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What are greenhouse gases

and what is their role?

The most common and effective greenhouse gases are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Common to these gases is that they all absorb outgoing radiation from earth to space, contributing to the greenhouse effect. In short, the greenhouse effect is that some of the heat radiated from earth towards space is absorbed by the greenhouse gases in the atmosphere and reemitted towards the earth, thus increasing the temperature. Without the atmosphere, earth would be much colder than it is. If the amount of greenhouse gases in the atmosphere increases from e.g. human activities, more radiation is absorbed into the atmosphere, thus increasing the temperature further.