A major part of the global climate change since the beginning of the industrial era is caused by human activities that have changed the radiation budget of the planet. This modifies a variety of climate parameters (temperature, precipitation, ice cover etc.) through a complicated chain of effects. Although there are many different polluters such as industry, private households, traffic and agriculture the institute focusses on emissions of the transport sector, in particular air, road and sea traffic, and their effects. In the transport sector carbon dioxide is only one of several influencing components. Other components like methane, nitrous oxides (via ozone), aerosols and clouds (indirectly through aerosols or directly by condensation trails) are of comparable significance. The effects have to be reliably quantified, not only in its entirety but also component by component. The latter is important in assessing the chances to reduce the climate effects, in particular when single effects are partly compensating. This is the case if certain measures, like the avoidance of the condensation trails, are accompanied by increased fuel consumption.
The most suited parameter to compare different influencing components is the radiative forcing, i.e. the influence of a disturbed radiation budget at the upper boundary of the troposphere. It can be determined quite easily and it is related with the expected temperature change near the ground. In the case of a changed concentration of well mixed greenhouse gases (like carbon dioxide, methane and nitrous oxide) the response of the globally averaged surface temperature is proportional to the value of the globally averaged radiative forcing. The proportional factor does not much vary from gas to gas (constant climate sensitivity). However, the climate sensitivity can significantly deviate from its normal values for spatially inhomogeneous disturbances of the radiation budget as it is typical of ozone and aerosols changes by traffic emissions. Following our investigations the climate sensitivity of condensation trails could be by 50 percent smaller than that of an equivalent radiative forcing by a CO2 increase.
In order to understand the differences in the climate sensitivities of different influencing components, the feedback mechanisms that are implemented in a climate model have to be analyzed. They have to be described in the model as correctly and as completely as possible. Some of the natural feedback processes, e.g. where ozone is involved, can be calculated in models only recently.
The figure shows how the ozone distribution changes as a function of latitude and height as a consequence of doubling the CO2 concentration. It is related with a radiation feedback which was not part of former model systems without interactive atmospheric chemistry. The figure depicts the temporally (over one year) and zonally averaged change of the ozone mixing ratio in the atmosphere as it would result from a CO2 doubling, provided as the percentage of the corresponding distribution for an unchanged climate. The result was generated by an ECHAM climate model simulation. The black line marks the tropopause.