Climate Engineering on Land: Comprehensive evaluation of Earth system impacts of terrestrial carbon dioxide removal (CE-LAND+)

• Applicants: Professor Dr. Dieter Gerten; Professorin Dr. Julia Pongratz
• Subject Area: Physical Geography ; Atmospheric Science
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 236906451

• Final Report Year: 2020

Final Report Abstract

Based on comprehensive global simulations with two global dynamic vegetation and Earth system models, the CE-LAND(+) project has quantified potentials, biogeophysical side-effects and trade-offs with food and water availability of terrestrial carbon dioxide removal (tCDR) via reforestation/afforestation and biomass plantations. Carbon uptake potentials of re-/afforestation were found to be 215 GtC (2 Gt per year) by 2100, reducing atmospheric CO2 levels by 85 ppm. These estimates, derived for a high-CO2 world, substantially revised up earlier studies that had not considered beneficial effects of increased CO2 levels on forest growth. The assumed re-/afforestation would lower global mean temperature rise by 0.3C in 2100, with large reductions in temperature extremes found for densely populated areas, hinting to potential beneficial side-effects for local adaptation. Planting herbaceous biomass plantations instead of reforesting was simulated to bring CO2 levels down by 47 to 177 ppm for fossil-fuel substitution levels of 0 and 100%, respectively. For the high-end estimates of potentials, biomass plantations become more effective per unit area than forests within 30 years in most regions of the globe. Both scenarios, however, assumed a high price for CO2, which made agricultural intensification profitable, which in turn freed up agricultural areas for tCDR. The results emphasise that tCDR potentials need to be put into the perspective of trade-offs with land (and water) requirements for agriculture or nature conservation. In further systematic trade-off studies for biomass plantations we found low to moderate potentials for achieving negative emissions that would be required for mitigating global warming or balancing excess fossil fuel emissions. These potentials are constrained by environmental side-effects and technological efficiencies analysed. For example, achieving tCDR volumes of 160–190 GtC within this century via dedicated biomass plantations, needed to complement strong mitigation keeping global warming below 2C, would only be possible by means of extensive irrigation and highly efficient conversion to stored carbon. In more in-depth analyses and comprehensive literature reviews we demonstrate that some scenarios imply global freshwater requirements in the order of current agricultural water use, suggesting massive additional pressure on freshwater systems. Under the assumption that planetary environmental boundaries (with respect to freshwater and nitrogen cycling as well as biodiversity) were to be maintained worldwide, we even find that only minimal negative emissions could be achieved through tCDR with biomass plantations. A collective conclusion of our studies is that due to the severe trade-offs with society and the biosphere, tCDR is no viable alternative to drastic greenhouse gas emissions reductions but could still support climate mitigation or climate engineering if sustainably deployed, and also provide benefits for the adaptation potential on the local scale. Project results were published in high-level journals and also recognized in the international press.

Publications

• Climate change reduces warming potential of nitrous oxide by an enhanced brewer-dobson circulation. Geophysical Research Letters, 43 (11):5851–5859, 2016
  D. Kracher, C. H. Reick, E. Manzini, M. G. Schultz, and O. Stein
  
• Collateral transgression of planetary boundaries due to climate engineering by terrestrial carbon dioxide removal. Earth System Dynamics, 7(4):783, 2016
  V. Heck, J. F. Donges, and W. Lucht
  
• Impacts devalue the potential of large-scale terrestrial CO2 removal through biomass plantations. Environmental Research Letters, 11(9):095010, 2016
  L. R. Boysen, W. Lucht, D. Gerten, and V. Heck
  
• Is extensive terrestrial carbon dioxide removal a ‘green’ form of geoengineering? a global modelling study. Global and Planetary Change, 137:123 – 130, 2016
  V. Heck, D. Gerten, W. Lucht, and L. R. Boysen
  
• Reforestation in a high-CO2 world—higher mitigation potential than expected, lower adaptation potential than hoped for. Geophysical Research Letters, 43 (12):2016GL068824, 2016
  S. Sonntag, J. Pongratz, C. H. Reick, and H. Schmidt
  
• Nitrogen-related constraints of carbon uptake by large-scale forest expansion: Simulation study for climate change and management scenarios. Earth’s Future, 5(11):1102–1118, 2017
  D. Kracher
  
• Potentials and Side-Effects of Herbaceous Biomass Plantations for Climate Change Mitigation. PhD thesis, Universität Hamburg, Geowissenschaften, 2017
  D. Mayer
  
• The limits to global-warming mitigation by terrestrial carbon removal. Earth’s Future, 5(5):463–474, 2017
  L. R. Boysen, W. Lucht, D. Gerten, V. Heck, T. M. Lenton, and H. J. Schellnhuber
  
• Trade-offs for food production, nature conservation and climate limit the terrestrial carbon dioxide removal potential. Global Change Biology, 23(10):4303–4317, 2017
  L. R. Boysen, W. Lucht, and D. Gerten
  
• Biogeochemical potential of biomass pyrolysis systems for limiting global warming to 1.5◦C. Environmental Research Letters, 13(4):044036, 2018
  C. Werner, H.-P. Schmidt, D. Gerten, W. Lucht, and C. Kammann
  
• Biomass-based negative emissions difficult to reconcile with planetary boundaries. Nature Climate Change, 8(2):151–155, 2018
  V. Heck, D. Gerten, W. Lucht, and A. Popp
  
• Land use options for staying within the planetary boundaries–synergies and trade-offs between global and local sustainability goals. Global Environmental Change, 49:73–84, 2018
  V. Heck, H. Hoff, S. Wirsenius, C. Meyer, and H. Kreft
  
• Quantifying and comparing effects of climate engineering methods on the earth system. Earth’s Future, 6(2):149–168, 2018
  S. Sonntag, M. Ferrer González, T. Ilyina, D. Kracher, J. E. M. S. Nabel, U. Niemeier, J. Pongratz, C. H. Reick, and H. Schmidt
  
• Freshwater requirements of large-scale bioenergy plantations for limiting global warming to 1.5◦C. Environmental Research Letters, 14(8):084001, 2019
  F. Stenzel, D. Gerten, C. Werner, and J. Jägermeyr
  
• Global scenarios of irrigation water use for bioenergy production: a systematic review. Hydrology and Earth System Sciences Discussions, 2020:1–24, 2020
  F. Stenzel, D. Gerten, and N. Hanasaki