Every scientific theory involves assumptions. Global warming theory starts with the assumption that the Earth naturally maintains a constant average temperature, which is the result of a balance between (1) the amount of sunlight the Earth absorbs, and (2) the amount of emitted infrared (“IR”) radiation that the Earth continuously emits to outer space.
In other words, energy in equals energy out. This is the same concept that governs the temperature of anything; if energy is gained faster than it is lost, warming occurs… but if energy is lost faster than it is gained, cooling occurs.
Averaged over the whole planet for 1 year, the energy flows in and out of the climate system are estimated to be around 235 to 240 watts per square meter. We don’t really know for sure because our global observations from spaceborne satellite instruments are not accurate enough to measure those flows of radiant energy.
“Greenhouse” components in the atmosphere (mostly water vapor, clouds, carbon dioxide, and methane) exert strong controls over how fast the Earth loses IR energy to outer space. Mankind’s burning of fossil fuels creates more atmospheric carbon dioxide. As we add more CO2, slightly less infrared energy is lost to outer space, strengthening the Earth’s greenhouse effect. This causes a warming tendency in the lower atmosphere and at the surface, and at the same time causes the upper atmosphere (especially the stratosphere) to cool. From an energy standpoint, it’s similar to adding insulation to the walls of a heated house in the winter; for the same rate of energy input (no thermostat), the result will be that the walls are warmer on the inside, and colder on the outside. This is analogous to the greenhouse effect of our atmosphere insulating the Earth’s surface from the “cold” depths of outer space.
It is believed (based upon theoretical calculations) that our global emissions of carbon dioxide have enhanced the Earth’s natural greenhouse effect by about 1%, thus reducing the rate at which IR energy is lost to outer space. Global warming theory (through conservation of energy) says that the lower atmosphere must then respond to this energy imbalance (less IR radiation being lost than solar energy being absorbed) by causing an increase in temperature. This warming then increases the IR escaping to space until the emitted IR radiation once again reaches a balance with absorbed sunlight, and the temperature stops rising. This is the basic explanation of global warming theory.
Manabe and Strickler (1964) calculated the global-average strength of the “greenhouse effect” on surface temperatures assuming all energy transfers were radiative (no weather processes), based upon the theory of how infrared energy courses through the atmosphere. They found that the surface of the Earth would average a whopping 75 deg. C warmer than if there was no greenhouse effect. But in reality, the surface of the Earth averages about 33 deg. C warmer, not 75 deg. C warmer than a no-greenhouse Earth. That’s because convective air currents (which create weather) carry excess heat away from the surface, cooling it well below its full greenhouse effect value represented by their imagined “pure radiative energy equilibrium” assumption.
Now, you might be surprised to learn that the amount of warming directly caused by us adding extra CO2 to the atmosphere is, by itself, relatively weak. It has been calculated theoretically that, if there are no other changes in the climate system, a doubling of the atmospheric CO2 concentration would cause about 1 deg C of surface warming. This is NOT a controversial statement…it is well understood by climate scientists. As of early 2019, we were about 50% of the way toward a doubling of atmospheric CO2.
