Useful piece in the Independent today.
The only alternative to man made GHGs as the explanation for the the global warming that we observe is the solar theory.
A study in 1991 by Friis-Christensen which showed a correlation between earth temperatures and the length of sunspot cycles. However, sunspots only increase solar intensity by 0.1%, so Christensen and Svensmark wrote a further paper in 1998 that added in an hypothetical effect of cosmic rays, which they argued would seed clouds, leading to cooling by increased reflectivity.
Peter Laut reworked their data, and pointed up mathematical flaws in their work. Other climate scientists endorse his criticisms. It is now agreed that sunspot activity is falling rather than rising, while temperatures continue to rise. This leaves CO2 as the only explanation for the continued rise.
Christensen now accepts that the 1991 study is now invalid, according to the Independent (paper version, 14th Dec 2009), although this is not certain. The link between cosmic rays and clouds is being studied, but latest data shows that cosmic rays are going up while cloudiness is going down - which contradicts Christensen's hypothesis.
More links provided by Student on the Daily Mail debate:
Lockwood and Frohlich: "we conclude there is no credible way that the recent rise in air surface temperature can be attributed to solar effects"
Erlykin, Sloane and Wolfendale: "we deduce that the maximum recent increase in the mean surface temperature of the Earth which can be ascribed to this activity is ~<14%>
Useful BBC summary of recent work which refutes the cosmic ray/cloud hypothesis here.