Maarten van den Berg

Alternative pathways to the 1.5 °C target reduce the need for negative emission technologies

Mitigation scenarios that achieve the ambitious targets included in the Paris Agreement typically rely on greenhouse gas emission reductions combined with net carbon dioxide removal (CDR) from the atmosphere, mostly accomplished through large-scale application of bioenergy with carbon capture and storage, and afforestation. However, CDR strategies face several difficulties such as reliance on underground CO2 storage and competition for land with food production and biodiversity protection. The question arises whether alternative deep mitigation pathways exist. Here, using an integrated assessment model, we explore the impact of alternative pathways that include lifestyle change, additional reduction of non-CO2 greenhouse gases and more rapid electrification of energy demand based on renewable energy. Although these alternatives also face specific difficulties, they are found to significantly reduce the need for CDR, but not fully eliminate it. The alternatives offer a means to diversify transition pathways to meet the Paris Agreement targets, while simultaneously benefiting other sustainability goals.Scenarios that constrain warming to 1.5 °C currently place a large emphasis on CO2 removal. Alternative pathways involving lifestyle change, rapid electrification and reduction of non-CO2 gases could reduce the need for such negative emission technologies.

Impact of the choice of emission metric on greenhouse gas abatement and costs

This paper analyses the effect of different emission metrics and metric values on timing and costs of greenhouse gas mitigation in least-cost emission pathways aimed at a forcing level of 3.5 W m?2 in 2100. Such an assessment is currently relevant in view of UNFCCC?s decision to replace the values currently used. An emission metric determines the relative weights of non-CO2 greenhouse gases in obtaining CO2-equivalent emissions. For the first commitment period of the Kyoto Protocol, the UNFCCC has used 100 year global warming potential (GWP) values as reported in IPCC?s Second Assessment Report. For the second commitment period, the UNFCCC has decided to use 100 year GWP values from IPCC?s Fourth Assessment Report. We find that such a change has only a minor impact on (the optimal timing of) global emission reductions and costs. However, using 20 year or 500 year GWPs to value non-CO2 greenhouse gases does result in a significant change in both costs and emission reductions in our model. CO2 reductions are favored over non-CO2 gases when the time horizon of the GWPs is increased. Application of GWPs with time horizons longer than 100 year can increase abatement costs substantially, by about 20% for 500 year GWPs. Surprisingly, we find that implementation of a metric based on a time-dependent global temperature potential does not necessary lead to lower abatement costs. The crucial factor here is how fast non-CO2 emissions can be reduced; if this is limited, the delay in reducing methane emissions cannot be (fully) compensated for later in the century, which increases total abatement costs.

The impact of technology availability on the timing and costs of emission reductions for achieving long-term climate targets

While most long-term mitigation scenario studies build on a broad portfolio of mitigation technologies, there is quite some uncertainty about the availability and reduction potential of these technologies. This study explores the impacts of technology limitations on greenhouse gas emission reductions using the integrated model IMAGE. It shows that the required short-term emission reductions to achieve long-term radiative forcing targets strongly depend on assumptions on the availability and potential of mitigation technologies. Limited availability of mitigation technologies which are relatively important in the long run implies that lower short-term emission levels are required. For instance, limited bio-energy availability reduces the optimal 2020 emission level by more than 4 GtCO2eq in order to compensate the reduced availability of negative emissions from bioenergy and carbon capture and storage (BECCS) in the long run. On the other hand, reduced mitigation potential of options that are used in 2020 can also lead to a higher optimal level for 2020 emissions. The results also show the critical role of BECCS for achieving low radiative forcing targets in IMAGE. Without these technologies achieving these targets become much more expensive or even infeasible.