles for several hundred thousand years. This close correlation attests to the tight coupling between climate and carbon cycles during the past 800,000 years, and has stimulated climate scientists to consider them to be part of the same global system.

Ancient air challenges prominent explanation for a shift in glacial cycles

Four decades of ‘dry’ sampling of ice-core bubbles have contributed to our current understanding of the succession of glacial–interglacial cycles. Lying somewhere between the initial suggestions of Arrhenius and Croll, these cycles are seen as the combined effects of orbital variations and greenhouse gases, amplified by a series of natural feedbacks. Insolation changes driven by Earth’s orbital and axial oscillations (Milankovitch cycles) set the timing of the glacial cycles. But greenhouse gases (mainly CO 2 ), together with the growth and retreat of the northern ice sheets, affect the radiation balance between absorbed sunlight and energy escaping back to space, thereby driving the size and shape of the changes.

The reliability of Antarctic ice as a natural sampler of past air has been confirmed by a reconstruction of atmospheric CO 2 levels over the past millennium14, which covers pre-industrial CO 2 levels (280 p.p.m.) and which partly overlaps with the period when direct atmospheric measurements were made. Caution should be taken in analyses of ice from Greenland, where Delmas noted that windborne carbonaceous dust could cause in situ CO 2 production15. This is not the case for Antarctic ice, especially farther inland; but because outer parts of ice cores are always slightly contaminated by dust (from handling, storage or the atmosphere), dry extraction is needed even when analysing Antarctic samples. Last but not least, we know that current atmospheric CO 2 levels (407 p.p.m. in 2018; see go.nature.com/2j4heej) are probably unprecedented for the past 800,000 years16.

One of the most intriguing puzzles in palaeoclimate science is why the periodicity of the climate cycle has changed: marine sediment records show that climate followed a 40,000-year cycle before 1.2 million years ago, but changed to 100,000-year cycles during the Mid-Pleistocene Transition, about 1 million years ago. Delmas and colleagues’ legacy can be seen in an ongoing international Antarctic project that might shed light on this17. Known as ‘Beyond EPICA Oldest Ice’, it aims to extend the ice record to 1.5 million years ago by drilling down to 3 kilometres, to investigate greenhouse-gas levels and climate at that time.