GHG emission pathways until 2300 for the 1.5 A°C temperature rise target and the mitigation costs achieving the pathways

The Paris Agreement of the 21st Conference of the Parties of the United Nations Framework Convention on Climate Change refers to the 1.5 A degrees C target as well as the 2 A degrees C target, and it is important to estimate the emission pathways and mitigation measures for the 1.5 A degrees C target for the discussions on the target. The possible emission pathways vary widely because of the uncertainties involved. We assumed three kinds of temperature trajectories for meeting below 1.5 A degrees C compared with the pre-industrial level, and three numbers for the climate sensitivity. The first trajectory remains below 1.5 A degrees C all the time until 2300, the second overshoots but returns to below 1.5 A degrees C by 2100, and the third overshoots but returns to below 1.5 A degrees C by 2300. There are large differences in terms of 2030 emissions between the estimate from the submitted Nationally Determined Contributions (NDCs) and any of assessed emission pathways involving climate sensitivity of 3.0 A degrees C or higher, and high emission reduction costs were estimated, even for 2030. With climate sensitivity of 2.5 A degrees C, only the third trajectory exhibits consistent emissions in 2030 with the NDCs. However, this case also appears very difficult to achieve, requiring enormous amounts of negative emissions after the middle of this century toward 2300. A climate mitigation strategy aiming for the 1.5 A degrees C target will be debatable, because we face serious difficulties in near- or/and long-term for all the possible emission pathways, and therefore, we should rather focus on actual emission reduction activities than on the 1.5 A degrees C target with poor feasibility.