Marginal Lands Gross Primary Production Dominate Atmospheric CO2 Interannual Variations

Abstract:

Since the 1960s terrestrial ecosystems have acted as a substantial sink for atmospheric CO₂, sequestering about one quarter of anthropogenic emissions in an average year. Variations in this land carbon sink are also responsible for most of the large interannual variability in atmospheric CO₂ concentrations. While most evidence places the majority of the sink in highly productive forests and at high latitudes experiencing warmer and longer growing seasons, the location and the processes governing the interannual variations are still not well characterised. Here we evaluate the hypothesis that the long-term trend and the variability in the land CO₂ sink are respectively dominated by geographically distinct regions: the sink by highly productive lands, mainly forests, and the variability by semi-arid or “marginal” lands where vegetation activity is strongly limited by water and therefore responds strongly to climate variability. Using novel analysis methods and data from both upscaled flux-tower measurements and a dynamic global vegetation model, we show that (1) the interannual variability in the terrestrial CO₂ sink arises mainly from variability in terrestrial gross primary production (GPP); (2) most of the interannual variability in GPP arises in tropical and subtropical marginal lands, where negative anomalies are driven mainly by warm, dry conditions and positive anomalies by cool, wet conditions; (3) the variability in the GPP of high-latitude marginal lands (tundra and shrublands) is instead controlled by temperature and light, with warm bright conditions resulting in positive anomalies. The influence of ENSO (El Niño-Southern Oscillation) on the growth rate of atmospheric CO₂ concentrations is mediated primarily through climatic effects on GPP in marginal lands, with opposite signs in subtropical and higher-latitude regions. Our results show that the land sink of CO₂ (dominated by forests) and its interannual variability (dominated by marginal lands) are largely decoupled in terms of both geographic locations and driving processes, and will respond in different ways to further increases in atmospheric CO₂ levels and changes in climate.