He points out that we design bridges based complicated models that apply Newton's laws of motion and the observed properties of materials, and these bridges are build and they (rarely) fall down.
We build airplanes based on the physics of fluid mechanics and the dynamical laws of motion and properties of substances, and they are rolled out of hanger, pointed down the runway, and take off.
And in exactly the same way, we project climate based on elementary laws of physics proven a century or more ago -- the Planck Law, the Stefan Boltzmann law, the measured absorption properties of gases. And many other laws of physics that represent the real world. Just look at all this physics.
Manmade global warming isn't rocket science. I mean, Arrhenius put the pieces together in the late 1800s, though not quite prefectly. Lots of improvements and additions have come since. This is basic, even obvious stuff.
All climate models ever constructed show warming from human emissions of greenhouse gases. Given CO2's role in the greenhouse effect, it is hardly surprising that 45% more CO2 in the atmosphere would cause more warming.
It'd be far more surprising if it didn't.
It is perfectly legitimate to make predictions based on observations combined with well established physical laws. That's how geologists discovered tectonic plates. How Neptune was discovered. How the atomic nucleus was inferred. And so on.
There is no experimental proof that smoking is harmful -- it's unethical to do such experiments, and impractical -- yet we know that it is. This conclusion isn't going to be reverse next year, or in 20 years.
Climate models are made out of theory. They are huge assemblies of equations that describe how sunlight warms the Earth, and how that absorbed energy influences the motion of the winds and oceans, the formation and dissipation of clouds, the melting of ice sheets, and many other things besides. These equations are all turned into computer code, linked to one another, and loaded into a supercomputer, where they calculate the time-evolution of the Earth system, typically in time steps of a few minutes. On time scales of a few days, we use these models for weather prediction (which works very well these days), and, on longer time scales, we can explore how the climate of the next few decades will develop depending on how much carbon dioxide we release into the atmosphere. There are three items of good news about this modelling enterprise: one, we can check on how well the models perform against the historical record, including the satellite data archive; two, we can calculate the uncertainty into the predictions; and three, there are more than twenty of these models worldwide, so we can use them as checks on one another. The conclusions drawn from a careful study of thousands of model runs are clear: the Earth is rapidly warming, and fossil-fuel burning is the principal driver.I think the prediction of manmade warming, based on our emissions and the known laws of physics, is perfectly legitimate and adequate to prove AGW. Sure, you have to get into some of the weeds to get all the details, but physicists have been doing this for decades and have the radiative transfer down pat.
But theories are abstract, after all, so it’s easy for people to get tricked into thinking that because something is based on theory, it could very likely be wrong or is debatable in the same way that a social issue is debatable. This is incorrect. Almost all the accepted theories that we use in the physical and biological sciences are not open to different interpretations depending on someone’s opinion, internal beliefs, gut feelings, or lobbying. In the science world, two and two make four. To change or modify a theory, as Einstein’s theories modified Newton’s, takes tremendous effort and a huge weight of experimental evidence.
And this is where politics and science can find themselves at cross purposes. In many political discussions, climate science gets treated like family planning or tax restructuring. When it comes to these social issues, convictions or personal views count for a lot, and rightly so. But the climate issue, and the business of climate prediction, is different. The changes we’ve seen over the past hundred and thirty years are incontrovertibly documented: they are facts.
I'm not sure how many people realize this, but the role of CO2's radiative effects (and the other main GHGs) is the best know part of climate science. Because it's the most amenable to a standard treatment by fundamental physics.
So deniers thinking that someday CO2's role in climate change is going to be negated based on some new observation or some future pause or something else are very wrong and very lost. The uncertainities all lie elsewhere, especially in the details of clouds and in the details of the carbon cycle and how it will change in the future.
So the arguments CO2 looks increasingly naive and foolish. CO2 from fossil fuels will never go back to being an innocent substance. Ever.
On the contrary -- future generations will look back on us as dumb greedy idiots for thinking we could dump massive amounts of CO2 in the atmosphere and expect there would be no result.
We may (and probably won't) zero in on the exact value of climate sensitivity -- too may uncertainties in too many variables. But knowing it's 2°C or more is perfectly sufficient to know we have a big problem.
Piers Sellers didn't have room, or the audience, to point out that this isn't all theory. There are many -- increasingly many -- observations that verify the vital parts of AGW theory -- the enhancing greenhouse effect, the radiative forcing of CO2 (which agrees with the predictions of climate models), the observed ocean warming and energy balance of the planet.
And more. There is so much evidence from so many different angles that, like the health effects of smoking, the result is surrounded by evidence and cannot escape it.
But I'm probably repeating myself.