Most of those who argue about whether the so-called greenhouse effect actually exists seem to be arguing about one or more explanations of the greenhouse effect that are not scientifically accurate or lead to false conclusions. Other sources of confusion include the atmospheric lapse rate and various climate feedback mechanisms.
Even the IPCC’s description of the greenhouse effect in AR4 WG1 has issues. Page 115 says “Much of [the] thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect.” If that were the greenhouse effect, then one would expect low clouds to have a much larger greenhouse effect than high clouds, since they radiate much more energy back to Earth. In fact, the opposite is true.
To fix this problem, I’ve come up with a succinct description of the greenhouse effect and the consequence of increasing the concentration of greenhouse gases. I think this explanation is fully self-consistent and leads to the proper inferences about the climate system. I call it “Best Ever” as a challenge: please unearth any flaws and suggest improvements so that it can indeed become the best ever. Additions and footnotes are in italics.
Meanwhile, the next time you read something that says that the greenhouse effect doesn’t exist or that carbon dioxide would only cause a tiny change in temperature, check it against the description below and see if it actually refutes any of it.
The Best Ever Description of the Atmospheric Greenhouse Effect
The so-called atmospheric greenhouse effect (or Tyndall gas effect) can be most directly quantified as the difference between the globally- and time-averaged rate of electromagnetic radiation emitted by the surface of the Earth (~396 W/m2) and the rate by which radiation emitted from the combined Earth/atmosphere system escapes to space (~239 W/m2). These two numbers are different because certain constituents in the atmosphere (mostly water vapor, CO2, clouds, and other Tyndall gases) intercept most of the radiation emitted by the Earth’s surface.
The mere interception of radiation emitted by the Earth’s surface is not the whole story though, because anything that can absorb radiation at a particular wavelength can also emit radiation just as easily at that same wavelength (Kirchhoff). Also, the intensity of emitted radiation is proportional to the fourth power of temperature (Stefan-Boltzmann) (1). So if the atmosphere everywhere were somehow required to have the same temperature as the underlying surface of the Earth(2), it would be emitting to space exactly as much radiation as it intercepted from the Earth’s surface and the greenhouse effect would be zero. Instead, we observe(3) that the atmosphere is mostly colder than the Earth’s surface, by several tens of Kelvins, and this accounts for the relatively lower intensity of radiation escaping the top of the atmosphere compared to radiation entering the bottom of the atmosphere from below.
The value that escapes to space (~239 W/m2) is constrained by energetics to be essentially equal to the amount of solar radiation absorbed by the Earth-atmosphere system, since heat escaping from the Earth’s interior or received from elsewhere in the universe are both orders of magnitude smaller.
One can calculate a simplified temperature version of the atmospheric greenhouse effect by comparing the temperature of a blackbody emitting at 396 W/m2 (289 K) with the temperature of a blackbody emitting at 239 W/m2 (255 K). (The relatively small difference in temperature corresponds to a large difference in emitted energy because the energy depends on the fourth power of the temperature.) This is sometimes explained to the general public as “the Earth would be 34 K cooler without the greenhouse effect” but the reality is not so simple. To eliminate the Tyndall gas effect, you’d have to eliminate water vapor, and to be physically consistent you’d also have to eliminate clouds, oceans, and plants, so the amount of solar radiation absorbed by the Earth-atmosphere system would no longer be 239 W/m2. Since clouds are highly reflective, for example, eliminating them would increase the amount of energy absorbed.
An increase in the concentration of a greenhouse (or Tyndall) gas such as CO2 would have two direct effects. First, the fraction of radiation emitted by the surface of the Earth that gets absorbed by the atmosphere would increase, thereby reducing the total amount of radiation that escapes into space. Second, the average height at which radiation emitted by CO2 can escape to space would also increase. Since most of the IR that’s emitted by CO2 and escapes emissions that escape to space comes from the troposphere (where temperature decreases with height) or the lower stratosphere (where temperature is nearly constant with height), such radiation would on average be of lower intensity, so this also reduces the total amount of radiation that escapes into space. Since the Earth/atmosphere system is now absorbing more energy than is escaping into space, its temperature increases. In principle, its temperature would increase until the Earth’s surface and atmosphere are warm enough that 239 W/m2 is once again escaping into space. In reality, maybe 1 W/m2 of the excess heat would be stored in the deep ocean, and something approaching true equilibrium would not be attained until the whole depth of the ocean readjusts.
This increase in temperature can cause several side effects, called “feedbacks”, which in turn could enhance or reduce the energy imbalance. These are caused by changes in the average global temperature of the Earth as well as in the horizontal temperature distribution (since the heating is known to be uneven from past climate changes). Among these would be changes in the water vapor concentration in the atmosphere (important since water vapor is a Tyndall gas), changes in cloud coverage and distribution, and changes in the equilibrium vertical temperature structure. Since clouds both reflect solar radiation and absorb and emit infrared radiation, the net effect of changes in cloud cover depends on altitude. Since low clouds would emit radiation at almost the same temperature as the underlying Earth, they have a relatively small greenhouse contribution and their net effect would be cool the climate system by reducing the amount of energy absorbed by from the Sun. Most high clouds, by contrast, make a powerful greenhouse contribution because they are so cold and therefore emit radiation at very low intensities.
The direct effect of doubling the concentration of CO2 is relatively easy to calculate from the laws of radiation and the observed distribution of clouds and other Tyndall gases: a reduction of 3.7 W/m2 in the amount of energy escaping into space. The Earth and its atmosphere would have to warm by about 1 K degree C to make up for this reduction.
The total effect, including feedbacks, is not known very accurately and thus is presently a subject of intense research. There are two complementary approaches to evaluating these feedbacks. One is to attempt to list and quantify each possible feedback individually. The other is to evaluate the total effect of all feedbacks. Based on multiple lines of evidence, including past climate changes including glacial-interglacial cycles, recent large climate system perturbations due to volcanic eruptions, and simulations of the climate system with computer models, the total change in average global temperature due to an energy imbalance of 3.7 W/m2 is somewhere between 1.5 K and 4.5 K, with the most common estimates being around 2.5-3.0 K.
(1) These laws apply to systems in thermodynamic equilibrium. We observe that under normal conditions the various bits of the climate system are in approximate local thermodynamic equilibrium so that these laws are observed to hold. Even so, systematic departures from local thermodynamic equilibrium conditions would only change some of the numeric values, not the physical processes that lead to changes in temperature described above.
(2) This description is strictly correct only if the emissivity of the Earth’s surface is exactly 1. In reality, the emissivity of the Earth’s surface is almost but not quite 1, and the resulting greenhouse effect with an isothermal air column would be slightly negative.
(3) This temperature difference is fundamentally a consequence of the radiative equilibrium temperature of the atmosphere being much colder than that of the Earth’s surface. Within the troposphere, such a radiative equilibrium would be strongly dynamically unstable, and the observed rate of decrease of temperature with height is maintained at a weakly-unstable state by a combination of moist convection (commonly known as thunderstorms) and mid-latitude baroclinic eddies (commonly known as low-pressure systems).