Shipping Emissions: From Cooling to Warming of Climate—and Reducing Impacts on Health

Abstract

International shipping has been a fast growing sector of the
\nglobal economy and its share of total anthropogenic emissions
\nis significant, having effects on climate, air quality, and human
\nhealth. The nature of the contribution to climate change is
\ncomplex: In addition to warming by CO2 emissions, ship
\nemissions 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.
\nGiven these reductions, shipping will, relative to present-day
\nimpacts, impart a “double warming” effect: one from CO2, and
\none from the reduction of SO2. Therefore, after some decades
\nthe net climate effect of shipping will shift from cooling to
\nwarming.