Sun's activity flies in face of climate expectations

IF NEW satellite data can be trusted, changes in solar activity warmed the Earth when they should have cooled it.

Joanna Haigh of Imperial College London studied satellite measurements of solar radiation between 2004 and 2007, when overall solar activity was in decline. The sun puts out less energy when its activity is low, but different types of radiation vary to different degrees. Until now, this had been poorly studied.

Haigh's measurements showed that visible radiation increased between 2004 and 2007, when it was expected to decrease, and ultraviolet radiation dropped four times as much as predicted.

Haigh then plugged her data into an atmospheric model to calculate how the patterns affected energy filtering through the atmosphere. Previous studies have shown that Earth is normally cooler during solar minima.Yet the model suggested that more solar energy reached the planet's surface during the period, warming it by about 0.05 °C (Nature, DOI: 10.1038/nature09426).

The effect is slight, but it could call into question our understanding of the sun's subtle effects on climate. Or could it? Stefan Brönnimann of the University of Bern in Switzerland says Haigh's study shows the importance of looking at radiation changes in detail but cautions that the results could be a one-off. He points out that the sun's most recent cycle is known to have been atypical