Talks to focus on most abundant greenhouse gas

Water vapor, the most abundant greenhouse gas, is also a part of the global hydrological cycle that produces rain and snow.

October 12, 1999
Web posted at: 2:53 p.m. EDT (1853 GMT)

Since the 1995 conference, scientists from around the world have made significant improvements in clarifying this issue. Their results will be presented at this year's conference, organized by the American Geophysical Union in Potomac, Md., today through Friday.

Not only is water vapor (water in the gaseous phase) the most abundant greenhouse gas, it is also involved in the global hydrological cycle. It participates in chemical reactions both in the troposphere, or lower atmosphere, and in the stratosphere, which is six to 10 miles above the Earth's surface, where it affects the quantity of ozone.

What scientists do know is that the climate of Earth is able to support life in large part because of the atmospheric greenhouse effect and the hydrological cycle and water vapor is a key element in both of these.

Like water vapor, some greenhouse gases occur naturally while others are the result of industrial activities. A greenhouse gas absorbs infrared radiation emitted by Earth's surface and atmosphere. Water vapor is classified as a greenhouse gas because it absorbs radiation and does not allow it to escape into space. Thus, the planet is warmer than it would be if there was no water vapor, said Dr. Dian Gaffen of the National Oceanographic and Atmospheric Administration.

Without water vapor acting as a greenhouse gas, Earth would be much colder.

Water vapor is involved in an important climate feedback loop as the temperature of the Earth's surface and atmosphere increases, the atmosphere is able to hold more water vapor. The additional water vapor, acting as a greenhouse gas, absorbs energy that would otherwise escape to space, causing further warming. This basic cycle is complicated by important interactions between water vapor, clouds, atmospheric motion and radiation from both the sun and the Earth. There are some aspects of the role of water vapor as a greenhouse gas that are not well understood, mainly because scientists lack the necessary observations to test theoretical models.

The hydrological cycle is the movement of water within and between the Earth's atmosphere, oceans and continents. Water vapor is constantly cycling through the atmosphere, evaporating from the surface of oceans and continents, condensing to form clouds blown by the winds and returning to the Earth as rain or snow.

Heat from the sun evaporates the water, and this heat is put into the air when the water condenses into clouds and precipitates. Increases in temperature can lead to greater evaporation and therefore more water vapor and hence to even more warming. The evaporation-condensation cycle is an important mechanism for transferring heat energy from the Earth's surface to the atmosphere and in moving heat around the Earth.

One of the conference goals is to bring together troposphere and stratosphere specialists, said Gaffen, who helped to organize the conference along with Rebecca Ross, also of NOAA, and Dr. John Gille of the National Center for Atmospheric Research in Boulder, Colo. More than 80 participants are expected from the United States and abroad.

At this year's Chapman Conference on Water Vapor in the Climate System, atmospheric scientists will focus on three themes:

Water vapor and the greenhouse effect. At the 1995 conference, the notion that water vapor might have a negative impact on the greenhouse effect evoked some controversy. Since then, considerable effort has been made to clarify this issue and results will be presented at the conference.

The global hydrological cycle precipitation and evaporation of water in the atmosphere. Extreme precipitation events, both flood-producing rains and prolonged drought, may result from an intensification of the cycle, in which water vapor plays a key role.

Water vapor in the upper troposphere and lower stratosphere. A region difficult to study, it has been the object of intense research in recent years in part because of the greenhouse effects of water vapor there and because water vapor patterns reveal aspects of the circulation of air in the stratosphere.

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