Interannual variation
Interannual climate variations, including droughts, floods, and other events, are caused by a complex array of factors and Earth system interactions. One important feature that plays a role in these variations is the periodic change of atmospheric and oceanic circulation patterns in the tropical Pacific region, collectively known as El Niño–Southern Oscillation (ENSO) variation. Although its primary climatic effects are concentrated in the tropical Pacific, ENSO has cascading effects that often extend to the Atlantic Ocean region, the interior of Europe and Asia, additionally the polar regions. These effects, called teleconnections, occur because alterations in low-latitude atmospheric circulation patterns in the Pacific region influence atmospheric circulation in adjacent and downstream systems. As a result, storm tracks are diverted and atmospheric pressure ridges (areas of high pressure) and troughs (areas of low pressure) are displaced from their usual patterns.
As an example, El Niño events occur if the easterly trade winds in the tropical Pacific weaken or reverse direction. This shuts down the upwelling of deep, cold waters off the west coast of South America, warms the eastern Pacific, and reverses the atmospheric pressure gradient in the western Pacific. As a result, air at the surface moves eastward from Australia and Indonesia toward the central Pacific as well as the Americas. These changes produce high rainfall and flash floods along the typically arid coast of Peru and severe drought in the typically wet regions of northern Australia and Indonesia. Particularly severe El Niño events lead to monsoon failure in the Indian Ocean region, resulting in intense drought in India and East Africa. At the same time, the westerlies and storm tracks are displaced toward the Equator, providing California together with desert Southwest of the United States with wet, stormy winter weather and causing winter conditions in the Pacific Northwest, which are typically wet, to become warmer and drier. Displacement of the westerlies also results in drought in northern China and from northeastern Brazil through sections of Venezuela. Long-term records of ENSO variation from historical documents, tree rings, and reef corals indicate that El Niño events occur, on average, every two to seven years. However, the frequency and intensity of these events vary through time.
The North Atlantic Oscillation (NAO) is another example of an interannual oscillation that produces important climatic effects within the Earth system and may influence climate throughout the Northern Hemisphere. This phenomenon results from variation in the pressure gradient, or the difference in atmospheric pressure between the subtropical high, frequently situated between the Azores and Gibraltar, in addition to Icelandic low, centred between Iceland and Greenland. If the pressure gradient is steep due to a strong subtropical high and a deep Icelandic low (positive phase), northern Europe and northern Asia experience warm, wet winters with frequent strong winter storms. At the same time, southern Europe is dry. The eastern United States also experiences warmer, less snowy winters during positive NAO phases, although the effect is not as great as in Europe. The pressure gradient is dampened when NAO is in a negative mode—that is, when a weaker pressure gradient exists from the presence of a weak subtropical high and Icelandic low. When this happens, the Mediterranean region receives abundant winter rainfall, while northern Europe is cold and dry. The eastern United States is typically colder and snowier during a negative NAO phase.
During years if the North Atlantic Oscillation (NAO) is in its positive phase, the eastern United States, southeastern Canada, and northwestern Europe experience warmer winter temperatures, whereas colder temperatures are found in these locations during its negative phase. If the El Niño/Southern Oscillation (ENSO) and NAO are both in their positive phase, European winters tend to be wetter and less severe; however, beyond this general tendency, the influence of the ENSO upon the NAO is not well understood.Encyclopædia Britannica, Inc.
The ENSO and NAO cycles are driven by feedbacks and interactions between the oceans and atmosphere. Interannual climate variation is driven by these and other cycles, interactions among cycles, and perturbations in the Earth system, such as those resulting from large injections of aerosols from volcanic eruptions. One example of a perturbation due to volcanism is the 1991 eruption of Mount Pinatubo in the Philippines, which led to a decrease in the average global temperature of approximately 0.5 °C (0.9 °F) the following summer.
Decadal variation
Climate varies on decadal timescales, with multiyear clusters of wet, dry, cool, or warm conditions. These multiyear clusters can have dramatic effects on person activities and welfare. As an example, a severe three-year drought in the late 16th century probably contributed to the destruction of Sir Walter Raleigh’s ‘Lost Colony’ at Roanoke Island in what is currently North Carolina, and a subsequent seven-year drought (1606–12) led to high mortality at the Jamestown Colony in Virginia. Also, some scholars have implicated persistent and severe droughts due to the fact main reason for the collapse of the Maya civilization in Mesoamerica between AD 750 and 950; however, discoveries in the early 21st century suggest that war-related trade disruptions played a role, possibly interacting with famines and other drought-related stresses.
Although decadal-scale climate variation is well documented, the causes are not entirely clear. Much decadal variation in climate is related to interannual variations. For example, the frequency and magnitude of ENSO change through time. The early 1990s were characterized by repeated El Niño events, and several such clusters happen identified as having taken place during the 20th century. The steepness of the NAO gradient also changes at decadal timescales; it has been particularly steep since the 1970s.
Recent research has revealed that decadal-scale variations in climate result from interactions between the ocean additionally the atmosphere. One such variation is the Pacific Decadal Oscillation (PDO), also referred to as the Pacific Decadal Variability (PDV), which involves changing sea surface temperatures (SSTs) in the North Pacific Ocean. The influence that is SSTs strength and position of the Aleutian Low, which in turn strongly affects precipitation patterns along the Pacific Coast of North America. PDO variation is composed of an alternation between ‘cool-phase’ periods, when coastal Alaska is relatively dry additionally the Pacific Northwest relatively wet ( e.g., 1947–76), and ‘warm-phase’ periods, characterized by relatively high precipitation in coastal Alaska and low precipitation in the Pacific Northwest ( e.g., 1925–46, 1977–98). Tree ring conclusion for global warming essay and coral records, which span at least the last four centuries, document PDO variation.
A similar oscillation, the Atlantic Multidecadal Oscillation (AMO), occurs in the North Atlantic and strongly influences precipitation patterns in eastern and central North America. a warm-phase amo (relatively warm North Atlantic SSTs) is associated with relatively high rainfall in Florida and low rainfall in much of the Ohio Valley. However, the AMO interacts aided by the PDO, and both interact with interannual variations, such as ENSO and NAO, in complex ways . Such interactions may lead to the amplification of droughts, floods, or other climatic anomalies. For example, severe droughts over much of the conterminous United States in the first few years of the 21st century were associated with warm-phase AMO combined with cool-phase PDO. The mechanisms underlying decadal variations, such as PDO and AMO, are poorly understood, but they are probably related to ocean-atmosphere interactions with larger time constants than interannual variations. Decadal climatic variations are the subject of intense study by climatologists and paleoclimatologists.
Climate Change Since The Emergence Of Civilization
Person societies have experienced climate change since the development of agriculture some 10,000 years ago. These climate changes have often had profound effects on person cultures and societies. They include annual and decadal climate fluctuations such as those described above, as well as large-magnitude changes that occur over centennial to multimillennial timescales. Such changes are believed to have influenced and even stimulated the initial cultivation and domestication of crop plants, as well as the domestication and pastoralization of animals. Person societies have changed adaptively in response to climate variations, although evidence abounds that certain societies and civilizations have collapsed in the face of rapid and severe climatic changes.
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Centennial-scale variation
Historical records as well as proxy records (particularly tree rings, corals, and ice cores) indicate that climate has changed during the past 1,000 years at centennial timescales; that is, no two centuries being exactly alike. During the past 150 years, the Earth system has emerged from a period called the Little Ice Age, which was characterized in the North Atlantic region and elsewhere by relatively cool temperatures. The 20th century in particular saw a substantial pattern of warming in many regions. Some of this warming may be attributable to the transition from the Little Ice Age or other natural causes. However, many climate scientists believe that much of the 20th-century warming, especially in the later decades, resulted from atmospheric accumulation of greenhouse gases (especially carbon dioxide, CO2).
The Little Ice Age is best known in Europe additionally the North Atlantic region, which experienced relatively cool conditions between the early 14th and mid-19th centuries. This was not a period of uniformly cool climate, since interannual and decadal variability brought many warm years. Furthermore, the coldest periods did not always coincide among regions; some regions experienced relatively warm conditions at the same time others were subjected to severely cold conditions. Alpine glaciers advanced far below their previous (and present) limits, obliterating farms, churches, and villages in Switzerland, France, and elsewhere. Frequent cold winters and cool, wet summers ruined wine harvests and led to crop failures and famines over much of northern and central Europe. The North Atlantic cod fisheries declined as ocean temperatures fell in the 17th century. The Norse colonies on the coast of Greenland were cut off from the rest of Norse civilization during the early 15th century as pack ice and storminess increased in the North Atlantic. The western colony of Greenland collapsed through starvation, additionally the eastern colony was abandoned. In addition, Iceland became increasingly isolated from Scandinavia.
The Little Ice Age was preceded by a period of relatively mild conditions in northern and central Europe. This interval, known as the Medieval Warm Period, occurred from approximately AD 1000 to the first half of the 13th century. Mild summers and winters led to good harvests in much of Europe. Wheat cultivation and vineyards flourished at far higher latitudes and elevations than today. Norse colonies in Iceland and Greenland prospered, and Norse parties fished, hunted, and explored the coast of Labrador and Newfoundland. The Medieval Warm Period is well documented in much of the North Atlantic region, including ice cores from Greenland. Like the Little Ice Age, this time was neither a climatically uniform period nor a period of uniformly warm temperatures everywhere in the world. Other regions of the globe lack evidence for high temperatures during this period.
Much scientific attention continues to be devoted to a series of severe droughts that occurred between the 11th and 14th centuries. These droughts, each spanning several decades, are well documented in tree-ring records across western North America and in the peatland records of the Great Lakes region. The records appear to be regarding ocean temperature anomalies in the Pacific and Atlantic basins, but they are still inadequately understood. The information suggests that much of the United States is susceptible to persistent droughts that would be devastating for water resources and agriculture.
Millennial and multimillennial variation
The climatic changes of the past thousand years are superimposed upon variations and trends at both millennial timescales and greater. Numerous indicators from eastern North America and Europe show trends of increased cooling and increased effective moisture during the past 3,000 years. For example, in the Great Lakes–St. Lawrence regions along the U.S.-Canadian border, water levels of the lakes rose, peatlands developed and expanded, moisture-loving trees such as beech and hemlock expanded their ranges westward, and populations of boreal trees, such as spruce and tamarack, increased and expanded southward. These patterns all indicate a trend of increased effective moisture, which might indicate increased precipitation, decreased evapotranspiration due to cooling, or both. The patterns usually do not necessarily indicate a monolithic cooling event; more complex climatic changes probably occurred. For example, beech expanded northward and spruce southward during the past 3,000 years in both eastern North America and western Europe. The beech expansions may indicate milder winters or longer growing seasons, whereas the spruce expansions appear related to cooler, moister summers. Paleoclimatologists are applying a variety of approaches and proxies to simply help identify such changes in seasonal temperature and moisture during the Holocene Epoch.
Just as the Little Ice Age was not associated with cool conditions everywhere, so the cooling and moistening trend of the past 3,000 years was not universal. Some regions became warmer and drier during the same time period. For example, northern Mexico additionally the Yucatan experienced decreasing moisture in the past 3,000 years. Heterogeneity of this type is characteristic of climatic change, which involves changing patterns of atmospheric circulation. As circulation patterns change, the transport of heat and moisture in the atmosphere also changes. This fact explains the apparent paradox of opposing temperature and moisture trends in different regions.
The trends of the past 3,000 years are just the latest in a group of climatic changes that occurred over the past 11,700 years or so—the interglacial period referred to given that Holocene Epoch. At the start of the Holocene, remnants of continental glaciers from the last glaciation still covered much of eastern and central Canada and parts of Scandinavia. These ice sheets largely disappeared by 6,000 years ago. Their absence— along with increasing sea surface temperatures, rising sea levels (as glacial meltwater flowed into the world’s oceans), and especially changes in the radiation budget of Earth’s surface owing to Milankovitch variations ( changes in the seasons resulting from periodic modifications of Earth’s orbit around the Sun)—affected atmospheric circulation. The diverse changes of the past 10,000 years across the globe are difficult to summarize in capsule, but some general highlights and large-scale patterns are worthy of note. These include the presence of early to mid-Holocene thermal maxima in various locations, variation in ENSO patterns, and an early to mid-Holocene amplification of the Indian Ocean monsoon.
Thermal maxima
Many parts of the globe experienced higher temperatures than today some time during the early to mid-Holocene. In some cases the increased temperatures were accompanied by decreased moisture availability. Although the thermal maximum has been described in North America and elsewhere as a single widespread event (variously known as the ‘Altithermal,’ ‘Xerothermic Interval,’ ‘Climatic Optimum,’ or ‘Thermal Optimum’), it is now recognized that the periods of maximum temperatures varied among regions. For example, northwestern Canada experienced its highest temperatures several thousand years earlier than central or eastern North America. Similar heterogeneity is seen in moisture records. As an example, the record of the prairie-forest boundary in the Midwestern region of the United States shows eastward expansion of prairie in Iowa and Illinois 6,000 years ago (indicating increasingly dry conditions), whereas Minnesota’s forests expanded westward into prairie regions at the same time (indicating increasing moisture). The Atacama Desert, located primarily in present-day Chile and Bolivia, on the western side of South America, is one of the driest places on Earth today, but it was much wetter during the early Holocene when many other regions were at their driest.
The primary driver of changes in temperature and moisture during the Holocene was orbital variation, which slowly changed the latitudinal and seasonal distribution of solar radiation on Earth’s surface and atmosphere. However, the heterogeneity of these changes was caused by changing patterns of atmospheric circulation and ocean currents.
ENSO variation in the Holocene
Because of the global importance of ENSO variation today, Holocene variation in ENSO patterns and intensity is under serious study by paleoclimatologists. The record is still fragmentary, but evidence from fossil corals, tree rings, lake records, climate modeling, and other approaches is accumulating that (1) ENSO variation was relatively weak in the early Holocene, (2) ENSO has undergone centennial to millennial variations in strength during the past 11,700 years, and (3) ENSO patterns and strength similar to those currently in place developed within the past 5,000 years. This evidence is particularly clear when comparing ENSO variation over the past 3,000 years to today’s patterns. The causes of long-term ENSO variation are nevertheless being explored, but changes in solar radiation owing to Milankovitch variations are strongly implicated by modeling studies.
Amplification of the Indian Ocean monsoon
Much of Africa, the Middle East, and the Indian subcontinent are under the strong influence of an annual climatic cycle known as summary of as you like it about the Indian Ocean monsoon. The climate of this region is highly seasonal, alternating between clear skies with dry air (winter) and cloudy skies with abundant rainfall (summer). Monsoon intensity, like other aspects of climate, is subject to interannual, decadal, and centennial variations, at least some of which are regarding ENSO and other cycles. Abundant evidence exists for large variations in monsoon intensity during the Holocene Epoch. Paleontological and paleoecological studies show that large portions associated with the region experienced much greater precipitation during the early Holocene (11,700–6,000 years ago) than today. Lake and wetland sediments dating to this period being found under the sands of the Sahara Desert. These sediments contain fossils of elephants, crocodiles, hippopotamuses, and giraffes, together with pollen evidence of forest and woodland vegetation. In arid and semiarid parts of Africa, Arabia, and India, large and deep freshwater lakes occurred in basins that are now dry or are occupied by shallow, saline lakes. Civilizations based on plant cultivation and grazing animals, such as the Harappan civilization of northwestern India and adjacent Pakistan, flourished in these regions, which have since become arid.
These and similar lines of evidence, together with paleontological and geochemical data from marine sediments and climate-modeling studies, indicate that the Indian Ocean monsoon was greatly amplified during the early Holocene, supplying abundant moisture far inland into the African and Asian continents. This amplification was driven by high solar radiation in summer, which was approximately 7 percent higher 11,700 years ago than today and resulted from orbital forcing ( changes in Earth’s eccentricity, precession, and axial tilt). High summer insolation resulted in warmer summer air temperatures and lower surface pressure over continental regions and, hence, increased inflow of moisture-laden air from the Indian Ocean to the continental interiors. Modeling studies indicate that the monsoonal flow was further amplified by feedbacks involving the atmosphere, vegetation, and soils. Increased moisture led to wetter soils and lusher vegetation, which in turn led to increased precipitation and greater penetration of moist air into continental interiors. Decreasing summer insolation during the past 4,000–6,000 years led to the weakening of the Indian Ocean monsoon.
Climate Change Since The Advent Of Humans
Examine glacial scratches on rocks from Switzerland to New York City for evidence of Earth’s icy pastEvidence of Earth’s glacial past.Encyclopædia Britannica, Inc.See all videos for this article
The history of humanity—from the initial appearance of genus Homo over 2,000,000 years ago to the advent and expansion of the modern personal species (Homo sapiens) beginning some 150,000 years ago—is integrally linked to climate variation and change. Homo sapiens has experienced nearly two full glacial-interglacial cycles, but its global geographical expansion, massive population increase, cultural diversification, and worldwide ecological domination began only during the last glacial period and accelerated during the last glacial-interglacial transition. The first bipedal apes appeared in a time of climatic transition and variation, and Homo erectus, an extinct species possibly ancestral to modern humans, originated during the colder Pleistocene Epoch and survived both the transition period and multiple glacial-interglacial cycles. Thus, it can be said that climate variation has been the midwife of humanity and its various cultures and civilizations.
Recent glacial and interglacial periods
The most recent glacial phase
With glacial ice restricted to high latitudes and altitudes, Earth 125,000 years ago was in an interglacial period similar to the one occurring today. During the past 125,000 years, however, the Earth system went through an entire glacial-interglacial cycle, only the most recent of many taking place over the last million years. The absolute most recent period of cooling and glaciation began approximately 120,000 years ago. Significant ice sheets developed and persisted over much of Canada and northern Eurasia.
After the initial development of glacial conditions, the Earth system alternated between two modes, one of cold temperatures and growing glaciers additionally the other of relatively warm temperatures (although much cooler than today) and retreating glaciers. These Dansgaard-Oeschger (DO) cycles, recorded in both ice cores and marine sediments, occurred approximately every 1,500 years. a lower-frequency cycle, called the Bond cycle, is superimposed on the pattern of DO cycles; Bond cycles occurred every 3,000–8,000 years. Each Bond cycle is characterized by unusually cold conditions that take place during the cold phase of a DO cycle, the subsequent Heinrich event ( which really is a brief dry and cold phase), additionally the rapid warming phase that follows each Heinrich event. During each Heinrich event, massive fleets of icebergs were released into the North Atlantic, carrying rocks picked up by the glaciers far out to sea. Heinrich events are marked in marine sediments by conspicuous layers of iceberg-transported rock fragments.
Lots of the transitions in the DO and Bond cycles were rapid and abrupt, and they are being studied intensely by paleoclimatologists and Earth system scientists to understand the driving mechanisms of such dramatic climatic variations. These cycles now appear to result from interactions between the atmosphere, oceans, ice sheets, and continental rivers that influence thermohaline circulation (the pattern of ocean currents driven by differences in water density, salinity, and temperature, rather than wind). Thermohaline circulation, in turn, controls ocean heat transport, such as the Gulf Stream.
The Last Glacial Maximum
During the past 25,000 years, the Earth system has undergone a series of dramatic transitions. The most recent glacial period peaked 21,500 years ago during the Last Glacial Maximum, or LGM. At that time, the northern third of North America was covered by the Laurentide Ice Sheet, which extended as far south as Des Moines, Iowa; Cincinnati, Ohio; and New York City. The Cordilleran Ice Sheet covered much of western Canada as well as northern Washington, Idaho, and Montana in the United States. In Europe the Scandinavian Ice Sheet sat atop the British Isles, Scandinavia, northeastern Europe, and north-central Siberia. Montane glaciers were extensive in other regions, even at low latitudes in Africa and South America. Global sea level was 125 metres ( 410 feet) below modern levels, because of the long-term net transfer of water from the oceans to the ice sheets. Temperatures near Earth’s surface in unglaciated regions were about 5 °C (9 °F) cooler than today. Many Northern Hemisphere plant and animal species inhabited areas far south of their present ranges. For example, jack pine and white spruce trees grew in northwestern Georgia, 1,000 km (600 miles) south of their modern range limits in the Great Lakes region of North America.
The last deglaciation
The continental ice sheets began to melt back about 20,000 years ago. Drilling and dating of submerged fossil coral reefs provide a clear record of increasing sea levels due to the fact ice melted. The absolute most rapid melting began 15,000 years ago. For example, the southern boundary of the Laurentide Ice Sheet in North America was north associated with the Great Lakes and St. Lawrence regions by 10,000 years ago, and it had completely disappeared by 6,000 years ago.
The warming trend was punctuated by transient cooling events, most notably the Younger Dryas climate interval of 12,800–11,600 years ago. The climatic regimes that developed during the deglaciation period in many areas, including much of North America, have no modern analog (i.e., no regions exist with comparable seasonal regimes of temperature and moisture). For example, in the interior of North America, climates were a lot more continental (that is, characterized by warm summers and cold winters) than they are today. Also, paleontological studies indicate assemblages of plant, insect, and vertebrate species that do not occur anywhere today. Spruce trees grew with temperate hardwoods (ash, hornbeam, oak, and elm) in the upper Mississippi River and Ohio River regions. In Alaska, birch and poplar grew in woodlands, and there were very few of the spruce trees that dominate the present-day Alaskan landscape. Boreal and temperate mammals, whose geographic ranges are widely separated today, coexisted in central North America and Russia during this period of deglaciation. These unparalleled climatic conditions probably resulted from the combination of a unique orbital pattern that increased summer insolation and reduced winter insolation in the Northern Hemisphere together with continued presence of Northern Hemisphere ice sheets, which themselves altered atmospheric circulation patterns.