Time for another physics blog.
For something that is supposed to be the most important issue facing mankind, it seems utterly amazing to me that so few people have any real understanding of how this thing called “the Greenhouse Effect” works. The explanation most people know, that is repeated endlessly in the media, is oversimplified to the point of being simply wrong.
That’s not a criticism of those people who have been deceived. That’s a criticism of those few who do know better, but who persist in using their reputations as experts to push this rubbish without explaining it. It does make it harder to challenge, but I don’t suppose that’s why they do it.
And before anybody jumps to conclusions about my ‘denier’ ravings, what I’m about to describe to you is the official, orthodox climate science version, as seen in peer-reviewed literature. (Behind a paywall, I’m afraid.) I will come on to its shortcomings on another day I’m sure, but it would be a good start if first we all understood the theory we’re supposed to be debating.
One thing we can get out of the way immediately is that it doesn’t work in the same way as a greenhouse. There used to be a theory, dating back to Joseph Fourier in 1824, that visible radiation could enter through the transparent glass, but because glass is opaque to infrared, when it is re-emitted it gets trapped. Fourier proposed that gases in the atmosphere could act the same way. This theory was proved wrong for actual greenhouses in 1909 by Professor Wood of John Hopkins University. An experiment comparing a pane of glass to a pane of crystallised rock salt (Sodium Chloride) which is totally transparent to infrared found no difference in temperature. In fact, greenhouses work by preventing convection, a mechanism that is of course impossible to freely floating CO2.
However, this idea that heat is “trapped” in the atmosphere by absorption by CO2 has persisted. While it is now more-or-less accepted that greenhouses don’t work this way, what isn’t so well known is that neither does the Greenhouse Effect.
We have a bit of a problem with terminology here. The Greenhouse Effect has nothing to do with with greenhouses, real or theorised. So what do we mean by the term, if not the mechanism of visible light in – outgoing infrared trapped? Here, I am using the term to describe a fact of which there is no doubt – the surface of the Earth is warmer than it should be on the grounds of energy balance alone. We know how much energy enters the Earth’s system from the sun. The Earth ought to settle at the temperature at which it radiates exactly the same amount to outer space. (Taking the average over time and latitude.) We can calculate this temperature fairly easily, and it turns out to be –24 C. But the average temperature of the Earth’s surface is about 14 C! This temperature difference and its dominant mechanism are what I’m choosing to call the Greenhouse Effect.
Right. Now here’s a puzzle for you. Everybody knows that hot air rises. So why are the tops of mountains so cold?
It’s one of those sort of questions that only geniuses and small children ask. We all know hot air rises. But if you measure the temperature of the atmosphere as you climb, it starts at about +14 C at the surface, and drops in a straight line to –54 C at about 10 km up. (I’m simplifying. It varies from about 8 km to 20 km, but 10 is the standard.) Then it levels out, and starts rising again with altitude.
We have another mystery, too. What keeps the layer at 10 km so cold? –54 C is far below the –24 C we expect on energy-balance grounds, so it can’t be by radiating to space. And the fact that there is a straight line all the way down to the ground suggests that whatever the mechanism is, it’s the same one that keeps the surface at +14 C. Straight lines don’t happen by accident.
I won’t keep you in suspense any longer. The answer is pressure. Because of the weight of air, the pressure at the surface is greater than it is higher up. This means that if air moves up and down, the pressure changes, and the air expands or is compressed. And when air is compressed its temperature increases.
Air is driven to circulate up and down by convection. As it rises, it expands and its temperature drops. As it descends, it is compressed and its temperature rises. This maintains a constant temperature gradient of about 6 C/km. (It would be bigger, but evaporation of water carries heat upwards too, which somewhat counteracts the effect.)
No heat passes in to or out of the air to effect this change. It is solely an effect of the changing pressure. (If you really want to know, the compression does ‘work’ on the gas, which increases its internal energy. It doesn’t come from any flow of heat or radiation.)
This temperature gradient is called the adiabatic lapse rate, and is an absolutely standard bit of physics.
Right, we’re half way there. There is a second piece to the puzzle, which is where those greenhouse gases come in. We’ve said there’s a slope, but we haven’t said what level the slope should be set at.
When we look at the Earth in infrared wavelengths, we see it merrily glowing away, like a coal ember, radiating all the heat it has absorbed from the sun. But unlike the view in visible light, where we can clearly see the surface, in infrared the atmosphere is fuzzy and opaque. It is full of water vapour, and a few other trace gases, that fog our view of the surface. And so when we ask what temperature the surface of the Earth should radiate at, the surface we see isn’t solid ground, but this fuzzy layer high up in the air. And therefore, it is this surface that settles down to –24 C, to radiate exactly the right amount of heat away.
It is about 4 km up, and held at –24 C by the heat rising from below balancing radiation directly to space. Below it, compression increases the temperature. Above it, decompression lowers it. The actual mechanism and explanation for the Greenhouse Effect is in fact pressure. To be specific, it is the pressure difference between the surface and the average altitude from which heat radiates to outer space. Moreover, it is the exact same mechanism by which the upper atmosphere is cooled to –54 C, and there is no way you can explain a massive cooling by heat being in any sense “trapped”.
Heat is not trapped by absorption by CO2. That is Wrong, Wrong, Wrong! Such trapping does go on, but it has no long-term effect on the temperature because the adiabatic lapse rate has overriding control. You can even theoretically get a greenhouse effect with no greenhouse gases at all! All you need is some high altitude cloud to radiate heat to space.
Take Venus, for example. Venus has a layer of thick cloud about 50-80 km above the surface, and a surface pressure of 92 (Earth) atmospheres. Can you tell me now, why do you think its surface is so hot? The cloud tops are at a roughly Earth-like temperature. (And an Earth-like pressure. A point that has some interesting possibilities for colonisation. Balloons, anyone?) It’s 50 km down to the surface at about 8 C/km. And there’s plenty of convection to keep things mixed up. It’s simple physics. No need for a ‘runaway’ anything.
Now supposedly, according to rather more complicated calculations, doubling CO2 levels in Earth’s atmosphere will raise the average altitude of emission about 150 m, which will therefore raise the pressure difference and hence the surface temperature about 1.1 C. If we raise CO2 by only 40%, surface temperature will go up about half that. So we had half a degree last century (an amount too small to reliably measure). We’ll have half a degree next century. And that’s all the standard Greenhouse Effect can give you.
You have to invent a bunch of other effects on top to turn it into a disaster, and that’s where it gets controversial. But I’m sure we’ll talk about that some other day.