What is the greenhouse effect?
The greenhouse effect is the warming of Earth’s surface and lower atmosphere as certain atmospheric gases absorb and re-emit infrared energy from the surface. The natural effect is important for life; its human-driven intensification is a separate issue.
How the energy balance works
Some incoming shortwave sunlight is reflected by clouds, air and the surface; the rest is absorbed. Warm Earth emits longer-wave infrared radiation toward space. Water vapor, carbon dioxide, methane and other greenhouse gases interact with that infrared radiation. Energy moves in different directions within the atmosphere and toward space, leaving the surface warmer than it would be without greenhouse gases. The mechanism is not identical to a glass greenhouse trapping warm air.
Human influence and feedbacks
Burning fossil fuels and changes in land use raise atmospheric carbon dioxide; several human activities also release methane. Higher concentrations change the energy balance and cause warming. Warmer air can hold more water vapor, so water vapor is usually a feedback that amplifies warming rather than its initial human-driven cause. Clouds and aerosols can warm or cool under different conditions. Ozone-layer depletion and an intensified greenhouse effect are distinct physical problems.