Climate and Climatology

Climate is the long-term combined effects of atmospheric variations. Over shorter periods of time, climate is referred to as weather. Climate always refers to a specific geographical location or region and is determined by many factors, including wind belts, topography, elevation, barometric pressure, the movement of air masses, the amount of solar radiation available, proximity to oceanic influences, and planetary and solar cycles.

Science of Climatology

From early times, humans have attempted to classify climates. The ancient Greeks in the sixth century BCE visualized Earth as having three temperature zones based on the sun’s elevation above the horizon. (The Greek word klima, meaning “inclination,” is the origin of the word “climate.”) They called these three zones torrid, temperate, and frigid. This system did not consider the differences between climates over land and over water, the effects of topography, or other atmospheric elements such as precipitation.

People tend to confuse weather with climate. The two phenomena are related, but weather is concerned with day-to-day atmospheric conditions, such as air temperature, precipitation, and wind. Climatology is concerned with the mean (average) physical state of the atmosphere, along with its statistical variations in both time and location over a period of many years. In addition to the description of climate, climatology includes the study of a wide range of practical matters determined by climate and the effects and consequences of climatic change. As a result, it has become an interdisciplinary science, relevant to and affected by a wide range of other fields, including geophysics, biology, oceanography, geography, geology, engineering, economics, statistics, solar-system astronomy, and political and social sciences.

For centuries, the content of the atmosphere and the amount of solar radiation reaching Earth’s surface have been fairly constant. Until the mid-1950s, Earth's climates were assumed to be relatively unchanging, but climate scientists now understand that climate is never constant. The global climate consists of effects generated by the oceans, the atmosphere, the cryosphere (areas of permanent ice), the land surface, and biomass. The various components of climate are linked to one another in a nonlinear feedback process, so that a change in any one of these factors produces a change or feedback effect in all of the other factors. Therefore, all or any of these processes can change the statistical state of the system called climate.

The science of climatology has developed along two main lines. Regional climatology studies the discrete and characteristic qualities of a particular region of the globe. The second approach of climatology is a physical analysis of the basic relationships among various atmospheric elements such as temperature, precipitation, air pressure, and wind speed. A third branch is dynamical climatology, which originated in the 1960s; this branch uses models to simulate climate and climatic change based on the averaged forms of the basic equations of dynamic meteorology.

Classification of Climates

There are two main classification systems for climates: genetic and empirical. A genetic system is based on air masses and global wind belts, which control climates. The empirical classification system is based on the observed elements of climate, such as temperature.

The Köppen system is an empirical classification system with five designated general categories: tropical forest climates; dry climates; warm, temperate, rainy climates with mild winters; cold forest climates with severe winters; and polar climates.

Another empirical classification system, which may be more useful in describing world climates, is based on decreasing temperature and increasing precipitation. This system of classification is convenient for use in many other sciences as well. The first category is the desert climate, which has the highest temperatures and the lowest precipitation. The second category is the savanna, which may be either temperate or tropical and is characterized in either case by nearly treeless grassland. The steppes of Eastern Europe and the prairie regions of Canada and the United States are examples of temperate savannas, with plants adapted to very hot conditions and extremely limited water supply. The grassy plains of western Africa are a typical tropical savanna. The third category or climate type is temperate and tropical Mediterranean climate. An example of temperate Mediterranean climate is the coast of Southern California; tropical Mediterranean climate is found typically in the northern region of Africa. The fourth climate type is the temperate and tropical rainforests. An example of a temperate rainforest is found on the Olympic Peninsula in Washington State, while the Amazon basin in Brazil is the standard example of a tropical rainforest.

The fifth and sixth climate types are the coldest and have a significant amount of precipitation in the form of snow. The first of these two, the taiga, is characterized by great forests of evergreen coniferous trees. Taigas occur only in the temperate zone or at great elevation. Large portions of Canada and Russia are typical of this climate type. The last region is the tundra, the coldest climate type, which may have small amounts of precipitation. Again, this region is found in either the temperate zone or the highest elevations on Earth. The best examples are the northern territories of Canada and much of eastern Alaska.

Factors of Climate Change

Climate is not a steady, unvarying cycle of weather; it fluctuates and varies over a period of time. One of the major factors causing these variations is the role of atmospheric circulation. The atmosphere extends outward from Earth’s surface for hundreds of kilometers and consists of five clearly defined regions. The troposphere, the region closest to Earth, extends about eleven kilometers above the surface. It is in this region that most weather phenomena affecting climate take place.

The tropics experience intense solar heating, while the polar regions have little solar heating. These differences of heating and cooling of the atmosphere result in a large-scale global circulation of air that carries excess heat and moisture from the tropical areas of intense heating into the higher latitudes, where there is little excess heat and moisture. The great movements of air masses that are produced create and alter various local climates.

Global cloud patterns are also critical factors in regulating the global climate. Heavy cloud cover reduces the amount of solar radiation reaching Earth’s surface, producing a cooling effect. It also acts as a kind of blanket, reducing the amount of heat lost from the ground by absorbing surface-emitted infrared radiation and directing about half of it back toward the surface. A clear sky in winter always results in colder temperatures at the surface than when there is cloud cover. Even so, only the solar heat that has already reached Earth’s surface can be kept under this protective cover. Extended cloud cover will result in less solar heat reaching the surface, leading to a cooler climatic condition.

The activity of the climate in various parts of the globe is also strongly influenced by the movement of ocean currents and global wind belts. An oceanic effect termed El Niño is periodically responsible for dramatic climate changes, possibly on a global scale. Scientists are not completely certain of the initial cause of this phenomenon, but it appears to be dependent on the accumulation of warmer surface water driven by prevailing surface winds.

It is important to remember that the climate has never been stable and unchanging. Many factors can cause changes in local and global climate; when one factor changes, it inevitably modifies or influences other factors affecting climate.

Factors Resulting from Human Activity

Some factors causing climate change are of natural origin, while many others result from human activity. The globe’s tropical rainforests are vital to maintaining the equilibrium of carbon dioxide in the global atmosphere. Heavy deforestation in South America and Asia has endangered this balancing mechanism by removing great numbers of the trees that remove vast quantities of carbon dioxide from the atmosphere. At the same time, industrial activity and automobile emissions release massive quantities of carbon dioxide, contributing to the buildup of carbon dioxide in the atmosphere and resulting in measurable increases in global temperatures. Carbon dioxide absorbs heat emitted from Earth’s surface and reradiates much of it back toward the surface, creating a greenhouse effect that normally raises the average global temperature to a level that supports the existence of life. Even a small increase of 2 degrees Celsius in the average global temperature, however, would have a potentially disastrous effect on climatic conditions. Global warming would cause the oceans to expand appreciably as their temperatures increased. In addition, ocean temperature and the direction of upper-level winds are the main factors in the development of hurricanes. Some scientists argue that the warming of the oceans has already increased the frequency as well as the power and destructive force of hurricanes.

One of the main dangers of a warming climate is flooding due to rising sea levels. Scientists have calculated that if the average temperature increases by about 3.7 degrees Celsius, the sea could rise approximately eighty-one centimeters—enough to flood huge areas of unprotected coastal land. Nearly 30 percent of all human beings live within about sixty kilometers of a coastline. A rise in sea level of only fifty centimeters would have a profound effect, as it would result in the flooding of many of the world’s most important cities and ports.

Large urban areas alter the local climate through the heat island effect, by which the average temperature of an urban area is raised significantly, forming rising air columns that would not otherwise exist, which in turn influence the activity of the larger air mass. Various forms of pollution over large urban areas such as Los Angeles, Tokyo, and London have also caused local climate changes in these areas. Concentrations of ozone that build up at ground level from industrial and automobile exhausts cause extreme eye and lung irritation. Nitrogen oxides, carbon dioxide, sulfur dioxide, and particle-laden air, which can cause a depletion in the amount of total solar radiation, also accumulate and alter climate conditions over local urban areas. Changes in surface characteristics, such as converting forests and prairies to agricultural fields, building cities, damming rivers to create lakes, and spilling oil at sea, all affect local climate conditions. Usually these surface changes appear to have only a minor global effect, though they do alter local conditions significantly, but the cumulative effects of these minor changes are not well understood and should, therefore, not be overlooked.

There is widespread national and international interest in how human activity is fundamentally altering the global climate. Organizations are seeking to inform the public regarding society’s role in maintaining a healthy and healthful climate. The Climate Institute in Washington, DC, sponsors international conferences where scientists and government leaders from many nations gather, providing a forum for information exchange among climate researchers and analysts, policymakers, planners, and opinion makers. Since 1992, 198 nations have joined the United Nations Framework Convention on Climate Change. The parties started meeting annually since 1995 and have reached international climate agreements, including the Kyoto Protocol in 1997 and the Paris Agreement in 2015.

Departures from the “Norm”

For centuries, the content of the atmosphere and the amount of solar radiation reaching Earth’s surface have been fairly constant. Scientists have come to accept that climate is never constant and that departures from the expected “norm” provide the greatest insight into climatic processes and have the greatest human impact.

The ice ages were a decided departure from the norm. Drought in central North America during the 1920s and 1930s resulted in the Dust Bowl disaster, which disrupted human life and crop production in the fertile middle region of the continent. In the early 1970s, the impact of the devastating drought of the Sahel region in Africa led to increased desertification. Loss of the anchovy fisheries along the Peruvian coast in the 1970s (probably as a result of as El Niño), severe droughts in the US farm belt in the 1980s, and large-scale flooding in many parts of the world all attest to the fact that dramatic climatic fluctuations and changes are the true norm. It is now understood by climatologists that climate modification may even result in microclimates as small as portions of a backyard garden.

Data-Gathering Tools

Because the science of climatology is the study of weather patterns for a geographic area over a long span of time, records of scientific data on the weather are valuable for interpreting climate trends. Because of the international agreements and standardized procedures concerning climate that were adopted in 1853, about one hundred million observations have been taken from ships since then. The quantities recorded were sea-surface temperature, wind direction and speed, atmospheric pressure and temperature, the state of the sea, and cloudiness.

A great variety of instruments are used by meteorologists in gathering weather information. The net radiometer, pyranometer, and Campbell-Stokes sunshine recorder are used for measuring radiation and sunshine. The sling psychrometer is used to determine relative humidity. For remote sensing, a humidity gauge is used in which the variations of electrical transmission by the device depend on the passage of an electrical current across a chemically coated strip of plastic that is proportional to the amount of moisture absorbed at its surface. This type of gauge is used in radiosondes for upper-air observations.

Measurement of wind velocity, its speed and direction, involves a simple device called an anemometer. Anemometers have four-bladed propellers that are driven by the wind to record wind speed. When the propellers are mounted at right angles, an anemometer responds to air movement in three dimensions, the sum of which reveals its true direction. Weather pilot balloons, or “pibals,” are released and then tracked visually using a theodolite, or a right-angled telescopic transit that is mounted in a way to make possible the reading of both azimuthal (horizontal) and vertical angles. The progress of the balloon is then plotted minute by minute, and the direction and speed are computed for each altitude. This technique does not work, however, for upper-air wind observations when there is a low cloud cover. Under these conditions, “rawin” observation is used. Here, a metal radar target is attached to the balloon and tracked by a radar transceiver or radio theodolite until the balloon breaks or is out of range.

Precipitation is measured in several ways. Rain measurements are made using a rain gauge. An improved version is the tipping-bucket gauge, which has a small divided metal bucket mounted so that it will automatically tip and empty measured quantities of rainfall. This tipping closes electrical contacts that activate a recording pen. Another type of improved rain gauge is a weighing-type gauge. Here, as precipitation falls on a spring scale, a pen arm attached to the scale makes a continuous record on a clock chart. Snow depth is determined by averaging three or more typical measured depths. Where snow depths become very great, graduated rods may be installed in representative places so that the depth can be read directly from the snow surface. Heat and air pressure are measured by thermometers and barometers, most of which record and transmit data automatically using digital electronics.

Aerial Photographs and Core Samples

Specially equipped airplanes and satellites can obtain previously inaccessible data of immeasurable value in determining climate changes. The National Aeronautics and Space Administration (NASA) has supplied a modified U-2 plane to carry instruments to an altitude of twenty kilometers for seven-hour flights up to 80 degrees north latitude. A DC-8 is also used at the same time because of its extended range. The Nimbus 7 satellite is also employed. Landsat satellites continually photograph Earth’s surface, then radio the images back to Earth stations, where they are computer enhanced to provide visual information about such things as the amount and type of vegetation, precipitation, and underground waterways. The enhanced photographs also provide information about changing conditions that may cause changes in local and global climates.

To gain information about the relationship between carbon dioxide and global temperature, scientists have drilled deep into Arctic and Antarctic ice to obtain core samples of the polar ice pack. By studying the amount of carbon dioxide trapped in air bubbles in ancient layers of ice pack, scientists have been able to chart the ebb and flow of carbon dioxide in the atmosphere. Fossilized plant tissues indicate how warm the air was during the same period as the bubbles in the core samples. This clue helps provide a picture of the warming and cooling trends that have occurred in the past, making comparisons with current climate conditions possible.

General Circulation Models

Climatologists have found mathematical and computer-modeling methods very useful, primarily regarding general circulation models (GCMs). The most important part of a GCM is the parameterization of a wide range of physical processes too small to be resolved by the model. These may include all the turbulent fluxes that occur in the surface boundary layer as well as the occurrence of convection, cloudiness, and precipitation. These relatively small processes must be parameterized in terms of the variables that are capable of being resolved by GCMs. It is primarily this parameterization that makes GCMs different from one another and leads to variations between the results of different GCM models. The value of the models, however, depends mostly on the accuracy with which the model simulates natural conditions. Assumptions sometimes must be made in modeling, and these assumptions can give a false picture under certain conditions. Models, therefore, are intensely tested against actual weather data as newer methods of observation and more data become available to ensure the accuracy of the models.

Principal Terms

air mass: a mass of air in the lower atmosphere that has generally uniform properties of temperature and moisture

atmosphere: the envelope of gases surrounding Earth, consisting of five clearly defined stratigraphic regions

general circulation models (GCMs): comprehensive, mathematical-numerical formulas in climate studies that attempt to express in equations the basic dynamics thought to govern the large-scale behavior of the atmosphere

greenhouse effect: the enhanced surface heating effect of solar radiation due to the absorption and redirection of infrared radiation by various gases in the atmosphere

parameterization: the arbitrary assignment of a value to physical processes that occur on scales too small to be resolved by a general circulation model

precipitation: phenomena such as rain, snow, and hail that form through condensation of atmospheric water vapor into liquid and solid forms that subsequently fall toward the surface due to gravity

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