Carbon Cycle Extremes Accelerate Weakening of the Land Carbon Sink in the Late 21st Century

Abstract. Rising atmospheric CO2 concentrations enhance vegetation growth through increased carbon fertilization and water-use efficiency. Terrestrial vegetation takes up more than one-quarter of global anthropogenic carbon emissions, and uptake is modulated by regional climate. Increasing surface temperature could lead to enhanced evaporation, reduced soil moisture availability, and more frequent droughts and heat waves. The spatiotemporal co-occurrence of such effects further drives extreme anomalies in vegetation productivity and net land carbon storage. However, the impacts of climate change on extremes in net biospheric production (NBP) over longer time periods are unknown. Here we show that due to climate warming, about 88 % of global regions will experience a larger magnitude of negative NBP extremes than positive NBP extremes toward the end of 2100, which accelerate the weakening of the land carbon sink. Our analysis indicates the frequency of negative extremes associated with declines in biome productivity was larger than positive extremes, especially in the tropics. While the overall impact of warming at high latitudes is expected to increase plant productivity and carbon uptake, high-temperature anomalies increasingly induce negative NBP TCEs toward the end of the 21st century. We found soil moisture anomalies as the most dominant individual driver of NBP extremes and the compound effect of hot, dry, and fire caused extremes at more than 50 % of the total grid cells. The larger proportion of negative NBP extremes raises a concern about whether the Earth is capable of increasing vegetation production with growing human population and rising demand for plant material for food, fiber, fuel, and building materials. The increasing proportion of negative NBP extremes highlights the consequences of not only reduction in total carbon uptake capacity but also of conversion of land to a carbon source.