Recent changes in the dominant environmental controls of net biome productivity.

In the last decades terrestrial ecosystems have reabsorbed on average more than one quarter of anthropogenic emissions (Le Quéré et al., 2018). However, this large carbon sink is modulated by climate and is therefore highly variable in time and space. The magnitude and temporal changes of the sensitivity of terrestrial CO₂ fluxes to climate drivers are key factors to determine future atmospheric CO₂ concentration and climate trajectories. In the literature there is so far a strong focus on the climatic controls of inter-annual variability, while less is known about the key drivers of the sub-annual variability of the fluxes. This latter temporal scale is relevant to assess which climatic drivers dominate the seasonality of the fluxes and to understand which factors limit the net ecosystem CO₂ exchange. Here, we investigated the global sensitivity of terrestrial CO₂ fluxes to three key climate drivers (i.e. global radiation, temperature and soil water content) from weekly to seasonal temporal scales, in order to explore the short-term interdependence between climate and the terrestrial carbon budget. We observed that the CO₂ exchange over most of the land surface is controlled by temperature during the carbon uptake period, while radiation is the most widespread dominant climate driver during the carbon release period. As expected, soil water content plays a key role in arid regions of the southern hemisphere. Looking at the decadal trend of these sensitivities we observed that the importance of radiation as a driver is increasing over time, while we observed a decrease in sensitivity to temperature in Eurasia. Overall, we show that the temporal variation of the fluxes due to a specific driver is dominated by the temporal changes in ecosystem sensitivity rather than to the temporal variability of the driver itself. Ultimately this analysis shows that the response of the ecosystem to climate drivers is significantly changing both in space and in time, with potential repercussion on the future terrestrial CO₂ sink and therefore on the role that land may play in climate mitigation. [ABSTRACT FROM AUTHOR]