1. Those experiments with a container filled with CO2
These date back to Eunice Foote and later John Tyndall in the 19th century. There are plenty of videos on YouTube. They fill one container with normal air and one with CO2 and expose them to sunlight or some other bright light. Inevitably, the one filled with CO2 warms considerably more.
Their conclusion: CO2 causes global warming, and it does this by absorbing radiation.
[Most of these experiments are fundamentally flawed;
- even if they prove that a 100% CO2 atmosphere (or air with 1% or 10% CO2) would be warmer than normal air, that is not relevant, what is relevant is whether an increase from 0.04% to 0.06% would make a measurable difference.
- apparently, it only requires a small amount of CO2 to block all the infra red anyway, and we are way past that point. Any more than that makes no difference. But he does a lot of stuff for the BBC, so he draws the opposite conclusion.
but let's gloss over those flaws. There is clearly a difference.]
There are actually four effects here, all pretty much undisputed:
1. CO2 absorbs and re-emits more infra red that N2 or O2*.
2. CO2 has a lower specific heat capacity that N2 or O2, so for a given amount of energy coming in, CO2 will warm up more than normal air.
3. CO2 has lower conductivity than N2 or O2, so once warmed up, won't cool down as fast.
4. On a very large scale, the lapse rate would be higher if we had a significant amount of CO2 in the atmosphere (let's say more than 10% CO2) because a its lower specific heat capacity means a higher lapse rate. But this effect is irrelevant in the laboratory.
So what is the relative importance of the first three effects in these laboratory experiments?
I stumbled across a write-up of a cool classroom experiment. Instead of filling one container and one with normal air, they filled one with CO2 and one with argon.
Those two gases have a similar molecular mass, higher than normal air (44 and 40, compared to 29). At room temperature, they have a lower specific heat capacity than normal air (0.846 J/g/K and 0.5203 at constant pressure, compared to 1.010); similar conductivity, lower than normal air (16.8 mW/m K and 17.9, compared to 26.2). The only major difference is that argon is mon-atomic, so completely unaffected by infra red and CO2 can absorb/re-emit some infra red wavelengths.
The results were that the two gases warmed more or less identically under a bright light, thus ruling out effect 1 (infra red absorption) as relevant. But the experimenters didn't want to lose their jobs, so they softens the conclusion by saying that the increase in atmospheric CO2 since the 19th might have caused a 0.3C surface temperature rise (i.e. 1% of the claimed Greenhouse Effect of 33C).
* Effect 1 is probably nonsense, or at least wildly overstated. For sure, CO2 can absorb and re-emit infra red, half of it downwards, by definition. But normal air warms up, and in turn it warms up things above or beneath it. Take a tray of ice cubes out of the freezer and put it on some polystyrene, what do you think will happen? It's all just "warmth" as far as the ice is concerned.
2. Climate sensitivity
From The Conversation:
The study, which was organised by the World Climate Research Programme (WCRP) looks at a measure called “equilibrium climate sensitivity”. This refers to how much global average temperatures will increase by in the long-term following a doubling of carbon dioxide concentrations. It can be estimated using three main lines of evidence:
1. Temperature measurements made with thermometers from 1850 (when enough global coverage began) to the near present. By comparing temperatures, CO₂ levels and the effect of other climate drivers in the past and present, we can estimate the longer-term changes.
2. Evidence from paleoclimate records from the peak of the last ice age 20,000 years ago, when CO₂ was lower than now, and a warm period around 4 million years ago when CO₂ was more comparable to today. We can tell how warm the climate was and how much CO₂ there was in the atmosphere based on the make-up of gases trapped in air bubbles in ancient ice cores.
3. Present-day observations – for instance from satellite data – and evidence from climate models, theory and detailed process models that examine the physics of interactions within the climate system.
That's not three lines of evidence, it's one at most!
Line 1 just assumes that CO2 drives temperatures, and skips the whole causation-correlation question.
Line 2 is cherry picking random dates and extrapolating from them and also has same weakness as Line 1.
Line 3 is partly just providing more accurate measurements for Line 1. The climate models and theories are in turn based on the same foregone conclusion, so this is just extrapolation based on questionably logic.
Extrapolations are always dodgy anyway. If you were 5' tall at age 10 and are 6' tall at age 20, it would be stupid to assume that you will be 7' by the age of 30. Interpolation is much safer, you can reasonably assume that you were about 5'6'" at age 15, give or take a bit for growth spurts etc.
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