Brighter Planet

The next time you book a flight, remember that the global warming effect of a daytime flight is less than half of that of a nighttime flight. This is because unlike cars, planes’ contributions to the greenhouse effect go well beyond just carbon dioxide emissions. Jet emissions high in the troposphere catalyze the development of contrails, which affect the rates that energy enters and leaves the atmosphere. This effect is known as radiative forcing. Contrails are essentially high ice clouds, and although they do reflect some solar radiation back into space, they reflect much more heat back down to earth, and thus have a net warming effect.

The IPCC estimates that on average, a commercial aircraft’s radiative forcing is 1.7 times as great a contributor to global warming as is its carbon emissions. But the impact of contrails fluctuates over the course of the day and year — the darker and colder it is outside, the greater the relative radiative forcing effect of contrails, so night and winter flights contribute much more to the greenhouse effect than do day and summer flights. Radiative forcing from a night flight is 7 times greater than a day flight, and from a winter flight is 3.5 times greater than a summer flight.