This post, the 3rd (1st here, 2nd here) in the series on Understanding the Science of CO2’s Role in Climate Change, discusses how water vapor, CO2, CH4, O3 and N2O absorb and emit the Earth’s longwave radiation, changing the Earth’s energy balance.
I’ve made a 5 panel chart that shows spectra data for 5 greenhouse gases (GHG). Molecules of these gases in the atmosphere absorb and emit the Earth’s infrared radiation at specific frequencies, trapping some of the Earth’s radiation, warming the planet.
I’ve included a link to my R script so that readers can access the online spectra data and generate your own GHG spectra.
Discovery of Greenhouse Effect
In 1824, Joseph Fourier, the famous mathematician who discovered the Fourier transform, is credited with discovering that atmospheric gases might increase the temperature of the Earth’s surface through a process later called the greenhouse effect.
John Tyndall, starting in 1858, was the 1st scientist to conduct reliable laboratory experiments on the role that atmospheric gases play in longwave radiation absorption. Spencer Weart, in his Discovery of Global Warming, describes John Tyndall’s contribution to climate science this way:
“In 1862 John Tyndall described the key to climate change. He had discovered in his laboratory that certain gases, including water vapor and carbon dioxide ( CO2), are opaque to heat rays. He understood that such gases high in the air help keep our planet warm by interfering with escaping radiation.” Source: Spencer Weart
Weart quotes from Tyndall’s 1862 paper
“As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth’s surface.” Source: John Tyndall, 1862
Wikipedia has an interesting description of the laboratory equipment that Tyndall used in the late 1850′s to measure the absorption of infrared radiation by atmospheric gases.
We have all heard about the greenhouse effect and we have experienced the effects of it when we open our car doors after it has been sitting in the sun with the windows closed. When we first open the door, we notice that the air inside the car is much warmer than the air outside, a good example of the greenhouse effect (link).
So let’s dig a little deeper into the greenhouse effect, lets review the physics behind the air warming in a car with closed windows on a sunny day.
Infrared Radiation and Atmospheric Gas Molecular Vibration
From post 2 we know that objects like a car’s interior absorb energy/heat from the sunlight. The seats, dashboard and steering wheel absorb the sun’s radiant energy. As blackbodies, these objects also emit radiation in the infrared region. What happens to this longwave radiation within the car?
While the car’s glass windows allow the incoming shortwave sunlight into the car, they block the longwave radiation from leaving, causing the car’s interior to heat up. Opening a window lets the heat quickly escape.
David Archer explains the GHG gas longwave radiation absorption mechanism like this:
“Gases are the simplest types of molecules, and they only vibrate in very particular ways. Vibration in a gas molecule is like the vibrations of a piano string in that they are fussy about frequency. This is because, like a piano string, a gas molecule will only vibrate at its ‘ringing frequency’.” Source: Understanding the Forecast, page 50.
IR Absorption Spectra for Atmospheric Greenhouse Gases
Interested readers can view the GHG gas spectra online at the NIST’s Chemistry Webbook. Here are the links for the individual gases:
Note that NIST’s default spectra plot shows wavenumbers on the X axis in reverse order. You can easily plot in micron’s in normal order by adjusting the pull down window settings.
I wrote an R script to download NIST’s spectra data for water vapor, CO2, CH4, O3 and N2O and generate this 5 panel chart. Click to enlarge
View/ download R script for this chart at this Google docs link.
These spectra show the wavelengths (0 – 22 microns) on the X axis and transmittance (% radiation passing through) on the Y axis. Regions where the transmittance drops to 0 are regions where the GHG completely absorbs the IR radiation. Regions where the transmittance is 1 allow the IR radiation to pass through unaffected. Notice the water vapor, shown as H2O in chart, is active through much of the 2-22 micron span. CO2 is active in the 4.3 and 15 micron regions. O3 is active in the 9 micron region, CH4 is active in the 3 and 7 micron region and N2O is active in the 4 and 7 micron regions.
Water vapor (H2O) is the most powerful GHG, followed by CO2, O3 and CH4.
CO2, O3, CH4′and N2O ‘s importance rests with the fill-in effect that they play to the water vapor spectra. Look at the water and CO2 spectra in the 4 and 15 micron regions. Notice how CO2 fills in the gaps in the water vapor spectra. Water vapor is inactive in the 4 micron region, while CO2 provides 100% absorption at 4.3 microns. CO2 augments the water vapor spectra in the 15 micron region to fully absorb IR.
Working in concert, the GHGs absorb and emit the Earth’s outgoing longwave radiation.
In the next post in this series, I will show several simple global energy equilibrium models that to help us understand how retention of outgoing longwave radiation raises the Earth’s temperature.
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