Biogeochemistry: agriculture and the global carbon cycle.

Abstract

Evolving agricultural practices dramatically increased crop production in the twentieth century. Two studies now find that this has altered the seasonal flux of atmospheric carbon dioxide. See Letters p.394 & p.398 The atmospheric CO2 record displays a seasonal cycle reflecting seasonal variations in CO2 uptake by terrestrial vegetation. An increase in the amplitude of this seasonal cycle over the past five decades cannot be fully explained at present. Two groups now report that the intensification of agriculture may have been a key contributor to the increase in atmospheric CO2 seasonal amplitude. Ning Zeng et al. used the VEGAS terrestrial biosphere model to show that enhanced mid-latitude agricultural productivity contributed 45% of the increasing amplitude of global net surface carbon fluxes for the period 1961 to 2010, compared to 29% from climate change and 26% from CO2 fertilization. Josh Gray et al. used crop production statistics from the UN Food and Agriculture Organization and a carbon accounting model to demonstrate that as much as a quarter of the observed change in atmospheric CO2 seasonality can be explained by elevated crop productivity, with maize, wheat, rice and soybean major contributors. These studies will contribute to a better understanding of the global carbon cycle, and highlight the extent to which human actions are changing large-scale biosphere–atmosphere interactions.