CO2 (carbon dioxide) is not a GH effect, but allegedly a GH gas that allegedly has a GH effect. I say allegedly because this is an old theory that was disproved by physicist Knut Angstrom (the leading expert on radiation spectra of his time) in 1901 as soon as it got proposed by Svante Arrhenius based on John Tyndall’s experiments and his own.
We must make things clear: while the answer from Lee Thè gives a good description of what the mainstream media and school books refer as Greenhouse Effect, the science behind it is completely wrong and nonsensical.
We must distinguish 2 types of ‘greenhouse effect’:
- radiative greenhouse effect
- physical (or thermic) greenhouse effect
The main idea is that most scientists who accept the theory compare the Earth / Atmosphere system as a Greenhouse, where the atmosphere would act as the glass walls of the greenhouse. Inside a greenhouse, what happens is what is called Physical (or Thermic) Greenhouse Effect, that is, the air is heated thru the glass by the sun, moves in convective motion, arrives to the glass walls that trap air impeding it to escape; pressure and temperature rise by increase of kinetic energy. You can try this going into a greenhouse in the hottest hours of the day and open 1 or 2 windows in the glass: you will see pressure and temperature drop.
Why won’t this apply to the Earth / Atmosphere system? For two main reasons:
Atmosphere is not solid and is not comparable to a glass wall.
The glass wall stops and traps air inside the greenhouse, augmenting the kinetic energy of the internal system because it limits convective transfer of heat inside the system. Atmosphere will not stop any convective movement because it is not solid, moreover in a non-limited multistrata atmosphere, heat always travels upward and from a center of higher temperature to one of lower temperature. This means that, by law of thermodinamics, in an open system the lower strata are always the warmer ones. This means also that if surface of Earth is heated by the Sun (or whatever source) it will transmit heat upwards to the atmosphere which will transfer the heat upwards. No single suspended stratum can send heat to the lower strata heating it.
Now, why do scholars say that CO2 sends heat back, then? Because they convert the concept in terms of Infrared Radiation (IR), saying that an absorbing and emitting object can radiate spherically to all directions, so CO2 molecules can also transmit IR downwards, heating the lower strata.
This is formally true in terms of theoretical transmission of both energy (IR radiation) and heat, but it is wrong when you apply this concept to real life and use numbers. And we here come to the second reason.
Actual temperature is given by the result of the calculation of Net heat transfer.
What ‘climate scientists’ are saying is basically that the Sun invests the Earth with a quantity X of IR radiation that causes the temperature to have e Temperature T, then the Earth emits back a part of radiation X1 (to which corresponds a temperature T1) which is blocked by the atmosphere and sent back as a quantity X2 that gives a temperature T2. They say that T2 is higher that T, because “were not for the greenhouse effect the earth temperature would be -18°C instead of +15°C” (this is a famous and universally expressed concept from most scholars and the IPCC as well). Thay call it ‘radiative greenhouse effect’ based on ‘back radiation’ from the atmosphere.
This is wrong, because the earth cannot ‘create’ energy and temperature, so everything it emits can only come from the original quantity X of IR radiation corresponding to the original T temperature. So X1 (T1) radiation emitted from the surface, and the X2 (T2) given back from the air to the surface, are and can only be A PART of the original radiation / temperature coming from the sun.
Now, we come to the ‘net heat transfer’. It is true that a particle transmits heat spherically, but let’s say a CO2 molecule stays in a stratum where the local temp is 10°C, it will emit let’s say 1°C in all directions, downward included. The upper stratum is, let’s say, at 9°C and the lower stratum is let’s say at 11°C. The 1°C given back to the lower stratum cannot heat up a stratum that is at a higher temperature, just like your body skin at 33°C cannot heat a pipe at 40°C. This is because the heat coming from a stratum is always less than that coming from a lower stratum, and more than that coming from a higher stratum. The concept is: temperatures result from the net value obtained averaging the heat from the lower warmer stratum and the upper cooler stratum.
There is no such thing like ‘back radiation’, there is no GH effect that ‘heats’ the surface. What heats the surface (letting alone the Sun, clouds, albedo, etc) are local activities (natural and non natural) that locally rise the temperature (i.e.: emissions from traffic, etc) but this is no Greenhouse effect. This is normal heating, just like when you boil water to make good italian Spaghetti (*) the environment nearby warms up.
Hope this helped.
(*)please use only real italian spaghetti, with italian Ragù (tomato sauce with meat).
