Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO2

Photosynthesis uses the energy in sunlight to bind CO2 to a five-carbon sugar, transferring CO2 from the atmosphere to plants (Calvin & Benson, 1948; Farquhar et al., 1980). Sugars produced by photosynthesis provide the building blocks and the primary fuel for much of life on Earth. Plant tissues, many microbes, animals, and dead organic matter are all composed of carbon-rich compounds formed from these photosynthetic sugars. In many environments, an increase in atmospheric CO2 concentration [CO2] increases photosynthesis. Thus an increase in [CO2] leads to greater plant sugar availability with the potential to increase the total amount of carbon stored in the live and dead organic matter in an ecosystem. These observations have led to the CO2-fertilisation hypothesis (Box 1): that plant responses to increasing atmospheric [CO2] drive increases in terrestrial-ecosystem carbon storage creating negative feedback on atmospheric [CO2] growth. Since the industrial revolution human activities have increased [CO2] by 48 % (1760-2019, 277-411 ppm), an increase in atmospheric CO2-carbon of 277 Pg C (Friedlingstein et al., 2019). However, global-scale carbon accounting quantifies anthropogenic emissions to the atmosphere at 645 Pg C and suggests a substantial ‘natural’ terrestrial carbon sink (a net flux of carbon from the atmosphere to intact terrestrial ecosystems) which currently removes the equivalent of 33±9 % of anthropogenic atmospheric CO2 (2009-2018 (Friedlingstein et al., 2019). Along with the ocean carbon sink, this terrestrial carbon sink is mitigating the rate of climate change. Process-based carbon-cycle models attribute increasing [CO2] (iCO2, Table 1) as the primary driver of the terrestrial carbon sink, albeit with substantial uncertainty (Huntzinger et al., 2017; Arora et al., 2019). However, iCO2 is not the only global-change factor that can influence terrestrial carbon stocks. Anthropogenic land-use and land-cover change (hereafter land-use change) and recovery (Pugh et al. 2019), nitrogen cycle changes (Fowler et al., 2013), and climate change all affect ecosystem carbon stocks (Keenan & Williams, 2018). A vast and overwhelming literature often disagrees on the size and duration of CO2- driven increases in terrestrial carbon storage and predictive understanding of this process is a longstanding and unresolved scientific goal.