Greenhouse effect: Absorbing energy
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Greenhouse gases such as water vapour, carbon dioxide and methane don’t absorb visible sunlight. But their complex molecules interact with the infrared heat energy given off by the Earth, absorbing it by vibrating and rotating before re-emitting it in a random direction, sometimes back down towards the ground. This temporarily traps the heat in the atmosphere, warming up the Earth’s surface in the process.
All objects with temperatures above absolute zero give off energy in the form of radiation which oscillates as it travels. Although all radiation travels at the same speed, the frequency at which it oscillates depends on the temperature. Objects at higher temperatures give off a band of higher frequencies of radiation, while objects at lower temperatures give off a band of lower frequencies. The Sun’s average surface temperature is over 5500 °C, so it gives off energy at relatively high frequencies – mostly as visible and ultraviolet light. In contrast, the Earth’s average surface temperature is about 15 °C, so it gives off energy at much lower frequencies – mostly as infrared heat energy.
Just as visible light comes in all the colours of the rainbow, infrared heat energy comes in different shades – frequencies. The colour of all objects depends on the frequencies of visible light their molecules absorb. For example, living grass is green because its molecules absorb visible light energy at frequencies corresponding to all colours except green, so green light is reflected from each blade of grass. Likewise, different molecules absorb different frequencies of infrared energy. Which frequencies a molecule can absorb depends on the geometry and size of the molecule and the atoms it’s made of. Greenhouse gas molecules have shapes and sizes that cause them to absorb certain frequencies of infrared energy.
The atoms in molecules are continually rotating and vibrating at certain frequencies. Energy in the form of visible light or infrared heat also oscillates at different frequencies. When the energy’s frequency matches that of a molecule’s vibration, the molecule can absorb the energy. Nitrogen and oxygen don’t absorb infrared energy in our atmosphere because their molecules – composed of two identical atoms – don’t vibrate or rotate at infrared frequencies. But the more complex greenhouse gas molecules have vibration and rotation frequencies matching those of the infrared heat energy given off by the Earth. They absorb this energy before re-emitting it in a random direction.
The Sun gives off energy primarily at frequencies corresponding to visible light – the colours of the rainbow that human eyes can see. Most of this energy passes straight through the atmosphere and is absorbed by the Earth’s surface, though some is reflected by aerosols, clouds, snow and ice. However, the Sun also gives off energy at other frequencies, including ultraviolet and infrared. Most ultraviolet sunlight is absorbed by ozone in the stratosphere, while greenhouse gases absorb some solar energy at infrared frequencies. In total, about 20% of the Sun’s energy is absorbed by gases in the atmosphere, 30% is reflected back to space, and 50% is absorbed by the surface.
The Earth’s surface, warmed by energy from the Sun, gives off energy at frequencies corresponding to infrared heat. Without an atmosphere surrounding the planet, this energy would be lost to space and the Earth would be more than 30 °C colder than it is today. But clouds and greenhouse gases in the atmosphere absorb most of the outgoing infrared energy, delaying its progress out to space and keeping the Earth’s surface warmer than it would otherwise be. The main greenhouse gases are water vapour, carbon dioxide and methane. Other greenhouse gases, such as nitrous oxide, ozone and chlorofluorocarbons are also present in the atmosphere, though in far smaller quantities.
Clouds and greenhouse gases absorb the Earth’s outgoing energy and re-emit it in all directions. So instead of travelling directly out to space, some of the energy travels back towards the ground. The same ‘parcel’ of energy can be reabsorbed and re-emitted many times, but eventually reaches an altitude where the atmosphere is so sparse that it becomes transparent and the energy finally escapes to space. Because temperatures at this altitude are lower than at ground level, energy escaping at high altitude carries less heat away from the Earth than energy escaping directly from the ground. This is the basic mechanism through which the greenhouse effect keeps the Earth warm.