Large influence of soil moisture on long-term terrestrial carbon uptake

The terrestrial biosphere absorbs about 25% of anthropogenic CO(2) emissions, yet the rate of land carbon uptake remains highly uncertain, leading to uncertainties in climate projections(1,2). Understanding the factors that are limiting or driving land carbon storage is therefore important for improved climate predictions. One potential limiting factor for land carbon uptake is soil moisture, which can reduce gross primary production due to ecosystem water stress(3,4), cause vegetation mortality(5), and further exacerbate climate extremes due to land-atmosphere feedbacks(6). Previous work has explored the impact of soil moisture availability on past carbon flux variability(3,7,8). However, the magnitude of the effect of soil moisture variability and trends on the long-term carbon sink and the mechanisms responsible for associated carbon losses remain uncertain. Here we use four global land-atmosphere models(9), and find that soil moisture variability and trends induce large CO(2) sources (~2–3 GtC/year) throughout the twenty-first century; on the order of the land carbon sink itself(1). Subseasonal and interannual soil moisture variability generates a CO(2) source as a result of the nonlinear response of photosynthesis and net ecosystem exchange to soil water availability and the increased temperature and vapour pressure deficit caused by land-atmosphere interactions. Soil moisture variability reduces the present land carbon sink while soil moisture variability and its drying trend reduce it in the future. Our results emphasize that the capacity of continents to act as a future carbon sink critically depends on the nonlinear response of carbon fluxes to soil moisture and on land-atmosphere interactions. This suggests that with the drying trend and increase in soil moisture variability projected in several regions, the current carbon uptake rate may not be sustained past mid-century and could result in an accelerated atmospheric CO(2) growth rate.