INCITE Researchers Explore How Aircraft Contrails Can Impact Climate

Cheryl Drugan

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When aircraft in the United States were grounded for three days after the Sept. 11, 2001 terrorist attacks, scientists had a singular opportunity to study the effects of contrails—ice clouds generated by water exhaust gases from aircraft engines—on climate. The scientists measured day and night temperatures to find out if contrails contributed to regional warming and/or global climate change.  Some studies indicated that the absence of contrails during this grounding period increased the daily temperature range at the Earth’s surface, but this result is still a subject of scientific debate. 

Nonetheless, there’s no question that the environmental impact of aviation represents a source of increasing concern among scientists and policymakers as the demand for air travel continues to grow.  Aviation is one of the fastest growing sectors, with a projected twofold increase by 2020.

Contrails are ice clouds that form by condensation of water vapor exhaust from aircraft engines and develop further in the aircraft wake as they are entrained by the airplane trailing vortices (see Fig. 1). When contrails spread to form cirrus clouds (see Fig. 2), they can persist for hours and extend over areas of several square kilometers. These “contrail cirrus,” which artificially increase Earth’s cloudiness and become almost indistinguishable from natural cirrus, are among the most uncertain contributors to the Earth’s radiative forcing. (Radiative forcing is defined as the change of the net radiating flux resulting from changes in the atmospheric composition. A measure of the perturbation of Earth-atmospher