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!

Physical Components

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:

• on seasonal, annual, and decadel time scales, an interactive upper ocean and sea ice, changes in atmospheric composition and aerosol loading, and changes induced by biospheric interactions with the earth's surface and atmospheric constituents;
• on the time scale of hundreds to thousands of years, changes in the deep ocean and variations of the continental ice sheets;
• and on geologic time scales, changes in topography and continental drift.

In general terms, the earth's climate system consists of five physical components:

1. The atmosphere, which has a characteristic response time to temperature forcing (imposed changes) of about one month in the troposphere.
2. The oceans, where most of the solar radiation that reaches the surface is absorbed, so that oceans represent an enormous heat reservoir, and where ocean currents transport vast amounts of heat from low latitudes polewards. The upper ocean layers respond to the overlying atmosphere or cryosphere with a response time of months to years, while deeper ocean waters have thermal adjustment times of the order of centuries. The oceans also exchange carbon dioxide with the atmosphere (in processes that are not fully understood) and thus are involved in the chemical balance of the climate system.
3. The cryosphere consists of continental ice sheets, mountain glaciers, sea ice, and surface snow cover. The response time to temperature forcing of the sea ice and snow cover is rapid, sometimes on the order of days, while the glaciers and continental ice sheets respond much more slowly.
4. The land surface consists of the continents and all that is geologically connected with them (mountains, soil, lakes, etc.). Lakes, rivers, and groundwater are important components of the hydrological cycle, but probably have little significance to the global climate system. With the exception of the rebound of the earth's crust as the weight of the ice sheets are removed, the change in shape and geographical location of the land masses is of little importance over nongeologic time scales.
5. The biosphere consists of all land and water plants and animals, all of which have widely different response times to forcing. The biosphere plays an important role in the carbon dioxide budgets of the atmosphere and oceans, on the production of aerosols, and in the chemical balances of other gases and salts. Biogenic atmospheric trace gases such as carbon dioxide, methane, and various nitrogen-containing species, play a major role in determining the radiative transfer characteristics of the atmosphere and hence the energy budget of the planet (Sellers, 1991). Natural changes in plants occur over periods ranging from seasons to thousands of years, in response to changes in temperature, precipitation, and radiation. These changes in turn affect surface albedo, surface roughness, evapotranspiration, and ground hydrology, all of which have a feedback or feedforward effect on climate. The feedback/feedforward interactions tend to be complex, and arguments in the scientific literature range back and forth, merely illuminating, in the end, the state of our ignorance regarding these issues. For example, one of the most contentious issues regarding biospheric interaction with climate is the effect of warming oceans and carbon dioxide enrichment on phytoplankton, their dimethylsulfide (DMS) waste metabolic product, and the role of DMS in a cloud-albedo feedback mechanism that may oppose global warming “forced” by anthropogenically-induced increases of atmospheric carbon dioxide (see Idso, 1992).

Principal Planetary Heat Exchanges

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.

Climate Feedback/Feedforward Mechanisms

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.

References

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.

This site last updated July 7, 1997.