S. Fred Singer, Ph.D.

Commercial and military air traffic is currently (1997) injecting water vapor (WV) into the atmosphere at the rate of more than 2.3x10¹¹ kg per year. About 40 percent of the WV, 10¹¹kg/yr, is injected into a narrow interval of altitude (10 to 11 km) and latitude (25° to 55°) in the Northern hemisphere (NH), where it is calculated to be comparable to the WV contribution from anthropogenic methane. Thus subsonic air traffic, growing at more than 5 percent per year, has become a major contributor to WV in the vicinity of the tropopause. Support for these calculations comes from observations of increasing trends of WV and of sulfate aerosols, presumably caused by aircraft exhaust.

Cirrus produced or induced by the increasing air traffic has important radiative effects, with infrared opacity outweighing visible albedo for thin clouds; the net effect should be a positive radiative forcing causing warming at the Earth's surface. Support comes from observations of emissions in the atmospheric window region, taken with high-resolution IR spectrometers; they show appreciable values of radiative forcing.

Evidence for such positive radiative forcing is presented here from "fingerprints" in the weather satellite record: a surface warming trend exists at NH midlatitudes, in contrast to cooling trends at all other latitudes. As expected from the rapid increase in air traffic, one also finds an acceleration of the effect when one compares the first decade of the climate record with the most recent years. These findings are supported by balloon radiosonde data. Further support comes from the finding that a decreasing trend in the diurnal temperature range exists only at NH midlatitudes.

Longer-term observations with spectrometers and lidars are necessary to construct a climatology as a step towards a more complete theory of the effect.