A lazy sun explains it. The solar magnetic shield was weak, and the Earth suffered a larger influx of swift atomic particles coming from exploded stars.

Those cosmic rays helped to make more low clouds, which cooled the world during the Little Ice Age. But in the 20th century the sun doubled its magnetic strength and cut the influx of cosmic rays. That meant fewer clouds and a warmer world.

This is no crackpot theory. A string of discoveries by the physicist Henrik Svensmark at the Danish National Space Institute backs it up. He and his small team have even traced the chemical action of cosmic rays involved in cloud-making. Evidence for the cosmic-ray theory is now far stronger than for the politically fashionable notion that carbon dioxide drives global warming (http://www.newsletter.co.uk/3425/OPINION-Manmade-glo...).