Sunshine just isn鈥檛 what it used to be. Since 2000 less sunlight has been reaching the Earth鈥檚 surface, say solar physicists. But paradoxically this drop in radiation hasn鈥檛 cooled our planet.
Philip Goode鈥檚 team at the New Jersey Institute of Technology in Newark measured the amount of sunlight reflected by the Earth鈥檚 atmosphere onto the moon and back again. They found that the Earth is reflecting 3 per cent more sunlight than it did between 1985 and 2000 (Eos, vol 87, p 37).
The effect could be caused by increasing cloud cover, says Goode. Recent data from the International Satellite Cloud Climatology Project (ISCCP) shows that the total cloud cover grew in this period. But most sunlight hitting clouds bounces off and never reaches the Earth鈥檚 surface, so the question troubling climate scientists is why global temperatures are still rising.
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The answer may lie in a curious rearrangement of cloud cover during the past five years, says Goode. The ISCCP study shows that the proportion of high-level cloud to low cloud has been climbing. Low-lying clouds help cool the Earth by reflecting sunlight, while high clouds act predominantly as blankets, trapping heat. So although the total cover has increased, the change in the type of cover has compensated, allowing temperatures to rise, says Goode.
鈥淭his is a reminder that climate change is subtle, and what seems obvious 鈥� that less sunlight means cooling 鈥� may not be right,鈥� he says.
