A greenhouse traps heat one way; Earth's atmosphere does it quite differently
Strip away the heat-absorbing gases and Earth's average surface would sit near minus 18 degrees Celsius. Instead it hovers around 15, a 33-degree gift from a thin fraction of the air. The name borrows from glass garden buildings, yet those stay warm by stopping air from moving, not by catching infrared light.
Everything above absolute zero glows with thermal radiation, and hotter bodies glow at shorter wavelengths. The Sun, with a surface near 5,500 degrees Celsius, pours out visible and near-infrared light. Earth, far cooler, sends energy back as mid- and far-infrared. The atmosphere absorbs just 23 percent of incoming sunlight but 90 percent of that outgoing longwave radiation. Each gas-laden layer soaks some up and re-emits it in every direction, including downward, so less heat escapes and the layers below stay warmer.
Another way to size the effect is in energy flow. Earth's surface sends out about 398 watts per square metre of longwave radiation, yet only 239 reach space. The missing 159, roughly 40 percent, is the greenhouse effect expressed in energy rather than degrees. Carbon dioxide leaves a clear fingerprint: a dip in outgoing radiation around a wavelength of 15 microns.
The idea is two centuries old. Joseph Fourier proposed it in 1824, and in 1856 Eunice Newton Foote showed that sunlight warms moist air more than dry air, and air laden with carbon dioxide more still. From 1859 John Tyndall measured how individual gases absorb infrared, finding that the main atmospheric gases do nothing while water vapour and small amounts of carbon dioxide matter a great deal. Svante Arrhenius in 1896 made the first numerical forecast of warming from doubled carbon dioxide, and Nils Gustaf Ekholm attached the word greenhouse in 1901.
Burning fossil fuels, making cement and clearing tropical forest have strengthened the effect. Mauna Loa measurements climbed from about 313 parts per million of carbon dioxide in 1960 past 400 in 2013, above the roughly 180 to 270 range ice cores record for the past 800,000 years. By 2023 the planet had warmed about 1.2 degrees since the Industrial Revolution. Earth's energy imbalance, about 0.7 watts per square metre in 2015, roughly doubled by 2024, with over 90 percent of the surplus heat going into the oceans.
Source: Greenhouse effect
