By now, it's pretty clear that carbon dioxide, released by burning fossil fuels, is responsible for most of the warming seen throughout the world in recent decades.
As the Intergovernmental Panel on Climate Change concluded in September, it is "extremely likely" that human activities have caused most of the warming of the planet's surface since the 1950s.
But theories to the contrary linger. One of the more persistent is that global warming is caused by cosmic rays and changes in levels of solar radiation. As the theory goes, cosmic rays - which are thought to emanate from supernovas, explosions of distant stars - can increase the number of clouds in Earth's atmosphere by filling the atmosphere with charged particles, upon which water vapor condenses. The clouds, in turn, reflect some sun and cool the globe.
However, during times of increased solar radiation, fewer cosmic rays enter the atmosphere, as they are deflected by charged particles spit out from the sun. When this happens, the globe warms, as the thinking goes, both because there is more solar radiation to heat up the atmosphere and because there are fewer cosmic rays and thus fewer clouds to reflect incoming light.