Increasing vulnerability of terrestrial carbon sinks to extreme heatwaves beyond critical thermal thresholds

Terrestrial ecosystems play a pivotal role in mitigating climate change by sequestering a significant portion of anthropogenic carbon dioxide emissions. However, the stability of these vital carbon sinks is increasingly threatened by the rising frequency, duration, and intensity of extreme weather events, particularly severe heatwaves. While it is widely recognized that extreme heatwaves are increasingly regulating interannual variability in terrestrial carbon uptake, the precise climatic thresholds at which ecosystem functions and carbon assimilation capacities break down remain poorly constrained. Traditional assessments relying on daily-aggregated data often obscure transient but critical physiological responses, limiting our predictive capabilities. To address this critical knowledge gap, we leverage high-frequency, sub-daily observations from the Himawari-8 geostationary satellite to track the real-time response of regional gross primary production (GPP) to escalating thermal stress across a temperate forest-dominated region. Our high-resolution analysis identifies a distinct, nonlinear tipping point in carbon assimilation behavior. Specifically, we observe that once canopy temperatures surpass a critical thermal threshold of 31.2 °C, GPP exhibits a rapid and sustained downturn. Beyond this tipping point, carbon assimilation decreases at a striking rate of 0.48 umol CO2 m-2 s-1 for every additional degree Celsius. This physiological degradation is most acute during peak midday hours. During this temporal window, the detrimental convergence of elevated solar radiation and critically high vapor pressure deficit forces rapid stomatal closure, inducing a sub-daily physiological collapse that is partially obscured in conventional daily-scale assessments. The severe 2018 heatwave served as a clear, landscape-scale demonstration of this underlying ecosystem instability. During this prolonged extreme event, the widespread exceedance of local thermal limits triggered massive spatial divergence in ecosystem carbon uptake, resulting in a substantial loss of event-scale regional carbon assimilation. Ultimately, these findings demonstrate that terrestrial carbon sinks can exhibit threshold-like instability under extreme heat. Our results underscore the urgent need to integrate sub-daily monitoring and resolve high-frequency climate-ecosystem interactions to accurately project carbon cycle responses in a warming world.