How CO₂ actually causes Warming and Climate Change
The chains of basic physics that bind our civilization
A lot of basic science explained.
It is radiation that moves energy in a vacuum
The first principle: a planet (or any object in space or in a vacuum) can only gain or lose heat through radiation. The energy from the Sun arrives in the form of radiation. Earth’s energy departs as radiation.
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Same thing happens with satellites and thermal management is a major topic in satellite design.
The energy radiated depends on the temperature of the surface as seen from space.
(technically this is called Planck or black body radiation and it is the fourth power of the absolute temperature but those details are not relevant at this level)
When it’s a satellite, the surface that radiates is clearly visible as the edge of the satellite. When it is a planet with an atmosphere, the surface is the atmosphere.
So, the radiating surface is fuzzy, and quite certainly not the surface you are standing on at present.
The Energy Balance
The second principle: For the temperature of the object in space to remain the same, the energy that goes in has to equal the energy that goes out. This is a fundamental law of thermodynamics. This energy is always radiation when it is in space.
(I learned it in Engineering as the gozintas have to equal the gozoutas. Engineers have weird ideas about what is funny sometimes).
If the energy in is different from the energy out, the temperature changes until a new stable level of incoming and outgoing radiation is achieved.
How energy is transferred
The third principle: There are 3 means of heat (energy) transfer here on earth.
Conduction - This happens when two objects at different temperatures are in direct contact with one another.
Convection - which allows a substance that is liquid or gaseous to transfer the energy as the substance itself is transferred, and
Radiation - which depends on the temperature of the object and works everywhere.
It’s all photons
The fourth principle: the energy that comes in or goes out as radiation is made up of photons. The tiny packets of energy have specific wavelengths, which determine how much energy each photon contains. I will not attempt to explain photons here.
CO₂, CH₄ and H₂O are different from the principal components of the atmosphere, O₂ and N₂. If you look at their representations, the difference is clear. O₂ and N₂, oxygen and nitrogen molecules which make up the bulk of the “million” in the parts per million (ppm) that we use to measure the concentrations consist of 2 of the same atomic element. They have one atomic bond, while the first 3 have more than one.
This is important. It gives CH₄, CO₂ and H₂O the ability to absorb low levels of incoming energy as vibrations of the atoms held in place by the atomic bonds. This is independent of temperature. It is latent energy rather than conduction or convection.
When any of these Greenhouse Gases absorbs a photon of IR it will have a little more energy which it can transfer by collision (conduction) to any other molecule including the non-greenhouse gases, and it will re-emit that energy in a random direction after a very short period of time measured in milliseconds. At sea level the energy is usually transferred to another molecule of the atmosphere in less than 1 millisecond, but it can also get energy back from those molecular collisions.
O₂ and N₂ do not have this ability. They cannot efficiently absorb IR photons, which pass through them without causing any changes.
Capturing photons at different energy levels
The light from the Sun is very energetic because the Sun has an internal source of energy, nuclear fusion, which is energy it also must get rid of to maintain a stable temperature. This means that the Sun is extremely hot. On the absolute (Kelvin) scale, it is usually regarded as being roughly 5778° Kelvin.1
As a result, the photons of energy it sends to Earth are mostly not Infrared (heat), but instead are visible light and ultraviolet light. These cannot be captured by a greenhouse molecule (The IR it sends contributes to the total energy absorbed by the planet, but never reaches the surface).
The Earth does not warm through fusion power, so it is far cooler; roughly 288° Kelvin at the surface. The Earth radiates almost entirely in the infrared.
So any greenhouse molecule is going to allow all the incoming visible and UV radiation (energy) to pass through to the surface, while all the outgoing radiation is captured. Each molecule can do this every millisecond, day and night, and there are 86,400,000 milliseconds every day.
Opaque in the IR
1 cubic metre contains 44.64 moles of gas, of which 0.0410% is CO₂ = 0.0183 moles = 11,020,519,860,000,000,000,000 molecules of CO₂ per Cubic Meter. Not counting H₂O feedback molecules.
The distance the IR photon is likely to travel in the atmosphere before it is captured by a molecule of CO₂ or H₂O, even at 400 ppm, is about 10 meters (35 ft). So for the wavelengths the CO₂ occupies it all gets captured. H₂O (Water Vapor) is a condensing gas and thus a feedback; it does not stay in the atmosphere without the CO₂, but it captures more photons than the CO₂ does. The feedback is actually stronger than the forcing CO₂.
In fact, the lower atmosphere is effectively opaque to IR except at some very specific wavelengths. This is well understood because we have to use those very specific wavelengths for things like heat-seeking missiles and infrared goggles.
All additional CO₂ in the global atmosphere is anthropogenic. This is clear from isotope ratios and from the increasing CO₂ entering the ocean. The additional CO₂ causes the IR to be blocked more completely.
How the energy gets out
The energy still has to make it to deep space though. Remember the requirement that gozintas=gozoutas.
So how does that work?
The Greenhouse Molecules that intercept the IR collide with all the other molecules including the O₂ and N₂ (conduction) and the temperature at the bottom of the atmosphere (ground level) increases.
As temperature increases the air expands and rises (convection), carrying the energy upward to where some radiation to space actually can reach space. This is regarded as the effective radiative surface and is about 5–10 km above the ground. The air cools as it rises as a function of pressure and the gas laws (this is called the adiabatic lapse rate or just the lapse rate) so the temperature at that altitude is a lot cooler than the temperature on the ground.
It is important to note that the adiabatic lapse rate defines a temperature difference between the base of the atmosphere and the radiating surface of the atmosphere. This is a key concept and it is a difficult one for most people to grasp.
The adiabatic temperature difference does not define the temperature at the bottom or the top.
The temperature at radiating surface of the atmosphere, at the top of the adiabatic transfer, is much less and its Black Body IR energy is much less than the IR and temperature that we have at the bottom of the atmosphere. Yet it also has to get hot enough to radiate enough to balance all the energy coming in.
If it is not balanced, the bottom of atmosphere must change temperature as required until that temperature minus the adiabatic temperature drop to reach the radiating surface, yields the black-body temperature required to radiate the energy away.
If CO2 is added to the atmosphere, this makes the effective radiating surface higher and thus cooler. This makes it less efficient at radiating energy so the temperature at the base has to rise to get the radiating surface warm enough to get rid of the incoming solar energy.
That rise in temperature is the Global Warming of the greenhouse effect.
It is our CO₂ per both mass balance equations and the isotopic analysis, so it is Anthropogenic Global Warming. (AGW)
Note: The warming that the CO2, CH₄, and H20 produce also causes reduced albedo (often melting ice), which amplifies the AGW by reducing the amount of visible light energy reflected back into space. This indirect change still has our Greenhouse Gases as its root cause, so it is included in the anthropogenic climate sums, but it is different from the “greenhouse effect.”
Pedants will vociferously protest that “° Kelvin” is wrong, as physicists demand that it is absolute. Yet, the use of K creates ambiguities for everyone else because the abbreviation is used in so many other contexts, such as “20 K down the road.” Unlike pedantic physicists, engineers actually have other uses for that letter, so I will continue to use the disambiguating ° sign.