The past century has been a time of unparalleled changes in global climate and global biogeochemistry. At the forefront of the study of these changes are regular time-series observations at remote stations of atmospheric CO₂, isotopes of CO₂, and related species, such as O₂ and carbonyl sulfide (COS). These records now span many decades and contain a wide spectrum of signals, from seasonal cycles to long-term trends. These signals are variously related to carbon sources and sinks, rates of photosynthesis and respiration of both land and oceanic ecosystems, and rates of air-sea exchange, providing unique insights into natural biogeochemical cycles and their ongoing changes. This review provides a broad overview of these records, focusing on what they have taught us about large-scale global biogeochemical change.

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