A couple of blogs (Bishophill and Jo Nova) direct you to a short 30 minute presentation by Prof Vincent Courtillot. The proceeding presentation by Prof. Nir Shaviv on cosmic ray theory, though more technically advanced, is worth a look, especially if you compare the strength of his argument with the IPCC greenhouse theory.

For the non-scientist, the Shaviv thesis of solar changes explaining the 20^th century warming episode is better than the IPCC greenhouse theory as

• Has some corroborating evidence to suggest that cosmic rays are affecting the climate, with the extent.
• Has a simple computer model that explains most of the twentieth century warming. In particular the two similar periods of warming from 1915 to 1940 and 1975 to 1998, and the pauses are all modelled quite well. Using Occams Razor  (the most succinct hypothesis, or that which needs the fewest assumptions), it beats the anthropogenic greenhouse gas theories. Alternatively, it is a better fit of the data, as AGW only fits the later warming. The early 20th century warming can only be explained by predominantly natural factors.
• Is happy flicking between the decadal time-scale that he is trying to explain to geological time scale of hundreds of millions of years and then to the influence of solar flares that last a few days. Neither does he have problems with natural variations.

The IPCC greenhouse gas models do have a number of models that concur. But this can be explained that they have similar assumptions and assumptions behind them. Indeed, given the strong coherence it is a weakness that they have such a wide variation in the data. The IPCC

• Lacks corroborating evidence, particularly of the tropical tropospheric hotspot.
• Relies on computer models are highly complex, rely on a two-stage process (see note below), and have many ad hoc adjustments.
• Yet these computer models that do not tie in very well with the data. To explain the lack of warming in the 1945 to 1978 period and post 1998 you have to resort to an ad hoc inclusion of aerosols. The early-twentieth century warming, so similar empirically, has to have a different explanations.
• Greenhouse gas theory is uncomfortable with looking beyond the twentieth. It cannot explain the medieval warm period, hence the amount of backing for the infamous hockey stick which suggests the twentieth century warming was unusual. Neither can it explain the other natural fluctuations in the current inter-glacial.

An opposite view that Shaviv’s work is insignificant can be referenced at Sourcewatch, a highly pro-AGW site. They state

“While he does believe the earth is warming, he contends that the sun’s rays, rather than human produced CO2, are the cause. But a 2009 analysis of data “on the sun’s output in the last 25 years of the 20th century has firmly put the notion to rest. The data shows that even though the sun’s activity has been decreasing since 1985, global temperatures have continued to rise at an accelerating rate.”

There are counters to this is that Sourcewatch is speaking about the wrong thing. Shaviv contends it is cosmic rays emanating from elsewhere in the galaxy that affect cloud cover and by this means temperature. Solar winds (determined by solar activity) heavily influence the levels of cosmic rays reaching the earth. A much smaller influence is the solar variability. Shaviv shows the following slide (at 17 mins) to show the difference in his measured magnitudes.

Note on IPCC Climate models

The IPCC climate models do not just rely on greenhouse gases directly impacting on the temperature to generate global climate catastrophe. This was nicely summarized by Prof Richard Lindzen in his Congressional testimony of November 17^th 2010. (Full pdf here, Warren Meyer comments here)

1. A doubling of CO2, by itself, contributes only about 1C to greenhouse warming. All models project more warming, because, within models, there are positive feedbacks from water vapour and clouds, and these feedbacks are considered by the IPCC to be uncertain.
2. If one assumes all warming over the past century is due to anthropogenic greenhouse forcing, then the derived sensitivity of the climate to a doubling of CO2 is less than 1C. The higher sensitivity of existing models is made consistent with observed warming by invoking unknown additional negative forcings from aerosols and solar variability as arbitrary adjustments.