Tragic as they were, the events of Sept. 11 provided an unexpected boon to climate science: They caused an unprecedented three-day interruption in U.S. air traffic that enabled scientists to assess the impact on the climate of condensation from jet planes. Those streaks of condensation, known as contrails, all but disappeared during the flight hiatus — and variations in high and low temperatures increased by 2 degrees Fahrenheit each day, according to meteorologists. The research establishes that contrails can affect temperatures, although whether they have a net effect on global warming remains a question. Contrails generally behave similarly to normal clouds, blocking solar energy from above and trapping heat below, thereby reducing the daily temperature range. Because certain species depend on specific daily temperature variations for survival, even slight changes in climate can have substantial, ecosystem-wide effects.