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

TL;DR

This study shows that climate warming will cause more frequent and severe negative land carbon cycle extremes by 2100, weakening the Earth's capacity to absorb carbon and potentially turning land into a carbon source.

Contribution

It introduces a novel percentile-based method to quantify NBP extremes and demonstrates their increasing magnitude and frequency under climate warming scenarios.

Findings

88% of global regions will experience larger negative NBP extremes by 2100.

Negative NBP extremes are more frequent than positive ones, especially in the tropics.

Soil moisture anomalies are the primary driver of NBP extremes.

Abstract

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. Using the percentile threshold on the probability distribution curve of NBP anomalies, we computed negative and positive extremes in NBP. 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 biospheric 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 extremes toward the end of the 21st century. Using regression analysis, we found soil moisture anomalies to be the most dominant individual driver of NBP extremes. 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.