Although governments and scientists are currently worried about the role which carbon dioxide and methane emissions play in global warming, perhaps somewhat surprisingly, the most important greenhouse gas today is actually water vapour. Recent research has confirmed that, although the rise in carbon emissions is expected to contribute to harmful levels of warming over the coming century, most of the greenhouse effect which the Earth currently experiences can be explained by the effects of water vapour.
A greenhouse gas, according to the U.S. National Oceanic and Atmospheric Administration (NOAA), is any gas which "allows direct sunlight to reach the Earth's surface" and absorbs heat from the sunlight radiating off the surface of the Earth. The effect is that, instead of radiating off of the surface of Earth back into space, heat gets trapped, warming the Earth. Most greenhouse gases, like carbon dioxide, water vapour and methane, occur in nature, but some human-made organic compounds, like chlorofluorocarbons (CFCs), also act as greenhouse gases. The present scientific concerns about climate change are motivated by observations that human activities have caused a dramatic increase in carbon dioxide and that this may result in an increase in average temperatures.
Some level of greenhouse effect is also naturally occurring, however. Presently, the most important greenhouse gas is water vapour, which is engaged in a continual cycle of evaporating from the surface of the oceans, being blown inland, falling as rain, and circulating back to the oceans via rivers. While in the atmosphere, water vapour functions as a powerful greenhouse gas. On the surface, the oceans also function as a sort of massive heat sink. This does not mean that water vapour would be equivalent, in terms of its greenhouse effect potential, to an equal amount of some other greenhouse gas. Instead, water vapour is the most important greenhouse gas today because its current levels in the atmosphere are much higher than any other gas.
Climate change news and education website Real Climate states that water vapour makes up 0.3 percent of Earth's atmosphere (about five times as much as carbon dioxide), and that "water vapour is the single most important" greenhouse gas, responsible for "between 36% and 66%" of Earth's natural greenhouse effect. The present-day greenhouse effect is estimated to be about 57 degrees Fahrenheit, or 33 degrees Celsius. That is, Earth is 57 degrees Fahrenheit warmer than it would be without its current greenhouse effect.
The effect of water vapour as a greenhouse gas has led some critics of the contemporary climate science consensus to argue that the effects of carbon dioxide, and therefore the danger posed by unchecked human-caused carbon emissions, has been exaggerated. That's the position taken, for instance, by climatologist Tim Ball. No climatologists deny that water vapour is a greenhouse gas; instead, the present debate is about how much additional warming will be caused by increasing the atmospheric levels of other greenhouse gases. NASA, like many other organizations, is also funding research that explores the ways in which different greenhouse gases interact. For instance, their researchers say, additional warming caused by carbon dioxide will result in greater evaporation of water into the atmosphere, which will in turn increase the greenhouse effect caused by water, as well, in what they call a positive feedback cycle.