There is much research that needs to be done to gain a comprehensive, quantitative understanding of these relationships.
Surface transportation is not the only contributor to atmospheric change — aviation constitutes the fastest growing component of the transportation sector, and it may contribute to the global scale atmospheric changes described above. Howard Wesoky, from the National Aeronautics and Space Administration, talked about the major research and assessment programs (both in the U.S. and Europe) designed to quantify the atmospheric impacts of subsonic and supersonic aircraft. Aircraft emissions of CO2, water vapor, and particulates can cause direct radiative forcing, although this effect is estimated to be quite small. A more recent concern is that contrails and particulate emissions may increase the abundance of cirrus clouds, which could, in turn, have significant climatic impacts. There is also concern that emissions of NOx and other species can perturb the concentrations of ozone in the troposphere and stratosphere.