Water can take the form of an invisible gas called water vapour. Water vapour is the most abundant natural greenhouse gas in the atmosphere and has a strong effect on water and climate.
Changes in water vapour concentration are a result of climate feedbacks related to the warming of the atmosphere rather than a direct result of human activities. As the temperature of the atmosphere rises, more water is evaporated from ground storages such as rivers, oceans, reservoirs, and soil. Water vapour being a greenhouse gas, more water vapour in the atmosphere leads to even more warming. This positive feedback loop in which water is involved is critically important to projecting future climate change, but as yet is still fairly poorly measured and understood.