What are climate sceptics actually saying?

They are shy of making any statements that are actually testable, so in a spirit of helpfulness, I will deduce them for them, by mirroring the scientists' case:

Essentially, climate sceptics are putting forward one or more of these key statements:

1 CO2 is not a greenhouse gas.

This would be very difficult to sustain, given that the effect is demonstrable in the laboratory, but one sceptic tweeter  (@TertiusIII) has already suggested that it is not, in fact maybe the opposite. I have challenged him for evidence, and no answer has been given.

2 CO2 has not increased since 1800

Again, this would be difficult to sustain, since there is a wealth of observational data to show that it has.

3 The climate sensitivity to CO2 is less than 1.5*C

It seems that this is emerging as their thesis, though Peiser was unwilling to actually state it. 

It probably is their key position, and the debate should therefore centre on this point.

Climate sensitivity is the degree of change in global temperature that will result from the doubling of any input factor - natural (solar, volcanoes, ocean currents) or man-made.

The IPCC states:

The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is defined as the equilibrium global average surface warming following a doubling of CO₂ concentration. Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values. {WGI 8.6, 9.6, Box 10.2, SPM}

Note the "Very unlikely to be less than 1.5*C".

So the challenge for the global warming skeptics is for them to try to defend their thesis that CO2 sensitivity is at that level or less.

The debate now gets pretty technical, but the skeptics claim to understand climate science, so the first approach would be to lock a few climate scientists and skeptics in a room together for a few days and see what they come up with.


One way of testing their thesis would be to put CO2 sensitivity of, say, 1*C into all the models, and see how good the match that produces is for observed temperatures.

On this point, their response might be that they want to disallow any computer models. The question then becomes, would they accept it if calculations were made by hand, with pencil and paper as they were in the early years of climate science? That seems pretty absurd.


 Or are they against calculation of any kind?

In the latter case, they would be not just anti-science, but also anti mathematics! Clearly an absurd position.


More:
What is the point of debating with sceptics?
Correspondence with a leading sceptic
Summary of the correspondence