Climatology

  Climatology is the study of CLIMATE over extended periods of time; ie, of both average and extreme WEATHER conditions. It can also be seen as the study of energy flow through the Earth-atmosphere system. Owing to the Earth's highly interconnected network of subsystems (eg, solar input, OCEANS, GLACIERS, VEGETATION, atmospheric composition), modern climatology is a multidisciplinary science. The complexity of the Earth's climate is almost certainly due to the co-existence of the 3 phases of WATER (vapour, liquid and ICE) and the manner in which they interact with radiation. The 2 main subfields of climatology are physical climatology, which seeks to understand the principles of mass and energy exchanges in the Earth-atmosphere system, and dynamic climatology, which examines factors influencing climate.

The study of modern climatology is primarily global in perspective. The main tools used by climatologists are computer models of the atmosphere, ocean and land surface, and data inferred from SATELLITE observations. Other divisions of climatology include descriptive climatology; synoptic climatology, which analyzes WEATHER OBSERVATIONS taken simultaneously at different locations; and applied climatology, which makes CLIMATE INFORMATION available to engineers, planners, vacationers, etc (seeCLIMATE AND HUMANKIND). The possibility that human activity can deliberately or inadvertently influence climate is also of interest to climatologists.

Physical Climatology

Physical climatology is the science of examining and modelling numerically the processes, magnitudes and directions of energy and mass exchanges within the atmosphere, as well as at and immediately below the Earth's surface. These factors are important in determining all scales of climate, from local to global and from diurnal to millennial.

Variability in the absorption of solar radiation by the Earth-atmosphere system is the driving force behind climate. Annually the system absorbs about 70% of the incoming solar radiation and emits a similar amount of infrared radiation to space. Small but cumulative changes to this quasi-equilibrium due to, for example, changes in solar emission, the Earth's orbital geometry or atmospheric composition can initiate chain reactions (feedback processes) that amount to climatic change. The effects of variations in orbital geometry are negligible in the short term, but on geological time scales, orbital variations produce changes to solar irradiance patterns that are believed to be the cause of ICE AGES. On the other hand, altering the atmosphere's ability to emit and transmit thermal radiation through the burning of fossil fuels (enhanced greenhouse effect) could force unprecedented rates of climatic change in the coming century.

Currently, the portion of the Earth equatorward of 40° latitude is an energy source, since radiant energy gains exceed losses here; poleward of 40° latitude, however, the system is an energy sink, where losses exceed gains. WINDS and OCEAN CURRENTS transport energy poleward to rectify this latitudinal imbalance, thereby ameliorating surface temperature differences between the tropics and the poles. Moreover, the Earth's surface has an annual surplus of radiant energy whereas the atmosphere has a deficit. This imbalance is caused largely by the thermal insulating effect of atmospheric water vapour, the Earth's most abundant and important greenhouse gas. Convective transport of heat from the surface prevents excessive surface warming and atmospheric cooling. Likewise, on a global basis, CLOUDS serve to cool the system, as their solar reflecting properties outweigh their thermal insulating properties.

Dynamic Climatology

Dynamic climatology is the study of climate as a branch of physics. The climate system is a physical system governed by reasonably well-known physical laws (fluid dynamics, thermodynamics, radiative transfer, etc). There are 5 major components of the climate system: atmosphere, hydrosphere (ie, oceans, lakes), land surface, cryosphere (sea ice, permanent ice caps, snow) and biosphere. The global system is interrelated in many complex ways, and the study of dynamic climatology requires expertise from many branches of science and data from all parts of the globe.

Some major aspects of the climate system may be appreciated by considering simpler, idealized systems. A nonrotating Earth with no topography or oceans would have a climate far different from that observed. If the atmosphere were not permitted to move in this idealized case, the resulting climate would not vary with longitude, but only with latitude; eg, temperatures would be high at equatorial latitudes, low at poleward latitudes. The resulting temperature structure at the surface, at various levels in the atmosphere and at various latitudes could be calculated using the equations of thermodynamics and radiative transfer. If the atmosphere were allowed to move on this nonrotating Earth, the warm air in equatorial regions would rise, flow polewards, cool, sink and flow equatorwards once more, giving rise to a Hadley circulation. The temperature difference from pole to equator provides the driving force for the motions and would be reduced as a consequence of the flow.

If the planet now began to rotate, the nature of the flow (hence, the climate) would differ remarkably. A simple Hadley circulation would no longer be found but rather a flow from west to east at middle latitudes and a flow from east to west at lower latitudes. Superimposed on this general flow would be large ripples and eddies. Without the oceans, winter cold and summer warmth would be extreme and the coldest and warmest times of the year would occur at the solstices. If moisture were introduced into the system as oceans and water vapour in the air, the resulting climate would again change dramatically; changes would include the appearance of major east-west asymmetries in the climate. The oceans have a profound effect on climate through storage and transport of heat. Topography, ice and snow, etc, add further degrees of complexity to the system.

Climated Models

This complicated 3-dimensional global system is studied observationally and by means of "climated models." Observational or diagnostic approaches proceed by the collection and study of many millions of observations of the atmosphere and oceans, not only at the surface but with height and depth. Such studies attempt to understand how the physical system operates and what the dominant mechanisms are. They also include attempts to understand past variations in climate and to detect and understand variations in the current climate.

Global climate models (also called general circulation models or GCMs) are increasingly used to study the climate and its past and future evolution. GCMs are physically based mathematical models that attempt to deduce the climate and its variations and changes from the "first principles" embodied in the governing equations. These mathematical equations are approximated on a grid of points over the globe and at many levels in the atmosphere and ocean. Physical processes, including convection, long- and short-wave radiative transfer, cloud formation and dissipation, precipitation and evaporation, and ice formation and melt, are calculated for each grid-square. New simulated values of temperature, wind, rainfall, snow cover, ocean salinity, SEA-ICE thickness and all the other variables that specify the state of the atmosphere, land and ocean in the model are calculated in steps of about 30 minutes. Multiyear averages and other statistics of these evolving variables provide the simulated climate.

Human's Effect on Climate

Human activities can change some aspects of the climate on a local scale. Some changes are deliberate; eg, use of trees or hedges as windbreaks to prevent frost hollows or to control snow drifting. Others are inadvertent; eg, the warming of the downtown cores of expanding cities, or the increase in rainfall downstream from cities caused by increased release of heat and of pollutants which promote condensation (seeAIR POLLUTION; URBAN EFFECT ON CLIMATE).

The evidence is growing that byproducts of human activity are affecting the large-scale climate. The recent scientific assessment by the Intergovernmental Panel on Climate Change states, "Observed global warming over the past 100 years is larger than our best estimates of the magnitude of natural climate variability over at least the last 600 years. More importantly, there is evidence of an emerging pattern of climate response in the observed climate record to forcings by greenhouse gases and sulphate aerosols. The evidence comes from the geographical, seasonal and vertical patterns of temperature change. Taken together, these results point towards a detectable human influence on global climate."

The greenhouse gas carbon dioxide (CO2) is produced by the burning of COAL, oil and gas to heat homes, operate equipment and generate ELECTRIC POWER. From 1880 to 1994 the levels of CO2 have increased by about 25%, about half of that increase coming in the past 25 years. Although CO2 represents much less than 1% of the total volume of the atmosphere, it is very important because it interferes with the flow of heat radiated from the Earth into space. Experiments using computer models of climate indicate that, if the volume of atmospheric CO2 were double its preindustrial level, the average temperature of the air at ground level would increase by about 2-3° C. Such a change would be larger than any climate change in recorded history and would be accompanied by much larger regional shifts in climate. The atmospheric concentration of greenhouse gases other than CO2 (such as chlorofluorocarbons and methane) is also on the rise. Should present trends continue, the total warming effect of such gases is projected to be comparable to the CO2 effect. On the other hand, aerosols in the lower atmosphere resulting from burning of fossil fuels and BIOMASS burning produce a cooling effect.

The possibility of deliberately modifying the large-scale climate is intriguing, but cursory examination of the question indicates that energies involved in the climate system are so large that a direct approach would be impossible. For example, the energy converted in a single one-hour THUNDERSTORM would supply Canada's entire electrical energy needs for one day. The energy converted in a single low-pressure system in one day is 10 000 times greater than that of a thunderstorm. Thus, it is improbable that any direct action could be taken to modify our climates, even if methods of doing so were known. One possibility would be to influence the climate by depositing long-lived dust clouds in the high atmosphere: there is some evidence that volcanoes affect the climate by injecting large amounts of sulfate aerosols at high altitudes. It is not known what effects such a deliberate modification would have on the climate at the surface. The present understanding of the climate system, however, leaves many uncertainties in accounting for the inadvertent "climate experiment" going on due to increasing greenhouse effects; making deliberate interventions is risky business indeed.

See alsoMETEROROLOGY.

P.E. MERILEES

Authors contributing to this article:

Energy Budgets

An energy budget can be drawn up for any volume or surface within the Earth-atmosphere system. The radiation component in this budget is governed mainly by incoming solar radiation, which in turn is controlled by the seasons, presence of clouds and surface albedo. The modelling of clouds and radiative transfer for cloudy atmospheres in numerical climate models represents one of the most difficult yet important areas of climate research. During winter in Canada, weak solar irradiance, frequent cloudiness and high albedo of snow produce a negative radiation balance everywhere. In midsummer, the radiation balance is positive everywhere because solar irradiance is greater, clouds are fewer and snow is absent for the most part. The midsummer budget is remarkably uniform between the Canada-US border and the High Arctic. Long daylight periods in the North compensate for a low SUN.

Details of the magnitudes of seasonal and geographical variations of the other energy budget components are not well known, although sensible and latent heat transfers are the major components. Sensible heat is the energy of the movements of molecules that make up a mass and is perceived as temperature. Latent heat is the energy required to change the phase of matter (as from ice to liquid and liquid to vapour in melting and evaporating water). During spring and early summer in much of Canada, melting snow and ice use a significant amount of energy. Latent heat transfer is always large (about 80-90% of the radiation balance) from open water, wetlands and transpiring vegetation and crops. In drier areas, or as wet surfaces become drier, sensible heat transfer becomes more important. Surface modifications may alter radically the relative magnitudes of energy budget components. For example, in many cities, replacement of vegetation with concrete and brick has reduced latent energy transfer and increased the transfer of sensible heat. Land management also has its effects, such as in dry areas where IRRIGATION produces moist, cool oases as a result of increased latent heat transfer and diminished sensible heating.

Satellite Data

Satellite data have become increasingly important for providing information about our climate. Not only can the top of the atmosphere's energy budget be monitored, but numerous other properties of the system can also be inferred, such as atmospheric and surface temperatures, cloud fraction and water content, and surface conditions such as moisture, vegetation abundance and albedo. Because satellite coverage ranges from tens of metres to entire continents, satellite-inferred quantities have become crucial for diagnosing the ability of numerical climate models to portray the current climate. Likewise, satellite data will undoubtedly be important for assessing environmental and climatic change.

HOWARD BARKER

Climate Change

GCMs are used to study the current workings of the climate system, but their other main use is the simulation of CLIMATE CHANGE. For instance, increases in greenhouse gases in the atmosphere are expected to result in GLOBAL WARMING. The GCM "translates" this change in atmospheric composition into estimates of the magnitude, 3-dimensional distribution and timing of potential climate change. The effects of atmospheric aerosols, volcanic activity, solar variability and other changes in "climate forcing" are also studied, as are past changes in forcing and their consequences in the study of paleoclimatology.

The climate system has undergone profound changes in the distant past and real but less extreme variations in historical times. Such changes are caused by natural phenomena (natural variability, VOLCANOES, changes in the sun, etc) and human-induced factors (increased greenhouse gases, aerosols, deforestation, etc). Recent simulations of the evolution of climate from the preindustrial period into the future, from about 1850 to 2050 and beyond, using historical and projected greenhouse gas (warming) and aerosol (cooling) forcing effects are consistent with observed records and indicate the potential of increased warming in the near future.

The climate system is global and must be studied from a global perspective. The World Meteorological Organization has established a World Climate Research Program for this purpose. The WMO and the United Nations Environment Programme have involved the international scientific community in the Intergovernmental Panel on Climate Change, which has recently produced a "Scientific Assessment on Climate Change" summarizing current knowledge of the climate system and its projected changes. In Canada, research in dynamic climatology and climate modelling and analysis is undertaken in the Climate and Atmosphere Research Directorate of the Atmospheric Environment Service and in other government laboratories and universities.

GEORGE J. BOER