While there are a number of scientific papers and books that look at this, I am hoping to do it without math or physics. Instead, I will look at what some of the sceptics say.
Any branch of science will have a number of people who are interested in the given topic. For this post I have (somewhat arbitrarily) divide the AGW sceptics into the following categories: the established expert, the up and coming new comer, and the talented amateur. From these people I was looking for a clear statement about the effects of CO2 only. Something like “X increase in CO2 concentration will cause Y change in global temperature”. For an established expert I took Richard Lindzen. Dr. Lindzen is the Alfred P. Sloan Professor of Meteorology at the Massachusetts Institute of Technology – you don’t get much more established than that. While many people disagree with Dr. Lindzen’s editorial comments, his scientific statements and contributions are still respected. In a presentation to CEI in November of this year, he stated that (on slide 44):
Remember, we are ultimately talking about the greenhouse effect. It is generally agreed that doubling CO2 alone will cause about 1C warming due to the fact that it acts as a ‘blanket.’ Model projections of greater warming absolutely depend on positive feedbacks from water vapor and clouds that will add to the ‘blanket’ – reducing the net cooling of the climate system.So that is the first statement – a doubling of CO2 will cause a 1C rise. (I suppose it could be argued that he is only saying this and that he doesn’t believe it – but nothing in his presentation contradicts that view. He is more concerned about the feedbacks that follow from this rise). For the new scientist on the scene, I am going to use Dr. Nir Shaviv. Dr. Shaviv was born in 1972 and received his PhD in 1996 so he is one of the newest people on the scene. He has received a number of prizes and awards based on his scholarship. In regards to the issue of global warming his contribution was the idea that climate is controlled by cosmic rays (through their ability to generate seed nuclei from which clouds can grow). Looking at his site, he goes through an analysis which shows “This sensitivity translates to an equilibrium CO2 doubling temperature of about 1.2°K.” His analysis is based on the earth being a “grey body” – i.e. albedo is taken into account - but he does not include feedbacks.
Finally, we can look at the work of a talented amateur and as far as I am concerned the best person for this role is Canada’s own Steve McIntyre. No one can doubt Mr. McIntyre’s mathematical skill or his persistence in defending what he believes as correct. In Mr. McIntyre’s case I could not find any actual explicit statement that he made in regards to the effect of doubling of CO2 alone - probably not surprising since he tends to avoid the physics and concentrate more on the statistics. However he has stated a number of times that he accepts a great deal of the IPCC work, while having serious issues with the statistics and handling of the data. Most recently he wrote on a blog at the Spectator (the UK magazine) .... but the most important outstanding scientific issue appears to me to be the amount of "water cycle" feedback, including clouds as well as water vapor. This controls the "climate sensitivity" to increased CO2. To me that would imply that he accepts the IPCC’s analysis of the effect of a doubling of CO2 alone (which is between 1 and 1.2C). However if anyone believes I have misrepresented his views feel free to provide a counter argument in the comments.
So, here we have 3 individuals who are among the best and the brightest in the sceptic world. They all seem to feel that the effect of CO2 alone is well established, it will cause warming and the warming will be between 1 and 1.2C. If you ever find yourself in a discussion about the effects of CO2, the starting point is that a doubling of CO2 alone will cause a 1C temperature rise. If anyone disagrees, tell them to take it up with Lindzen.