I have been wanting to do a complete thread on the science of global warming for a long time. Starting it today and hope to continue it over time. I will discuss the foundations of the relevant science and the evidence collected that backs up the current conclusions.
Some ground rules:-
1) Questions are welcome but need to pertain to the post I have actually made. I will not answer questions about topics that I am yet to cover. Others are free to do so.
2) Philosophical questions about the validity of the scientific process etc. will not be answered by me.
3) Others can contribute as they see fit.
The basic science behind man-made global warming is that of the atmospheric greenhouse effect and how it is increasing due to emissions from human activity. The basic concept of the greenhouse effect has been known for several centuries now. It was Fourier in 1824 who first noted how atmospheric gases trap heat making earth hotter than usual. To quote Fourier,
"The earth receives the rays of the Sun, which penetrate its mass, and are converted into non-luminous heat; it likewise possesses an internal heat with which it was created, and which is continually dissipated at the surface; and lastly the earth receives rays of light and heat from innumerable stars, in the midst of which is placed the solar system. These are the three general causes which determine the temperature of the earth.
The transparency of the waters appears to concur with that of the air in augmenting the degree of heat already acquired, because luminous heat flowing in, penetrates, with little difficulty, the
interior of the mass, and non-luminous heat has more difficulty in finding its way out in a contrary direction. "
While Fourier made this (correct) observation, the fact that only some of the gases act as resistances to heat as it tries to escape earth would be discovered a bit later. In a series of papers published in 1860, John Tyndall experimented on the heat-absorbing capacity of various gases. Tyndall reported on how he was able to measure the absorption of radiant heat by oxygen, nitrogen, hydrogen, water vapor, carbon dioxide, ozone, and even the fragrances of flowers. The first three had no capacity to absorb energy, but all of the latter did. He calculated that a single molecule of water vapor had a capacity to absorb 16,000 times as much radiant energy as nitrogen or oxygen molecules. He realized that water vapor was critical to the regulation of the surface temperature of the Earth. He argued that more than 10 % of the radiation from the soil is stopped by vapor within ten feet of the surface. Thus he discovered the importance of water vapour and CO2 and ozone in the greenhouse effect discussed by Fourier 30 years before him.
Still, however, it was not known how the net heat coming to earth and going out of earth can be calculated and how that heat budget can be connected to earth's surface temperature. It was Gustav Kirchhoff who made the crucial breakthrough of the "blackbody radiation relations" that paved the way towards calculating this. A black body is a hypothetical object that absorbs all the electromagnetic radiation incident upon it. To maintain a specific constant temperature the black body re-radiates as much energy as it receives back into space. It is both a perfect absorber of energy and a perfect emitter of energy. However, the wavelength at which the blackbody emits radiation is different from the wavelength of the incoming radiation that it absorbs. Kirchhoff found that the emission wavelengths of the blackbody depends on the temperature of the blackbody. Thus knowing the temperature, one can predict the emission wavelengths for the blackbody (and vice-versa). The blackbody radiation is the basis of infrared thermometer which can determine the temperature of an emitting surface by detecting the infra-red radiation being emitted by it.
Infrared thermometer - Wikipedia
But apart from (very useful) engineering applications, the blackbody relations also helped in finding the mean temperature of the sun and the earth (as well as other planets) by measuring the frequency of emitted radiation from their surfaces and adding a correction factor that takes into account that these are not perfect blackbodies and do reflect part of the radiation. The Sun, however, is a perfect blackbody.
The wavelength (lambda_max) at which maximum emission occurs is related to the body's temperature as follows:-
where b is a constant (b=10 micrometer).
Also, the total radiative energy R emitted by the black body at temperature T is given by
is a constant as well. Temperature T is measured in Kelvins.
From these relations, we can measure the temperature of the sun as well as earth, including the wavelengths at which they emit radiation. The figure below shows the emitted radiation (y axis) vs wavelength (x axis) curves for bodies having different surface temperature
These ideas, all discovered in the 19th century, and much much before climate change became known...would prove crucial in the science of global warming that will develop in the 20th century atmospheric physics.
TO BE CONTINUED......
Some ground rules:-
1) Questions are welcome but need to pertain to the post I have actually made. I will not answer questions about topics that I am yet to cover. Others are free to do so.
2) Philosophical questions about the validity of the scientific process etc. will not be answered by me.
3) Others can contribute as they see fit.
The basic science behind man-made global warming is that of the atmospheric greenhouse effect and how it is increasing due to emissions from human activity. The basic concept of the greenhouse effect has been known for several centuries now. It was Fourier in 1824 who first noted how atmospheric gases trap heat making earth hotter than usual. To quote Fourier,
"The earth receives the rays of the Sun, which penetrate its mass, and are converted into non-luminous heat; it likewise possesses an internal heat with which it was created, and which is continually dissipated at the surface; and lastly the earth receives rays of light and heat from innumerable stars, in the midst of which is placed the solar system. These are the three general causes which determine the temperature of the earth.
The transparency of the waters appears to concur with that of the air in augmenting the degree of heat already acquired, because luminous heat flowing in, penetrates, with little difficulty, the
interior of the mass, and non-luminous heat has more difficulty in finding its way out in a contrary direction. "
While Fourier made this (correct) observation, the fact that only some of the gases act as resistances to heat as it tries to escape earth would be discovered a bit later. In a series of papers published in 1860, John Tyndall experimented on the heat-absorbing capacity of various gases. Tyndall reported on how he was able to measure the absorption of radiant heat by oxygen, nitrogen, hydrogen, water vapor, carbon dioxide, ozone, and even the fragrances of flowers. The first three had no capacity to absorb energy, but all of the latter did. He calculated that a single molecule of water vapor had a capacity to absorb 16,000 times as much radiant energy as nitrogen or oxygen molecules. He realized that water vapor was critical to the regulation of the surface temperature of the Earth. He argued that more than 10 % of the radiation from the soil is stopped by vapor within ten feet of the surface. Thus he discovered the importance of water vapour and CO2 and ozone in the greenhouse effect discussed by Fourier 30 years before him.
Still, however, it was not known how the net heat coming to earth and going out of earth can be calculated and how that heat budget can be connected to earth's surface temperature. It was Gustav Kirchhoff who made the crucial breakthrough of the "blackbody radiation relations" that paved the way towards calculating this. A black body is a hypothetical object that absorbs all the electromagnetic radiation incident upon it. To maintain a specific constant temperature the black body re-radiates as much energy as it receives back into space. It is both a perfect absorber of energy and a perfect emitter of energy. However, the wavelength at which the blackbody emits radiation is different from the wavelength of the incoming radiation that it absorbs. Kirchhoff found that the emission wavelengths of the blackbody depends on the temperature of the blackbody. Thus knowing the temperature, one can predict the emission wavelengths for the blackbody (and vice-versa). The blackbody radiation is the basis of infrared thermometer which can determine the temperature of an emitting surface by detecting the infra-red radiation being emitted by it.
Infrared thermometer - Wikipedia
But apart from (very useful) engineering applications, the blackbody relations also helped in finding the mean temperature of the sun and the earth (as well as other planets) by measuring the frequency of emitted radiation from their surfaces and adding a correction factor that takes into account that these are not perfect blackbodies and do reflect part of the radiation. The Sun, however, is a perfect blackbody.
The wavelength (lambda_max) at which maximum emission occurs is related to the body's temperature as follows:-
where b is a constant (b=10 micrometer).
Also, the total radiative energy R emitted by the black body at temperature T is given by
From these relations, we can measure the temperature of the sun as well as earth, including the wavelengths at which they emit radiation. The figure below shows the emitted radiation (y axis) vs wavelength (x axis) curves for bodies having different surface temperature
These ideas, all discovered in the 19th century, and much much before climate change became known...would prove crucial in the science of global warming that will develop in the 20th century atmospheric physics.
TO BE CONTINUED......
