Originally Posted by
fizzissist
In the first graph is the blackbody spectrum of the Sun and the absorption spectrum of various gases. The Sun emits radiation mostly from the UV to IR, with a peak in the visible region around 500 nanometers (half a micron, a green color wavelength), though the majority of the energy emitted by the Sun is in the IR (the area under the curve to the right of the peak). Most of the shorter UV is absorbed by molecular oxygen and ozone in the in the mesosphere and the stratosphere, respectively. The small amount of near UV gets through to the troposphere and is what we must worry about in terms of sunburn. Visible light is mostly reflected from soil, rocks, water, etc. with only a little being absorbed (the color of most objects is largely due to reflection with a small amount due to absorption). Absorption of visible light usually involves electron transitions in most elements, a high temperature phenomenon, but not always. The Earth's surface mostly absorbs what is called "solar infrared", radiation with a wavelength of about 0.8-5 microns, represented by most of the area under the blackbody curve to the right of the peak. This absorption is mostly responsible for the Earth being warmed. Some of this incoming radiation is absorbed by water vapor and CO2 in the troposphere and is radiated both downward and back up into space before it reaches the ground. Most soil and rock is crystalline material, and these short IR wavelengths cause the crystal lattice to vibrate (no electron transitions to speak of). This vibrational energy is converted and radiated away as longer wavelength "terrestrial infrared" with a peak of around 10 microns, with some of this energy at shorter wavelengths, but more at longer ones greater than 10 microns, though the shorter stuff between 1-5 microns is more strongly absorbed by water vapor as you can see in the lower parts of the figure [/*There's a simple law called Wien's law that describes the maximum wavelength, in microns, emitted by a blackbody at some temperature, as equal to roughly 2900 divided by the temperature in Kelvins, and since the Earth's surface has an average temperature of around 290 K (about 15 C), this figures to be about 10 microns.*/] It is this longer wavelength radiation that is strongly absorbed by the water vapor and CO2, again with water vapor being the stronger absorber of the two. Because of the asymmetric shape the blackbody curve, there is always more area (and energy) under the part of the curve to the right of the peak. [/*There's another law called the Stephan-Boltzmann law that says an object radiates energy at a rate or power proportional to its temperature raised to the fourth power, so a little difference in temperature goes a long way. More about this later.*/]
Due to gravity, the density of the Earth's atmosphere drops off exponentially with increasing height, such that about 80% of the atmosphere is below the stratosphere i.e. in the troposphere, below about 40,000 feet (see atmos structure images). Hence, the majority of the heat absorbed by greenhouse gases occurs in the troposphere, and the absorption also varies exponentially with height according to a quantity called optical depth, which varies for each absorbing gas. For CO2 the optical depth is such that most of the heat absorbed occurs in the mid to upper troposphere (or an optical depth of about 1, i.e. I = Io exp(-1); and optical depth of 2 accounts for 86% of the absorption, and this happens in the first 50, 000 feet or so, down to a pressure of about 250 mb - see "Christy" graph ), with about half re-radiated down towards the ground, and the rest radiated up into space. As CO2 concentrations increase, the height at which an optical depth of 1 or 2 is encountered is not quite so high, so in this sense, the re-radiating level is lowered towards the ground. But the amount of heat trapped does not increase dramatically, and it does not increase unchecked. CO2 has been increasing approximately in a linear fashion since the late 20th Century. Because of the exponential variation of density with height, the response by the atmosphere is a logarithmic increase in temperature, a much slower change than a linear one (CO2 would have to increase exponentially in order to cause a linear increase in temperature). There are a number of estimates about the warming effects of the first 280 ppm of CO2 in the atmosphere, the long term global average for the last million years or so. However, a doubling of this leads to a relatively small additional increase in temperature (see ...-x2.png, and ...-x4.png). Doubling, tripling, or quadrupling CO2 does not lead to a proportional increase in temperature, because most of the heat is absorbed in the first two optical depths, such that adding more CO2 doesnt significantly change the optical depth. Its sort of like piling blankets on a block of ice. After the first two or three, the next five or six don't do much for you because, while the inner blankets are cool due to their proximity to the ice, the outer ones are at room temperature, and this heat eventually conducts inwards towards the ice (the room temp blankets are radiating according the fourth power of room temperature as opposed to the ice which is much cooler). If it didnt work like this, you cold preserve a block of ice indefinitely simply by piling enough blankets on it. This is what Lindzen has been trying to point out for some time now. We are already about 68% of the way towards a doubling of CO2 in the atmosphere, so it has already provided most of the warming that it can.
The other side of this argument is the sensitivity of the atmosphere to CO2, hence the different temperatures predicted for 280 ppm in ...-x2.png, and ...-x4.png (however, once estimated, an increase in CO2 still has a small effect). Lindzen I believe has the best (and lowest) estimate of sensitivity based on observations, which is why his predictions are at the bottom of both the 2nd and third graphs. John Christy has been arguing that the amount of heat predicted to be trapped in the mid to upper troposphere by the IPCC models has not appeared ("Christy"). Microwave sounding units (MSU) on satellites, and weather balloons, have only observed about 1/2 to1/3 of the predicted warming, and Lindzen has been claiming that radiometers on satellites have been indicating two to three times as much outgoing radiation, escaping to space, than the models assume, so both of these assessments agree. So the sensitivity of the atmosphere is grossly overestimated by the models.
I hope this makes sense. I recently read a little science blip article that mentioned satellite measurements of the upper atmosphere, the "thermosphere ", indicate that it is cooling. The temperature of the thermosphere is largely due to solar radiation, not re-radiated terrestrial radiation, but the author of course concluded that this was another indication of the far reaching effects of AGW, not a decline in solar activity. The trapping of more heat by the troposphere means less energy is radiated into and absorbed by the stratosphere, so the stratosphere cools slightly, but the stratosphere is well below the thermosphere. Dont let the apparent temperature of the thermosphere fool you. The "temperatures" are an indication of molecular speeds, and not bulk temperature, as there is no bulk atmosphere here. The density of the atmosphere is so low in the thermosphere, such that if you could stick your hand into it, you wouldn't sense any heat. Astronauts have to be cooled in their space suits because they absorb solar radiation and the "atmosphere" around them cannot conduct away any of this, or their own body heat, space acting like a perfectly evacuated Dewar, such that astronauts would roast in the heat they cant shed to their surroundings.
........There.
Now, that was too much like work, and I'm here for fun. So, back to reading about G.P. Bear in Washington.
(a very special thanks to my good friend Matt for clarification on this one)