Global Critical Drought Thresholds of Terrestrial Carbon Sink-Source Transition

Drought is considered a major contributor to carbon sink fluctuations in terrestrial ecosystems and is expected to lead to more frequent carbon sink-source transitions under future climate change. The drought threshold for carbon sink-source transition reflects the critical inflection point at which the carbon sequestration capacity of vegetation is affected by water deficit. However, the spatiotemporal patterns of the global drought threshold and their underlying mechanisms remain poorly understood. Here, we use three independent datasets from vegetation dynamics models, inversion modeling, and observational data to map and explore the drought thresholds expressed by the standardized precipitation evapotranspiration index (SPEI) during the growing season over the past four decades. Sink-source transition is indicated by changes of sign for net ecosystem productivity (NEP). The drought thresholds were identified across 66.3% of global land, with an average threshold of -1.08 ± 0.68. Regions with lower thresholds are primarily located in the Northern Hemisphere at middle and high latitudes, whereas Australia, Africa, western South America, and southern North America exhibit higher thresholds. The dominant factor influencing the spatial pattern of drought thresholds is potential evapotranspiration. Our dynamic results show that 36.4% of the thresholds increased, while 55.8% decreased. We found that disproportionate decreases in photosynthesis and respiration caused by drought in South America led to decreased thresholds and increased drought resilience in this region. Under conditions of reduced soil moisture, lower radiation, increased vapor pressure deficit, and enhanced heatwave intensity, drought in North America had a greater effect on reducing photosynthesis than it did on respiration. This resulted in an increasing threshold trend, where even relatively low levels of drought can induce a carbon sink-source transition. In addition, CO₂ fertilization plays a major role in reducing thresholds and mitigating climate change. Our findings emphasize that the risk of carbon sink-source transition is more acute in regions with rising thresholds. This implies that the stability of ecosystem carbon sequestration in these regions may decrease under persistent water stress.