climate change, periodic modification ofEarth’sclimate brought about as a result of changes in theatmosphere as well as interactions between the atmosphere and various other geologic, chemical, biological, and geographic factors within the Earth system.

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The atmosphere is adynamicfluid that is continually in motion. Both its physical properties and its rate and direction of motion are influenced by a variety of factors, includingsolar radiation, the geographic position ofcontinents,ocean currents, the location and orientation ofmountain ranges, atmosphericchemistry, and vegetation growing on the land surface. All these factors change through time. Some factors, such as the distribution ofheat within theoceans, atmospheric chemistry, and surface vegetation, change at very short timescales. Others, such as the position of continents and the location and height of mountain ranges, change over very long timescales. Therefore, climate, which results from the physical properties and motion of the atmosphere, varies at every conceivable timescale.

Climate is often defined loosely as the averageweather at a particular place, incorporating such features astemperature,precipitation,humidity, andwindiness. A more specific definition would state that climate is the mean state and variability of these features over some extended time period. Both definitions acknowledge that the weather is always changing, owing to instabilities in theatmosphere. And as weather varies from day to day, so too does climate vary, from daily day-and-night cycles up to periods ofgeologic time hundreds of millions of years long. In a very real sense,climate variation is aredundant expression—climate is always varying. No two years are exactly alike, nor are any two decades, any two centuries, or any two millennia.

This article addresses the concept of climatic variation and change within the set ofintegrated natural features and processes known as the Earth system. The nature of the evidence for climate change is explained, as are the principal mechanisms that have caused climate change throughout the history of Earth. Finally, a detailed description is given of climate change over many different timescales, ranging from a typical human life span to all of geologic time. For a detailed description of the development of Earth’s atmosphere,see the articleatmosphere, evolution of. For full treatment of the most critical issue of climate change in the contemporary world,seeglobal warming.

TheEarth system

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The atmosphere is influenced by and linked to other features ofEarth, includingoceans, ice masses (glaciers andsea ice), land surfaces, and vegetation. Together, they make up an integrated Earth system, in which all components interact with and influence one another in often complex ways. For instance, climate influences the distribution of vegetation on Earth’s surface (e.g.,deserts exist in arid regions,forests in humid regions), but vegetation in turn influences climate by reflecting radiantenergy back into the atmosphere, transferringwater (andlatent heat) fromsoil to the atmosphere, and influencing the horizontal movement ofair across the land surface.

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Earth scientists and atmospheric scientists are still seeking a full understanding of the complex feedbacks and interactions among the various components of the Earth system. This effort is beingfacilitated by the development of an interdisciplinaryscience calledEarth system science. Earth system science is composed of a wide range ofdisciplines, includingclimatology (the study of the atmosphere),geology (the study of Earth’s surface and underground processes),ecology (the study of how Earth’s organisms relate to one another and their environment),oceanography (the study of Earth’s oceans),glaciology (the study of Earth’sice masses), and even thesocial sciences (the study ofhuman behaviour in its social and cultural aspects).

A full understanding of the Earth system requires knowledge of how the system and its components have changed throughtime. The pursuit of this understanding has led to development ofEarth system history, an interdisciplinary science that includes not only the contributions of Earth system scientists but alsopaleontologists (who study thelife of past geologic periods),paleoclimatologists (who study pastclimates), paleoecologists (who study pastenvironments andecosystems),paleoceanographers (who study the history of the oceans), and other scientists concerned with Earth history. Because different components of the Earth system change at different rates and are relevant at different timescales, Earth system history is adiverse and complex science. Students of Earth system history are not just concerned with documenting what has happened; they also view the past as a series of experiments in whichsolar radiation,ocean currents, continental configurations, atmospheric chemistry, and other important features have varied. These experiments provide opportunities to learn the relative influences of and interactions between various components of the Earth system. Studies of Earth system history also specify the full array of states the system has experienced in the past and those the system is capable of experiencing in the future.

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Undoubtedly, people have always been aware of climatic variation at the relatively short timescales of seasons, years, and decades. Biblical scripture and other early documents refer todroughts,floods, periods of severe cold, and other climatic events. Nevertheless, a full appreciation of the nature and magnitude of climatic change did not come about until the late 18th and early 19th centuries, a time when thewidespread recognition of the deep antiquity of Earth occurred. Naturalists of this time, including Scottish geologistCharles Lyell, Swiss-born naturalist and geologistLouis Agassiz, English naturalistCharles Darwin, American botanistAsa Gray, and Welsh naturalistAlfred Russel Wallace, came to recognize geologic and biogeographic evidence that made sense only in the light of past climates radically different from those prevailing today.

Geologists and paleontologists in the 19th and early 20th centuries uncovered evidence of massive climatic changes taking place before thePleistocene—that is, before some 2.6 million years ago. For example, red beds indicated aridity in regions that are now humid (e.g.,England andNew England), whereasfossils ofcoal-swampplants andreef corals indicated that tropical climates once occurred at present-day high latitudes in bothEurope andNorth America. Since the late 20th century the development of advanced technologies fordating rocks, together with geochemical techniques and otheranalytical tools, have revolutionized the understanding of early Earth system history.

The occurrence of multiple epochs in recent Earth history during which continentalglaciers, developed at high latitudes, penetrated into northern Europe and eastern North America was recognized by scientists by the late 19th century. Scottish geologistJames Croll proposed that recurring variations in orbital eccentricity (the deviation of Earth’s orbit from a perfectly circular path) were responsible for alternating glacial and interglacial periods. Croll’s controversial idea was taken up by Serbian mathematician and astronomerMilutin Milankovitch in the early 20th century. Milankovitch proposed that the mechanism that brought about periods ofglaciation was driven bycyclic changes ineccentricity as well as two other orbital parameters:precession (a change in the directional focus of Earth’s axis of rotation) and axial tilt (a change in the inclination of Earth’s axis with respect to the plane of its orbit around theSun). Orbital variation is now recognized as an important driver of climatic variation throughout Earth’s history (see belowOrbital [Milankovitch] variations).