Exhausts of industry, cars and aircraft are investigated since a long time because they significantly change the Earth’s atmosphere and the global climate. Only ship emissions remained unregarded for a long time. Since 2004 the impact of ship emissions on atmosphere and climate was investigated by the Helmholtz-University Young Investigators Group SeaKLIM, a cooperative project of the DLR Institute of Atmospheric Physics and the Institute of Environmental Physics of the University of Bremen. Now the final report is available: fine particles (aerosols) that are released into the atmosphere counteract the global warming, but contribute to air pollution.
The results of the SeaKLIM group are indeed surprising. In the year 2000, about 800 million tons of carbon dioxide (CO2), i.e. 2.7 percent of all anthropogenic CO2 emissions originated from ship engines. For nitrogen oxides (NOx) and sulphur dioxides (SO2) the share amounted even to 15 and 8 percent, respectively. Hence, navigation emits CO2 into the atmosphere nearly as much as aviation. The NOx and SO2 emissions of ships exceed those of aircraft by even the tenfold. Effects on the climate are therefore inevitable.
With the help of SCIAMACHY satellite date the SeaKLIM group was the first to determine the contribution of shipping to air pollution by increased nitrogen dioxide concentrations along major shipping routes. Ships contribute to the global warming by emitting the greenhouse gas CO2, but they overcompensate this contribution by the cooling effect of their SO2 emissions today. This is possible because sulphur dioxide and other sulphur compounds react in the atmosphere to form sulphur acid which, together with water, forms very small droplets, so called aerosols. The aerosols in turn reflect sun light back into space. Moreover, they act as nuclei for condensation of water vapour, thus leading to the formation of clouds. In some parts of the world this process can be clearly observed. Satellite images in regions with high ship traffic often show elongated curves of higher reflectance in cloud fields, which are not of natural origin. These so called ‘ship tracks’ are caused by the emissions of ships. Ship tracks are only the visible consequences of ship emissions. Some of them mix with natural clouds which then intensify, other dissolve. The aerosols remain longer in the atmosphere and may change the properties of clouds on larger scales, thus leading to cooling through the processes described above. However, unlike aerosols, CO2 remains in the atmosphere for a long time and thus contributes to global warming long after its emission.
Priv.-Doz. Dr. habil. Veronika Eyring
German Aerospace Center
Institute of Atmospheric Physics
Department "Atmospheric Dynamics"
Tel.: +49 8153 28-2533
Fax: +49 8153 28-1841
Contact for special requests regarding SCIAMACHY measurements:
Dr. Heinrich Bovensmann
University of Bremen
Institute for Environmental Physics
Tel.: +49 421 218-62102
Fax: +49 421 218-4555
SeaKLIM evaluation report: http://www.pa.op.dlr.de/SeaKLIM/SeaKLIM_DLR_EvaluationReport.pdf
SeaKLIM final report: http://www.pa.op.dlr.de/SeaKLIM/SeaKLIM_Nachwuchsgruppe_Schlussbericht_FINAL.pdf