The Rising Pulse of Land Carbon Uptake

Because long-term observations are sparse and heavily affected by inter-annual variability, the detection of forced trends in land carbon uptake has been remarkably difficult. While changes in aggregated monthly or yearly carbon uptake have been heavily studied, changes in the diurnal cycle of carbon uptake remain uninvestigated. Here, we evaluate long-term changes in three-hourly Net Ecosystem Production (NEP) and the contribution of different times of the day to long-term changes in the terrestrial carbon sink. We first show that between 1950 and 2014, five CMIP6 models show a significant increasing trend in the diurnal NEP amplitude (DNA). Second, we show that DNA trends have a much higher signal-to-noise ratio compared to trends in NEP itself, and linearly scale with long-term annual NEP. Evaluating these model-based results against observations, we show that positive DNA trends are also found at a majority of observational eddy-covariance sites between 1990 and 2022. The positive correlation between DNA and long-term NEP is also confirmed by these observations. Our results reveal a widespread but previously undocumented emergent climate signal in terrestrial carbon exchange at the diurnal scale with the potential to serve as a new observational constraint for Earth system models.