Shipping emissions: From cooling to warming of climate – and reducing impacts on health

International shipping has been a fast growing sector of the global economy and its share of total anthropogenic emissions is significant, having effects on climate, air quality, and human health. The nature of the contribution to climate change is complex: In addition to warming by CO2 emissions, ship emissions of sulfur dioxide (SO2) cause cooling through effects on atmospheric particles and clouds, while nitrogen oxides (NOx) increase the levels of the greenhouse gas (GHG) ozone (O3) and reduce the GHG methane (CH4), causing warming and cooling, respectively. The result is a net global mean radiative forcing (RF) from the shipping sector that is strongly negative, leading to a global cooling effect today. However, new regulations of SO2 and NOx, while reducing air pollution and its harmful effects on health and water/soil acidification, will reduce this sector’s cooling effects. Given these reductions, shipping will, relative to present-day impacts, impart a “double warming” effect: one from CO2, and one from the reduction of SO2. Therefore, after some decades the net climate effect of shipping will shift from cooling to warming