 Earth's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%. Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space. These gases keep the Earth warm in a similar (but not identical) way as a greenhouse keeps plants warm. So these gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be. This blanketing is known as the natural greenhouse effect. Earth's average surface temperature is kept at about 15C by the blanket of atmosphere that surrounds it. Without the greenhouse gases the temperature would be about minus 18C - too cold for life. Greenhouse gases are mixed throughout in the atmosphere.
The climate of the Earth is always changing. In the past it has altered as a result of natural causes. The changes we've seen over recent years and those which are predicted over the next 100 years are thought to be mainly as a result of human behaviour rather than due to natural changes in the atmosphere.
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The Earth has a natural temperature control system. The greenhouse gases are critical to this system. The Earth receives its life sustaining warmth from the Sun. On its way to the Earth's surface most of the heat energy passes through the Earth's atmosphere, On average, about one third of the 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 infrared radiation. The greenhouse gases trap the infrared radiation, thus warming the atmosphere. Naturally occurring greenhouse gases include water vapour, carbon dioxide, ozone, methane and nitrous oxide, and together create a natural greenhouse effect. However, human activities are causing greenhouse gas levels in the atmosphere to increase.
Although most of the greenhouse gases occur naturally in the atmosphere, some are man-made and the most well-known of these are the fluorocarbons. Since the industrial revolution, human activities have also resulted in an increase in natural greenhouse gases, especially.
An increase in these gases in the atmosphere enhances the atmosphere's ability to trap heat, which leads to an increase in the average surface temperature of the Earth.
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