CLIMATE

conditions of the atmosphere at a particular location over a long period of time; it is the long-term summation of the atmospheric elements (and their variations) that, over short time periods, constitute weather. These elements are solar radiation, temperature, humidity, precipitation (type, frequency, and amount), atmospheric pressure, and wind (speed and direction). From the ancient Greek origins of the word (klma, an inclination or slopee.g., of the Sun's rays; a latitude zone of the Earth; a clime), and from its earliest usage in English, climate has been understood to mean the atmospheric conditions that prevail in a given region or zone. In the older form, clime, it was sometimes taken to include all aspects of the environment, including the natural vegetation. The best modern definitions of climate regard it as constituting the total experience of weather and atmospheric behaviour over a number of years in a given region. Climate is not just the average weather (an obsolete, and always inadequate, definition). It should include not only the average values of the climatic elements that prevail at different times but also their extreme ranges, variability, and the frequency of various occurrences. Just as one year differs from another, decades and centuries are found to differ from one another by a smaller but sometimes significant amount: climate is, therefore, time-dependent, and climatic values or indices should not be quoted without specifying what years they refer to. This article treats the factors that produce weather and climate and the complex processes that cause variations in both. Other major points of coverage include global climatic types and microclimates. The article also considers both the impact of climate on human life and the effects of human activities on the climate. For details concerning the disciplines of meteorology and climatology, see earth sciences. See also the article atmosphere for further information about the properties and behaviour of the atmospheric system. Relevant data on the influence of the oceans and of atmospheric moisture on climate can be found in hydrosphere. The Editors of the Encyclopdia Britannica condition of the atmosphere at a particular location over a long period of time; it is the long-term summation of the atmospheric elements (and their variations) that, over short time periods, constitute weather. These elements are solar radiation, temperature, humidity, precipitation (type, frequency, and amount), atmospheric pressure, and wind (speed and direction). The idea of climate has broadened greatly in recent years. To the general public the word retains the meaning of expected or habitual weather, which is heavily dependent on place and time of year. To the specialist, however, it connotes an ensemble that transcends the simple notion of average weather. Climate is now perceived as part of a larger system that includes not only the atmosphere but also the hydrosphere (all liquid and frozen surface waters), the lithosphere (all solid land surfaces, including the ocean floors), the biosphere (all living things), and such extraterrestrial factors as the Sun. These interconnected subsystems are governed by known physical laws, but each nonetheless behaves in a complex, unpredictable manner. Thus, it is not surprising that the climatic system is also characterized by complexity and variability. Distinct rhythms (such as the seasons) or certain prevailing conditions may be evident, but within these constraints any number of variations may occur. Descriptions of climate therefore should include not only the average values of such elements as temperature and atmospheric pressure but also measurements of variability, such as frequency of occurrence and extreme ranges. Large-scale fluctuations that result in new and apparently lasting conditions are known as climatic changes. Such changes may take place over millennia or even over millions of years. Evidence of such changes and of past climates is understandably limited. Full-scale measurement and analysis of all climatic variables has been possible only in recent decades as a result of technological advances. Fairly reliable instrumental records of temperature and precipitation are available for the past century or so, but these measurements were usually taken only over land areas, which constitute only a fraction of the Earth's surface. Other variables were not even measured. Older instrumental records are even less complete, and they date only to about 1700. Archaeological evidence and sporadic, primarily anecdotal, documentary records exist for the 2,000 years or so before that, but knowledge of earlier climates depends on the ability to interpret indirect sources. Major sources of this kind are sediments deposited by wind, water, or glaciers, the plant and animal fossils often contained in these rocks, and tree-ring records of annual growth. In order for these indirect sources to be of use in reconstructing past climates, the age of the materials must first be accurately determined. The oldest and least-precise dating method involves comparing sediment contents for similarities to known deposits, thus establishing a time equivalence between the two. More accurate techniques developed in the 20th century involve radiocarbon dating of organic remains, correlating visible annual bands (such as tree rings or seasonal glacial deposits), and comparing the geomagnetic polarity of a sample to the known chronology of changes in the polarity of the Earth's magnetic field. Once the age of a source has been determined, the sample can be studied for climatic evidence. The texture, structure, and markings of sedimentary rocks reveal much about the climatic conditions under which they were formed and deposited. Evidence of erosion, for example, can provide information about temperature and precipitation. Inferences about the living conditions of fossil organisms can be made based upon the known living conditions of the organisms' modern relatives. Less-speculative information can be derived from analyzing the isotopic composition of fossil remains. Oxygen isotope analysis of marine fossils, for example, provides a record of the temperature of the waters in which the organisms lived as well as evidence of changes in sea level. Fossil pollen and spores are especially useful because they are abundant, widespread, well-preserved, and easily identifiable. Finally, the relative size of tree rings can be studied to determine fluctuations in temperature and precipitation. The study of paleoclimates reveals that the present is an abnormal time for the climates of the world. Ice sheets cover Greenland and Antarctica, and permanent pack ice blocks the Arctic Ocean. The deep waters of the oceans are frigidly cold in all latitudes (mostly below 3 C ). The vegetation map is strongly zonal, with treeless tundra on high-latitude land surfaces and luxuriant rain forests near the equator. Few of these conditions were typical of most of the Earth's history. Information about climates more than 540 million years ago is fragmentary. Little is known except that temperatures were probably between 0 and 50 C (32 and 122 F). From the beginning of the Paleozoic era (about 540 million years ago) to the beginning of the Cenozoic era (about 66.4 million years ago), it appears that the Earth's climate was relatively warm and moist, with latitudinal zonations. Evidence indicates that deserts existed and that the area around both poles was ice-free. Temporary variations did occur, including a prolonged glacial episode about 280 million years ago during which ice sheets extended into the southern high latitudes. In general, cooler, drier conditions prevailed from about 320 million to 208 million years ago. Then, warm, equable conditions returned, with mean annual surface temperatures about 6 C (11 F) higher than those of today. These mild conditions appear to have ended abruptly about 65 million years ago, when it is thought that rapidly cooling conditions may have contributed to the extinction of the dinosaurs as well as of many other forms of life. Warm, moist conditions returned fairly soon thereafter, giving rise to latitudinal climatic zones similar to those of today. The oceans, especially the deep waters, began to experience slow cooling at this time, with occasional sharp decreases in temperature. Atmospheric cooling began in the middle Cenozoic, and by about 36 million years ago large ice sheets covered much of Antarctica, which has remained at least partly glaciated ever since. Glaciation of the Northern Hemisphere began much later, probably in the Late Miocene epoch (about 10 million years ago). The cooling continued into the Pleistocene epoch, which began about 1.6 million years ago. In fact, the Pleistocene is sometimes referred to as the Ice Age, especially in older literature, because it is characterized by the advance of great continental glaciers from a number of centres, of which the most important were Scandinavia and the Alps in Europe and the Cordilleras and various other regions in North America. It was once believed that there were four glacial episodes, each followed by a warm interglacial interval during which the ice receded. The Holocene epoch, which constitutes roughly the past 10,000 years, was viewed as the fourth of these interglacials. Recent evidence, however, suggests that the number of glacial and of corresponding shorter interglacial episodes is actually much greater. Differences in planetary temperatures between the glacial and interglacial episodes may have been as much as 15 or 20 C (27 or 36 F). It is thought that during full glaciation global temperatures were lower than present-day values by 5 C (9 F) or more. The last glacial episode, called the Wisconsin/Wrm glaciation, reached its climax about 18,000 years ago. A warming trend followed the Wisconsin/Wrm maximum. By about 9,000 years ago, summer temperatures and precipitation were above current values in much of the land areas of the Northern Hemisphere. This warm, moist phase was especially widespread in the Middle East and Asia, where it gave early humans the opportunity to develop pastoral and agricultural civilizations. A similar case of climatic fluctuation affecting human development occurred during the period AD 600 to 1400, when a slight warming in Europe and the North Atlantic enabled Viking and other northern cultures to spread and flourish. This warm phase was followed by a period during which the glaciers advanced and cooler, harsher conditions prevailed in most parts of the world. Known as the Little Ice Age, this period lasted as late as the mid-19th century in some areas. Meteorologic instrument records show that a gradual global warming trend has set in since that time. Precipitation measurements show much greater variability. Large-scale climatic variations have been attributed to various causes. The most likely theories involve variables that produce changes in the pattern of solar heating. These are described hereafter. Variations in the solar constant, the power per unit area of incoming solar energy, would seem to be the simplest process. Although it appears that minor fluctuations in solar irradiance have occurred, they do not seem to correlate with observed climatic changes. Variations in the Earth's orbital behaviour can result in significant redistribution of solar heating, with attendant climatic effects. The three modes of variation are the obliquity (or tilt) of the Earth's axis, the precession of the equinoxes, and the eccentricity of the Earth's orbit around the Sun. All three occur over long time scales (from 20,000 to 100,000 years) and thus can only be used to explain climatic changes on the millennial time scale. Variations in atmospheric composition have been shown to correlate with changes in the amount of solar radiation that reaches the Earth. It is thought that carbon dioxide and certain other atmospheric trace gases act as selective screens, admitting incoming shorter-wave radiation from the Sun (rays of visible light) but obstructing outgoing long-wave infrared radiation. Thus, an increase in the concentration of such components in the atmosphere would result in higher planetary temperatures (see greenhouse effect). Volcanic activity can also introduce an atmospheric screen. The gases and dust particles emitted into the stratosphere by a major eruption can spread worldwide and remain in suspension for several years. Such dust veils can reduce the amount of solar energy reaching the lower atmosphere, resulting in lower temperatures. Other factors that result in climatic changes are continental drift, continental uplift, and mountain building. The changes in landsea distributions caused by these processes influence temperature and atmospheric and oceanic circulation patterns, but they operate on such long time scales that they can only help explain climatic variations on the order of millions of years. No single theory can adequately account for all past climatic variations. Short-term fluctuations on the order of 10 years or less and the internal variability of climate are especially difficult to explain. The effects of human activities upon climate, which are incompletely understood, further complicate the study of short-term climatic fluctuations. Additional reading General works Introductory works include A.S. Monin, An Introduction to the Theory of Climate (1986; originally published in Russian, 1982); Paul E. Lydolph, Weather and Climate (1985); John T. Houghton (ed.), The Global Climate (1984); and Louis J. Battan, Weather in Your Life (1983). Two excellent comprehensive reference works are John E. Oliver and Rhodes W. Fairbridge (eds.), The Encyclopedia of Climatology (1987); and David D. Houghton (ed.), Handbook of Applied Meteorology (1985). John E. Hobbs, Applied Climatology: A Study of Atmospheric Resources (1980); and John F. Griffiths, Applied Climatology: An Introduction, 2nd ed. (1976), are also useful. Definitions of meteorological terms are provided in Ralph E. Huschke (ed.), Glossary of Meteorology (1959, reprinted 1970); and World Meteorological Organization, International Meteorological Vocabulary (1966), including nomenclature in English, French, Russian, and Spanish.Current research is reported in the following journals: Bulletin of the American Meteorological Society (monthly); Climate Monitor (quarterly); Climatic Change (6/yr.); International Journal of Biometeorology (quarterly); Journal of Climate and Applied Meteorology (monthly); Journal of Climatology (bimonthly); Journal of Meteorological Research (bimonthly); Journal of Meteorology (10/yr.); Monthly Weather Review; Quarterly Journal of the Royal Meteorological Society; Soviet Meteorology and Hydrology (monthly); Weather (monthly); Weatherwise (bimonthly); and W.M.O. Bulletin (quarterly). The Editors of the Encyclopdia Britannica Solar radiation and temperature Introductory discussions of these basic elements of climate can be found in Glenn T. Trewartha and Lyle H. Horn, An Introduction to Climate, 5th ed. (1980); John F. Griffiths and Dennis M. Driscoll, Survey of Climatology (1982); and Stanley David Gedzelman, The Science and Wonders of the Atmosphere (1980). G.W. Paltridge and C.M.R. Platt, Radiative Processes in Meteorology and Climatology (1976), provides more advanced treatment. Roger Davies Atmospheric humidity and precipitation Discussions of water vapour in the atmosphere and global water budgets are found in Neil Wells, The Atmosphere and Ocean: A Physical Introduction (1986); and F.H. Ludlam, Clouds and Storms: The Behavior and Effect of Water in the Atmosphere (1980). Forms of precipitation are surveyed in B.J. Mason, Clouds, Rain and Rainmaking, 2nd ed. (1975); W.E. Knowles Middleton, A History of the Theories of Rain and Other Forms of Precipitation (1965); and D.M. Gray and D.H. Male (eds.), Handbook of Snow: Principles, Processes, Management & Use (1981). B.J. Mason, The Physics of Clouds, 2nd ed. (1971), is an authoritative text. See also the cloud atlas by Richard Scorer, Clouds of the World: A Complete Color Encyclopedia (1972). The Editors of the Encyclopdia Britannica Atmospheric pressure and wind General textbooks with effective discussions of wind and pressure are Frederick K. Lutgens and Edward J. Tarbuck, The Atmosphere: An Introduction to Meteorology, 3rd ed. (1986); C. Donald Ahrens, Meteorology Today: An Introduction to Weather, Climate, and the Environment, 2nd ed. (1985); Louis J. Battan, Fundamentals of Meteorology, 2nd ed. (1984); and Stanley David Gedzelman, The Science and Wonders of the Atmosphere (1980). More sophisticated treatment of the wind/pressure relationship is provided by John A. Dutton, The Ceaseless Wind: An Introduction to the Theory of Atmospheric Motion, enl. ed. (1986); James R. Holton, An Introduction to Dynamic Meteorology, 2nd ed. (1979); John M. Wallace and Peter V. Hobbs, Atmospheric Science: An Introductory Survey (1977); Horace R. Byers, General Meteorology, 4th ed. (1974); and E. Palmn and C.W. Newton, Atmospheric Circulation Systems: Their Structure and Physical Interpretation (1969). Phillip J. Smith Climatic variations and changes Overviews are given by M.I. Budyko, The Earth's Climate, Past and Future (1982; originally published in Russian, 1980), a masterly account by the founder of contemporary climatology; H.H. Lamb, Climate, History, and the Modern World (1982); T.M.L. Wigley, M.J. Ingram, and G. Farmer, Climate and History: Studies in Past Climates and Their Impact on Man (1981); and Michael R. Rampino et al., Climate, History, Periodicity, and Predictability (1987), on the relationship between climate cycles and their causes, with an extensive bibliography. A.B. Pittock et al. (eds.), Climatic Change and Variability: A Southern Perspective (1978), offers an excellent antipodean analysis. Emmanuel Le Roy Ladurie, Times of Feast, Times of Famine: A History of Climate Since the Year 1000 (1971, reissued 1988; originally published in French, 1967), is an interdisciplinary study using many data sources to document climatic variations. R.S. Bradley, Quaternary Paleoclimatology: Methods of Paleoclimatic Reconstruction (1985), summarizes research techniques. F. Kenneth Hare Climatic classification Useful introductory discussions can be found in Arthur N. Strahler and Alan H. Strahler, Modern Physical Geography, 3rd ed. (1987); and Hermann Flohn, Climate and Weather (1969; originally published in German, 1968). More advanced treatment is provided by A. Henderson-Sellers and P.J. Robinson, Contemporary Climatology (1986); John G. Lockwood, World Climatic Systems (1985); Roger G. Barry and Richard J. Chorley, Atmosphere, Weather, and Climate, 4th ed. (1982); B.W. Atkinson, Dynamical Meteorology: An Introductory Selection (1981); and R.G. Barry and A.H. Perry, Synoptic Climatology: Methods and Applications (1973). Specific treatment of the topic is given by John E. Oliver and L. Wilson, "Climatic Classification, in John E. Oliver and Rhodes W. Fairbridge (eds.), The Encyclopedia of Climatology (1987), pp. 221237; and John E. Oliver, Climate and Man's Environment: An Introduction to Applied Climatology (1973). The global distribution of major climate types is the subject of Glenn T. Trewartha and Lyle H. Horn, An Introduction to Climate, 5th ed. (1980); John F. Griffiths and Dennis M. Driscoll, Survey of Climatology (1982); Howard J. Critchfield, General Climatology, 4th ed. (1983); John G. Lockwood, World Climatology: An Environmental Approach (1974); S. Nieuwolt, Tropical Climatology: An Introduction to the Climates of the Low Latitudes (1977); and Herbert Riehl,Climate and Weather in the Tropics (1979). Particular regions are examined in H.E. Landsberg (ed.), World Survey of Climatology (1969 )15 vol. have appeared to 1987; and Glenn T. Trewartha, The Earth's Problem Climates, 2nd ed. (1981). Climatic data are covered in Howard J. Critchfield, Climatic Data, Sources of, in Oliver and Fairbridge (op. cit.), pp. 272276. Discussion of meso- and microclimates are found in T.R. Oke, Boundary Layer Climates (1978); Rudolf Geiger, The Climate near the Ground (1965; originally published in German, 1961); Masatoshi M. Yoshino, Climate in a Small Area: An Introduction to Local Meteorology (1975); and Helmut E. Landsberg, The Urban Climate (1981). A. John Arnfield Climate and life Comprehensive introductions include Stephen H. Schneider and Randi Londer, The Coevolution of Climate and Life (1984); and James Lovelock, The Ages of Gaia: A Biography of Our Living Earth (1988), a restatement of a controversial view of interactions between the living and inorganic parts of Earth. Causes of climatic change are explained in detail in B. Bolin and R.B. Cook (ed.), The Major Biogeochemical Cycles and Their Interactions (1983); B. Bolin et al., The Greenhouse Effect, Climatic Change, and Ecosystems (1986); Norman Myers, Conversion of Tropical Moist Forests (1980), a clearly argued but controversial account; and Robert E. Dickinson (ed.), The Geophysiology of Amazonia: Vegetation and Climate Interactions (1987). The impact of climate on human life is treated in Michael Glantz, Richard Katz, and Maria Krenz (eds.), The Societal Impacts Associated with the 198283 Worldwide Climate Anomalies (1987), a report on the effects of the 198283 El Nio/Southern Oscillation, published by the National Center for Atmospheric Research; Wilfrid Bach, Jrgen Pankrath, and Stephen H. Schneider (eds.), FoodClimate Interactions (1981); Robert W. Kates, Jesse H. Ausubel, and Mimi Berberian, Climate Impact Assessment: Studies of the Interaction of Climate and Society (1985); and William W. Kellogg and Robert Schware, Climate Change and Society: Consequences of Increasing Atmospheric Carbon Dioxide (1981). The impact of human activities on the climate is presented in William C. Clark and R.E. Munn (eds.), Sustainable Development of the Biosphere (1986); Michael C. MacCracken and Frederick M. Luther (eds.), Projecting the Climatic Effects of Increasing Carbon Dioxide (1985); National Research Council (U.S.), Carbon Dioxide Assessment Committee, Changing Climate (1983); and P.S. Liss and A.J. Crane, Man-Made Carbon Dioxide and Climatic Change: A Review of Scientific Problems (1983). Climate model predictions are explored in Stephen H. Schneider, Climate Modeling, Scientific American, 256(5):7280 (May 1987); and A. Berger et al., Milankovitch and Climate: Understanding the Response to Astronomical Forcing, 2 vol. (1984). Stephen H. Schneider A. Brewster Rickel