Water is a compound of hydrogen and oxygen having the chemical formula H2O and is found everywhere on the surface of Earth, even in deserts and high in the atmosphere. Depending on temperature and other factors, water is present on different parts of Earth's surface and at different times as a liquid, solid, or vapor. (The gas form of a substance, when it coexists with the liquid or solid form, is termed a vapor.) Most of the water in the atmosphere is vapor; clouds and precipitation consist of particles of liquid or solid water, but water vapor is present at some level in all air.
Although water vapor is a trace gas in Earth's atmosphere, making up only about .25% of it, it is the most important greenhouse gas. Water vapor absorbs more of the infrared light (heat radiation) emitted by Earth's surface than carbon dioxide (CO2) or methane (CH4). About 60% of the natural greenhouse effect under a clear sky is due to water vapor, and water vapor is the most important feedback term in all mathematical models of climate change.
Water vapor receives less attention in discussions of human-caused (anthropogenic) climate change, but for good reason. Human activities have only a slight effect on the amount of water vapor in the atmosphere. Rather, water vapor prvides feedback to climate change. Warmer air contains, typically, more water vapor than cooler air, so as Earth warms, more water vapor is mixed with the
atmosphere. This increases the greenhouse effect, leading to further warming.
Historical Background and Scientific Foundations
The greenhouse effect was first recognized by French scientist Joseph Fourier (1768–1830) in 1827. Fourier described the atmosphere's role as a one-way passage for solar energy: sunlight passes easily through the atmosphere and is absorbed by land and sea, which warm up as a result. Land and sea re-emit most of this energy as infrared light, which the atmosphere is not fully transparent to. The atmosphere absorbs much of this infrared radiation and is warmed by it. This warming of the air keeps the surface of Earth warmer than it would be otherwise, acting like a blanket wrapped around the whole planet. Without this natural greenhouse effect, Earth would be too cold to support most life.
In 1861, Irish scientist John Tyndal (1820-1893) noted that the two main ingredients of Earth's atmosphere, nitrogen (N2, 78%) and free oxygen (O2, 21%), are transparent to infrared radiation and so do not contribute to the greenhouse effect. Tyndal identified water vapor as the main greenhouse absorber, once writing that it serves as “a blanket, more necessary to the vegetable life of England than clothing is to man.”
By the beginning of the twentieth century, scientists were also aware of the role of carbon dioxide in greenhouse warming and of the possibility that anthropogenic increases in atmospheric carbon dioxide could warm Earth's climate. They were also aware of water vapor's positive feedback role. Water vapor warms Earth, releasing more water vapor, mostly by evaporation from the oceans. This warms Earth further, releasing even more water vapor, and so on. The amount of carbon dioxide in the atmosphere is only slightly dependent on temperature, while the amount of water vapor is significantly dependent on temperature. Water vapor's positive feedback approximately doubles the amount of greenhouse warming caused by a given amount of carbon dioxide.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
INFRARED: Wavelengths slightly longer than visible light, often used in astronomy to study distant objects.
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.
POSTDICTION: Prediction of past climate events by a computer model using information that predates the events; also called hindcasting. For example, postdiction of the climate of the late 1990s would be performed by feeding data on climate prior to 1990 into a global circulation model. Postdiction tests the realism of computerized climate models.
TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.
However, despite this early awareness, attempts to calculate the effect of fossil-fuel use on climate in the 1930s through the 1950s neglected the role of water vapor. This was corrected starting in 1963, and for the last few decades, water-vapor feedback has been an important part of all climate models. Data on atmospheric water vapor of a quality suitable for climate studies are available from the 1950s onward. These measurements show that as of 2005 there had been little change in water vapor at the surface; a 5% increase in the lower troposphere (the first few miles of atmosphere) over the twentieth century, 4% of which has happened since 1970; and little change in the upper troposphere (up to about 7 mi or 11 km altitude). Climate models predict that by the end of the twenty-first century, the amount of water vapor in the upper troposphere will have approximately doubled as a result of global warming, enhancing that warming.
Water vapor is important in climate not only as an infrared greenhouse blanket but as the source of clouds. Clouds, which are masses of water droplets or ice crystals suspended in the atmosphere, have a paradoxical double effect on the global heat balance. First, they are highly effective blankets, preventing infrared radiation from escaping Earth's surface. In this role, they warm Earth. Second, their whiteness makes them effective reflectors of solar energy, and so tend to cool Earth. The role of clouds as part of the feedback role of water vapor is therefore complicated. Scientists have long questioned: will global warming increase global cloud cover, or decrease it? And whether clouds increase or decrease, will this act as a positive feedback that accelerates warming, or a negative feedback that slows it down?
Scientific understanding of the relationships between water vapor, climate, aerosol particles, deforestation, clouds, and other factors has steadily improved in recent years. As of early 2007, the scientific consensus view, as described by the United Nations Intergovernmental Panel on Climate Change (IPCC), was that in the present state of the world's climate, clouds exert a negative feedback (cooling effect) on climate, but that with further warming, this negative feedback may diminish and possibly become positive (a warming effect). Cloud cover was predicted to decrease by up to 4% over most of the world by the late twenty-first century, while increasing by up to 4% over the polar regions. However, the effects of clouds on climate remain the largest source of uncertainty in computerized global climate models.
The uncertainties surrounding clouds are not large enough to undermine the basic conclusions of the global climate models, namely, that the present warming of the world is anthropogenic and will worsen. The magnitude, effects, and speed of this warming trend can be predicted only within some range of likely error.
Impacts and Issues
In the 1990s through the early 2000s, there was considerable scientific debate about the role of water vapor feedback in climate. Many scientists argued that water vapor might actually produce a negative or cooling climate feedback rather than a positive one. For instance, increased vapor levels might increase cloud cover, which might cool Earth more than it warms it. In this case, global climate models, which assume positive feedback, would be representing the role of water vapor incorrectly.
This debate was largely resolved from 2002 to 2005 by a series of measurements and tests. In 2002, global climate models were used to predict the effects of an event that had already happened, the 1991 eruption of Mount Pinatubo, a volcano in the Philippines. Using computerized models to predict what has already happened, based only on data from before the event, is often called postdiction (as opposed to prediction) or hind-casting (as opposed to forecasting), and serves to test the realism of the models.
Mt. Pinatubo ejected vast dark clouds of dust that cooled the world's weather for several years and reduced the amount of water vapor in the atmosphere. When a standard computerized global climate model was used to predict the climate effects of such an eruption, the model could correctly describe the timing and amount of the observed drying and cooling—but only if water vapor was assigned its usual positive-feedback role.
IN CONTEXT: FEEDBACK FACTORS
According to the National Academy of Sciences: “Uncertainties remain about the magnitude, rate, and impacts of future climate change, largely due to gaps in understanding climate and the difficulty in predicting future societal choices….”
“One of the major scientific uncertainties currently being investigated is how climate could be affected by what are known as ‘climate feedbacks.’ Feedbacks can either amplify or dampen the climate response to an initial radiative forcing. During a feedback loop, a change in one factor, such as temperature, leads to a change in another factor, such as water vapor, which then causes a change in the first factor.”
SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.
Further confirmation of water vapor's positive feedback role in climate came in 2005, when researchers used satellite data to show that water vapor had indeed increased in the upper troposphere, in agreement with global climate models. This offered the strongest evidence to date that such models are correctly representing the climate effect of water vapor. Clouds, however, remain a significant source of uncertainty.
Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Cess, Robert D. “Water Vapor Feedback in Climate Models.” Science 310 (2005): 795-796.
Del Genio, Anthony D. “The Dust Settles on Water Vapor Feedback.” Science 296 (2002): 665-666.
Held, Isaac M., and Brian J. Soden. “Water Vapor Feedback and Global Warming.” Annual Review of Energy and the Environment 25 (2000): 441-475.
Soden, Brian J., et al. “Global Cooling After the Eruption of Mount Pinatubo: A Test of Climate Feedback by Water Vapor.” Science 296 (2002): 727-730.