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Paul_G comments on Open Thread, December 1-15, 2012 - Less Wrong Discussion
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Why do LWers believe in global warming? The community's belief has changed my posterior odds significantly, but it's the only argument I have for global warming at the moment. I saw the CO2 vs temperature graphs, and that seemed to sell it for me... Then I heard that the temperature increases preceded the CO2 emissions by about 800 years...
So why does the community at large believe in it?
Thanks!
I believe it is true as an environmental engineer engaged in atmospheric modeling. Atmospheric modeling is a field in which the standard scientific method seems to be working well, that is, there is a large benefit to researchers who are right and/or can prove others wrong. This means that there is a lot of effort going into improving models that are already quite accurate, to the limits of the data you input. For example, the 1990 model of climate change does quite well if you give it better data, and at least correctly predicts the temperature trend with bad data. http://www.huffingtonpost.com/2012/12/10/1990-ipcc-report_n_2270453.html Similar to comments below, the IPCC is an enormous body, and I find invalidating their arguments to require an implausible conspiracy theory. You can look up the executive summary for the various reports at your leisure, they are quite readable.
Can't speak for the community at large.
CO2 blocks some frequencies of infrared. This is known and uncontested by even the craziest deniers. Without an atmosphere the earth's average temperature would be around -20 C. You can calculate this based on radiation theory. (that specific number may be wrong, but it's around there). An atmosphere with CO2 (and some other major ones I don't remember) blocks a higher proportion of the radiation from earth than from the sun (because the earth radiation is mostly infrared near the range blocked by CO2). With a model for that, you can recalculate the surface temperature. It will be much higher.
edit: (on the other hand, now that I think about it, I can't prove to myself that absorbant CO2 will actually cause a greenhouse effect. Maybe it's reflective, which would cause greenhouse...) /edit
edit2: ok I just read the wiki article. Everything they tell you about how the greenhouse effect works is wrong. It's not that the atmosphere somehow blocks the outgoing radiation, as that would violate the second law by allowing the earth to heat up relative to it's surroundings. The real mechanism is that the absorbtion surface (the ground) and the emission surface (roughly tropopause) is seperated by a mechanism that enforces a temperature difference (adiabatic lapse rate). I need to think about this more. /edit
That analysis does not include things like the effect of temperature on albedo (clouds and snow), which changes things, and other effects, but it gives you rough bounds for what must happen. The model establishes a causal link from CO2 to temperature (there are also links the other way, like forest fires and desertification).
Beyond that, though, climate science is mostly empirical I think.
My rough belief is that global warming is a thing, but is probably hyped up a bit too much for political reasons.
That can't be right. The atmosphere does block most of the outgoing radiation — its transmissivity for the Earth's longwave radiation is only about 20% — and if it were transparent to radiation it couldn't exert a greenhouse effect at all. Also, a thought experiment: if we had an electric oven plugged into a solar panel orbiting the Sun, the oven could heat itself relative to the surrounding space just by using light from the Sun, and that wouldn't violate the second law.
Maybe the second law is the wrong way to look at it. The second law says that the sun can't cause you to heat up hotter than the sun on average. (you can do tricks with eat pumps to make parts of you hotter than the sun, though)
It also says you can't do tricks with surface properties to change your temperature. (in the absence of heat pumps)
Ok I'm still a bit confused about this. I suspect that this effect alone is not enough to cause a greenhouse effect. Let's think it through:
Assume the 0.2 missing from transmissivity is all absorptivity (t, a, and r add up to one). And that we model it as simply an optical obstruction in thermal equilibrium.
The sun's radiation comes, some of it goes to the atmosphere, some to the earth. If the atmosphere magically ate heat, the earth would get less radiation. However, it does not magically eat heat; it heats up until it is emitting as much as it absorbs. The longwave from earth also gets eaten and re-emitted. About half of the emitted goes to earth, the rest out to space.
So our greenhouse layer prevents some power P1 from getting to earth. Earth emits, and P2 is also eaten. The emitted P3 = P1+P2. Earth gets P3/2. The sun is hotter than earth so the power at any given wavelength will be higher, so P1 > P2, therefor P3/2 > P2, which means on net, heat is flowing from the greenhouse layer to earth. However the earth is receiving P1 less from the sun, and P1 > P3/2. So the earth cools down relative to the similar earth without "greenhouse" effect. This makes sense to me because the earth is effectively hiding behind a barrier.
Therefor, if a greenhouse effect exists, it cannot be explained by mere atmospheric absorption. Unless I made some mistake there...
The assumption that is not true in that model is the atmosphere being in independent thermal equilibrium.
If we instead make the atmosphere be in thermal eq with earth, there is no effect; the earth acts as a single body, and absorption by atmosphere is the same as absorption by ground.
If we instead model the atmosphere realistically as a compressible fluid, things become more interesting. I'm not going to do the math here, but the model goes like this: the atmosphere at ground is eq with the ground. If a piece of air gets heated up at ground, it expands and floats up. As it goes up, there is less pressure from the air above it, so it expands, which does work, which cools it down. It cools down at the adiabatic lapse rate, which is the temperature gradient in a well mixed compressible fluid in a gravitational field. At the tropopause, our piece of air has reached about -40. The tropopause is where the atmosphere stops being opaque to greenhouse rays. Therefor, the tropopause is approximately the emission/absorption surface for greenhouse radiation, and the earth gets the other stuff.
So what does this mean for greenhouse? If we make the atmosphere absorb more, it shifts the average radiation surface upwards to colder air. If the surface of our body is too cold, it must heat up to maintain thermal eq. So it heats up. The lapse rate enforces a certain temperate rate between earth and atmosphere, so you can see if you move the equilibrium point up, the earth has to heat up. As for the second law, the atmosphere is acting as a heat pump.
Therefore global warming.
Even this model is a bit broken. if you heat up some air at the top of the atmosphere, it stays up there and stops mixing. I think this is what the tropopause is. I have no idea how to model this.
I think so. In practice, changing the surface properties of a body in orbit can affect its temperature. If we coated the Moon with soot it would get hotter, and if we coated it in silver it would get colder.
Two key complications break this toy model:
P1 > P2 doesn't follow from the Sun having higher spectral power. The Sun being hotter just means it emits more power per unit area at its own surface, but our planet intercepts only a tiny fraction of that power.
The atmosphere likes to eat Earth's emissions much more than it likes to eat the Sun's. This allows P1 to be less than P2, and in fact it is. P2 > P1 implies P3/2 > P1, which turns the cooling into a warming.
The barrier metaphor's a bit dodgy because it suggests a mental picture of a wall that blocks incoming and outgoing radiation equally — or at least it does to me! (This incorrect assumption confused me when I was a kid and trying to figure out how the greenhouse effect worked.)
It's a false assumption, but it's not the assumption breaking your (first) model. It's possible to successfully model the greenhouse effect by pretending the atmosphere's a single isothermal layer with its own temperature.
The second model you sketch in your last 4 paragraphs sounds basically right, although the emission/absorption surface is some way below the tropopause. That surface is about 5km high, where the temperature's about -19°C, but the tropopause is 9-17km high. (Also, there's mixing way beyond the top of the troposphere because of turbulence.)
Yeah, understanding the real reason for the greenhouse effect was tricky for me. CO2 makes the atmosphere opaque to infrared even on the scale of meters, so it's not like a regular greenhouse. If the CO2 already absorbs all the infrared emitted from the ground, why does increasing CO2 decrease the amount of energy reaching space? Because what space sees is the temperature of the last atom to emit infrared, and as you add more CO2, the last atom gets higher and higher on average, and thus colder and colder.
This is more like a "warm, clear blanket" effect than a greenhouse effect. (That is, more like diffusion than reflection).
Though note that neither greenhouses nor warm blankets violate the second law - they just can't get any warmer than the sun, which is pouring in energy at wavelengths for which the atmosphere is mostly transparent. Good ol' sun.
This is one of those things you should probably just take on authority, like relativity or the standard model of particle physics. That is to say, it's an exceedingly complex topic in practice, and any argument stated for either side which can readily be understood is likely to be wrong. You have two or three options: study the field long enough to know what's going on, or trust the people who have already done so. (The third option, 'form an opinion without having any idea what's going on', is also commonly taken.)
In short: I believe it's happening because this is what scientists tell me, and it's not worth putting in the time required to understand the field well enough that I could trust my opinion over theirs.
You might want to look at Skeptical Science which lists a large number of arguments raised by skeptics of global warming, and what climate science has to say about them. "CO2 lags temperature" is number 11 on the list. Here is the basic response:
This is exactly what I was looking for! Thank you kindly, looking through it as soon as I find time.
Source?
I have lots of reasons for believing in climate change I could quote at you, but they can mainly be found on the relevant wikipedia pages (so I assume you've already looked at them). So why am I putting more credence on those arguments than you? (Assuming we're both equally rational/sane/intelligent).
What it comes down to when you abstract from individual arguments, is that those who have most domain specific expertise strongly believe it to be true. In general it is best to trust experts in a particular domain unless you have strong reasons to believe that field is flawed. Absent improbable conspiracy theories I have no reason to in this case.
Teacher in a geology class who is decidedly non-rationalist mentioned that 800 years thing, without a source. Something about thickness of a line.
This is the first topic I've found in which I have no idea how to dissect this and figure out what's going on. It appears that there are incredibly powerful arguments for both sides, and mountains of strong evidence both for and against human caused climate change... Which shouldn't be possible. A lot of the skeptics seem to have strong arguments countering many of the "alarmist" ideas...
I'm not a good enough rationalist for this, yet. If it weren't for this community's famous support of global warming, there is no way I'd believe in it, given the data I have. Strange.
I'm not sure it's worth posting sources and the like, counter-counter arguments become difficult to follow, and it could easily cause a kurfuffle that I would rather avoid.
Thank you all greatly!
The lag is a phenomenon of the ice age cycle, which is caused by orbital shifts but amplified by emission or absorption of carbon dioxide by the ocean. It takes the ocean about a thousand years to respond to changed atmospheric temperature.