By Javier Vinos
Most people do not have a clear idea of the greenhouse effect (GHE). It’s not complicated to understand, but it’s usually not explained well. It’s often referred to as “heat trapping,” but that’s incorrect. Greenhouse gases (GHGs) do not trap heat even when there is more heat in the climate system due to their presence in the atmosphere. The truth is that after the planet has adapted to a change in GHG levels, it still returns all the energy it receives from the sun. Otherwise it would continue to heat up indefinitely. So there is no change in the returned energy. How do greenhouse gases produce GHE?
Greenhouse gases cause the atmosphere to be more opaque to infrared radiation. Because solar radiation primarily warms the planet’s ocean and land surfaces, greenhouse gases absorb heat emissions from the surface in the lower troposphere and immediately pass that energy on to other molecules (typically N2 and O2) through collisions that occur much faster than the time it would take to emit the radiation again. This warms the lower troposphere. Density and temperature decrease rapidly in the troposphere, so molecules in the upper troposphere are colder and more separated. Now greenhouse gases have the ability to emit IR radiation, so when they eventually collide with another molecule, they are colder, so greenhouse gases have a cooling effect in the upper troposphere and stratosphere.
Because greenhouse gases make the atmosphere more opaque to IR radiation, emission from the planet to space is not usually from the surface (as is the case with the moon) when they are present. Some of this is still happening from the surface through the atmospheric window, but most is happening from higher up in the atmosphere. We can define a theoretical effective emission height as the average height at which the Earth’s long-wavelength radiation (OLR) is emitted. The temperature at which the Earth emits is the temperature at the effective emission level in the atmosphere. This temperature measured from space is 250 K (-23 °C), not 255, which is the calculated temperature for a theoretical black body Earth. This temperature corresponds to an altitude of about 5 km, which we call the effective emission altitude.
The last piece we need to understand the GHE is the lapse rate, which is positive in the troposphere, meaning temperature decreases with altitude. The GHE does not work without a positive cancellation rate. Because greenhouse gases cause the planet to emit from higher altitudes because the atmosphere becomes more opaque to IR radiation, that altitude is colder due to the rate of decay. Earth still has to give back all the energy it receives from the sun, but colder molecules give off less. So the planet will go through a period of emitting less than it should, heating the surface and lower troposphere until the new emission level reaches the temperature required to return all the energy, at which point the planet stops warming.
The GHE simply states that the temperature at the surface (Ts) is just the emission temperature (Te) plus the decay rate (Γ) times the emission height (Ze).
Ts = Te + ΓZe
Held & Soden (2000) illustrated it in Figure 1:
This is how the GHE actually works. An increase in CO2 means an increase in emissions. Since the emission temperature must remain the same, the temperature must increase from the surface to the new emission height. The increase is small but significant. As Held and Soden say:
“The increase in opacity due to a doubling of CO2 causes Ze to increase by ≈150 meters. This leads to a reduction in the effective temperature of the emission over the tropopause by ≈(6.5 K/km) (150 m) ≈ 1 K.”
Hero and Soden
The temperature at the surface must therefore rise by 1K. This is the direct warming caused by the doubling of CO2 before the feedbacks (mainly water vapor) kick in and further increase emission levels.
This also has an interesting prediction. If the warming is due to an increase in CO2, when the increase occurs and the level of emissions increases, the planet should emit less OLR since the new altitude is colder and a reduced OLR is the warming mechanism. Once the warming takes place, the OLR will be the same as it was before the increase in greenhouse gases. As Held and Soden’s caption states: “Note that the effective emission temperature (Te) remains unchanged.” Same Te, same OLR. So if CO2 is responsible for the increase in surface temperature, we should first expect less OLR and then the same OLR. If we detect more OLR at any point in time, it would point to a different cause of the warming. Anything that makes the surface warmer, except greenhouse gases, increases the emission temperature and raises the OLR.
So, this is the test:
– Surface heating but less or the same OLR: CO2 is to blame as loaded
– Surface warming and more OLR: CO2 is innocent
And the test results can be evaluated with Derwitte and Clerbaux 2018, for example:
“Decadal changes in outgoing longwave radiation (OLR) measured by the Clouds and Earth’s Radiant Energy System from 2000 to 2018, the Earth Radiation Budget Experiment from 1985 to 1998, and the High-Resolution Infrared Radiation Sounder from 1985 to 2018 are analyzed. The OLR has been increasing since 1985 and correlates well with rising global temperature.“
Derwitte and Clerbaux 2018
CO2 is innocent. His fingerprint is not found at the crime scene. Something else is warming the planet and causing the OLR to rise.
Bibliography:
Dewitte, S. and Clerbaux, N., 2018. Decadal changes in longwave radiation emanating from the Earth. Remote Sensing, 10(10), p.1539.
https://www.mdpi.com/2072-4292/10/10/1539/pdf
Held, IM and Soden, BJ, 2000. Water Vapor Feedback and Global Warming. Annual Review of Energy and the Environment, 25(1), pp.441-475.
https://www.annualreviews.org/doi/pdf/10.1146/annurev.energy.25.1.441
Stephens, GL, O’Brien, D., Webster, PJ, Pilewski, P., Kato, S., and Li, JL, 2015. The Earth’s Albedo. Reviews of Geophysics, 53(1), pp. 141-163.
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