Global Warming I - The Physical Science in a nutshell
When I was working on my Master's thesis, it suddenly occurred to me one night as I wrestled with ModTran that one aspect of it closely corresponded to the science involved in global warming. I was trying to calculate the effects of the atmosphere on light; light is electromagnetic energy; global warming is the result of what happens when energy tries to radiate from earth to space.
My problem had to do with what happens when light from the sun, which can be approximated as a black body of approximately 6600 K (or other values, depending on what you're doing), comes through the atmosphere. A black body emits light over a broad range of wavelengths in a manner first successfully described by Planck (his breakthrough that led to the development of quantum theory). Only three things can happen as energy moves from one medium (space) through another (the atmosphere): it can be reflected, absorbed, or transmitted. It turns out that there are several "windows" in the atmosphere that selectively pass light of certain wavelengths. The largest window happens to correspond to the wavelengths to which our eyes respond (roughly 0.4-0.7 microns), so it is called the visible window or band. This also happens to correspond to the peak wavelengths of the sun. Other windows lie in the ultraviolet spectrum (shorter wavelengths than visible) and the infrared (longer wavelengths).
Planck's curve for a 6600 K blackbody
Atmospheric transmission curve such as the one above are commonplace (this one from here)
When the earth warms, it also has characteristics of a black body, but it is considerably cooler than the sun (nearer 300 K). As a result of being cooler, the peak wavelengths are much longer (see also Wien's Displacement Law). Thus, the ability of the earth to radiate energy back to space depends more (though not exclusively) on the windows in the IR than those in the visible spectrum. The extent of those windows are governed by the gases in the atmosphere which selectively absorb in specific wavelengths.
Planck curve at 300 K, about room temperature (note the vertical scale has changed but the horizontal has not)
CO2 is mostly active in the 3-5 micron region called Midwave Infrared or MWIR, though it is also active in the extremely long wave IR (LWIR). Methane has two very strong spikes in the Short-to-Mid wave IR. CO, O2, and O3 are also active in a few areas. However, the number one most important gas is the vapor of dihydrogen monoxide, which is potentially lethal in its liquid form, has been found in many lakes in North America, and has been found in cancer cells. Also known as water vapor, this gas forms between 0 and 2% of the atmosphere depending on local conditions (as opposed to CO2's 0.04% and methane's 0.005%). That is, water is absent in very hot deserts and in very cold regions (such as the poles). Water vapor is active in every region of the electromagnetic spectrum, from light to long wave IR (LWIR). No other molecule comes close to water's ability to respond to incident radiation because of water's unique molecular shape. Whereas CO2 is laid out symmetrically on an axis, H2O is shaped more like Mickey Mouse's head. This gives it the ability to vibrate in many modes at many frequencies (think of the molecules as masses attached by springs, like John Belushi's bee antennae); since frequency is inversely related to wavelength, this means H2O interacts at many wavelengths.
Each of the charts above shows where that gas by itself is active in absorbing energy, except the last which shows the cumulative effect. Note the similarity to the atmospheric transmission near the top (this one is inverted, since it is showing absorption rather than transmission). Also, note that the shape of the cumulative curve is far and away dominated by water's contribution. This curve comes from the excellent, 8 Volume The Infrared and Electro-Optical Systems Handbook, Joseph S. Accetta and David L. Schumaker, eds., specifically out of vol. 2, Atmospheric Propagation of Radiation.
Water is thus the greatest contributor to the Greenhouse Effect, though not to the phenomenon known as Global Warming, or more precisely, Anthropogenic (man-made) Global Warming (AGW) or "Forced" global warming (see also this from RealClimate) . Without the Greenhouse Effect, we would likely not exist. The Greenhouse Effect is what keeps Earth from being an icy ball in space; I have read that without the atmosphere, the average temperature of Earth would be about -18 degrees C. Using only the Greenhouse Effect, the average temperature would, however, be about +30 degrees C, way too hot. In fact, the average temperature is about +16 degrees C (average around the whole world, including the poles). I have read that the difference is apparently accounted for by convection and the energy required to melt and evaporate ice and water, respectively, (which was why I said that the the earth's ability to radiate back to space did not depend exclusively on atmospheric transmission windows in the IR spectrum), though that may overstate the case for convection and state change.
I have no doubt that the atmospheric content of CO2 is increasing. The Mauna Loa data (here) and even the difference between what we have sampled lately and the default ModTran values shows that.
Given just these facts, we can surmise that an increase in CO2 will make the most difference where its contribution are relatively greater than those of the sun and water. Those are at night (when the sun is not contributing) and in deserts and over poles (where water content is lower).
My problem had to do with what happens when light from the sun, which can be approximated as a black body of approximately 6600 K (or other values, depending on what you're doing), comes through the atmosphere. A black body emits light over a broad range of wavelengths in a manner first successfully described by Planck (his breakthrough that led to the development of quantum theory). Only three things can happen as energy moves from one medium (space) through another (the atmosphere): it can be reflected, absorbed, or transmitted. It turns out that there are several "windows" in the atmosphere that selectively pass light of certain wavelengths. The largest window happens to correspond to the wavelengths to which our eyes respond (roughly 0.4-0.7 microns), so it is called the visible window or band. This also happens to correspond to the peak wavelengths of the sun. Other windows lie in the ultraviolet spectrum (shorter wavelengths than visible) and the infrared (longer wavelengths).
Planck's curve for a 6600 K blackbody
Atmospheric transmission curve such as the one above are commonplace (this one from here)
When the earth warms, it also has characteristics of a black body, but it is considerably cooler than the sun (nearer 300 K). As a result of being cooler, the peak wavelengths are much longer (see also Wien's Displacement Law). Thus, the ability of the earth to radiate energy back to space depends more (though not exclusively) on the windows in the IR than those in the visible spectrum. The extent of those windows are governed by the gases in the atmosphere which selectively absorb in specific wavelengths.
Planck curve at 300 K, about room temperature (note the vertical scale has changed but the horizontal has not)
CO2 is mostly active in the 3-5 micron region called Midwave Infrared or MWIR, though it is also active in the extremely long wave IR (LWIR). Methane has two very strong spikes in the Short-to-Mid wave IR. CO, O2, and O3 are also active in a few areas. However, the number one most important gas is the vapor of dihydrogen monoxide, which is potentially lethal in its liquid form, has been found in many lakes in North America, and has been found in cancer cells. Also known as water vapor, this gas forms between 0 and 2% of the atmosphere depending on local conditions (as opposed to CO2's 0.04% and methane's 0.005%). That is, water is absent in very hot deserts and in very cold regions (such as the poles). Water vapor is active in every region of the electromagnetic spectrum, from light to long wave IR (LWIR). No other molecule comes close to water's ability to respond to incident radiation because of water's unique molecular shape. Whereas CO2 is laid out symmetrically on an axis, H2O is shaped more like Mickey Mouse's head. This gives it the ability to vibrate in many modes at many frequencies (think of the molecules as masses attached by springs, like John Belushi's bee antennae); since frequency is inversely related to wavelength, this means H2O interacts at many wavelengths.
Each of the charts above shows where that gas by itself is active in absorbing energy, except the last which shows the cumulative effect. Note the similarity to the atmospheric transmission near the top (this one is inverted, since it is showing absorption rather than transmission). Also, note that the shape of the cumulative curve is far and away dominated by water's contribution. This curve comes from the excellent, 8 Volume The Infrared and Electro-Optical Systems Handbook, Joseph S. Accetta and David L. Schumaker, eds., specifically out of vol. 2, Atmospheric Propagation of Radiation.
Water is thus the greatest contributor to the Greenhouse Effect, though not to the phenomenon known as Global Warming, or more precisely, Anthropogenic (man-made) Global Warming (AGW) or "Forced" global warming (see also this from RealClimate) . Without the Greenhouse Effect, we would likely not exist. The Greenhouse Effect is what keeps Earth from being an icy ball in space; I have read that without the atmosphere, the average temperature of Earth would be about -18 degrees C. Using only the Greenhouse Effect, the average temperature would, however, be about +30 degrees C, way too hot. In fact, the average temperature is about +16 degrees C (average around the whole world, including the poles). I have read that the difference is apparently accounted for by convection and the energy required to melt and evaporate ice and water, respectively, (which was why I said that the the earth's ability to radiate back to space did not depend exclusively on atmospheric transmission windows in the IR spectrum), though that may overstate the case for convection and state change.
I have no doubt that the atmospheric content of CO2 is increasing. The Mauna Loa data (here) and even the difference between what we have sampled lately and the default ModTran values shows that.
Given just these facts, we can surmise that an increase in CO2 will make the most difference where its contribution are relatively greater than those of the sun and water. Those are at night (when the sun is not contributing) and in deserts and over poles (where water content is lower).
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