At The Guardian's series How to talk to a climate sceptic they have an 'interactive' bit which allows you to click though to this:
The greenhouse effect is one of the main factors determining the temperature of a planet. It's the phenomenon by which certain gases – so-called greenhouse gases – in the atmosphere trap heat that would otherwise escape to space, thereby keeping the planet warm.
The greenhouse effect is not a man-made phenomenon. The Earth's atmosphere has always contained greenhouse gases, such as CO2, and they have always caused warming. If there was no greenhouse effect, the planet would be uninhabitably cold – more than 30C colder than the hospitable current average of 15C.
I like the use of the expression "one of the main factors". I refer once again to the fine article by Pa Annoyed at Counting Cats which explains it all properly, to summarise:
1. Greenhouses themselves do not work by 'trapping infra red', they just prevent warm air inside the greenhouse being blown away.
2. The main influence on global temperatures is the Sun (the other is good old fashioned clouds made of water droplets). If the Earth had no atmosphere at all, we would indeed expect the average surface temperature to be -24C.
3. The main reason why surface temperatures are much higher that this is because we have an atmosphere and gravity, which produce pressure. Gravity pulls down all the atoms and molecules in the atmosphere on top of each other and so pressure at sea level is much higher than at the top of a mountain.
4. Remember Charles' Law, if you put a gas under pressure, it will warm up. So surface temperatures are thirty-to-forty degrees warmer than expected, and up at ten kilometres altitude, the temperature is thirty-to-forty degrees cooler than expected because that air is de-pressurised (so to speak) and this cools it.
5. Overall, the average temperature of the atmosphere averages out to the expected average surface temperature of -24C. The true 'surface' of the Earth is not ground level, it's somewhere 4 km up.
6. The author reckons that massively increasing the amount of CO2 in the atmosphere will increase surface temperatures a bit (doubling CO2 -> increase of 1C) because CO2 is heavier than oxygen or nitrogen and so increases pressure at ground level, but it won't increase the average temperature of the atmosphere, hence there won't be any runaway anything.
Saturday, 27 August 2011
Greenhouse Effect Fun
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13 comments:
Spot on - CO2 is not a problem we should worry about even if we could do anything about it. Unlike house prices.
"because that air is de-pressurised (so to speak) and this cools it."
AFAICR from doing A-level physics, a gas only warms or cools when its pressure changes, so a system in equilibrium, like the Earth's atmosphere, can easily be the same temperature throughout, whilst being at different pressures. The fact that it is not is, AFAIK, because of the fact that solar radiation doesn't heat the air very much, it mainly heats the ground, therefore the air closest to the ground receives the most heat from the ground, and hence tends to be hotter.
This is not why the CO2 "greenhouse effect" is bollocks. It is bollocks because the main job of reflecting infra red radiation back to the earth is done by clouds. CO2 only has a bit part in this. The main "greenhouse gas" is water vapour. It is also bollocks, because the greenhouses gases don't reflect infra red like a mirror, they scatter it, so that only a proportion eventually comes back towards the earth. Also it is bollocks because the main effect of greenhouse gases is precisely like a greenhouse - they prevent the warm air drifting up and cooling down by trapping it near the surface, which is why it gets bloody cold at night in the desert. And finally it is bollocks because the long term correlation between the earth's temperature and CO2 levels is assumed to mean that high CO2 causes high temperature and a mechanism has been invented to account for this, when the reverse could easily be true (and probably is), that high temperatures cause high CO2 levels.
AKH, I like this explanation because intuitively it is correct and it is easy to understand.
B, as to your second para, clearly the temperature of the atmosphere does not even out between sea level and 10km up, that's a matter of fact.
I take your point that if you compress a gas it heats up and if you just leave it, it will cool down again, but that is only because heat is transferred to the container and so on. But the atmosphere is its own container! Warmth can't be transferred up from warm lower layers to cooler upper layers because that would involve warm molecules going up (and hence cooling) and cold molecules coming down (and hence warming).
Your list is very good though, this is also stuff which we know from observation to be true - it's noticeably warmer on a cloudy night than on a clear night (for a given day time temperature).
Venus is hotter because it's
a) Closer to the Sun.
b) Got 100 times more atmosphere.
Mars is colder for the opposite reasons.
IF you swapped their orbits I wonder what the temperature would be?
Anon, indeed. If you swapped their orbits they might end up with the same surface temperature.
But it is more instructive to compare Earth and Moon, as they are the same distance from the Sun, the only difference being the Moon has no atmosphere (and much less gravity) BUT the Moon is darker than the Earth, on average (lower albedo).
"B, as to your second para, clearly the temperature of the atmosphere does not even out between sea level and 10km up, that's a matter of fact."
Did I imply that? I meant that at sea level (closest to the "warm" sea) the air is warmer and 10km up, the air is further away from the sea and hence cooler.
"Warmth can't be transferred up from warm lower layers to cooler upper layers because that would involve warm molecules going up (and hence cooling) and cold molecules coming down (and hence warming)."
I am sorry to say that this is completely wrong. Hot air rises, cold air falls. This is how hot air balloons and chimneys work. Yes, in rising it cools, so it eventually stops rising, but rise it does, initially. The tendency for the lower, warmer layers of the atmosphere to rise, cool, and then fall is what causes the weather.
B, Counting Cats explains it better than I can, i.e. the point that superficially, warm air rises etc, but that the temperature difference between high up and sea level is a more or less permanent thing and has to do with gravity and hence pressure and so on.
I've read his piece now (the link was broken last time I tried it) and I can see what you mean, though it seemed you were implying that the heating and cooling was something static (the gas near the surface is hotter because it's under higher pressure) when it's actually something dynamic (cold air falling becomes hotter as the pressure increases (but still remains colder than the surrounding air, which is why it continues to fall)).
The "greenhouse infa red trap" (disproving) experiment was R Wood John Hopkins university 1909
I was going to say, as Bayard has, that it is not a static thing - it's dynamic and there could well be other factors as well but that having been said, population crammed into zoned areas is far more of a problem.
B, I can understand it but can't explain it, hence read what the expert says.
Den, yup. It is amazing that this idea still persists, it's like the notion that aeroplane wings have to be curved, which is clearly nonsense or else they wouldn't be able to fly upside down and paper planes wouldn't work.
JH, per capita, people in towns and cities cause less pollution than those in the countryside - smaller houses, less heating, shorter journeys, more public transport and refuse collection is cheaper as well.
Dreadful subject, made infinitely worse by the use of simplistic analogies rather than explanations (not you M, I'm talking about the greenhouse proponents.)
I also liked Pa A's post when I first saw it, much to commend it.
It's important to distinguish between the real world that 'is', i.e. the one you see when you look out the window, and an abstract 'modelled' world used for scientific study/philosophical musings.
The real world atmosphere is a hugely complex, dynamic, non linear, possibly chaotic system, which we understand poorly and imperfectly.
In order to study it we isolate specific aspects and try to improve our understanding of that area and it's behaviour. This does not mean that we then better understand the overall behaviour, or even the contribution of our area of study to the whole, but we hope we might.
To answer Bayard above, Pa A was indeed talking about the fact that the increased pressure at lower altitudes causes the air temp to be higher. Convection etc has nothing to do with it (in fact it is a mechanism to cool the surface in the real world). Remember though, that he is not talking about our real world atmosphere, he is talking about a physical fact. Increasing the pressure of a gas, without adding any energy (adiabatically), will cause the temp of the gas to rise. We see the opposite effect when we allow a gas under pressure in a cylinder to escape through a nozzle - the nozzle rapidly freezes up due to the temp drop in the expanding gas.
If we take a calm day at sea level, at e.g. 20deg C ground temp, and go straight upwards the surrounding air will cool as we get higher. The actual rate varies with moisture content, but a useful number is the one used in aviation, which is 3.56deg F or 1.98deg C per 1000 ft. rise. At 1000 ft up, it will therefore be just over 18 deg C, and it will continue to drop at that rate up to 36000 ft or so, when it hits the -56 deg C or so noted in the post. Again, this is a physical property of the total atmosphere, completely independent of any atmospheric circulation, but NOT unaffected by convection, radiation and other local or larger effects.
Ch, ta, that's how I understood it.
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