Trade-offs between Black Carbon and CO2 - Building a model to analyze the importance of black carbon abatement for cumulative emissions of CO2 compatible with a 2°C climate target

In addition to the greenhouse gases that cause radiative forcing (RF), the global climate is also affected by particles that absorb or reflect light. Black carbon aerosols (BC) are the most important anthropogenic aerosols that enhance global warming. However, the contribution to RF from BC is considerably more uncertain than the RF of the well-mixed greenhouse gases that are present in the international climate policy discussions. The atmospheric lifetime of BC is only a few days, which suggests that reductions in emissions of BC would give a fast climatic response although BC also has indirect effects when it is deposited. It changes the albedo of snow or ice when deposited on for example Himalayan glaciers. The magnitude of the RF from BC is considerable, central estimates of the current black carbon global average radiative forcing lie in the range 0.1-0.8 W/m2. For this reason and the sizeable costs of reducing CO2 emissions there is a policy interest in reducing BC emissions. However, comparing benefits of reducing BC to reducing CO2 is a difficult task since there are many different types of aspects to the trade-off. In this thesis a reduced complexity coupled carbon cycle climate model, constructed primarily for studying the climatic impact of BC and such possible trade-offs, is presented. The model, which is called Physical Prediction Model for Future Radiative forcing and Temperature (PPM-FRT), includes virtually all of the anthropogenic climate forcers and the subsequent global annual average temperature change. In order to find policy relevant questions to ask and experiments to conduct, a series of different rounds of experiments were conducted. The final set of experiments investigate the effect on CO2 emissions pathways compatible with a certain climate target of the timing of black carbon emissions reductions start as well as the rate of decline of these emissions. The aspects of the pathway studied are the cumulative CO2 emissions up to 2100 and the rate of CO2 emissions reductions from a certain peak year. The climate target used is defined as a maximal increase of the global average surface temperature to 2°C above the pre-industrial level, a target acknowledged in the Copenhagen Accord signed by 138 countries in January 2010. For mean values of climate sensitivity and BC RF we find that the difference between zero and rapid reductions of BC emissions (red. of 4% per year) corresponds to a difference of between 60 and 140 GtC emitted during the 21st century. The variation in this result depends on the level of coupling between reductions in black carbon and reductions in a frequently co-emitted cooling carbonaceous aerosol called organic carbon (OC). This difference of between 60 and 130 GtC corresponds to a delay of the start year of CO2 emissions reductions by a few years (for a given CO2 emissions decline rate of 2.3% per year), illustrating the tradeoff between BC emission pathways and maximum cumulative CO2 emissions. A few years delay of CO2 emissions reductions represents an opportunity for the world to start taking measures against the single most important driver of the climate change: use of fossil fuels (without carbon capture and storage) and unsustainable land use changes, the two main reasons behind our emissions of carbon dioxide