The most abundant greenhouse gas in the atmosphere is water vapour. Its atmospheric concentration is determined by temperature, land use and the effects of aerosol particles. Water in the earth system cycles rapidly between vapour in the atmosphere, clouds, rain and liquid in, say, river and lakes. So water has a short lifetime in the atmosphere, and its large-scale concentration is only indirectly affected by human activity. Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. Once emitted is stays in the atmosphere for a long time. About one third of the atmosphere's CO2 has come from human activities, mainly burning fossil fuels (oil, coal and gas), but also from deforestation.
