Humidity and greenhouse gases
Humidity refers to the amount of water vapor present in the atmosphere and plays a crucial role in Earth's climate system. Water vapor, as the most abundant greenhouse gas, significantly influences the planet's radiation budget by trapping infrared radiation, contributing to warming. This gas exists in equilibrium with its liquid and solid forms, and its behavior is governed by temperature and local atmospheric pressure. The concept of relative humidity, which compares the current water vapor pressure to the maximum possible at a given temperature, is key to understanding atmospheric conditions.
In the context of climate change, water vapor's role is complex. It can amplify warming effects through feedback loops, as increases in temperature lead to more evaporation and subsequently more water vapor. However, the presence of water vapor in the upper atmosphere can disproportionately affect climate dynamics due to its ability to trap heat. Additionally, clouds and aerosols influence the amount of solar energy that reaches the Earth's surface, thus affecting albedo and potentially moderating temperature rises. Understanding humidity and its interactions with greenhouse gases is essential for comprehending larger climate patterns and changes.
Subject Terms
Humidity and greenhouse gases
Definition
Humidity is the amount of water present in the atmosphere in the form of vapor. As a gas, water vapor contributes to the local atmospheric pressure in accordance with Dalton’s law of partial pressures: In any mixture of gases, the partial pressure of any one component is equal to the total pressure of the mixture multiplied by the fraction of the gas present in the mixture. For example, molecular oxygen constitutes 20 percent of the atmosphere, so the partial atmospheric pressure of oxygen is 20 percent of Earth’s total atmospheric pressure. The total pressure is about 1.03 kilograms per square centimeter, so the partial atmospheric pressure of oxygen is about .20 kilograms per square centimeter.
Water normally exists in liquid and solid as well as vaporous form. Its vapor pressure is the pressure at which pure water vapor coexists in equilibrium with either the liquid or the solid state. At equilibrium, the liquid would not evaporate, the solid would not sublimate, and the vapor would not condense. By contrast, if the local partial pressure of water vapor is greater than its vapor pressure, the vapor condenses; if the local partial pressure is less than the vapor pressure, then the liquid evaporates and the solid sublimates. Vapor pressure is not a constant but rather is a function of temperature.
If the local partial pressure of water is exactly equal to its vapor pressure, the air is said to be saturated. This state is defined as 100 percent humidity, and the corresponding temperature is water’s dew point. If the vapor pressure of water is equal to the total local atmospheric pressure, the water will evaporate without limit, and the corresponding temperature is water’s boiling point. Relative humidity is the ratio, expressed as a percentage, of the local partial pressure of water vapor to the vapor pressure associated with the local temperature.
Humidity can exceed 100 percent, a condition known as supersaturation. In supersaturation, water vapor’s partial pressure exceeds the theoretical vapor pressure at that temperature. Condensation cannot take place, however, unless condensation nuclei are present. Water droplets exceeding a certain critical size act as such nuclei, absorbing water vapor and growing; water droplets below the critical size evaporate. If no droplets larger than the critical size exist and no other condensation nuclei are present, then the supersaturated vapor is stable. Fine, dry particles, such as dust or pollutants, also act as condensation nuclei in supersaturated air.
Evaporation is an endothermic process, or one that requires an input of energy in order to occur. The change of phase from liquid to gas takes place at constant temperature. The energy consumed by the process is stored in the water vapor in the form of latent heat of vaporization. When the vapor condenses, all of the latent heat is released, which means that condensation is exothermic. Water has a latent heat of vaporization of 2,256,000 joules per kilogram, an unusually high value for such a simple compound.
Significance for Climate Change
Water vapor is the most abundant greenhouse gas (GHG) in Earth’s atmosphere, exceeding the amount of CO2 by a factor of one thousand. It is transparent to visible radiation but opaque to infrared radiation of 5.25 to 7.5 micrometers in wavelength, which is a lower frequency range than that of visible light. Incoming solar radiation peaks in the visible spectrum. Energy re-emitted by the Earth peaks in the infrared portion of the spectrum. Thus, incoming energy is better able to penetrate water vapor than is outgoing energy.
In order for the Earth to maintain thermal equilibrium, radiating as much energy back to space as it receives from the Sun, the global average temperature must rise higher than it would if there were no humidity in the atmosphere. The balance of the amounts of radiation received from the Sun and emitted back into space is called the radiation budget.
No simple statements about the effect of total atmospheric water vapor on climate change are possible, because the atmosphere is a nonequilibrium system. Water vapor resides in the air for fairly short periods before precipitating out as rain, snow, or dew. As a result, the amount of water vapor in the atmosphere itself responds quickly to changes in climate. Water vapor in turn affects Earth’s radiation budget and, through it, surface temperatures, closing the loop and generating feedback. Layers of air near the planet’s surface are warm enough and close enough to the oceans to stay relatively saturated. Their effect on the radiation budget is small, because they are nearly as warm as the surface itself. Upper layers of the atmosphere are cooler and moistened only by the water vapor that convects or diffuses upward from the layers below. Small quantities of water in these upper layers can have a disproportionate effect on the radiation budget, trapping enough infrared energy to significantly warm the climate.
This feedback is complicated, however, by the presence of water in the air as suspended droplets in the form of clouds and fog. These droplets scatter visible radiation in all directions, preventing an appreciable fraction of the incident solar energy from reaching the ground and contributing to Earth’s albedo (the fraction of incident solar energy reflected back into space). Ice and snow have the same effect. Increases in albedo have a cooling effect and act to moderate any global average temperature increases.
Bibliography
Colman, B. R., and T. D. Potter, eds. Handbook of Weather, Climate, and Water. Hoboken, N.J.: Wiley-Interscience, 2003. Concise and thorough treatment of the hydrologic cycle and its effect on climate. Illustrations, figures, tables, references, index.
Schneider, T., and A. H. Sobel, eds. The Global Circulation of the Atmosphere. Princeton, N.J.: Princeton University Press, 2007. Chapter 6, “Relative Humidity of the Atmosphere,” may be difficult for the nonexpert to read in depth, but there is value in skimming it to understand the authors’ positions on the subject. Figures, tables, and references.
Taylor, F. W. Elementary Climate Physics. New York: Oxford University Press, 2005. Readers without significant prior knowledge of climate science should consult this book for help in understanding difficult topics. Illustrations, figures, tables, bibliography, index.