By Javier Vinós

Most people don’t have a clear understanding of the greenhouse effect (GHE). It is not complicated to understand, but it is usually not well explained. It is often described as “heat-trapping,” but that is incorrect. Greenhouse gases (GHG) do not trap heat, even if more heat resides within the climate system due to their presence in the atmosphere. The truth is that after adjusting to a change in GHG levels, the planet still returns all the energy it receives from the Sun. Otherwise, it would continue warming indefinitely. So, there is no change in the energy returned. How do GHGs produce GHE?

GHGs cause the atmosphere to be more opaque to infrared radiation. As solar radiation heats mainly the ocean and land surface of the planet, GHGs absorb thermal emission from the surface at the lower troposphere and immediately pass that energy along to other molecules (typically N2 and O2) through collisions that occur much faster than the time it would take to re-emit the radiation. This warms the lower troposphere. The density and temperature decrease rapidly through the troposphere, so molecules are colder and more separated at the upper troposphere. Now GHGs have a chance to emit IR radiation so when they finally collide with another molecule, they are colder so GHGs have a cooling effect in the upper troposphere and stratosphere.

Because GHGs make the atmosphere more opaque to IR radiation, when they are present the emission to space from the planet normally does not take place from the surface (as happens in the Moon). Part of it still takes place from the surface through the atmospheric window, but most of it takes place from higher in the atmosphere. We can define a theoretical effective emission height as the average height at which the Earth’s outgoing longwave radiation (OLR) is being emitted. The temperature at which the Earth emits is the temperature at the effective emission height in the atmosphere. That temperature, when measured from space is 250 K (-23°C), not 255 which is the calculated temperature for a theoretical blackbody Earth. That temperature corresponds to a height of about 5 km, which we call the effective emission height.

The last piece we need to understand the GHE is the lapse rate, which in the troposphere is positive, meaning that temperature decreases with height. Without a positive lapse rate, the GHE does not work. Since GHGs cause the planet to emit from a higher altitude, due to making the atmosphere more opaque to IR radiation, that altitude is colder due to the lapse rate. The Earth still needs to return all the energy received from the Sun, but colder molecules emit less. So, the planet will go through a period when it will emit less than it should, warming the surface and the lower troposphere until the new height of emission achieves the temperature necessary 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 temperature of emission (Te) plus the lapse rate (Γ) times the height of emission (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 the height of emission. Since the temperature of emission must remain the same, the temperature from the surface to the new height of emission must increase. The increase is small but significant. As Held and Soden say:

“The increase in opacity due to a doubling of CO2 causes Ze to rise by ≈150 meters. This results in a reduction in the effective temperature of the emission across the tropopause by ≈(6.5K/km) (150 m) ≈1 K.”

Held and Soden

So, the temperature at the surface must increase by 1K. That’s the direct warming caused by the doubling of CO2, before the feedbacks (mainly water vapor) kick in, further raising the height of emission.

This also has an interesting prediction. If the warming is due to an increase in CO2 when the increase takes place and the altitude of emission increases, the planet should emit less OLR as the new altitude is colder and a reduced OLR is the warming mechanism. Once the warming takes place, the OLR will become the same as before the GHG increase. It says so in Held and Soden’s figure 1 caption: “Note that the effective emission temperature (Te) remains unchanged.” Same Te, same OLR. So, if CO2 is responsible for the surface temperature increase, we should first expect less OLR and then the same OLR. If at any time we detect more OLR that would indicate another cause for the warming. Anything that makes the surface warmer, except GHGs, will increase the temperature of emission, increasing OLR.

So, this is the test:

– Surface warming but less or same OLR: CO2 is guilty as charged

– Surface warming and more OLR: CO2 is innocent

And the test results can be evaluated for example with Derwitte and Clerbaux 2018:

“decadal changes of the Outgoing Longwave Radiation (OLR) as 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 rising since 1985, and correlates well with the rising global temperature.“

Derwitte and Clerbaux 2018

CO2 is innocent. Its fingerprint is not found at the crime scene. Something else is warming the planet and causing the increase in OLR.

Bibliography:

Dewitte, S. and Clerbaux, N., 2018. Decadal changes of earth’s outgoing longwave radiation. Remote Sensing, 10(10), p.1539.
https://www.mdpi.com/2072-4292/10/10/1539/pdf

Held, I.M. and Soden, B.J., 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, G.L., O’Brien, D., Webster, P.J., Pilewski, P., Kato, S. and Li, J.L., 2015. The albedo of Earth. Reviews of geophysics, 53(1), pp.141-163.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014RG000449