The temperature of the Earth is controlled by the balance between the input from energy of the sun and the loss of this back into space. Certain atmospheric gases are critical to this temperature balance and are known as greenhouse gases. The energy received from the sun is in the form of short-wave radiation, i.e. in the visible spectrum and ultraviolet radiation. On average, about one-third of this solar radiation that hits the Earth is reflected back to space. Of the remainder, some is absorbed by the atmosphere, but most is absorbed by the land and oceans. The Earth's surface becomes warm and as a result emits long-wave 'infrared' radiation. The greenhouse gases trap and re-emit some of this long-wave radiation, and warm the atmosphere. Naturally occurring greenhouse gases include water vapour, carbon dioxide, ozone, methane, and nitrous oxide, and together they create a natural greenhouse or blanket effect, warming the Earth by 35°C. Despite the greenhouse gases often being depicted in diagrams as one layer, this is only to demonstrate their 'blanket effect', as they are in fact mixed throughout the atmosphere (see Figure 1).
Another way to understand the Earth's natural 'greenhouse' is by comparing it to its two nearest neighbours. A planet's climate is decided by several factors: its mass, its distance from the sun, and of course the composition of its atmosphere and in particular the
amount of greenhouse gases. For example, the planet Mars is very small, and therefore its gravity is too small to retain a dense atmosphere; its atmosphere is about a hundred times thinner than Earth's and consists mainly of carbon dioxide. Mars's average surface temperature is about -50°C, so what little carbon dioxide exists is frozen in the ground. In comparison, Venus has almost the same mass as the Earth but a much denser atmosphere, which is composed of 96% carbon dioxide. This high percentage of carbon dioxide produces intense global warming and so Venus has a surface temperature of over + 460°C.
The Earth's atmosphere is composed of 78% nitrogen, 21% oxygen, and 1% other gases. It is these other gases that we are interested in, as they include the so-called greenhouse gases. The two most important greenhouse gases are carbon dioxide and water vapour. Currently, carbon dioxide accounts for just 0.03-0.04% of the og atmosphere, while water vapour varies from 0 to 2%. Without the | natural greenhouse effect that these two gases produce, the Earth's j| average temperature would be roughly -20°C. The comparison 3 with the climates on Mars and Venus is very stark because of the different thicknesses of their atmospheres and the relative amounts of greenhouse gases. However, because the amount of carbon dioxide and water vapour can vary on Earth, we know that this natural greenhouse effect has produced a climate system which is naturally unstable and rather unpredictable in comparison to those of Mars and Venus.
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