Sources, sinks and seasons

Abstract

Changes in the phasing of seasonal cycles of carbon dioxide in the atmosphere mark the time when a region becomes a source or a sink of CO2. One study of such changes prompts thought-provoking conclusions. An analysis of variations in atmospheric CO2 and ecosystem CO2 fluxes in the Northern Hemisphere shows that warmer autumns have been associated with an earlier autumn-to-winter CO2 build-up in the atmosphere. This seems counter-intuitive: warm autumns surely imply long growing seasons and a beneficial effect on terrestrial carbon sinks as trees and plants make more biomass. An explanation is provided by satellite observations and numerical modelling. Enhanced respiration caused by higher temperatures causes carbon losses that offset photosynthetic gains, limiting the potential of these ecosystems to act as carbon sinks. And CO2 loss due to autumn warming may offset most of the increased CO2 uptake during spring. If future warming occurs more rapidly in autumn than in spring, the ability of northern ecosystems to sequester carbon may diminish more rapidly than previously predicted.