Aircraft strengthen cooling effect of clouds

Contrails provide better reflectivity

Vapor trails on airplanes do not change much when the sky is clear, but when clouds are already there. © Contrail / CC-by-sa 2.5
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White veils with far-reaching effects: contrails of aircraft could have a hitherto neglected impact on the climate - and a positive one. As researchers report in the journal Nature Communications, in the presence of the stripes, the proportion of solar radiation that certain clouds can reflect increases significantly. This reinforces their cooling effect for the earth.

Clouds play a significant role in the climate. Depending on the nature and height, they reflect a not inconsiderable part of the incident solar radiation - and thus work during the day as a cooling screen for the earth. On the other hand, clouds influence how much thermal radiation the earth can send back into space. They therefore also have a greenhouse effect.

Whether the airy structures of the finest water droplets or ice crystals in the net result, the warming or cooling the climate, depends mainly on their so-called optical thickness. The thicker and lighter a cloud is, the more sunlight it reflects - and airplanes, of all places, can positively influence this cooling effect, say scientists around Matthias Tesche from the University of Stockholm.

Many planes are the same lighter clouds

Researchers have studied how contrails created by air traffic affect cirrus clouds in the atmosphere. Using sophisticated laser techniques and satellite observations, they analyzed the properties of these high-altitude ice clouds. They compared clouds in the area of ​​the much frequented flight routes between the west coast of the USA and Hawaii with clouds outside of these routes.

The results showed that where many aircraft were traveling, cirrus clouds had a significantly greater optical thickness. Say: They were brighter and had more reflexive power than clouds beyond the large air traffic. On average, their ability to reflect sunlight increased by an impressive 22 percent. display

Aircraft trace (gray background) in a cloudy region over the Pacific. The colored dots indicate the optical density. T Matthias Tesche

How does the phenomenon come about?

However, Tesche and his colleagues can only speculate how exactly this effect is achieved. In a cloudless sky, the contrails themselves have rather a minor effect on the climate. However, possibly the smallest particles of soot emitted by the aircraft together with the cloud vapor could provide an explanation. They could act as condensation nuclei that stimulate the growth of the clouds.

However, further studies are needed to understand the mechanisms behind this phenomenon. However, one thing they think is certain: "Apart from the harmful CO2 emissions, air traffic is likely to have another important impact on the climate, " they write. This contribution made by the contrails should be considered in more detail in the future. (Nature Communications, 2016; doi: 10.1038 / ncomms12016)

(Stockholm University, 22.06.2016 - DAL)