THE GREENHOUSE EFFECT
Have you read the previous page: The greenhouse effect.
How the greenhouse effect works
Imagine a greenhouse as we know it here on earth[1]. Radiation from the sun passes virtually unimpeded through the glass roof and is absorbed by the plants and soil inside. These then emit their own thermal radiation which rises upwards reaching the glass roof. A small amount is emitted outwards by the glass, but most descends back down into the greenhouse. So here there are two forces operating: radiation (the absorption and emission of light and energy as heat, and convection by means of which the less dense warm air moves upwards and the more dense colder air downwards.
These principles also operate in our atmosphere, though on a much grander scale. When the air close to the surface is heated, it rises because of its lower density. As it rises it expands and cools. While this is going on, other colder heavier air masses descend, so the air is constantly turning over as different movements balance each other out. In the process, radiation is transferred. Because of its temperature, the Earth’s surface emits radiation in the 4 to 100 µm[2] region. At some wavelengths in the infrared, roughly between 8 and 14 µm the atmosphere is largely transparent just as in the visible part of the spectrum emanating from the sun. These transparencies are referred to as ‘windows’. At these wavelengths all the radiation originating from the earth’s surface leaves the atmosphere. At other wavelengths, the radiation from the surface is strongly absorbed by the GHGs, in particular water vapour and carbon dioxide. All this is depicted in the energy budget diagram on the previous page.
The amount of thermal radiation emitted into space by the absorbing gases is dependent on their wavelength and temperature. When the rising gases reach the upper limits of the troposphere, up to a height of about 10 ks, where the temperature falls with height and where convection is the dominant process for transfer of heat in the vertical, the temperature is so cold that very little radiation is or can be emitted.
Lower down, the GHGs assist in retaining heat at the Earth’s surface, and the atmosphere effectively acts as a radiation blanket, keeping it warmer than would otherwise be the case - on average some 15 to 20⸰ C warmer. This radiation is absorbed and re-emitted in all directions, warming the Earth's surface and the lower atmosphere. So before we even begin to talk about “global warming” in the current context, these natural greenhouse gases have already done their job by keeping us warmer than we otherwise would be, making our lives that much more comfortable in the process. This is the natural greenhouse effect.
If the concentration of CO2 in the atmosphere were suddenly to double, and it is predicted to do so (at least on a base of pre-industrial levels) in the next hundred years, everything else remaining the same, the greater amount of carbon dioxide in the atmosphere means that the thermal radiation emitted will originate on average from a higher and colder level in the troposphere[3] than before meaning less is emitted, resulting in a net imbalance in the thermal radiation budget of about 4 watts per square metre. More energy is coming in than going out.
This radiation imbalance is an example of radiative forcing: a change in average net radiation at the top of the troposphere which occurs because of a change in the concentration of a greenhouse gas or some other some other change in the overall climate system, for example a change in the incoming solar radiation. To restore the balance, over time the earth’s surface and lower atmosphere will warm up resulting in increased trapped heat causing a rise in the earth’s surface temperature. This is the enhanced greenhouse effect or, to use the common parlance, “climate change”.
It is estimated that to date the increase in carbon dioxide has contributed about 74% of the enhanced greenhouse effect, methane 19% and nitrous oxide about 7%[4].
Empirical evidence for the connection
The connection between CO2 and rising temperatures can be found in the spectrum of greenhouse radiation. Using high-resolution Fourier-transform infrared spectroscopy spectroscopy (FTIR), scientists can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.
The properties of CO2 also mean that it adds to the greenhouse effect in a way that other emissions do not, thanks to its ability to absorb wavelengths of thermal energy that things like water vapor can’t.[5]
We know CO2 absorbs and re-emits longwave radiation (Tyndall), and the theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur (Arrhenius). Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation.
Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths. This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.[6]
How the greenhouse effect works
Imagine a greenhouse as we know it here on earth[1]. Radiation from the sun passes virtually unimpeded through the glass roof and is absorbed by the plants and soil inside. These then emit their own thermal radiation which rises upwards reaching the glass roof. A small amount is emitted outwards by the glass, but most descends back down into the greenhouse. So here there are two forces operating: radiation (the absorption and emission of light and energy as heat, and convection by means of which the less dense warm air moves upwards and the more dense colder air downwards.
These principles also operate in our atmosphere, though on a much grander scale. When the air close to the surface is heated, it rises because of its lower density. As it rises it expands and cools. While this is going on, other colder heavier air masses descend, so the air is constantly turning over as different movements balance each other out. In the process, radiation is transferred. Because of its temperature, the Earth’s surface emits radiation in the 4 to 100 µm[2] region. At some wavelengths in the infrared, roughly between 8 and 14 µm the atmosphere is largely transparent just as in the visible part of the spectrum emanating from the sun. These transparencies are referred to as ‘windows’. At these wavelengths all the radiation originating from the earth’s surface leaves the atmosphere. At other wavelengths, the radiation from the surface is strongly absorbed by the GHGs, in particular water vapour and carbon dioxide. All this is depicted in the energy budget diagram on the previous page.
The amount of thermal radiation emitted into space by the absorbing gases is dependent on their wavelength and temperature. When the rising gases reach the upper limits of the troposphere, up to a height of about 10 ks, where the temperature falls with height and where convection is the dominant process for transfer of heat in the vertical, the temperature is so cold that very little radiation is or can be emitted.
Lower down, the GHGs assist in retaining heat at the Earth’s surface, and the atmosphere effectively acts as a radiation blanket, keeping it warmer than would otherwise be the case - on average some 15 to 20⸰ C warmer. This radiation is absorbed and re-emitted in all directions, warming the Earth's surface and the lower atmosphere. So before we even begin to talk about “global warming” in the current context, these natural greenhouse gases have already done their job by keeping us warmer than we otherwise would be, making our lives that much more comfortable in the process. This is the natural greenhouse effect.
If the concentration of CO2 in the atmosphere were suddenly to double, and it is predicted to do so (at least on a base of pre-industrial levels) in the next hundred years, everything else remaining the same, the greater amount of carbon dioxide in the atmosphere means that the thermal radiation emitted will originate on average from a higher and colder level in the troposphere[3] than before meaning less is emitted, resulting in a net imbalance in the thermal radiation budget of about 4 watts per square metre. More energy is coming in than going out.
This radiation imbalance is an example of radiative forcing: a change in average net radiation at the top of the troposphere which occurs because of a change in the concentration of a greenhouse gas or some other some other change in the overall climate system, for example a change in the incoming solar radiation. To restore the balance, over time the earth’s surface and lower atmosphere will warm up resulting in increased trapped heat causing a rise in the earth’s surface temperature. This is the enhanced greenhouse effect or, to use the common parlance, “climate change”.
It is estimated that to date the increase in carbon dioxide has contributed about 74% of the enhanced greenhouse effect, methane 19% and nitrous oxide about 7%[4].
Empirical evidence for the connection
The connection between CO2 and rising temperatures can be found in the spectrum of greenhouse radiation. Using high-resolution Fourier-transform infrared spectroscopy spectroscopy (FTIR), scientists can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.
The properties of CO2 also mean that it adds to the greenhouse effect in a way that other emissions do not, thanks to its ability to absorb wavelengths of thermal energy that things like water vapor can’t.[5]
We know CO2 absorbs and re-emits longwave radiation (Tyndall), and the theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur (Arrhenius). Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation.
Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths. This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.[6]
[1] This is an edited version of Sir John Houghton’s explanation in Global Warming – The Complete Briefing, 5th Ed, Cambridge, 2015, 22-28, 33.
[2] µm refers to the wave’s spatial distance (the distance over which it repeats) in micrometres. A micrometre is a unit of length equal to one millionth of a metre, or about a tenth of the size of a droplet of mist or fog. It is also commonly known as a micron, although that term is now officially outdated.
[3] The troposphere is the region of the lower atmosphere up to a height of about 10 kms where the temperature falls with height and where convection is the dominant process for transfer of heat in the vertical.
[4] Houghton, op cit 33.
[5] https://techcrunch.com/2019/05/12/co2-in-the-atmosphere-just-exceeded-415-parts-per-million-for-the-first-time-in-human-history/
[6] “How do we know more CO2 is causing warming?”: https://skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse-effect.htm; Houghton, op cit. 21-22.
In summary therefore, of the visible light that enters the atmosphere in the shorter-wave length UV spectrum, about 30% is reflected back out into space by clouds, snow and ice-covered land, sea surfaces, and atmospheric dust[1], and the rest is absorbed by the liquids, solids, and gases that constitute our planet. The energy absorbed is eventually re-emitted, but not as visible light because the Earth’s radiated heat is not hot enough.
Instead it's emitted as longer-wavelength light, also called "heat" radiation, because although we cannot see in the infrared, we can feel its presence as heat. This is what we feel when you put our hand near the surface of a hot saucepan. The trace gases absorb this outgoing infrared radiation, in effect trapping its heat energy. This trapped heat energy makes the earth warmer than it would otherwise be without them. The ability of trace gases such as CO2 to be relatively transparent to incoming visible light from the sun yet opaque to the energy radiated from earth is one of the best-understood processes in atmospheric science.
Brian Cox encapsulates all this beautifully in a disarmingly simple resume of the greenhouse effect:
“The greenhouse effect is pretty simple physics. Gases like CO2 and water vapour in the planetary atmosphere are transparent to visible light. That’s obvious because I can see the sun. When this radiation falls to the earth it heats it up, and then relays that radiation back into the atmosphere not as visible light but in the infrared, which I can’t see because it’s invisible to the naked eye. Carbon dioxide and water vapour absorb this infrared radiation and trap that energy and the planet heats up. That’s not necessarily a bad thing. The earth would have an average temperature of -18 degrees centigrade without the greenhouse effect. But there’s a very thin line between warming the planet up and frying it”.[2]
It has long been known that increased levels of greenhouse gases cause the Earth to warm in response, and there is now 97-98% consensus among climate researchers worldwide that human caused greenhouse gas emissions caused by the burning of fossil fuels are the primary cause of global warming[3].
Measuring the greenhouse effect
There are two ways of measuring the greenhouse effect. The first is the 33 degree difference between the Earth’s “effective temperature” (itself being the difference between its incoming and outgoing radiation (-18 degrees)) and its actual (global average) surface temperature of 15 degrees. The second is (harking back to our graphic) the difference between the 398.2 w/m2 surface emission on the Earth’s energy budget diagram above, and the 239.9 w/m2 which eventually emits to space, that is 158 w/m2. (Recall that w/m2 refers to the radiant flux (irradiance) received by a surface whose power comes from an energy source like our sun per unit area). Of this 158 w/m2, H20 (water vapour) accounts for about 94 and CO2 (carbon dioxide) for about 55 w/m2 and the other gases a small amount each. Any increase in greenhouse gas concentrations will trap the Earth’s longwave radiation and warm the planet on a cumulative and continuing basis, cumulative because once emitted they stay in the atmosphere for long periods of time.
It is estimated that the earth’s radiation budget is currently out of balance by around 1 w/m2, and that in recent decades the earth has accumulated significant amounts of ‘excess’ energy, so where has it gone?
- Ocean warming accounts for 93%;
- melting ice accounts for 3%;
- continental warming accounts for 3%, and
- the atmosphere accounts for the remaining 1% - but only because the vast majority of trapped energy has ended up in the oceans.[4]
A very good presentation of the process may be found here, courtesy of the Skepical science site: "Getting skeptical about global warming skepticism".
[1] Source: The U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC, 39 – 44 Global Climates - Past, Present, and Future, S. Henderson, S. Holman, and L. Mortensen (Eds.), EPA Report No. EPA/600/R-93/126. U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC, 39 - 44., found on the site http://www.ucar.edu/learn/1_3_2_12t.htm
[2] Brian Cox, The Planets, Episode 1 “A moment in the Sun. The Terrestrial planets”, BBC/ABC Documentary, 2019, at 30.00/58.18 mins
[3] This 97, 98% consensus is analysed in Appendix C.
[4] Michael Box’s “Our Atmospheric Environment”, op cit, 5.1.5; 5.2.4.
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