A Cloudy View of Cloudiness

Moving satellites may have caused falling measurements of cloud cover.

Satellite evidence that cloud levels are decreasing could just be pie in the sky. The trend might simply be a result of where the satellites are positioned.

Data from the International Satellite Cloud Climatology Project (ISCCP) have shown that cloud levels have decreased by up to 4% over the past 20 years. Clouds increase the Earth's ability to reflect sunlight back into space, cooling the planet. So reduced cloud cover has been linked to global warming.

But Amato Evan at the University of Wisconsin at Madison and his colleagues have taken a closer look at the ISCCP data. Cloud cover decreases abruptly when satellites are moved, the team reports in Geophysical Research Letters1.

As more satellites were launched from the mid 1980s through the 1990s, each satellite could narrow its field of view, looking straight down rather than at an angle. And when observed straight on, clouds appear less cloudy.