The climate impact of ship NO x emissions: an improved estimate accounting for plume chemistry
Nitrogen oxide (NO x) emissions from maritime shipping produce ozone (O 3) and hydroxyl radicals (OH), which in turn destroy methane (CH 4). The balance between this warming (due to O 3) and cooling (due to CH 4) determines the net effect of ship NO x on climate. Previous estimates of the chemical impact and radiative forcing (RF) of ship NO x have generally assumed that plumes of ship exhaust are instantly diluted into model grid cells spanning hundreds of kilometers, even though this is known to produce biased results. Here we improve the parametric representation of exhaust-gas chemistry developed in the GEOS-Chem chemical transport model (CTM) to provide the first estimate of RF from shipping that accounts for sub-grid-scale ship plume chemistry. The CTM now calculates O 3 production and CH 4 loss both within and outside the exhaust plumes and also accounts for the effect of wind speed. With the improved modeling of plumes, ship NO x perturbations are smaller than suggested by the ensemble of past global modeling studies, but if we assume instant dilution of ship NO x on the grid scale, the CTM reproduces previous model results. Our best estimates of the RF components from increasing ship NO x emissions by 1 Tg(N) yr −1 are smaller than that given in the past literature: + 3.4 ± 0.85 mW m −2 (1σ confidence interval) from the short-lived ozone increase, −5.7 ± 1.3 mW m −2 from the CH 4 decrease, and −1.7 ± 0.7 mW m −2 from the long-lived O 3 decrease that accompanies the CH 4 change. The resulting net RF is −4.0 ± 2.0 mW m −2 for emissions of 1 Tg(N) yr −1. Due to non-linearity in O 3 production as a function of background NO x, RF from large changes in ship NO x emissions, such as the increase since preindustrial times, is about 20% larger than this RF value for small marginal emission changes. Using sensitivity tests in one CTM, we quantify sources of uncertainty in the RF components and causes of the ±30% spread in past model results; the main source of uncertainty is the composition of the background atmosphere in the CTM, which is driven by model formulation (±10 to 20%) and the plausible range of anthropogenic emissions (±10%).