Triple-isotopic oxygen record reveals global productivity during last eight glacial cycles

Over the last eight glacial cycles – a period spanning 800,000 years – the amount of carbon dioxide absorbed by the oceanic and terrestrial ecosystems combined has been inextricably linked to the rise and fall of atmospheric carbon dioxide, according to a new study. Global atmospheric carbon dioxide absorption by land and sea, also known as global primary productivity (GPP), represents the largest uptake of atmospheric carbon dioxide and is important in the global carbon cycle. To date, estimating how GPP will respond to changes in atmospheric carbon dioxide (CO₂) remains challenging. Understanding this response, however, is critical to forecasting projections of future climate change. Glacial cycles offer a natural laboratory to study the large-scale response of ecosystems to changing CO₂ concentrations. Much of the data on glacial cycles is based on measurements of air trapped in ice cores. Ji-Woong Yang and colleagues reconstructed changes in global biosphere productivity during the past 800,000 years by analyzing the isotopic signature of the oxygen trapped in ancient air bubbles locked within ice cores. The findings show that over the past eight glacial cycles, GPP has always been lower during glacial intervals compared to interglacial, and that, in most cases, productivity starts to rise several thousand years before glacial terminations. What’s more, these changes appear to occur in tandem with atmospheric CO₂ concentrations, suggesting a dominant impact of CO₂ on global biosphere productivity. According to the authors, the results support the presence of a pervasive negative feedback mechanism in which low CO₂ depresses global productivity, thus reducing further CO₂ drawdown. “The analysis of Yang et al. confirms the global importance of CO₂ fertilization and provides constraints on the size of its effect,” write Corinne Le Quéré and Nicolas Mayot in a related Perspective. “Although many uncertainties remain, it is clear that damping feedback of CO₂ fertilization will decrease in the coming decades while the amplifying feedback of climate on carbon storage continues to grow.”