The greenhouse effect is one of the main factors determining the temperature of a planet. It's the phenomenon by which certain gases – so-called greenhouse gases – in the atmosphere trap heat that would otherwise escape to space, thereby keeping the planet warm.
The greenhouse effect is not a man-made phenomenon. The Earth's atmosphere has always contained greenhouse gases, such as CO2, and they have always caused warming. If there was no greenhouse effect, the planet would be uninhabitably cold – more than 30C colder than the hospitable current average of 15C.
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However, humans are changing the strength of the greenhouse effect by increasing the proportion of greenhouse gases in the air. For example, CO2 concentration in the atmosphere has increased from around 315 parts per million (ppm) to 387 ppm since 1959.
Although scientists are still researching many details of greenhouse warming, the basic principles of the greenhouse effect were discovered in the nineteenth century and are accepted by virtually all scientists.