Greenhouse effect

Schematic of the atmospheric energy flow causing the greenhouse effect.

The greenhouse effect is caused by the varying absorption and transmission of electromagnetic radiation (mostly visible and infra-red) at different wavelengths. The gases which make up the atmosphere tend to be transparent at visible frequencies (which is why we can see though it, duh) and to absorb infrared frequencies.

People find it remarkably hard to understand the greenhouse effect, and unhelpful "blanket" analogies abound. An easy, and very simple, way to understand it is this: the earth is heated by two sources: the sun, and the sky. It is therefore warmer than if it were heated by the sun alone. Increasing the amount of greenhouse gases increases the warming from the sky, without decreasing the heat from the sun, and therefore makes us warmer. Unfortunately this is completely incorrect as the sky is colder than the earth, and colder objects can't heat up warmer objects (ice cubes can't warm up your hands). It's even more problematic once you realize the sky is heated up by the earth's heat in the first place, and thus provides a picture where an externally-heated object is heating its heat source back up with its own heat, which is not possible.

The more technically correct explanation is that the earth's surface loses heat by multiple modes, one of which is radiative exchange of infrared radiation with the air. Increasing the amount of greenhouse gases decreases the rate of this radiative exchange, so although the earth is still losing heat to the colder sky, it is losing heat at a slower rate, which results in a higher equilibrium temperature. This simple model is easily formalised and analysed, but the actual atmospheric greenhouse effect is more complex than this simple description.

To a first approximation, visible light from the sun passes through the atmosphere to be absorbed by the Earth's surface, where it is converted to heat and re-emitted as thermal (infrared) radiation. Near the surface, the atmosphere is largely opaque to infra-red radiation, and heat is mostly carried upwards by convection not radiation. Higher up the atmosphere is colder and has less water vapour and so is less opaque, and radiation becomes more important (from the point of view of the greenhouse effect, it is more helpful to think of it beginning at a "surface" halfway up the atmosphere). Upwelling thermal infrared is absorbed and re-emitted both upwards and downwards, leading to a warming of the surface.

The greenhouse effect is sometimes erroneously connected with ozone layer depletion; in fact, ozone is a greenhouse gas, but present in small quantities.

The greenhouse effect is not the same effect as that which keeps greenhouses warm (greenhouses are mostly warmed by the physical barrier of the glass or plastic preventing physical movement of the warmed air by convection). This is well known, although prominent examples abound of mistaken descriptions of greenhouses as working according to the greenhouse effect (a recent notable mention is Sabine Hossenfelder, a theoretical physicist, who made a YouTube video with 812,000 views describing the greenhouse effect and giving the wrong explanation for how greenhouses work).

Every now and again some cranks will deny that the greenhouse effect exists and claim that they have disproved global warming. A remarkably straightforward refutation of their crankery would be to point them to a laboratory experiment demonstrating the greenhouse effect. Unfortunately, none have been successfully carried out to this day. It's trivial to show that greenhouse gases absorb infrared radiation, but nobody has been able to demonstrate a heat source becoming warmer as a result of increasing greenhouse gas concentrations. Many benchtop experiments do purport to show this, but they actually work equally well with Argon, which doesn't absorb or emit infrared radiation; these demonstrations are due to carbon dioxide's differing gas density and conductive & convective properties, not the greenhouse effect, which is radiative. (This author suggests that the only valid edit to this paragraph will be one with a direct link to a laboratory experiment demonstrating the greenhouse effect.)

Absent experimental verification, Venus is frequently provided as an example of the greenhouse effect at work. The probative value of this is properly put into perspective by following the peer-reviewed literature to Pekeris 1932. (Hansen et al 1983 cites Wang 1976 which cites Goody 1964 who cites Pekeris 1932.) The latter, informed by the radiative models at the time, wrote that “it becomes plausible the temperatures on […] Venus, Earth, and Mars are about the same”. We know better now that Venus is far hotter than Earth; this just demonstrates that model can be made to fit observations and it doesn't necessarily mean the model is correct.

For an example of a place without any greenhouse effect (more properly: without any atmosphere at all), just look at our moon, where temperatures reach the triple digits below and above zero Celsius in the course of one "day" (i.e. roughly a month). It is relevant to note that the greenhouse effect predicts a strict increase in temperature whereas the moon is both hotter at day and colder at night than the Earth.

In 2023, a study was done analyzing the transitional stage between what Earth or a similar exoplanet is like now and a Venusian hellhole. What was found was that warming of several tens of degrees would cause the atmosphere to collect more water vapor. Water vapor is an "incredible insulator," and so this creates a feedback loop. The planet warms up, it collects more water vapor, dysregulating planetary thermal regulation as the planet can no longer efficiently cool, causing the planet to warm up and collect more water vapor. This will eventually cause the oceans to evaporate and transform the Earth into a Venusian hellhole.

See also

• Global warming
• Environmentalism

External links

• Greenhouse effect, University Corporation for Atmospheric Research
• Stephen Hawking and Carl Sagan on the Greenhouse Effect

References

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