Is aviation 'whitening' the sky?

by Lucas Joel
Wednesday, December 16, 2015

Clear blue skies may not be as clear as they appear. Skies are actually becoming less clear, causing incoming sunlight to scatter in different directions, rather than striking the planet directly — and aviation may be to blame, according to research presented this week at the fall meeting of the American Geophysical Union (AGU) in San Francisco.

“Skies we would typically describe as clear appear to have gotten a little whiter over the past few decades,” said Charles Long, a senior research scientist with NOAA, who along with several other researchers presented findings at a press conference on Tuesday.

At high altitudes, aircraft exhaust, or contrails, produce ice crystals that researchers suggest could be "whitening" the sky. Credit: U.S. Air Force

Cloud edges can grade into the air around them, and, in skies that appear to be clear, condensed water vapor — the stuff of clouds — can persist but not form visible clouds. These clear but moist skies form invisible clouds that are increasing in prevalence, “whitening” the skies and altering the way sunlight travels through the atmosphere.

The whiter, or more humid, portions of the sky appear to be composed of ice crystals similar to those found in cirrus clouds, which are some of the highest clouds on Earth, usually occurring above about 6,000 meters. Studying ground-based measurements of incoming solar radiation collected between 1996 and 2007, “we found an increase in the amount of sunlight that was being scattered,” Long said.

Why this whitening is occurring has been somewhat of a mystery, one that extends back several decades, but the researchers suspect the ice crystals are the result of increasing air traffic, particularly over North America.

Between the 1950s and the 1980s, air pollution was much higher than it is today, said Martin Wild of the Institute for Atmospheric and Climate Science in Zürich, Switzerland, who also presented the new research at the press conference. This pollution includes aerosols — particles like sulfates that are emitted by burning coal or oil — that, when airborne, scatter incoming solar radiation much like ice crystals. These particulates suppressed the effects of global warming, an effect referred to as “global dimming.”

However, air quality standards implemented in the 1970s helped decrease air pollution, which, presumably, should have resulted in less scattered, or diffuse, sunlight — but this has not been the case, Long said. “In order to account for the increase in scattering, there needs to be some other particle aside from pollutants,” he said.

The culprit could be an increase in water vapor due to increasing air traffic in the last few decades. The water vapor, which is being emitted at the same altitudes where cirrus clouds tend to occur, condenses and freezes. In addition to water vapor, Long said, “jet exhaust spits out chemicals that can form small particles that act as condensation nuclei, which are the ‘seeds' around which water vapor condenses and freezes.”

“It is quite reasonable that small ice crystals in this part of the atmosphere are forming because of changes in air traffic and aircraft,” Kevin Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the research, told EARTH. “Still, there are variations that still need to be accounted for,” such as the potentially different impact of aircraft in clear skies following a clear day versus following a storm. In the latter case, the air would tend to be less humid because the storm would remove most of the moisture, he says, meaning fewer ice crystals would be likely to form.

Wild and Long referred to the climatic effects of air pollution and aviation as an unintentional geoengineering experiment; some researchers have previously suggested injecting particulates into the atmosphere to block sunlight as a potential geoengineering method to slow global warming.

The decrease in air pollution since the 1970s means more solar radiation reaches Earth’s surface now than several decades ago, a phenomenon known as “global brightening” that also results in warming, Wild said.

While the ice crystals are not altering the total amount of energy Earth’s surface receives by much, Long said, they are repartitioning it, by scattering it and changing it from direct to diffuse light, which can have impacts on plant life.

“Humans are depositing aerosols high in the atmosphere, where they do not normally appear,” Long said. “It’s further evidence of the effect humans can have on the planet.”

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