Complicating scientific efforts to predict how much warming of the Earth's climate will occur in the future is the discovery of a large, previously unknown, zone of particles around clouds, transitioning from cloud droplets to dry particles. Assessing the role of clouds in the Earth's climate is complicated by the speed at which they change and their many sizes and types, and their influence has become even more complex with the discovery of the "twilight zone" surrounding them. Scientists at the Weizman Institute, Rehovat, Israel and NASA's Goddard Space Flight Center, Greenbelt, MD, have documented the presence of particles which are not cloud droplets or typical dry aerosols, such as dust or air pollution, around clouds in what was formerly considered clear air. Consequently, up to 60 per cent of the world's atmosphere labeled cloud-free in satellite observations is in reality full of these transitional particles.

Since 2000 NASA's highly sensitive instruments can differentiate between aerosols and clouds in greater detail, and had indicated something in the area around clouds. However, scientists were unable to identify it since it didn't match their understanding of either clouds or aerosols. What they believe they are seeing is a zone where clouds are forming or dying and where humidity is causing dry particles to absorb water and grow larger.

Predicting the future climate of Earth requires accounting precisely for anything in the atmosphere that can impact global temperatures. The effect of clouds and aerosols on the amount of heat retained in the atmosphere is still uncertain, and the discovery of a new factor, while complicating the calculations, could also help to resolve lingering questions encountered in research on climate change. Most current computer models do not include this "twilight zone" effect to estimate the effect of aerosols on climate, which might be one of the reasons why current measurements of this effect do not match model estimates.

The region around the clouds affected by this transitional zone extends to tens of kilometers beyond the identified edge of the cloud, suggesting from 30 to 60 percent of the atmosphere considered cloud-free is affected by processes that reflect solar energy back into space. This new information could lead to a reassessment of the Earth's ability to retain or reflect solar energy and thereby increase the accuracy of the predictions about global warming. This summer new measurements by the Aerosol Robotic Network and NASA aircraft will provide more information about the transitional particles in the "twilight zone".

Learn more about this author, Nancy L. Young-Houser.
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