The figure below is a schematic representation of the coupled atmosphere - ocean - cryosphere - land - biosphere climate system. Filled arrows are examples of external processes, and open arrows are examples of internal processes in climatic change (adapted from GARP, 1975). Read on to learn more about the Earth’s Climate System!
In trying to understand the mechanisms that result in the earth's climate and its variability, we are faced with an enormously complex physical system that includes not only the relatively well-known behavior of the atmosphere but also the less well-known behavior of the world's oceans, cryosphere, and land surface-atmosphere interactions. In addition to the purely physical factors, there are complex chemical and biological feedback and feedforward processes affecting climate. Each of these processes undergoes complex interactions with some of the others, often over a wide range of space and time scales (GARP, 1975).
Most scientists understand that we are merely at the threshhold of fully grasping the complex relationships between all the processes involved in generating the earth's climate. These processes include:
In general terms, the earth's climate system consists of five physical components:
Solar radiation is the source of the principal process that results in climate (and its nonintegrated analog, weather) – the rate at which heat is added to the system shown in the figure above. Note that the atmosphere is heated “from below,” i.e., the sun's shortwave energy must first strike the solid surface of the earth, be absorbed, and finally be reemitted as longwave (infrared, or “heat”) energy. The atmosphere and oceans transport heat along a temperature gradient from regions where there is a relative surplus to regions where there is a relative deficit. Defining different spatial scales, we refer to these transports as the trade winds and ocean currents (the global scale), as tropical and extratropical cyclones and ocean eddies (the synoptic scale), and as local storms and individual clouds (the subsynoptic scale). All three scales participate in the transport of momentum, mass, energy, and water vapor.
The net heating rate depends on the distribution of temperature, water vapor, and trace gas concentrations in the atmosphere, on the release of heat during the formation of clouds (latent heat of condensation), and on the uptake of heat when water evaporates (latent heat of evaporation). Once formed, clouds significantly influence the magnitude of incoming (solar) and outgoing (terrestrial) radiation. Water, in its various phases, dominates the planetary response to temperature forcing. For example, the reflection of shortwave energy and the emission of longwave energy by clouds accounts for about one-half of the total radiation leaving the atmosphere, and in terms of shortwave radiation alone, clouds account for roughly two-thirds of the planetary albedo. The largest single heat source for the atmosphere is the release of latent heat of condensation during cloud formation; this energy is partly responsible for the circulation of the upper troposphere (Salati, 1987; Paegle, 1987). Water vapor from the tropics is transported poleward by the general circulation; where it finally condenses, it releases its energy. The surface energy balance over the oceans and a large part of the Amazon Basin (and probably other wet, tropical forest areas; Dickinson, 1980, Salati, 1987, Paegle, 1987) is dominated by evaporation. Water vapor, as do trace gases such as carbon dioxide and methane, traps longwave energy in the atmosphere. And finally, ice and snow serve as effective heat sinks, both through their high albedo and through the heat required for melting.
Jones and Mitchell (1991) elaborate on the interaction between increased levels of atmospheric carbon dioxide and water vapor by first reiterating that water vapor is the most important “greenhouse” gas. As increases in carbon dioxide warm the surface and the atmosphere, more water evaporates from the surface and remains in the atmosphere. In fact, the amount of water vapor that can be held by the atmosphere increases exponentially with temperature. Because water vapor is such a strong greenhouse gas, this increase traps more longwave radiation, further warming the surface and the troposphere, and so amplifies the initial warming.
There are processes that may act as internal controls of the climate system, processes which may have response times ranging from fractions of a year to thousands of years. These processes couple specific variables of the system, or mutually interact among them. These interactions may act either to amplify anomalies of one of the interacting elements (feedforward, or positive feedback) or to damp them (feedback; GARP, 1975). Scientists are confident that there are a large number of such interactions between all segments of the climate system, but remain uncertain regarding the direction or magnitude of change of many.
I mentioned the temperature-carbon dioxide-dimethylsulfide-cloud feedback mechanism above. Some other plausible mechanisms operate between elements of the radiation balance and the surface temperature. For example, a perturbation of the ocean surface temperature will probably modify the transfer of sensible and latent heat to the overlying atmosphere, thereby influencing the atmospheric circulation and cloudiness. These changes in turn will affect the ocean surface temperature through changes in radiation, wind-induced mixing, advection, and convergence. That these processes can conceivably result either in the enhancement (feedforward) or reduction (feedback) of the initial anomaly (i.e., the perturbation in ocean surface temperature) illustrates the uncertainty that is attached to qualitative arguments (GARP, 1975). Moreover, any feedforward mechanism must, at some point, be neutralized by a feedback interaction, else the system would exhibit “chain-reaction” type growth – reactions that seemingly are not part of earth's history.
Dickinson, R.E., 1980. Effects of tropical deforestation on climate. In: Blowing in the Wind: Deforestation and Long-Range Implications. Studies in Third World Societies 14. Department of Anthropology, College of William and Mary, Williamsburg, Virginia, 411-442.
GARP, 1975. The Physical Basis of Climate and Climate Modelling. Report of the International Study Conference in Stockholm, 29 July - 10 August 1974. Global Atmospheric Research Program (GARP) 16, World Meteorological Organization, Geneva, Switzerland, 265 pp.
Idso, S.B., 1992. The DMS-cloud albedo feedback effect: Greatly underestimated? Climatic Change 21: 429-433.
Jones, R.L. and Mitchell, J.F.B., 1991. Is water vapor understood? Nature 353: 210.
Paegle, J., 1987.Interactions between convective and large-scale motions over Amazonia. In: The Geophysiology of Amazonia. John Wiley and Sons, New York, 347-387.
Salati, E., 1987. The forest and the hydrological cycle. In: The Geophysiology of Amazonia. John Wiley and Sons, New York, 273-296.
Sellers, P., 1991. Modeling and observing land-surface-atmosphere interactions on large scales. Surveys in Geophysics 12: 85-114.
Physical Components
Principal Planetary Heat Exchanges
Climate Feedback/Feedforward Mechanisms
References
The correct reference for this page is:
Milich, L., 1997. The Earth's Climate System. http://ag.arizona.edu/~lmilich/ecs.html
This site last updated July 7, 1997.