Greenhouse effect, a warming of Earth’s surface and troposphere i.e. the lowest layer of the atmosphere caused by the presence of water vapour, carbon dioxide, methane, and certain other gases in the air. Of those gases, known as greenhouse gases, water vapour has the largest effect.

The atmosphere allows most of the visible light from the Sun to pass through and reach Earth’s surface. As Earth’s surface is heated by sunlight, it radiates part of this energy back toward space as infrared radiation. This radiation, unlike visible light, tends to be absorbed by the greenhouse gases in the atmosphere, raising its temperature. The heated atmosphere in turn radiates infrared radiation back toward Earth’s surface.

Some of the re-emitted energy remains within the atmosphere or returns to the surface and warms the lower atmosphere and surface. The rest of the re-emitted energy leaves the atmosphere and goes into space. The outcome of this absorption-emission process by the greenhouse gases is that less energy leaves the atmosphere than is emitted by the Earth below. Despite its name, the greenhouse effect is different from the warming in a greenhouse, where panes of glass transmit visible sunlight but hold heat inside the building by trapping warmed air.

The increased amounts of greenhouse gases human activities are adding to the atmosphere have upset the balance that has been in place since the end of the last ice age. Adding more greenhouse gases decreases the amount of infrared radiation energy leaving the atmosphere. To get the energy back in balance, the surface of the Earth has to warm up, so that it will emit more infrared energy, some of which will leave the atmosphere and compensate for the effect of the added greenhouse gases. Thus, the greenhouse effect, which is essential for creating the climate for life on Earth, is also responsible for the Earth getting warmer than it was before we started burning large amounts of fossil fuels.