Integrating the evidence for a terrestrial carbon sink caused by increasing atmospheric CO<sub>2</sub>

Walker, Anthony P.; Kauwe, Martin G. De; Bastos, Ana; Belmecheri, Soumaya; Georgiou, Katerina; Keeling, Ralph F.; McMahon, Sean M.; Medlyn, Belinda E.; Moore, David J.P.; Norby, Richard J.; Zaehle, Sönke; Anderson-Teixeira, Kristina J.; Battipaglia, Giovanna; Brienen, Roel J.W.; Cabugao, Kristine G.; Cailleret, Maxime; Campbell, Elliott; Canadell, Josep G.; Ciais, Philippe; Craig, Matthew E.; Ellsworth, David S.; Farquhar, Graham D.; Fatichi, Simone; Fisher, Joshua B.; Frank, David C.; Graven, Heather; Gu, Lianhong; Haverd, Vanessa; Heilman, Kelly; Heimann, Martin; Hungate, Bruce A.; Iversen, Colleen M.; Joos, Fortunat; Jiang, Mingkai; Keenan, Trevor F.; Knauer, Jürgen; Körner, Christian; Leshyk, Victor O.; Leuzinger, Sebastian; Liu, Yao; MacBean, Natasha; Malhi, Yadvinder; McVicar, Tim R.; Penuelas, Josep; Pongratz, Julia; Powell, A.S.; Riutta, Terhi; Sabot, Manon E.B.; Schleucher, Juergen; Sitch, Stephen; Smith, William K.; Sulman, Benjamin; Taylor, Benton; Terrer, César; Torn, Margaret S.; Treseder, Kathleen K.; Trugman, Anna T.; Trumbore, Susan E.; Mantgem, Phillip J. van; Voelker, Steve L.; Whelan, Mary E.; Zuidema, Pieter A.


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.