Transition from positive to negative indirect CO2 effects on the vegetation carbon uptake

The increase in the vegetation carbon uptake, stimulated in the last decades by the elevated CO₂ concentration (eCO₂), has substantially contributed to the enhancement of terrestrial carbon sink, thus playing a crucial role in mitigating climate change. Changes in the vegetation carbon uptake are affected by eCO₂ through two distinct pathways. The first is the direct CO₂ effect through the stimulation of the photosynthetic carbon fixation and the increase in water-use efficiency. The second is the indirect CO₂ effect through the change in climate and related environmental conditions. Recent studies documented a declining trend in the direct physiological effect of eCO₂ on the vegetation carbon sink because of the increasing role of other limiting factors (e.g., nutrients and water availability). Consequently, the indirect effects of eCO₂ via associated climate change are expected to become increasingly important in controlling the terrestrial carbon budget. However, the current and future dynamics of such indirect CO₂ effects and the underlying ecological mechanisms remain unclear. Here we investigate how the impacts of eCO₂-driven climate change on growing-season gross primary production (GPP) have changed globally during the period 1982-2014, using both satellite observations and a suite of CMIP6 Earth system models, and evaluated their evolution until the year 2100 under the high emission scenario SSP5-8.5. We show that the initial positive effect of eCO₂-induced climate change on global vegetation carbon uptake has declined significantly during recent decades. In this respect, this indirect effect has shifted to negative in the early 21st century, and is expected to turn firmly negative in the future. Such a decrease in the indirect effect of eCO₂ appears more pronounced in northern high latitudes and occurs in combination with a concomitant decrease of the direct physiological effect of eCO₂. Together, these changes will likely determine a sharp reduction of the current strong growth benefits induced by climate warming and CO₂ fertilization in boreal ecosystems. The progressive weakening of the indirect CO₂ effect on vegetation carbon uptake can be partially attributed to the widespread climate drying, except for some non-humid regions where the CO₂ and drought-related increase in water-use efficiency potentially relaxes the water limitation to vegetation growth. These results imply that eCO₂ may exert a less positive up to negative role on the terrestrial carbon uptake in the near future, ultimately reducing the ecosystems’ capacity to sequester atmospheric CO₂. All together these findings contribute to a better understanding of the factors controlling the negative feedback between atmospheric CO₂ concentration and the natural terrestrial sink and highlight a worrying decline in its strength that might ultimately lead to an acceleration of climate warming. Consequently, stronger reductions in anthropogenic emissions will be required to meet climate goals.