The greenhouse effect, explained simply
The greenhouse effect is how certain gases in the atmosphere keep the planet warm. It is natural and we depend on it. Adding more greenhouse gas strengthens it, and that is what is warming the planet now.
What the greenhouse effect is
Sunlight passes through the atmosphere and warms the surface. The surface then radiates heat back out as infrared. Greenhouse gases, mainly water vapour, carbon dioxide and methane, absorb some of that outgoing heat and send part of it back down. The result is that the surface is warmer than it would be otherwise.
Without any greenhouse effect, Earth\'s average temperature would be well below freezing. The effect itself is not the problem. The problem is making it stronger by adding more CO2.
Why a small amount of CO2 still matters
CO2 is measured in parts per million, which sounds tiny. But a gas does not need to be abundant to matter. CO2 absorbs infrared at specific wavelengths, and changing its concentration changes how much heat escapes to space. Small by share, large by effect.
This point is covered on its own page: is CO2 too small to matter?
How we know it is really happening
This is not only theory. Satellites measure less heat escaping to space at exactly the wavelengths CO2 absorbs. The upper atmosphere is cooling while the surface warms, which is the specific fingerprint of greenhouse warming rather than a brighter Sun. The physics and the measurements agree.
Frequently asked
It is the way greenhouse gases such as CO2 absorb heat radiating from Earth's surface and keep the lower atmosphere warmer than it would otherwise be.
CO2 absorbs infrared heat at specific wavelengths. Adding more CO2 means less heat escapes to space, so the planet stores more energy and warms.
Concentration is not the same as effect. CO2 is a small share of the air but a strong absorber of infrared, so changes in it have a large effect on Earth's heat balance.