A variety of clever approaches has been developed to obtain proxy records of what the Earth was like in previous eras. Ocean cores, lake sediments, pollen, corals, palaeosoils and the annual rings of trees have all been used to infer changes in climate and productivity. But these records are essentially local in coverage, and they need to be extended to regional or global scales. Such a planetary-scale analogue to the annual rings of trees would be a boon for Earth-system research, and the study by Luz et al. on page 547 of this issue may provide just such information. Their work indicates that, by analysing the abundance of the three stable isotopes of oxygen — ~(16)O, ~(17)O and ~(18)O — from air trapped in ice cores, we can obtain estimates of gross photosynthetic production of oxygen, integrated to a global scale and averaged over millennia.
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