How the Greenhouse Effect Keeps Earth Warm

Most incoming solar energy arrives as visible light and nearby wavelengths. Some is reflected by clouds, ice and bright surfaces, while the rest is absorbed by land, oceans and the atmosphere. Absorbed energy warms the planet.

A warm surface releases energy mainly as infrared radiation. Greenhouse gas molecules can absorb particular infrared wavelengths because the radiation makes their bonds vibrate or rotate. The molecules then transfer energy through collisions or emit infrared radiation in different directions.

Some of that emitted energy travels downward, so the surface and lower atmosphere lose heat more slowly than they would in an atmosphere without greenhouse gases. The gases do not form a solid roof and do not simply reflect all heat. They participate in a continuing exchange of radiation.

Water vapour, carbon dioxide, methane and nitrous oxide are important greenhouse gases. Without the natural greenhouse effect, Earth's average surface temperature would be far colder. Oceans would contain much more ice, and present ecosystems could not function in the same way.

Water vapour acts mainly as a feedback. Warmer air can contain more water vapour, which adds greenhouse warming and strengthens an initial temperature change. Carbon dioxide remains in the climate system much longer and can drive that change when its concentration rises.

Clouds complicate the picture because they can reflect sunlight and absorb infrared energy. Their net effect depends on altitude, thickness, location and time. Climate science contains details because the atmosphere has declined repeated requests to behave like one tidy classroom diagram.

Burning fossil fuels, producing cement and clearing forests increase atmospheric carbon dioxide. Agriculture, waste and fossil fuel systems add methane and nitrous oxide. These gases reduce the rate at which energy escapes to space from lower levels of the atmosphere.

The planet responds by warming until outgoing energy once again balances incoming energy, but oceans, ice and ecosystems adjust over different timescales. This creates delayed effects and continuing change.

Keep the main sequence clear:

• •Sunlight enters the Earth system.
• •The surface absorbs energy and warms.
• •The surface emits infrared radiation.
• •Greenhouse gases absorb and re-emit some infrared energy.
• •Higher greenhouse gas levels strengthen the warming effect.

The basic physics has been understood for well over a century. In 1859 the Irish physicist John Tyndall built apparatus to pass heat radiation through tubes of different gases and measure how much got through. He found that the main gases of air, nitrogen and oxygen, let infrared pass almost freely, while water vapour and carbon dioxide absorbed it strongly. A few years earlier in the United States, Eunice Foote had reported that carbon dioxide warmed more than ordinary air in sunlight, though her experiment could not separate out the infrared effect.

In 1896 the Swedish chemist Svante Arrhenius used such findings to estimate how much Earth's temperature would change if carbon dioxide levels changed, a long, painstaking calculation done by hand. Direct, continuous measurements came later. From 1958, Charles David Keeling began recording carbon dioxide at the Mauna Loa observatory in Hawaii. His record shows a yearly zigzag, as northern hemisphere plants absorb carbon dioxide in summer and release it in winter, riding on a steady rise that has continued ever since.

A glass greenhouse stays warm mainly because it traps air. Sunlight heats the soil and plants inside, they warm the air, and the glass stops that warm air from rising away and mixing with cooler air outside. In 1909 the physicist Robert Wood built boxes with different kinds of covers and found that blocking infrared radiation made little difference compared with simply stopping air from escaping. The atmosphere has no lid of this kind, so the greenhouse effect works through radiation rather than by trapping air.

A helpful way to picture it is to think about where Earth releases energy to space. Most outgoing infrared escapes from fairly high in the atmosphere, where the air is thinner and colder. Adding greenhouse gases makes the lower air more opaque to infrared, so the level from which energy escapes moves higher, to where it is colder. Colder air radiates less, so the planet temporarily sends out less energy than it receives and warms until the balance is restored. This is a separate issue from the ozone hole, which concerns ultraviolet light.

Comparing the rocky planets shows how much an atmosphere matters. Simple calculations based on how much sunlight Earth absorbs suggest that without any greenhouse effect its average surface would sit at around minus 18 degrees Celsius. The actual average is roughly 15 degrees, so the natural greenhouse effect adds about 33 degrees. Mars has an atmosphere made mostly of carbon dioxide, but it is extremely thin, with surface pressure less than one per cent of Earth's. There are simply too few molecules to hold back much infrared, so Mars warms only slightly above what its distance from the Sun alone would give.

Venus sits at the other extreme. Its atmosphere is also mostly carbon dioxide but is so dense that the pressure at the surface is around ninety times that on Earth. Thick clouds of sulphuric acid reflect most incoming sunlight, so Venus actually absorbs less solar energy than Earth does. Even so, its surface stays at roughly 460 degrees Celsius, hot enough to melt lead, day and night and from pole to equator. That makes it hotter than Mercury, which is much closer to the Sun. The comparison is a useful reminder that the strength of the effect depends on how much greenhouse gas an atmosphere holds.

Measuring rising carbon dioxide is one thing; showing where it comes from is another. Carbon atoms carry fingerprints. Carbon-14 is radioactive, and about half of any sample decays in roughly 5,700 years, so coal, oil and gas, formed from living things many millions of years ago, contain essentially none. In the 1950s the chemist Hans Suess noticed that burning these fuels was diluting the carbon-14 in the air, an effect now named after him. Plants also take up the lighter isotope carbon-12 slightly more readily than carbon-13, so fossil fuels are low in carbon-13, and the share of carbon-13 in the atmosphere has been falling as expected.

Oxygen gives a further check. Burning fuel uses oxygen, so if the extra carbon dioxide comes from combustion, oxygen in the air should be falling slightly in step. Precise measurements begun in the late 1980s by Ralph Keeling, son of Charles David Keeling, show exactly that, at a rate matching estimates of how much fuel is burned. Volcanoes cannot explain the rise. Their carbon lacks the low carbon-13 signature of plant-derived fuels, their gases do not use up oxygen in this way, and they emit less than one per cent of the carbon dioxide released by human activity each year.

The greenhouse effect is a natural process that keeps Earth warmer by slowing the loss of infrared energy. Human activity strengthens it by increasing long-lived greenhouse gases, especially carbon dioxide. Separate incoming sunlight from outgoing infrared radiation, and the basic mechanism becomes much easier to understand without confusing it with the ozone layer or ordinary air pollution.