A persistent element of the climate debate is the claim that "there is no proof" that CO2 and other greenhouse gases warm the atmosphere. This has generated a number of amateurish demonstrations of how the greenhouse effect works A good example of how NOT to carry out such a demonstration is shown above. Nobody told this poor kid how to perform a scientific measurement. If he had switched his jars he would have discovered that the effects he observes are only the result of one of the two jars being closer than the other to the light source. And they didn't even explain to him how the greenhouse effect actually works: you cannot see any warming in this set-up unless you place a light adsorber (e.g. a piece of black cloth) in the jar. I discuss this and other disasters in a post of mine (in Italian), see also here.
Imagine that someone asks you to prove that the Moon orbits around the Earth because it is pulled by the force of gravity. Your first reaction would be to say something like "huh?" But - assuming you are in a good mood - you might try to explain how Newton's law of universal gravitation works and how it can be used to describe the moon's trajectory.
"So," your opponent could say, "is it correct to say that nobody ever measured the gravity pull of the Earth at the Earth-Moon distance?"
"Aw... No. What for? It would be terribly expensive. And useless, too."
"Then, you have no proof that the Moon goes around the Earth because of gravity. So, can you prove that the Moon is not being pushed by invisible angels, instead?"
That would end the civil conversation, but there is some logic in this kind of questions. You can't reproduce the Moon's motion in a lab, here on the Earth. Instead, to prove that Newton's law is valid you create a model based on the law and then use it to describe the movement of moons, planets, and stars. The model works, hence the law behind it is correct. No need of measuring the Earth's gravity pull at the Earth-Moon distance (and no need of invisible angels pushing).
Now, let's translate all this to a question often asked in the climate debate. What proof do we have that greenhouse gases, and CO2 in particular, warm the Earth's atmosphere? As a question, it is similar to the one about the Moon orbiting the Earth, in the sense that we can't reproduce the properties of a whole planetary atmosphere in the lab.
To answer the question, we can start from laboratory experiments showing that CO2 absorbs infrared radiation - they were done already by Tyndall in the 19th century. We don't need amateurs today to remake those experiments, doing the job poorly. Then, we create models that describe the Earth's atmosphere and we use them to fine-tune the parameters of the warming effect. The models follow reasonably well the warming trends of the past so that we confirm that CO2 warms the Earth.
In addition, we even have direct experiments showing that, as the atmosphere warms, more infrared radiation is radiated to space, while at the same time less infrared radiation escapes at the wavelengths where CO2 absorbs radiation. It is what we expect a greenhouse gas should do.
Case closed, then? Not really. The Moon-Earth system is relatively simple and, unless you really want to believe in invisible angels, there is no doubt that the Moon is kept in its orbit by gravity, and by gravity only. Instead, the Earth's ecosystem system is a tangle of subsystems interacting with each other in ways that - in many cases - we have troubles in quantifying exactly. Newton could never have described the Earth's atmosphere by means of a single universal equation. So, how to disentangle the various contributions to the Earth's temperature: albedo, atmospheric particulate, vegetation cover, and more?
This is the task of models, coupled with various kinds of measurements. For what we can say nowadays, the rising CO2 concentration is the main factor in the observed warming, but that's subjected to changes and refinements as we have new data. Among other examples, a very recent paper appeared in Nature quantified the effects of the vegetation cover. This is mainly a local effect and changes little to the overall picture, but it shows how dangerous it can be to say that "the science is settled." Of course, that doesn't mean jumping to the conclusion that angels are warming the Earth with their breath. And it doesn't mean to engage in silly games of data torturing. It just means recognizing the complexity of the problem.
Then, there is an even deeper problem: complex systems such as the Earth's ecosystem are just that: complex. They may react to perturbations in a strongly non-linear way, amplifying or dampening the perturbation. In a complex system, it is always difficult to say what causes what. You can say that in a complex system there are no causes and no effects, only forcings and feedbacks. So, when people say that the increase in CO2 concentration is not a cause but an effect of global warming, are they wrong? Yes, they are basically wrong, but it is also true that increasing concentrations of greenhouse gases may be both a cause and an effect of global warming. Complex systems are dominated by feedbacks. And that means they can amplify the effect of a small forcing, turning it into a disaster. This is, by the way, the origin of the "Seneca Effect"
In the end, it all means that we have to recognize that the Earth's climate could react to perturbations in ways we can't even imagine. And more than seven billion humans on the planet have been a huge perturbation, no matter how we want to see the relative importance of what we have been doing. We ignore that at our risk.
