Effects of Black Carbon Mitigation on Arctic Climate

Abstract

We use the aerosol-climate model ECHAM-HAMMOZ to assess the effects of black carbon (BC) mitigation measures on Arctic climate. To this end we constructed several mitigation scenarios that implement all currently existing legislation and then implement further reductions of BC in a successively increasing global area, starting from the eight member states of the Arctic Council, expanding to its active observer states, then to all observer states, and finally to the entire globe. These scenarios also account for the reduction of the co-emitted organic carbon (OC) and sulphate (SU). We find that, even though the additional BC emission reductions in the member states of the Arctic Council are small, the resulting reductions in Arctic BC mass burdens can be substantial, especially in the lower atmosphere close to the surface. This in turn means that reducing BC emissions only in the Arctic Council member states can reduce BC deposition in the Arctic by about 30 % compared to the current legislation, which is about 60 % of what could be achieved if emissions were reduced globally. Emission reductions further south affect Arctic BC concentrations at higher altitudes and thus only have small additional effects on BC deposition in the Arctic. The direct radiative forcing scales fairly well with the total amount of BC emission reduction, independent of the location of the emission source, with a maximum direct radiative forcing in the Arctic of about 0.4 W/m 2 for a global BC emission reduction. On the other hand, the Arctic effective radiative forcing due to the BC emission reductions, which accounts for aerosol-cloud interactions, is small compared to the direct aerosol radiative forcing. This happens because BC and OC containing particles can act as cloud condensation nuclei, which affects cloud reflectivity and lifetime, and counter-acts the direct radiative forcing of BC. Additionally the effective radiative forcing is accompanied by very large uncertainties that origin from the strong natural variability of meteorology, cloud cover, and surface albedo in the Arctic. We further used the model TM5-FASST to assess the benefits of the aerosol emission reductions on human health. We found that a full implementation in all Arctic Council member and observer states could reduce the annual global amount of premature deaths by 339 000 by 2030.