Which greenhouse gas is released in the greatest quantity by human activities?

Carbon dioxide is the greenhouse gas released in the greatest quantity by human activities.

Human sources include burning coal, oil, and natural gas, cement production, and changes in land use such as deforestation. Carbon dioxide is also exchanged naturally among the atmosphere, oceans, soils, and living organisms, but human emissions have increased its atmospheric concentration substantially since the Industrial Revolution.

Carbon dioxide is not the most powerful greenhouse gas molecule on a molecule-for-molecule basis. Water vapor has a major natural greenhouse effect, while gases such as methane can absorb infrared radiation strongly. Carbon dioxide is especially important because people emit large amounts of it and because it remains influential through long-lived carbon-cycle effects.

The gas absorbs outgoing infrared radiation, reducing the rate at which Earth loses heat to space. This is why carbon dioxide is central to modern discussions of anthropogenic climate change, even though it forms only a small fraction of the atmosphere.