Fuss, S.

Obersteiner, M.

Reuter, W.

Szolgayova, J.

International Institute for Applied Systems Analysis (IIASA)

The Impact of Carbon Sink Uncertainty on Mitigation Strategies• Optimal composition of a mitigation portfolio given the uncertainty surrounding key variables• Natural sink modelled stochastically and as a function of atmospheric Greenhouse Gas(GHG) concentration accounting for possible climate feedbacks• Analyze different emissions targets and identify best hedging strategies in the technospheregiven the uncertainty in the biosphere with explicit focus on their interrelation• Examine the role of bioenergy with carbon capture and storage (BECCS) when accountingfor natural sinks and the potential climate feedbacksWolf Heinrich Reuter*, Sabine Fuss, Jana Szolgayová, Michael Obersteiner Ecosystems Services and Management Program (ESM), International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria* Corresponding author: Tel: +43 2236 807 518, Fax: +43 2236 807 299, E-mail: reuter@iiasa.ac.at• A global decision maker plans abatement and negative emission technology (NET)deployment bound by a cumulative emission target over the 21st century• Three periods: 1) Short-term (2010-2030), 2) Medium-term (2030-2050), and 3) Long-term(2050-2100)• Select most cost-effective policy plan with two available options: (1) abatement and (2)deployment of NETs consisting of BECCS and direct air capture (DAC). The technologies differin their cost structure and mitigation levels.• CO2 sink modelled similar to Friedlingstein et al. (2006): feedback between atmospheric CO2concentration and the sink, with additional stochastic shocks representing extreme events likeforest fires, pests, etc. (Probability of shock occurrence modelled as being positivelydependent on the cumulative emissions)ObjectivesModel• Bio-energy in combination with carbon capture andstorage• Using biomass to produce bio-energy, then capturingand diverting the CO2 produced duringcombustion/processing into a long-term geologicalstorage facility• CO2 fixation by photosynthesis (i.e. bio-energy undercertain criteria, is considered to be carbon neutral) pluscapture and storage of CO2 from biomass combustion(negative emissions)BECCS• Deployment of BECCS (and DAC) very much dependent on sink’s sensitivity to atmosphericCO2 (β)• For high CO2 fertilization effect: NETs are a relatively stable share in the mitigation portfolio,which is equal to the maximum BECCS potential.• For a lower effect: amount of CO2 abated with NETs increases, i.e. deployment of DAC startsand increases.• Result as expected because “free” mitigation through uptake by natural sinks decreases andNETs have to make up for the differenceSink Sensitivity to Atmospheric CO2• Even though BECCS is used only in the third period, its potential has direct impact on theoptimal abatement already in the first and second periods.• Increase of 50% in potential of BECCS is reflected in an over 40% (20%) decrease in the first(second) period abatement.• If BECCS potentials are higher than in our baseline the present value (PV) of the portfoliocosts decrease relatively little, as we deploy more BECCS.• If BECCS potential is lower than in the baseline, however, the PV of the costs almost doublesbecause of the increased deployment of DACBECCS potential• Direct Air Capture• Using chemical reactions to remove CO2 directly fromthe air• Currently expensive, i.e. DAC is much higher on theabatement curve than BECCS and will only bedeployed if all other potentials are exploitedDAC• Least stringent cumulative emissions target (2,900 GtCO2): NETs are not part of the optimal portfolio anymore.• Most stringent target (88 GtCO2): in last period maximum amount of BECCS (1,605 GtCO2) plus 1,035 GtCO2 of DAC, as so much abatement in the last period needed that DAC becomes cheaper than marginal unit of abatement• Abatement need in first period similar with different targets (140 GtCO2 for the 88 and 880 GtCO2 targets to only 80 GtCO2 for the 1,500 and 2,900 GtCO2 targets)• The present value (PV) of the portfolio cost increases exponentially with the stricter cumulative emissions targetsCumulative Emission Target• With increasing shock size, more mitigation needs to be carried out to meet the target in the last period in total.• The larger part of this increase is achieved through NETs deployment. As the potential of BECCS is already fully exploited in the baseline case, the increase can here only happen via a deployment in DAC.• With a higher probability of a shock to the sink, this increase gets steeper with the shock sizeShocks to the Carbon SinkSequestrationFeedstockEmissionCapture

The impact of carbon sink uncertainty on mitigation strategies

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