Summary: Atmospheric carbon dioxide concentration ([CO₂]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO₂] increase and thus climate change. However, ecosystem CO₂ responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO₂]‐driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO₂] (iCO₂) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre‐industrial times. Established theory, supported by experiments, indicates that iCO₂ is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO₂ responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO₂ responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO₂, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change.

Fungi relieve nitrogen limitation: Rising concentrations of atmospheric CO ₂ stimulate plant growth; an effect that could reduce the pace of anthropogenic climate change. But plants also need nitrogen for growth. So far, experimental nitrogen addition has had equivocal effects on the magnitude of CO ₂ fertilization. Terrer et al. explain that the impact of nitrogen on plant growth depends on the relationship between nitrogen availability and symbioses with mycorrhizal soil fungi. Only plants with ectomycorrhizal fungi associated with their roots can overcome nitrogen limitation. Science , this issue p. 72

Elevated CO₂ (eCO₂) experiments provide critical information to quantify the effects of rising CO₂ on vegetation^1,2,3,4,5,6. Many eCO₂ experiments suggest that nutrient limitations modulate the local magnitude of the eCO₂ effect on plant biomass^1,3,5, but the global extent of these limitations has not been empirically quantified, complicating projections of the capacity of plants to take up CO₂^7,8. Here, we present a data-driven global quantification of the eCO₂ effect on biomass based on 138 eCO₂ experiments. The strength of CO₂ fertilization is primarily driven by nitrogen (N) in ~65% of global vegetation and by phosphorus (P) in ~25% of global vegetation, with N- or P-limitation modulated by mycorrhizal association. Our approach suggests that CO₂ levels expected by 2100 can potentially enhance plant biomass by 12 ± 3% above current values, equivalent to 59 ± 13 PgC. The future effect of eCO₂ we derive from experiments is geographically consistent with past changes in greenness⁹, but is considerably lower than the past effect derived from models¹⁰. If borne out, our results suggest that the stimulatory effect of CO₂ on carbon storage could slow considerably this century. Our research provides an empirical estimate of the biomass sensitivity to eCO₂ that may help to constrain climate projections.