Aviation contributes significantly to anthropogenic climate change by emissions of greenhouse gases, particles and nitrogen oxides as well as by changes of high (cirrus) cloudiness. An important but poorly explored component of the latter are contrail cirrus, comprising the familiar line shaped contrails and the irregularly shaped cirrus clouds that evolve from them. The contrail cirrus radiative forcing (a measure of Earth’s radiative imbalance due to contrail cirrus) has been simulated for the first time with a climate model in the Institute of Atmospheric Physics at DLR Oberpfaffenhofen and published as a research article in Nature Climate Change.
|Net contrail-cirrus radiative forcing and optical depth at 250 hPa pressure level for the year 2002. |
A new method has been developed and used in the ECHAM climate model to represent the physical processes causing the formation, evolution and decay of contrail cirrus. In the model, contrail cirrus are treated as a separate class of man-made clouds interacting with atmospheric moisture and natural clouds. Their spatial coverage, optical properties and radiative forcing are calculated, for which no information was hitherto available. In addition, the study highlights for the first time that contrail cirrus are capable of reducing natural cloud coverage and optical depth by consuming moisture that would otherwise be available for growth and maintenance of the natural clouds. Together, the direct radiative effect of contrail cirrus and the response of natural clouds are termed contrail-induced cloudiness (CIC).
By increasing the cloud coverage, contrail cirrus clouds exert a radiative forcing of 38 mW/m2 in the year 2002. The response of natural cloudiness introduces an offset (with a much larger degree of inherent model uncertainty) of about 20%. The study shows that CIC is the largest component (~31 mW/m2) of aviation-induced radiative forcing. Therefore, the current climate is more strongly warmed by CIC than by all the carbon dioxide emitted by aircraft that accumulated in the atmosphere since the beginning of aviation up to the year 2005 (28 mW/m2).
The results can be used in scientific assessments such as those from the Intergovernmental Panel on Climate Change (IPCC) as well as in studies attempting at reducing the climate impact of aviation. For the latter studies it is important to consider the time scale over which the aviation climate impact is to be reduced, because the short lived perturbations from contrail cirrus compete with the long term effects of carbon dioxide emissions.
In order to judge the overall uncertainties of the new estimates, it is necessary to investigate the climate impact of CIC using results from several independent models, as it is standard practise in the IPCC process. At the same time, more scientific research is required to identify sources of uncertainties in the climate predictions.
The study was performed within the DLR-Project ‚Klimaverträgliches Lufttransportsystem (CATS)’.
Ulrike Burkhardt, Tel. 08153/28-2561
Bernd Kärcher, Tel. 08153/28-1462
Burkhardt U. Kärcher B., 2011: Global radiative forcing from contrail cirrus. Nature Climate Change, 1, 54-58.