The source paper (Joos et al., 2001, Global Biogeochemical Cycles) drove a coupled climate–carbon-cycle model containing the LPJ dynamic global vegetation model with the six IPCC SRES nonintervention emission scenarios and projected atmospheric CO2 of 540–960 ppm by 2100, with a process-driven uncertainty of −10% to +30% in 2100 CO2 for a given emission scenario. The paper attributed this spread to uncertainties in modeled land and ocean processes, and acknowledged six limitations bearing on its projections: omitted land-use dynamics, one-way land–atmosphere coupling, unconstrained strength and saturation of CO2 fertilization, conflicting evidence on the multiannual temperature dependence of soil and litter respiration, uncertain aerosol radiative forcing, and ocean-transport sensitivity. Two decades of Earth system model development have closed the three structural threads: fully coupled carbon–climate models, harmonized dynamic land-use data sets, and observationally constrained ocean transport are now standard, and ocean feedback spread is small. However, the two terrestrial process sensitivities — the strength and saturation of global CO2 fertilization and the multiannual temperature response of soil and litter organic matter decomposition — together with aerosol forcing remain the dominant sources of intermodel spread: across models in the Coupled Model Intercomparison Project Phase 6 (CMIP6, the standard multi-model climate-projection protocol) the land carbon–concentration feedback spans 0.97 ± 0.40 PgC ppm−1, the land carbon–climate feedback −45.1 ± 50.6 PgC °C−1, and the cumulative land carbon sink spread is about 150 PgC even under a low-emissions scenario, while the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (2021) states that significant uncertainty remains in the future of the global land carbon sink. The surviving open problem is the quantitative, observationally grounded constraint of these process sensitivities and the resulting material narrowing of the scenario-conditional 2100 CO2 projection range.

Background

Coupled carbon–climate projection. Projections of future atmospheric CO2 and climate require models that link anthropogenic emissions, atmospheric chemistry and radiative forcing, ocean and land carbon uptake, and vegetation dynamics. Joos et al. (2001) built such a system by coupling the LPJ dynamic global vegetation model (DGVM — a model that simulates vegetation structure, productivity, and soil carbon pools dynamically rather than prescribing a fixed vegetation map) to a computationally efficient climate–carbon model, using a 'substitute climate' derived by pattern-scaling the response of the ECHAM3/LSG coupled atmosphere–ocean general circulation model (AOGCM, a three-dimensional model of atmospheric and ocean circulation). They drove it with the six 'marker' scenarios of the IPCC Special Report on Emission Scenarios (SRES), which assume no new climate-policy interventions, and projected CO2 to rise to 540 ppm (lowest scenario) to 960 ppm (highest scenario) by 2100, with radiative forcing increasing by 3–8 W m−2. Their headline uncertainty statement was that projected CO2 at 2100 is uncertain by −10% to +30% for a given emission scenario because of uncertainties in modeled land and ocean processes.

The six acknowledged uncertainties. (1) Land-use change was not simulated dynamically: land-use carbon fluxes were prescribed externally because gridded future land-use patterns were unavailable, an approximation the authors noted probably overestimates terrestrial carbon storage. (2) The coupling was one-way: simulated land-surface changes did not feed back on atmospheric circulation or the hydrological cycle. (3) CO2 fertilization — the stimulation of plant photosynthesis and water-use efficiency by rising atmospheric CO2, which most models make the dominant cause of the current and future land carbon sink — had (and has) no direct proof of its global strength or saturation behavior (whether the stimulation weakens or levels off as CO2 rises). (4) The multiannual temperature dependence of soil and litter respiration (the microbial decomposition of dead organic matter, which returns CO2 to the atmosphere) was contested: Giardina and Ryan (2000) argued decomposition rates of organic carbon in mineral soil are essentially temperature-insensitive, while other syntheses found strong warming responses. (5) Radiative forcing by sulfate aerosols (tiny atmospheric particles from sulfur emissions, which cool the climate directly by scattering sunlight and indirectly by altering cloud properties) was highly uncertain, affecting the warming trajectory. (6) Ocean carbon uptake depended on ocean circulation and overturning parameterizations (the surface-to-deep transport of carbonated water), with different transports producing substantial spread in ocean carbon uptake.

The sensitivity experiments that bracket the spread. In the source paper, capping CO2 fertilization at its year-2000 level after 2000 (experiment C) turned the terrestrial biosphere into a carbon source releasing about 130 GtC by 2100, while removing the warming response of soil and litter respiration (experiment R) increased terrestrial carbon storage by 188 GtC by 2100; scaling ocean transport parameters by 0.5 and 1.5 changed cumulative ocean uptake by ±90 GtC (roughly 20%). Combining these gave 2100 CO2 of, for example, 620–920 ppm under scenario A1B — the quantitative basis of the −10% to +30% range.

Later formalism and successors. Cox et al. (2000) had already run the first fully coupled three-dimensional carbon–climate model, showing carbon-cycle feedbacks could accelerate warming. The C4MIP (Coupled Climate–Carbon Cycle Model Intercomparison Project) exercise (Friedlingstein et al., 2006) standardized the analysis of these feedbacks in terms of the carbon–concentration feedback parameter β (how much carbon storage increases per ppm rise of CO2) and the carbon–climate feedback parameter γ (how much carbon storage changes per degree of warming). Free-air CO2 enrichment (FACE) experiments — open-air field experiments in which entire ecosystems are exposed to elevated CO2 — provide the main experimental evidence on fertilization (Norby et al., 2005; Hickler et al., 2008). Emergent constraints (observable present-day quantities statistically related, across a model ensemble, to an uncertain future projection) have been used to narrow fertilization estimates (Wenzel et al., 2016). Land-use harmonization data sets (Hurtt et al., 2011) supplied gridded land-use transitions for coupled models. In the latest phase of the Coupled Model Intercomparison Project (CMIP6, the current standard multi-model protocol for climate projections; Arora et al., 2020), the spread in land feedback parameters still dominates the carbon-cycle spread, and the IPCC Sixth Assessment Report (Canadell et al., 2021) assesses the current status of all these threads.

Problem Statement

For a given anthropogenic emission scenario, the coupled climate–carbon-cycle projections of Joos et al. (2001) carried a process-driven uncertainty of −10% to +30% in atmospheric CO2 at 2100, attributed by the paper to uncertainties in modeled land and ocean processes within a broader set of six acknowledged limitations. Later literature has closed the three structural threads (dynamic land use, two-way land–atmosphere coupling, ocean transport) and narrowed, but not resolved, the others. The surviving problem, stated at the source's own objective, is: to what extent can the remaining process sensitivities — chiefly the strength and saturation behavior of global CO2 fertilization of plant productivity and the multiannual temperature dependence of soil and litter organic matter decomposition, together with the magnitude of aerosol radiative forcing — be constrained by observational, experimental, and modeling evidence, so that the intermodel spread in projected land carbon storage is reduced relative to the current CMIP6 baseline (land carbon–concentration feedback 0.97 ± 0.40 PgC ppm−1, land carbon–climate feedback −45.1 ± 50.6 PgC °C−1, cumulative land-sink spread about 150 PgC) and the scenario-conditional uncertainty in projected atmospheric CO2 at 2100 is narrowed below the −10% to +30% range established by Joos et al. (2001)? The sensitivities at issue are the global, century-scale ones that the source paper's sensitivity experiments identified as the dominant land terms (the strength and saturation of CO2 fertilization and the temperature dependence of soil and litter respiration), together with the surviving open aerosol-forcing term, rather than regional detail; constraining them from observational, experimental, and modeling evidence, and tracing their propagation to the projected 2100 CO2 range, would narrow the scenario-conditional uncertainty.

References

  1. Fortunat Joos, I. Colin Prentice, Stephen Sitch, Robert Meyer, Georg Hooss, Gian-Kasper Plattner, Stefan Gerber, Klaus Hasselmann (2001). Global warming feedbacks on terrestrial carbon uptake under the Intergovernmental Panel on Climate Change (IPCC) Emission Scenarios. Global Biogeochemical Cycles 15(4), 891–907. doi:10.1029/2000GB001375, https://doi.org/10.1029/2000GB001375
  2. Peter M. Cox, Richard A. Betts, Chris D. Jones, Steven A. Spall, Ian J. Totterdell (2000). Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature 408, 184–187. doi:10.1038/35041539, https://doi.org/10.1038/35041539
  3. Christian P. Giardina, Michael G. Ryan (2000). Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature. Nature 404, 858–861. doi:10.1038/35009076, https://doi.org/10.1038/35009076
  4. P. Friedlingstein, P. Cox, R. Betts, L. Bopp, W. von Bloh, V. Brovkin, P. Cadule, S. Doney, M. Eby, I. Fung, G. Bala, J. John, C. Jones, F. Joos, T. Kato, M. Kawamiya, W. Knorr, K. Lindsay, H. D. Matthews, T. Raddatz, P. Rayner, C. Reick, E. Roeckner, K.-G. Schnitzler, R. Schnur, K. Strassmann, A. J. Weaver, C. Yoshikawa, N. Zeng (2006). Climate–Carbon Cycle Feedback Analysis: Results from the C4MIP Model Intercomparison. Journal of Climate 19(14), 3337–3353. doi:10.1175/JCLI3800.1, https://doi.org/10.1175/JCLI3800.1
  5. Richard J. Norby, Evan H. DeLucia, Birgit Gielen, Carlo Calfapietra, Christian P. Giardina, John S. King, Joanne Ledford, Heather R. McCarthy, David J. P. Moore, Reinhart Ceulemans, Paolo De Angelis, Adrien C. Finzi, David F. Karnosky, Mark E. Kubiske, Martin Lukac, Kurt S. Pregitzer, Giuseppe E. Scarascia-Mugnozza, William H. Schlesinger, Ram Oren (2005). Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102(50), 18052–18056. doi:10.1073/pnas.0509478102, https://doi.org/10.1073/pnas.0509478102
  6. W. Knorr, I. Colin Prentice, J. I. House, E. A. Holland (2005). Long-term sensitivity of soil carbon turnover to warming. Nature 433, 298–301. doi:10.1038/nature03226, https://doi.org/10.1038/nature03226
  7. Thomas Hickler, Benjamin Smith, I. Colin Prentice, Kristina Mjöfors, Paul Miller, Almut Arneth, Martin T. Sykes (2008). CO2 fertilization in temperate FACE experiments not representative of boreal and tropical forests. Global Change Biology 14(7), 1531–1542. doi:10.1111/j.1365-2486.2008.01598.x, https://doi.org/10.1111/j.1365-2486.2008.01598.x
  8. G. C. Hurtt, L. P. Chini, S. Frolking, R. A. Betts, J. Feddema, G. Fischer, J. P. Fisk, K. Hibbard, R. A. Houghton, A. Janetos, C. D. Jones, G. Kindermann, T. Kinoshita, Kees Klein Goldewijk, K. Riahi, E. Shevliakova, S. Smith, E. Stehfest, A. Thomson, P. Thornton, D. P. van Vuuren, Y. P. Wang (2011). Harmonization of land-use scenarios for the period 1500–2100: 600 years of global gridded annual land-use transitions, wood harvest, and resulting secondary lands. Climatic Change 109, 117–161. doi:10.1007/s10584-011-0153-2, https://doi.org/10.1007/s10584-011-0153-2
  9. Sabrina Wenzel, Peter M. Cox, Veronika Eyring, Pierre Friedlingstein (2016). Projected land photosynthesis constrained by changes in the seasonal cycle of atmospheric CO2. Nature 538, 499–501. doi:10.1038/nature19772, https://doi.org/10.1038/nature19772
  10. T. W. Crowther, K. E. O. Todd-Brown, C. W. Rowe, W. R. Wieder, J. C. Carey, M. B. Machmuller, B. L. Snoek, S. Fang, G. Zhou, S. D. Allison, J. M. Blair, S. D. Bridgham, A. J. Burton, Y. Carrillo, P. B. Reich, J. S. Clark, A. T. Classen, F. A. Dijkstra, B. Elberling, B. A. Emmett, M. Estiarte, S. D. Frey, J. Guo, J. Harte, L. Jiang, B. R. Johnson, G. Kröel-Dulay, K. S. Larsen, H. Laudon, J. M. Lavallee, Y. Luo, M. Lupascu, L. N. Ma, S. Marhan, A. Michelsen, J. Mohan, S. Niu, E. Pendall, J. Peñuelas, L. Pfeifer-Meister, C. Poll, S. Reinsch, L. L. Reynolds, I. K. Schmidt, S. Sistla, N. W. Sokol, P. H. Templer, K. K. Treseder, J. M. Welker, M. A. Bradford (2016). Quantifying global soil carbon losses in response to warming. Nature 540, 104–108. doi:10.1038/nature20150, https://doi.org/10.1038/nature20150
  11. J. M. Melillo, S. D. Frey, K. M. DeAngelis, W. J. Werner, M. J. Bernard, F. P. Bowles, G. Pold, M. A. Knorr, A. S. Grandy (2017). Long-term pattern and magnitude of soil carbon feedback to the climate system in a warming world. Science 358(6359), 101–105. doi:10.1126/science.aan2874, https://doi.org/10.1126/science.aan2874
  12. César Terrer, Robert B. Jackson, I. Colin Prentice, Trevor F. Keenan, Christina Kaiser, Sara Vicca, Joshua B. Fisher, Peter B. Reich, Benjamin D. Stocker, Bruce A. Hungate, Josep Peñuelas, Ian McCallum, Nadejda A. Soudzilovskaia, Lucas A. Cernusak, Alan F. Talhelm, Kevin Van Sundert, Shilong Piao, Paul C. D. Newton, Mark J. Hovenden, Dana M. Blumenthal, Yi Y. Liu, Christoph Müller, Klaus Winter, Christopher B. Field, Wolfgang Viechtbauer, Caspar J. Van Lissa, Marcel R. Hoosbeek, Makoto Watanabe, Takayoshi Koike, Victor O. Leshyk, H. Wayne Polley, Oskar Franklin (2019). Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nature Climate Change 9, 684–689. doi:10.1038/s41558-019-0545-2, https://doi.org/10.1038/s41558-019-0545-2
  13. Vivek K. Arora, Anna Katavouta, Richard G. Williams, Chris D. Jones, Victor Brovkin, Pierre Friedlingstein, Jörg Schwinger, Laurent Bopp, Olivier Boucher, Patricia Cadule, Matthew A. Chamberlain, James R. Christian, Christine Delire, Rosie A. Fisher, Tomohiro Hajima, Tatiana Ilyina, Emilie Joetzjer, Michio Kawamiya, Charles D. Koven, John P. Krasting, Rachel M. Law, David M. Lawrence, Andrew Lenton, Keith Lindsay, Julia Pongratz, Thomas Raddatz, Roland Séférian, Kaoru Tachiiri, Jerry F. Tjiputra, Andy Wiltshire, Tongwen Wu, Tilo Ziehn (2020). Carbon–concentration and carbon–climate feedbacks in CMIP6 models and their comparison to CMIP5 models. Biogeosciences 17, 4173–4222. doi:10.5194/bg-17-4173-2020, https://doi.org/10.5194/bg-17-4173-2020
  14. N. Bellouin, J. Quaas, E. Gryspeerdt, S. Kinne, P. Stier, D. Watson-Parris, O. Boucher, K. S. Carslaw, M. Christensen, A.-L. Daniau, J.-L. Dufresne, G. Feingold, S. Fiedler, P. Forster, A. Gettelman, J. M. Haywood, U. Lohmann, F. Malavelle, T. Mauritsen, D. T. McCoy, G. Myhre, J. Mülmenstädt, D. Neubauer, A. Possner, M. Rugenstein, Y. Sato, M. Schulz, S. E. Schwartz, O. Sourdeval, T. Storelvmo, V. Toll, D. Winker, B. Stevens (2020). Bounding Global Aerosol Radiative Forcing of Climate Change. Reviews of Geophysics 58, e2019RG000660. doi:10.1029/2019RG000660, https://doi.org/10.1029/2019RG000660
  15. Josep G. Canadell, Pedro M. S. Monteiro, Marcos H. Costa, Leticia Cotrim da Cunha, Peter M. Cox, Alexey V. Eliseev, Stephanie Henson, Masao Ishii, Samuel Jaccard, Charles Koven, Annalea Lohila, Prabir K. Patra, Shilong Piao, Joeri Rogelj, Stephen Syampungani, Sönke Zaehle, Kirsten Zickfeld (2021). Global Carbon and Other Biogeochemical Cycles and Feedbacks. Chapter 5 in Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press. doi:10.1017/9781009157896.007, https://doi.org/10.1017/9781009157896.007
  16. Ryan S. Padrón, Lukas Gudmundsson, Laibao Liu, Vincent Humphrey, Sonia I. Seneviratne (2022). Drivers of intermodel uncertainty in land carbon sink projections. Biogeosciences 19, 5435–5448. doi:10.5194/bg-19-5435-2022, https://doi.org/10.5194/bg-19-5435-2022