Wednesday, February 21, 2007

A Brief Primer on Global Warming

Global warming skeptics hide behind the notion that the effects of greenhouse gases are “very complicated,” which is true, but that shouldn’t stop anyone from knowing the easy parts.

The first easy part has to do what’s called radiative equilibrium, which may be expressed with a Firesign Theater reference to Teslacles’ Deviant to Fudd’s Law: “It goes in; it must come out.” In other words, the energy input to the Earth from the Sun must be re-radiated, otherwise the Earth would continue getting hotter without bound, and that would violate the Second Law of Thermodynamics.

In fact, the Earth, along with most other planets, radiates slightly more energy than it gets from the Sun, because the Earth has a natural, internal heat source, caused by radioactive decay. But that’s a small correction for the Earth (Jupiter’s excess heat is much greater, but Jupiter has more than just radioactive sources for its internal heat source).

Now the first thing that happens to light hitting a planet is that some of it gets reflected immediately. That’s called albedo, and it’s not a small effect. The albedo of the Earth is about 37%, which is pretty high compared to the other inner planets. Only Venus, at 65% is higher, whereas Mercury’s reflectivity is only 11%, the Moon’s is 12% and Mars’ 15%. These are averages, of course, although Venus is pretty uniform, because of the constant clouds.

The Gas Giants (Jupiter, Saturn, etc.) hover around 50%; some icy moon’s (I’m looking at you, Enceladus!) have very high albedos, despite not having atmospheres.

But it’s atmospheres that boost albedo, no real surprise there, because molecular scattering of light by gases (Raleigh scattering) is pretty efficient. Then there are those spiffy sulfuric acid droplets in the atmosphere of Venus.

Venus is particularly instructive, because its atmosphere is both thick and reflective. The result is that, by one measure, the place is surprisingly cool. That measure is the temperature at 55 km above the surface, where you have about one-half Earth’s atmospheric pressure and a temperature of a balmy 80 F. Up the O2 in your air supply a bit, and you could live there quite comfortably (in my story “Aphrodite’s Children,” I have it a bit cooler, owing to a reduction in the planetary greenhouse, but that just means that people can live further down). If Venus had a lower albedo, you wouldn’t be able to do that, because even at that height it would have to be much hotter to get rid of all the absorbed sunlight.

But the surface of Venus is hundreds of degree hotter, because increasing pressure heats things up. This is called the adiabatic lapse rate, and it’s a simple consequence of Boyle’s Law. Put some more pressure on a quantity of gas and its volume gets smaller, and its temperature increases.

So what does this have to do with the Earth’s greenhouse effect?

Well, first, Earth’s atmosphere is mostly transparent. Oxygen photodissociates in the hard ultraviolet (below about 280 nm, as I recall), and ozone is produced in the upper atmosphere. Ozone, in turn, absorbs certain wavelengths of infrared, so the stratosphere warms, and because there’s a lot of ozone in the stratosphere, it’s warm enough to stratify, i.e. form a thermal inversion.

Other gases absorb some IR, most notably water vapor. Water vapor is mostly confined to the lower atmosphere, because it gets colder as you go up (adiabatic lapse rate/Boyle’s law again), and the water rains and freezes out.

The Sun puts out a fair approximation of black body radiation, and at the temperature at the surface of the Sun, most of the energy is in the visible and near IR region. So the atmosphere is mostly transparent to sunlight. Some of it does get scattered (“Why is the sky blue, Daddy?”) and that has a lot to do with the Earth’s relatively high albedo. And some gets reflected away by clouds, more on that in a bit.

Most sunlight reaches the surface of the Earth, and most of it is absorbed. So the surface warms. Then what?

The surface of the Earth only gets up to, at most, a bit over three hundred Kelvin; a hundred degrees F is mighty hot for an Earth surface temperature, and most of the surface is way below that. At those temperatures, the radiation emitted is in the microwave, very long IR region. A lot of gases have absorption bands in that region; these are the so-called “greenhouse gases,” water vapor, CO2, methane, ozone, etc.

So when the Earth emits radiation, a good bit of it is absorbed by the air above it.

Also, and this is very important, air is warmed by contact with the Earth. So again, air near the ground gets warmed by the Earth.

Warm air rises, and as it rises, it cools. Eventually, it reaches the same temperature as the air whose level it has risen to. But there are some things that interfere with the idea of a “dry adiabat,” as it’s called. For one thing, the air probably contains some water vapor. As the air cools, it loses the capacity to hold water, and so you get water or ice formation (and precipitation). The condensation of the water vapor emits heat, so the air gets some extra “oomph” as it rises.

Then there is the matter of the air continuing to emit thermal radiation. The atmosphere is transparent to some of the bands in the radiation that the air is emitting, so that energy escapes rapidly into space. That cools the air more quickly than you would expect. Eventually, the cooling air begins to descend; usually it has moved toward higher latitudes when it does this, because of the large scale circulation patterns in the atmosphere.

If you add more greenhouse gases to the atmosphere, that radiative cooling phenomenon that I just mentioned happens at a higher altitude than if the GHGs weren’t there. The “optical thickness” of the atmosphere is greater to IR, so the rate of radiative loss is less at any given height. So the air stays warmer longer—and goes higher.

But Boyle’s Law still applies, and when the air begins to sink again, by the time it reaches the ground, it’s a bit warmer. That’s the “Greenhouse Effect” in a nutshell.

Now if that were all there was to it, estimates of global warming would be about half of what they currently are at. So current models of GW have a positive feedback term in them. That positive feedback term is water vapor. Warm air hold more water vapor.

Back in the 1980s, before the GW signal was really clear in the statistics, I thought that the water vapor effect was a loophole. In fact, I thought that increased water vapor would mean increased clouds, and that would result in a negative feedback term, so GW estimates might be high by as much as a factor of four.

Then came Pinatubo.

The eruption of Pinatubo put huge amounts of sulfuric acid droplets into the stratosphere, increasing Earth’s albedo by an easily measured amount. And the standard Global Climate Models just nailed the effect. The water vapor/clouds effect isn’t a long time constant effect. If it were a loophole, it would have shown up after Pinatubo. It didn’t; water vapor is a positive feedback effect, end of story.

Actual scientists (as opposed to political operatives, who are thick on the ground with corporate money from the carbon lobby) have been searching for negative feedback effects to offset GW for many years now. One was the notion that more water vapor meant increased precipitation at higher latitudes, more snow and ice, and that would have an effect on albedo. Nope, glaciers have been in retreat. Others think that added CO2 will cause greater plant growth, and this will somehow do something, but CO2 just keeps going up.

In fact, only about half of emitted CO2 stays in the atmosphere; most think the rest goes into the ocean. Heaven help us if that negative feedback loop goes positive.

And there have been some negative loops that have flipped. Sometime in the 1980s, the Arctic tundra went from being a net sink to a net source of greenhouse gases.

As things continue to heat up, the GW denialists are looking more and more like a faith-based initiative. The science just keeps looking more and more certain, while the denials get more and more strident. It’s already ugly. It’s going to get worse.

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