Interannual variability in the oxygen isotopes of atmospheric CO₂ driven by El Niño

Abstract

The stable isotope ratios of atmospheric CO₂ (¹⁸O/¹⁶O and ¹³C/¹²C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere–atmosphere exchange fluxes affect the different atomic masses in a measurable way¹. Interpreting the ¹⁸O/¹⁶O variability has proved difficult, however, because oxygen isotopes in CO₂ are influenced by both the carbon cycle and the water cycle². Previous attention focused on the decreasing ¹⁸O/¹⁶O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration³; a global increase in C₄ crops at the expense of C₃ forests⁴; and environmental conditions, such as atmospheric turbulence⁵ and solar radiation⁶, that affect CO₂ exchange between leaves and the atmosphere. Here we present 30 years’ worth of data on ¹⁸O/¹⁶O in CO₂ from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the ¹⁸O/¹⁶O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO₂ by biosphere–atmosphere gas exchange. We show how the decay time of the El Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño events, implying a shorter cycling time of CO₂ with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year⁷, may be too low, and that a best guess of 150–175 petagrams of carbon per year better reflects the observed rapid cycling of CO₂. Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.