Airline traffic may help create an icy haze that's brightening U.S. skies
SAN FRANCISCO, CALIFORNIA — The data just didn't seem to make sense. That's often the story right before scientists make a leap in understanding. In this case, scientists had some evidence that skies in the continental United States have been brightening, after several decades of so-called "dimming."
Brightening and dimming are overly simplified words that signify increases and decreases in how much light from the Sun (measured as "irradiance" in watts/m2) reaches the planet's surface—and these measurements are often analyzed under cloud-free conditions.
For the observed dimming under clear skies, convention would point to aerosols. Levels of these tiny particles, associated with pollution, had been rising for decades prior to the 90s and began falling after that thanks to pollution controls. That could make today's skies brighter than those in the 70s or 80s—and it could also warm the climate, as more direct radiation reaches the surface.
If the recent clear sky "brightening" trend were due to cleaner air and fewer aerosols alone, it should be accompanied by an increase in direct downwelling shortwave radiation, one part of solar radiation reaching the surface directly from the Sun. That didn't happen, Long reported during the American Geophysical Union fall meeting in San Francisco.
Instead, Long and his colleagues found that at the continental United States sites they analyzed, direct downwelling shortwave radiation remained roughly steady between 1995 and 2007, under cloud-free skies. Rather, it was the diffuse shortwave radiation that increased. That simply couldn't happen if fewer aerosols alone were the reason behind the brightening. If anything, fewer aerosols should mean less diffuse shortwave radiation, because particles in the atmosphere can bounce light around and back to space.
So the scientists dug deeper, and in a provocative new analysis, not yet published, Long suggests that a high-altitude "ice haze," created by water and other emissions from aircraft, is responsible. "I'm talking about a sub-visual contrail-generated haze of ice, which we do not classify as a cloud but gives blue sky more of a whitish tint." Long said.
The finding—if verified—could mean that we are in essence already conducting a geoengineering experiment on the atmosphere, adding ice particles that change the way solar radiation reaches Earth's surface. Understanding the overall impact of those changes on warming or cooling at the surface will take more research, Long said.
The hypothesis has some circumstantial support in other datasets, Long and his colleagues have found: The brightening trend is closely correlated with U.S. Federal Aviation Administration commercial flight hours during 1995-2007; those aircraft emit both water and the particles necessary to crystalize that high-altitude water into ice.
Moreover, a preliminary study using spectral solar data from an Oklahoma site shows that the clear skies had an overall "whitening" trend during the study years, an indication of increased scattering.
Professor Martin Wild of the Institute for Atmospheric and Climate Science at ETH Zurich, Switzerland, has been tracking Earth's changeable energy budgets. He and his colleagues detected upswings in sunlight reaching the Earth surface (i.e., "brightening") since the mid-1980s, which marked a recovery from substantial downswings in prior decades, a discovery published in Science. "We care about dimming and brightening because these phenomena may not only affect global warming, but also affect plant growth, glacier melt, the water cycle, solar power, and much more," Wild said.
Wild said he's interested in the new hypothesis, which will require more investigation, but which could help researchers to better understand the origins of dimming and brightening, a phenomenon with broad environmental and socioeconomic implications.