Abstract
Year-to-year changes in carbon uptake by terrestrial ecosystems have an essential role in determining atmospheric carbon dioxide concentrations1. It remains uncertain to what extent temperature and water availability can explain these variations at the global scale2–5. Here we use factorial climate model simulations6 and show that variability in soil moisture drives 90 per cent of the inter-annual variability in global land carbon uptake, mainly through its impact on photosynthesis. We find that most of this ecosystem response occurs indirectly as soil moisture–atmosphere feedback amplifies temperature and humidity anomalies and enhances the direct effects of soil water stress. The strength of this feedback mechanism explains why coupled climate models indicate that soil moisture has a dominant role4, which is not readily apparent from land surface model simulations and observational analyses2,5. These findings highlight the need to account for feedback between soil and atmospheric dryness when estimating the response of the carbon cycle to climatic change globally5,7, as well as when conducting field-scale investigations of the response of the ecosystem to droughts8,9. Our results show that most of the global variability in modelled land carbon uptake is driven by temperature and vapour pressure deficit effects that are controlled by soil moisture.
Highlights
A measure of atmospheric dryness that depends on air temperature and humidity)
Previous studies have shown that suppressing the non-seasonal soil moisture variability in experiment A strongly reduces the magnitude of temperature and VPD extremes compared to the control simulation[6,27,35] (Extended Data Fig. 3)
Without soil moisture variability, the IAV of net land carbon uptake is almost eliminated. This primarily occurs because of a reduction in the IAV of gross primary production (GPP) (Fig. 1b, c, Supplementary Table 1) and to a lesser extent because of a reduction in the IAV of ecosystem respiration and disturbance fluxes. Both direct soil moisture effects and indirect temperature and VPD effects related to land–atmosphere coupling (LAC) can be responsible for the widespread reduction of net biome production (NBP) variability occurring in experiment A (Fig. 2a)
Summary
Vincent Humphrey1 ✉, Alexis Berg[2], Philippe Ciais[3], Pierre Gentine[4], Martin Jung[5], Markus Reichstein[5], Sonia I. We find that most of this ecosystem response occurs indirectly as soil moisture–atmosphere feedback amplifies temperature and humidity anomalies and enhances the direct effects of soil water stress The strength of this feedback mechanism explains why coupled climate models indicate that soil moisture has a dominant role[4], which is not readily apparent from land surface model simulations and observational analyses[2,5]. These findings highlight the need to account for feedback between soil and atmospheric dryness when estimating the response of the carbon cycle to climatic change globally[5,7], as well as when conducting field-scale investigations of the response of the ecosystem to droughts[8,9]. This is achieved by forcing the soil moisture in experiment A to follow the mean seasonal soil moisture cycle calculated from a reference
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