Fr.: effet de serre
An increase in → temperature caused when incoming → solar radiation is passed but outgoing → thermal radiation is trapped by the → atmosphere. The major factors for this effect are → carbon dioxide and → water vapor. The greenhouse effect is very important on Venus and Earth but very weak on Mars. On average, about one third of the solar radiation that hits the Earth is reflected back to space. The Earth's surface becomes warm and emits → infrared radiation. The → greenhouse gases trap the infrared radiation, thus warming the atmosphere. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. However, human activities are causing greenhouse gas levels in the atmosphere to increase.