A constraint on historic growth in global photosynthesis due to increasing CO.

The global terrestrial carbon sink is increasing, offsetting roughly a third of anthropogenic CO released into the atmosphere each decade, and thus serving to slow the growth of atmospheric CO. It has been suggested that a CO-induced long-term increase in global photosynthesis, a process known as CO fertilization, is responsible for a large proportion of the current terrestrial carbon sink. The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models. Here we quantify the historic effect of CO on global photosynthesis by identifying an emergent constraint that combines terrestrial biosphere models with global carbon budget estimates. Our analysis suggests that CO fertilization increased global annual photosynthesis by 11.85 ± 1.4%, or 13.98 ± 1.63 petagrams carbon (mean ± 95% confidence interval) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global photosynthesis to CO, and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.