A constraint on historic growth in global photosynthesis due to rising CO2

Abstract

Theory predicts that rising CO2 increases global photosynthesis, a process known as CO2 fertilization, and that this is responsible for much of the current terrestrial carbon sink. The estimated magnitude of the historic CO2 fertilization, however, differs by an order of magnitude between long-term proxies, remote sensing-based estimates and terrestrial biosphere models. Here we constrain the likely historic effect of CO2 on global photosynthesis by combining terrestrial biosphere models, ecological optimality theory, remote sensing approaches and an emergent constraint based on global carbon budget estimates. Our analysis suggests that CO2 fertilization increased global annual terrestrial photosynthesis by 13.5 ± 3.5% or 15.9 ± 2.9 PgC (mean ± s.d.) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global terrestrial photosynthesis to CO2 and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.

Neutral Tropical African CO2 Exchange Estimated From Aircraft and Satellite Observations

“…Emergent constraints are powerful tools to reduce model spread and narrow uncertainty (e.g., K. W. Bowman et al., 2018; Eyring et al., 2019; Simpson et al., 2021; M. S. Williamson et al., 2021). They offer a promising way to further improve the quantification of carbon fluxes and the overall scientific understanding of the carbon cycle (e.g., Barkhordarian et al., 2021; Cox, 2019; Keenan et al., 2023; Long et al., 2021; Stephens et al., 2007). Overall, our approach here is to take advantage of the large model spread to derive robust relationships between the airborne observations and land fluxes.…”

Stand age diversity and climate change affects forests’ resilience and stability, although unevenly

“…At the same time, however, the effects of climate change on the carbon budget are highly uncertain and largely debated with models that simulate an overall increase in NPP, possibly due, among other things, to the fixed relation between NPP and GPP (e.g., Reyer 2015) and those models that return an increase in tree and stand respiratory costs over, e.g., the positive effects on GPP because of CO 2 fertilization and prolonging vegetative seasons (Peano et al, 2019; Keenan et al, 2023). Forest stability and resilience under climate change are even more uncertain.…”

“…However, in many of the models investigated, as also in other studies considered (e.g., Vitale et al, 2003; Jansson et al, 2008), factors such as photosynthetic acclimation (saturation) to increasing atmospheric CO 2 concentration, acclimation to rising temperature in autotrophic respiration and drought were not fully and always accounted for, and, often, NPP is considered a fixed fraction of the GPP (see Collalti & Prentice, 2019) or empirically constrained based on current climate conditions (e.g., de Wergifosse et al, 2022). The main consequence of considering NPP as a fixed fraction of GPP instead of prognostically simulating it under climate change scenarios is the increases in NPP via increasing GPP, thanks to the CO 2 fertilization effect (Keenan et al, 2023) and the lengthening of the growing season (Peano et al, 2019). On the other hand, the negative feedback on NPP due to increased autotrophic respiration (for maintenance respiration at tree and stand level) is not considered.…”

Global Vegetation‐Temperature Sensitivity and Its Driving Forces in the 21st Century

“…This includes For terrestrial ecosystems, we predicted that most terrestrial vegetation productivity will continue to increase globally by the end of the 21st century, especially in medium-and high-emission scenarios, and particularly in low-latitude regions such as the Amazonian forests and the tropical forests of Africa. This possibly link to the CO 2 fertilization effect, where elevated atmospheric CO 2 concentrations stimulate plant photosynthesis rates, thus enhancing productivity (Keenan et al, 2023). In high-latitude regions, the slow increase in vegetation productivity may relate to earlier tree activity or prolonged growing season by warmer temperature (S. Gao et al, 2022;Grossiord et al, 2022;Wang et al, 2021).…”