Climate researchers say that the Earth is a greenhouse, warmer than a lump of stone in space. This is mainly due to 'greenhouse gas' in the air. It radiates 342 Watt/m2 energy to the sea and the solid ground, according to the IPCC. This is more than double the amount the Sun contributes (161 Watt/m2).
Other researchers scoff at this statement. The air is colder than the ground and the sea; heat never flows from cold to warm by itself.
What is going on here? Sensible people in both camps, yet such a difference of opinion?
Energy is energy. Not heat, not electricity, not oil, not coal and not solar cells. It is a physical concept. This is very useful, because no changes occur between heaven and Earth without energy playing a role in it.
It is the integral of the equations of motion. Few understand this. The others mumble about the thermal radiation with which the air heats the Earth.
Energy is invisible and not palpable. It manifests itself indirectly. In a closed system it never increases or decreases, we say energy is conserved(2). Whoever throws up a ball, gives it kinetic energy. It decreases on the way up and changes into the same amount of potential energy. On the way back, the opposite happens. When you catch the ball all energy is converted into thermal energy, namely the random movement of molecules in the ball and the hand. When measured, the amount of energy has not changed.
Energy is transferrable as in the example of the ball. The kinetic energy that a cyclist gives to the pedal travels to the gear via the pedal. It transfers the energy to the chain, which then passes it on to the rear wheel. It is then returned to the cyclist and his bicycle. In factories, driver belts are used to transfer energy from an engine to another tool, such as a pump, a saw, etc.
Energy can thus be moved and also converted from one form to another. Energy can be stored in motion, in a flywheel for instance. It is also possible to store it in potential energy, such as the water that you pump up in a mountain lake. It can be converted in electrical energy with a dynamo. And then again stored in a battery. An electric motor can be used to convert electrical energy into kinetic energy again. Utilising a filament, this energy can be turned into thermal energy and radiant energy, light and other radiation. Radiant energy travels without loss through empty space. No driver belt is required for this. It can interact with matter over empty space at a great distance and be converted back into other types of energy.
Nuclear and chemical energy are different forms. Nuclear energy emerges at the expense of mass. It is released as heat of matter and as radiation.
Chemical energy, like nuclear energy, is a type of potential energy. It can be stored for a long time and is only released during a reaction, in the form of heat, radiation or other chemical energy.
A special law applies to the transport of thermal energy. We even call it a 'main law', just like that of the conservation of energy. The thermal energy transport law states: heat only flows from hot to cold. It can go the opposite way, but then an instrument is required, such as a heat pump, like a refrigerator, which needs extra energy.
Such a rule does not apply to the transport of other forms of energy. A motor in a cool room can transmit its energy via a shaft, or a driver belt, to a machine in another, warmer room. Other rules apply to energy transport via radiation.
Radiant energy comes from accelerating or decelerating matter. The movement and collision of molecules makes all matter radiate. Conversely, matter converts received radiation into another energy form. The warmer an object, the more it radiates.
The Sun is warm. It loses its energy through radiation, which is compensated for by nuclear fusion. Radiation only manifests itself in interaction with matter. A completely black body converts received radiant energy into heat and starts radiating itself. Materials that are less black, such as solar cells, convert part of the radiant energy into electricity, or chlorophyll from plants into chemical energy. In practice, there are no bodies that are entirely black. For instance, X-rays are able to pass straight through a pitch-black plank. Light is also able to travel long distance, seemingly unaffected, through air and water. So, matter can transmit radiation. It does not necessarily heat up, and the chemistry does not have to change. Matter is also able to reflect radiation. In this case, the radiation returns to space without any change occurring within the matter(3). No energy is lost. (Energy loss is an impossible word. Energy is not lost. It can only be converted into another form.)
In the same way that there are no absolutely black bodies, completely transparent ones do not exist. It depends on the material and the nature of the radiation. Water transmits light well; snorkelers can see each other and the corals they admire during the day. However, deep divers must bring lights. The solar radiation gradually decreases with depth. In the deep sea it is permanently night. The radiant energy is gone. It is of course not lost, but converted into a different form, mostly heat. The same thing happens in air. The radiant energy from the Sun reaching the solid Earth is less than at the top of the atmosphere. Sea and sky are neither transparent nor opaque. They are turbid(4) to radiation. If the layer is thick enough, nothing will pass through entirely.
If (radiant) energy flows in somewhere and the radiation decreases, there are only two possibilities: it is stored, or it flows out in another form. The disappearing rays of the Sun deep in the sea heats up the water there. If the heat could not escape, the water would boil after a while. You may think that the heated water itself will radiate more and thus keep its temperature constant. This is not the case, as the water acts as an entirely black body for the kind of radiation at that temperature. It does not allow this radiation to pass through. Therefore there must be another mechanism to transport the heat. In the sea, this is mainly done by ocean turbulence and currents and heat conduction. In flow it is matter, containing the thermal energy, that moves. This transport goes both from warm to cold and from cold to warm. But the cold air will cool a warmer room, although the cold air also contains heat energy.
A completely black spherical body in space heats up in the Sun's rays until the temperature rises to such a degree that it it radiates as much energy as it receives from the Sun. When the sphere conducts heat ideally, the sphere's temperature remains uniform. At equal distance from the Sun as the Earth and corrected for the Earth's reflection, the sphere would have a temperature of -18 °C. When the sphere's heat conduction is less than ideal, and the temperature differs in different places and times, an average temperature can be determined. It is then even further below zero. The fact that we have an average temperature of 15 °C above 0 on Earth is called the 'greenhouse effect'. This effect exists because the Earth is not a black sphere. It is largely a turbid body that receives radiant energy from the Sun, which it converts into heat energy far from the place where it reradiates the energy back into space. If the heat transport between place of reception and place of exit is not done by radiation, such as in water, or for parts of the radiation in air, then it must be through heat transport or mass transport. That can only be done from warm to cold. (Additionally in mass transport, the thermal energy it contains can only be transferred to a colder body.)
Thus a turbid body in a radiation field must be warmer on the inside than the outside, from which it returns the energy received from the Sun to the universe. We reside within such a turbid body, where it is fortunately warmer than on or within a black sphere.
The more challenging the energy transport from the inside to the outside, the warmer it gets inside. Air and water act like a blanket on a bed with an electric blanket. The electrical energy enters the bed through a wire; it is not obstructed by blankets. However, the thermal energy obtained has to exit through the upper blanket. Furthermore, the better that blanket insulates, the warmer it must become inside in order to push the thermal energy out.
Similarly, the Sun deposits its radiant energy almost unimpeded below the surface. That being said, much of it has to significantly heat matter up inside before the heat transport is large enough to allow the same amount of energy to reach a place from which the matter can radiate it back into the universe.
Does atmospheric CO2 contribute to this deceleration of energy transport, and if so, by how much? I do not think it is worth mentioning. Everything is influenced by everything. Fortunately, usually immeasurably little. Due to their low density, gases are poor emitters. You can easily find out for yourself by doing an experiment with a candle and a radiator of your central heating system. Close to the radiator (a solid body) at 60 °C your hand feels the radiation. The radiation from the hot air 20 cm above and beside of the flame of the candle cannot be felt, while the air itself is hot enough to burn your hand. (The flame, of course, does radiate appreciably. It is not a gas. It is a plasma.)
A small part of the out going radiation is absorbed and converted to heat by CO2 in the air. The air gets warmer and it radiates again about the same kind of radiation. Just as described above for water, the air with CO2 is quite dark, if not pitch black. We therefore mostly deal with local thermal energy, moving from warm to cold. Thus heating the warmer Earth from cold air is out of the question. If this were the case, as certain climatologists claim, only the radiation residue would have to do the job. They say at a rate stronger than that of the Sun (!)
Energy transport by radiation is also subject to certain laws. These laws cannot be explained using classical physics, and sometimes go against an intuitive expectation. Radiation is bound to quanta. They are radiation packages; photons, with a wave character. The energy of such a photon is hν where ν is the frequency of the radiation, and h is a constant of nature.
Any object with a temperature above absolute zero emits such packets. Their energy can be transferred to another object, even if the packets travel from a cold object to a warmer one.
Radiation is both a package and a wave. Mathematics describes this duality, and imposes a limitation on it. It can only be understood quantum physically. This limitation prevents the total overthrow of macroscopic thermodynamics. If calculated properly, it is not possible to further heat a warm object by unmodulated radiation(5), which is emitted by a cold object; similarly to the transport of thermal energy. If the objects only have radiation contact, then the net energy only travels from hot to cold, regardless of the shape of those objects. It is therefore not possible to heat a warmer Earth by means of the unmodulated radiation from cold air.
Naturally, the photons from the cold do reduce the net energy transfer from warm to cold.
The turbid sky and the turbid sea make the Earth warmer than an opaque black sphere. That is the much talked about greenhouse effect. These bodies inhibit energy transport back to the universe.
Back radiation is measurable. It must be noted that if you do measure the radiation, and your meter is pointed in the opposite direction, an amount of radiation will be going in the reverse direction, in exactly the same spot. If a difference in radiation occurs, the net energy only goes from hot to cold. A calculation using fundamental laws of radiation would give the same net result. In the IPCC CO2 model of radiation, the air not only radiates back to the Earth, it radiates even more downwards, 342 W/m2, than upwards, 239 W/m2 (Wild e.o. 2013). Radiant heaters that have the ability(5) to do this have yet to be invented in the laboratory ☻.
Note: Naturally, warmer air can heat colder sea or ground through radiation or conduction. This is a cumbersome process. Due to its low density, air is a poor radiator, and the heat capacity is low. It is unlikely that, on average, more energy would go from air to land and sea over a year on that route than vice versa.
I suspect that something went wrong when averaging the measurements. The greenhouse effect works differently.
Ken B. Gregory pointed out to me an erroneous thought experiment in an earlier version of this article, in which I tried to illustrate the impossible contradictory energy transport. I have changed the text relating to it.
I am grateful for the help of Nidanu O'Shea and Albert Stienstra with the editing of the English text. Remaining flaws are mine.
C. (Kees) le Pair
Revised 2020 09 20.
Translated from a Dutch article, dated 2020 05 15, revised 2020 09 20.
- The theory of relativity makes mass and energy equivalent. In fact, you have to say: Energy is preserved if the mass is unchanged.
- Radiation pressure is also present, however pressure is not energy. This only occurs when the pressure displaces something.
- 'Turbid' usually refers to liquids. I use it to transit of radiation through matter, be it solid, liquid or gas, to indicate they are neither opaque, nor transparent. In stead, when travelling some distance, the radiation is gradually absorbed and converted into other forms of energy.
- Radiation can be modulated. We can do it with a laser, or with lenses and mirrors. For modulated radiation rules are different.