Updated Study on Greenhouse Gas Emissions from Ships: Climate Impacts

Abstract

Increases in well-mixed greenhouse-gases such as carbon dioxide lead to positive radiative forcing (RF) and to global warming. CO2 remains in the atmosphere for a long time and will continue to have a warming effect long after its emission. Therefore, emissions data from ships starting as early as 1870 has been used when calculating the RF of shipping CO2 emissions. Since the historic data does not distinguish between international and domestic shipping, RF calculations are based on total shipping emissions rather than international shipping only. In order to calculate the RF from shipping we use a linear climate response model to calculate the contribution of CO2 emissions to marginal CO2 concentrations and the consequential radiative forcing. This model takes emission rates, calculates the resultant atmospheric concentrations of CO2 and then the RF which arises from changes in CO2 concentration. The RF from shipping CO2 for 2005 was calculated to be 46 mW m-2, contributing approximately 2.8% to the total anthropogenic CO2 RF. For comparison, aviation has a similar – if slightly smaller – present day annual emission rate (733 Tg CO2, 2005) but the RF is only 28 mW m-2. The somewhat larger forcing from shipping in this comparison may be easily explained by both the residence time of CO2 in the atmosphere and the time period of the activity.