Scientists have long suspected that contrails, the trails of white exhaust that jets leave in their wake, can affect climate. Quantifying their effects was extremely difficult, however, because the nearly constant air traffic over most of Europe and North America made it impossible to study clear skies for comparison. But the grounding of commercial aircraft in the wake of the September 11 terrorist attacks provided just such an opportunity. Now a report published today in the journal Nature confirms that contrails' affect the range of temperatures recorded on the ground.

David J. Travis of the University of Wisconsin-Whitewater and his colleagues collected three decades of temperature data from nearly 4,000 weather stations across North America for the three-day periods before, during and after the air traffic moratorium. By comparing the so-called diurnal temperature range (DTR), which is the difference between daily highs and nightly lows, the team calculated that the range of temperatures was more than one degree Celsius greater in the absence of commercial air traffic. This increase was even more pronounced in regions reported to have the most contrail abundance, such as the midwest, northeast and northwest sections of the country.

Cloud cover (particularly high-level cirrus clouds that are primarily composed of ice crystals) insulates the planet by reflecting incoming radiation from the sun and trapping outgoing infrared radiation. Contrails, the researchers conclude, exacerbate this effect.