Effect of CDR on emissions and mitigation costs
All climate policies lead to substantial emissions reductions relative to reference emissions (Fig. 2) indicating the large mitigation potential in the energy system particularly from reducing the use of coal (see Fig. S5). The reference scenario represents a world without climate policy, where emissions from fossil fuel combustion increase substantially until 2060, followed by a decline thereafter. This is due to increasing fossil fuel prices with cumulative extraction, and decreasing costs of learning technologies such as wind and solar power, so that renewable energies become competitive at the end of the century even without carbon pricing.
The additional gain in emissions reductions from strengthening the carbon tax scenarios diminishes as a result of the non-linear increase of marginal abatement costs in the energy sector. Achieving the 450 ppm CO2e stabilization target requires the strongest near term emissions reductions given the long lifetime of CO2 in the atmosphereFootnote 1.
The availability of CDR in the three tax scenarios leads to lower emissions and slower diminishing returns from an increase of the carbon tax. This shows the additional flexibility that BECCS adds to the portfolio of mitigation options in the energy sector. The situation is markedly different for a climate stabilization target (450 ppm) which largely determines the cumulative emissions budget. It leads to cumulated Kyoto gas emissions of 2000–2100 GtCO2e for the period 2005–2100. The CO2 emissions trajectories with and without BECCS diverge from each other after 2070. In the absence of BECCS, higher reductions in CH4 and N2O emissions partly compensate for the higher long-term CO2 emissions (cf. van Vuuren and Riahi 2011). Cumulative CO2 emissions from 2005–2100 amount to 1,160 GtCO2, about 8 % higher than in the case with BECCS. The difference in CO2 emissions between the two cases shows that higher long term emissions are also compensated by lower short term emissions in the absence of BECCS. The additional up and down of the emissions difference in the intermediate period is due to a switch in the use of BECCS technologies, in particular the phasing in and out of BioIGCCs with CCS during 2020–2060 (see Fig. 6). We note that this particular result is sensitive to the consideration of BECCS technologies and their capture rates.
The variation of the mitigation costs across the tax and 450 ppm scenarios shows a reverse image of the emissions results (Fig. 3). In the tax scenarios, the availability of the BECCS option results in higher mitigation costs, because at a given carbon price greater emission reductions are performed. The situation is substantially different for the 450 ppm stabilization target. The mitigation costs more than double without BECCS. The doubling arises from strongly increasing mitigation costs for generations in the more distant future. Costs for the period 2070–2090 quadruple without carbon dioxide removal. If BECCS is not available, very costly mitigation options need to be tapped in the 2nd half of the 21st century to achieve near zero emissions in order to stay below the climate target (see next section). Thus, the flexibility provided by CDR eases the high costs for future generations and balances the inter-generational distribution of costs in the 450 ppm scenario.
Effect of CDR on sectoral mitigation efforts
The availability of CDR has significant implications for the mitigation effort in different end use sectors (Fig. 4). As mentioned above, a small carbon price signal already brings large emissions reductions in all sectors, indicating a range of low cost mitigation options unrelated to CDR. The magnitude of the effect is to a certain extent baseline dependent, and is dominated by the large coal use in the REMIND reference scenario.
The electricity sector reacts most strongly to the increasing carbon price until mid-century, while the transportation sector is hardest to decarbonize beyond the utilization of low cost options, particularly in the short to mid-term. This can also be seen when switching off BECCS. Transport emissions in the tax cases are markedly higher. For the 450 ppm case, transport emissions are reduced by relying on transport demand reductions and non-BECCS technologies at significantly higher costs. Negative emissions in the transport and non-electric stationary sector are obtained from the use of hydrogen produced from biomass with CCS. Additional negative emissions in the transport sector are generated by liquefaction of biomass with CCS. While the electricity sector carries the bulk of near to midterm emissions reductions with BECCS, its reductions are much more gradual in the absence of BECCS and do no longer increase with increasing carbon prices beyond a $30 tax path. This indicates that the lifetime of existing fossil fuel infrastructure limits the emissions reductions in this sector (the model does not allow for early retirement).
Figure 5 analyses the breakdown of emissions reductions on individual sectors and BECCS. It can be seen that BECCS is not dominating the mitigation portfolio (the larger part is direct sector mitigation), but plays a crucial role in adjusting the mitigation requirements on sectors to increase efficiency. A comparison of the cases with and without BECCS reveals that BECCS is used to compensate emissions from the stationary non-electric and transport sectors. In the absence of BECCS, a larger amount of near to mid-term emissions reductions comes from non-electric energy use in the industry, residential and commercial sectors. In the 450 ppm CO2e scenario where the mitigation effort cannot be relaxed due to the absence of BECCS, the transport sector also has to carry a larger mitigation effort. The additional burden on the transport sector, much of which realized through the reduction of transport energy demand (45 % w.r.t. the case with BECCS and even 65 % w.r.t. to the reference case), is the main driver for the high cost increases in the long run. We note, however, that electrification of transport is another significant option to reduce emissions from the transport sector which can reduce the need for BECCS to compensate transport emissions. This option is not modeled in the present analysis, although factor substitution of non-electric energy with electricity is accounted for.
Effect of BECCS on the technology portfolio
The availability of BECCS has a profound impact on the energy technology portfolio in the climate policy scenarios. BECCS is utilized in all climate policy scenarios reaching a similar share of ca. 22 % of primary energy supply in 2100. There is only a slow expansion of BECCS in the low tax scenario, but high carbon price signals in the short term lead to rapid expansion. The primary energy share of BECCS increases from 5 % in 2020 to 16 % in 2030 by roughly 80 EJ in the 450 ppm CO2e scenario.
The mix of deployed BECCS technologies strongly depends on the stringency of mitigation policy (Fig. 6). The two BECCS technologies with the highest capture rates (BioIGCC and hydrogen production) dominate the use of bioenergy in the $30 and $50 tax cases and the 450 ppm CO2e scenario. The ReMIND model shows a distinct shift from BioIGCC to Bio-hydrogen production in the second half of the century due to the use of the hydrogen in sectors that are more difficult to decarbonize than the electricity sector, and because of the slightly higher capture rate (90 % in B2H2 vs 80 % in BioIGCC). In the case of carbon tax policies, BECCS displaces some of the CCS at fossil fuel power plants due to the trade-off induced by the flow constraint on annual sequestration. The opposite result is obtained for imposing a stringent climate stabilization target. Deployment of fossil fuel CCS increases significantly if BECCS is available. This result depends partly on our assumption that up to 10 % of the oxidized carbon in fossil fuel power plants is not captured, but released to the atmosphere (see Table S1). The residual emissions from fossil fuel use with CCS are large enough to make this option unattractive under a 450 ppm CO2e target without BECCS. This result would no longer be valid, if the capture rate of CCS was increased over time to eliminate residual emissions. More generally, the availability of BECCS helps to prolong the use of fossil fuels in the energy sector, which also makes fossil fuel CCS more attractive.
In all climate policy cases, biomass use is somewhat larger if BECCS is available, but remains significant in the absence of BECCS. Without BECCS, biomass is used primarily for the production of biofuels instead of electricity (Fig. S5, left panel). The reduction of bio-electricity in the absence of BECCS (Fig. S5, right panel) is mostly compensated by other renewable power, while nuclear power does not expand significantly beyond its utilization in the reference scenario due to a uranium constraint. In the carbon tax cases, compensation is also provided by an increased deployment of fossil fuel combustion with CCS. The situation is markedly different in the 450 ppm CO2e scenario. In the absence of BECCS, the electricity sector not only has to compensate the re-direction of bioenergy, but also the reduction of fossil fuel CCS. The sector responds by a large increase in the deployment of solar energy, which pushes up costs.