Hypotheses, organizing principles, of this sort emerge from the fabric of a science as a consequence of a search for unifying principles. The organizing principles of climatology come from various threads, but I’d mention the oceanographic sysyntheses of Sverdrup and Stommel, the atmospheric syntheses of Charney and Lorenz, paleoclimatological studies from ice and mud core field work, and computational work starting with no less than Johnny von Neumann.
The expectation of AGW does not organize this work. It emerges from this work. It’s not a theory, it’s a consequence of the theory.
Admittedly it’s a pretty important consequence, and that’s why the governments of the world have tried to sort out what the science says with the IPCC and its predecessors. That tends to color which work gets done and which doesn’t, and I think it should. As Andy Revkin pointed out, it may be time to move toward a service-oriented climatology, or what I have called applied climatology. The point is that this amounts to application of a theory that emerged and reached mathematical and conceptual maturity entirely independent of worry about climate change.
So attacks on climate change as if it were a “theory” make very little sense. Greenhouse gas accumulation is a fact. Radiative properties of greenhouse gases are factual. The climate is not going to stay the same. It can’t stay the same. Staying the same would violate physics; specifically it would violate the law of energy conservation. Something has to change.
The simplest consequence is that the surface will warm up. That this is indeed most of what happens is validated pretty much in observations, in paleodata, in theory and in simulation. Further, all those lines of evidence converge pretty much about how much warming: about 2.5 C to 3C for each doubling of CO2. (It’s logarithmic in total CO2, not in emitted CO2, guys, by the way.) There’s no single line of reasoning for this. There are multiple lines of evidence.
If you want to convince me that the sensitivity is less than 2 or more than 4, you will have to provide quite a good deal of evidence, but I don;t think this is what the denialists have in mind when they ask me what would “falsify the hypothesis”. In fact, though, they haven’t defined their terms. If the sensititivity is less than 1, is the supposed hypothesis falsified? What if it is more than 6? If the onset time is a hundred years rather than ten?
They want to know what it would take to pry my free of my “beliefs”, but they are not beliefs, they are estimates. Estimates of the sensitivity (2.8 C per doubling). Estimates of the built-in delays (about twenty years for full effect of current concentrations). Estimates of the threshold of excessively high social risk (some range here but I go with 2 C ~ 450 ppmv).
What would it take to change my opinion of the threshhold to 451 ppmv? A nice dinner at Fonda San Miguel, margaritas included, would surely do it. If that constitutes a falsification, bring it on.
Really, though, I don’t understand the question. If these numbers wobble around a bit that might shift the optimum policy a bit, but we’re so far from the optimum now that it’s not worth putting much thought into it yet. The numbers, however, are never going away. There will always be a sensitivity, a response curve, a risk threshold. If you are asking what evidence could make me believe that there are no such numbers, I can’t actually imagine it.
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