Irving Berlin wrote those lyrics in 1926, before jet planes existed. Those deep blue skies are now whitening, and some scientists blame jet engine exhaust. They presented their findings at the December 2015 Fall Meeting of the American Geophysical Union in San Francisco, US.

Martin Wild of the Swiss Federal Institute of Technology (ETH) in Zurich described the "substantial decadal changes" in the amount of sunlight that reaches Earth’s surface – the phenomenon known as global dimming and brightening. From the 1940s to around 1980, Europe experienced a period of dimming, with less sunlight reaching the surface. Since then, until the near-present, it has experienced brightening.

Sunlight, the ultimate source of energy in the climate system, is affected by greenhouse gases, air pollution, and the presence of aerosols in the atmosphere, Wild noted. The easy explanation for the dimming and brightening, according to Wild, is that the Sun’s output varies, as evidenced by its 11-year sunspot cycle.

But there is no correlation between solar output and the sunlight received on Earth’s surface, so the answer must lie within the atmosphere, Wild explained. The increase and decrease of air pollution, particularly sulphur emissions, correlates well with brightness. Periods of increased pollution experienced dimming, which suppressed global warming, while periods of reduced pollution, resulting from new environmental controls and regulations, experienced brightening and enhanced warming.

The findings pertain mainly to the northern hemisphere. The southern hemisphere, whose atmosphere Wild described as relatively pristine, warmed gradually over the last half century, but experienced no significant brightening or dimming.

Relating those findings to the blue skies question, Charles Long of the Cooperative Institute for Research in Environmental Sciences, US, suggested an additional source of brightening and dimming: exhaust from jet engines. He detailed two components of solar shortwave radiation – direct and diffuse. The former arrives directly from the Sun, like a spotlight, while the latter is scattered through the atmosphere.

Direct sunlight stayed the same as dry aerosol pollution diminished, yet diffuse radiation increased in the decade 1996–2007, Long said, making the unclouded sky less blue, or whiter, than in the past. He attributed this to the presence of larger particles that scatter sunlight in the atmosphere. Modeling suggested that small ice crystals would produce the observed light scattering.

Long charted a steady increase in US commercial air traffic during that period, with the exception of the months following 11 September 2001. Jet exhausts produce water vapour that freezes into contrails, which eventually fade into a cirrus haze, he said. The result, though not visible from the ground as a cloudlike structure, is a slight lessening of the blueness of the sky, called clear-sky whitening.

Long characterized the whitening effect of jet aviation as unintentional or accidental geoengineering, defined as the large-scale manipulation of an environmental process that affects Earth’s climate. Long distinguished jets from such human activities as automobile use and biomass burning because they affect the high atmosphere, adding that the other activities do contribute to brightening and dimming.

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