Sensitivity of Aerosol Radiative Effects to Shipping Emissions
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In 2020, The International Maritime Organization (IMO) will enforce significant restrictions on SO2 emissions from global shipping to improve public health. SO2 forms sulfate aerosols, which have a cooling effect on climate. A reduction in sulfate leads to atmospheric warming. In order to explore making this policy climate-neutral, we investigated the sensitivity of radiative forcing to potential restrictions on black carbon (BC) emissions. BC emissions are also closely tied to organic carbon (OC) emissions: reductions of one often leads to reductions of the other. The direct and indirect radiative effects from sulfate, BC, and OC aerosols emitted by ships were calculated using the chemical transport model GEOS-Chem. The TwO-Moment Aerosol Sectional (TOMAS) module was used to calculate particle number and mass of shipping aerosols, and the Rapid Radiative Transfer Model for General Circulation Models (RRTMG) module was then used to calculate the radiative fluxes at the top of the atmosphere. Simulations were performed for the year 2013 with 1-month December 2012 spinup. We investigated 4 different scenarios: the `business-as-usual' (BAU) scenario with no changes to emissions, an `IMO' scenario with 85% less SO2 emissions from ships, an `IMO + BC reduction' scenario with 85% less BC and SO2 emissions from ships, and an `IMO + BC + OC' reduction scenario, with 85% less BC, OC, and SO2. It was determined that the IMO scenario leads to a global average of 45.3 mWm^-2 of increased warming compared to the BAU scenario. The `IMO + BC reduction' leads to 44.0 - 44.5 mWm^-2 of increased warming, suggesting that a BC reduction is not sufficient to offset the warming from the 2020 SO2 restrictions. The `IMO + BC + OC' scenario leads to 53.8 - 54.3 mWm^-2 of increased warming. This is even larger than the warming introduced by the `IMO' scenario and is an undesirable scenario from a climate perspective. In order to make each scenario climate-neutral, CO2 from ships would have to be reduced by 47 to 65% on a 100-year timescale.