LAUSR

Global biosphere productivity is the largest uptake flux of atmospheric carbon dioxide (CO2), and it plays an important role in past and future carbon cycles. However, global estimation of biosphere productivity remains a challenge. Using the ancient air enclosed in polar ice cores, we present the first 800,000-year record of triple isotopic ratios of atmospheric oxygen, which reflects past global biosphere productivity. We observe that global biosphere productivity in the past eight glacial intervals was lower than that in the preindustrial era and that, in most cases, it starts to increase millennia before deglaciations. Both variations occur concomitantly with CO2 changes, implying a dominant control of CO2 on global biosphere productivity that supports a pervasive negative feedback under the glacial climate. Description Uncovering an uncoupling The amount of CO2 in the atmosphere and sea level are closely related, but sometimes they do not covary closely. Yang et al. present a triple-isotope record of oxygen for the last 800,000 years, which reveals that global biosphere productivity has varied over that time and that it provides a negative feedback that can help atmospheric CO2 fluxes decouple from sea level, typically before deglaciations (see the Perspective by LeQuéré and Mayot). This finding supports the idea that low CO2 depresses global photosynthesis and reduces further CO2 drawdown. —HJS Global photosynthesis has coevolved with atmospheric CO2 during the last eight glacial intervals.