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water vapor
The role of water vapor
Water vapor is a naturally occurring greenhouse gas and accounts for the largest
percentage of the greenhouse effect. Water vapor concentrations fluctuate
regionally, but human activity does not directly affect water vapor
concentrations except at very local scales.
In climate models an increase in atmospheric temperature caused by the
greenhouse effect due to anthropogenic gases will in turn lead to an increase in
the water vapor content of the troposphere, with approximately constant relative
humidity. The increased water vapor in turn leads to an increase in the
greenhouse effect and thus a further increase in temperature; the increase in
temperature leads to still further increase in atmospheric water vapor; and the
feedback cycle continues until equilibrium is reached. Thus water vapor acts as
a positive feedback to the forcing provided by human-released greenhouse gases
such as CO2 (but has never, so far, acted on Earth as part of a runaway
feedback). Changes in water vapor may also have indirect effects via cloud
formation.
Intergovernmental Panel on Climate Change (IPCC) IPCC Third Assessment Report
chapter lead author Michael Mann considers citing "the role of water vapor as a
greenhouse gas" to be "extremely misleading" as water vapor can not be
controlled by humans. The IPCC report has discussed water vapor feedback in more
detail.
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