To be scientific, a position must be refutable. I asked Dr Benny Peiser, Director of "Lord" Nigel Lawson's Global Warming Policy Foundation, what testable statement underpinned his case.
He replied that we should wait 20-30 years until we know the magnitude of AGW. "If we were to experience a decadal warming trend of 0.3 to 0.5 in the next 20-30 years, I would consider global warming to pose a potential long-term problem".
However, that is not a test of their position regarding Man-Made Greenhouse Gases (MMGHGs).
They could attribute an increase that to an underlying long term increase in solar output, or conversely, if there were a long term solar minimum over the next 30 years, the decadal trend might be less, but still more than it would have been without increased GHGs, with rapid global heating to be expected after the minimum.
So an actual global temperature trend would not be conclusive.
Also, in taking this approach, the sceptic is performing a massive experiment on the entire planet. An experiment with potentially disastrous consequences. No formal Ethical Committee (if such existed) would sanction it, and it certainly goes against the Precautionary Principle, which itself places the burden of proof on the sceptics.
Peiser says “I am agnostic on the theory of anthropogenic global warming, I do not hold a position that can be refuted”.
This will not do. Agnosticism applies to belief systems, not to science. Scientific statements must be refutable.
Implicit in what he says is the idea that the effect of CO2 on climate is weaker than the 2*C +/- 1.5*C that climate science points to. Some work done by a few sceptical scientists comes up with a figure in the region of 0.5*C.
It is clear therefore that the thesis of the climate sceptics stands or falls on the truth or otherwise of the statement:
Climate sensitivity is well below 1.5*C
That is a statement which the climate sceptics should be pressed to sustain. They will put forward a few papers by Lindzen, Spencer and others. These papers have already been criticised, but they should be methodically scrutinised. In way, they are the most important papers in the climate debate.
At the same time, the sceptics should also answer every question posed to their position by the many papers that set climate sensitivty at 2 +/- 1.5*C.
The whole debate, and therefore the future wellbeing of humanity, hinges on this tiny, resolvable point. That sounds overblown, but it is not.
The high value accepted by the overwhelming majority of climate means that we have to transform our global economy, decarbonising and in all other ways greening our economy.
The low value means we can pretty much continue business as usual, at least until the oil runs out.
This is a major lifestyle choice for everyone on the planet... and it all hinges on the question of the value we should attach to climate sensitivity.