Encyclopedia of Global Change
Climate
Encyclopedia of Global Change
Environmental Change and Human Society
2 volumes
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Climate is typically defined to be "average weather" and is often described in terms of mean conditions and their levels of variability over certain preset time periods. It involves processes within the atmosphere, oceans, and cryosphere, caused both by interactions within the climate system and by external factors. These processes act to redistribute energy provided by radiation from the Sun, or insolation. The climate system may be considered as consisting of external and internal components. External components include the Sun and its solar output, the rotation of the Earth, and the geometric arrangement of the Earth in relation to the Sun. Internal components of the system include the atmosphere (its mass and composition), oceans, land and sea ice, and the physical properties of the land surface (including reflectivity or albedo, extent of snow cover, hydrological regimes, vegetation, and biomass). Over short time scales (less than millions of years), the physical components of the Earth system–such as the distribution of land and oceans, continental relief, and the configuration of ocean basins–may be considered as fixed, but over longer time scales these too may be considered as internal variables within the climate system.
Climatic and Environmental ArchivesAlthough climate change has become a byword of our time, past instrumental observations of climatic variables, such as temperature and precipitation, have demonstrated that variations in climate are the norm. This pattern of variation is confirmed by a variety of proxy data sources derived from geologic records spanning considerably longer periods of time. Land-based data sources include ecological records from peat bogs, growth patterns of trees with seasonal rings (dendroclimatological data), the orientation of desert dunes, the growth and geochemistry of cave speleothems (calcareous, crystalline deposits, including stalactites and stalagmites), changes in fluvial (river) regimes, the extensive loess deposits that cover vast areas of continental Europe, Asia, and the Americas, and deposits related to high-altitude mountain glaciers. Glacial varves, annual layers that are found in proglacial lakes and result from the summer melting of glaciers, are another significant source of data. The sediments preserved in deep ocean basins have proved to be an extremely significant source of climate change data. In this case the data relate both to the marine organisms (principally radiolarians, foraminifera, and other microorganisms) and the inorganic sedimentary detritus supplied from continental areas as river sediment, eolian (wind-borne) dust, and ice-rafted detritus transported from high latitudes by icebergs. Over the past two decades the records of snow chemistry, quantity, and aerosol components (such as dust, salt, and volcanic ash), and greenhouse gases as recorded in ice cores from the high-latitude and high-altitude cryosphere have provided some of the most detailed records of the past few hundred thousand years of climate change. While such proxy records rarely provide a direct index of climate-related parameters, they are frequently capable of generating qualitative or quantitative indices of climatic characteristics. These records have indicated that the climate changes that have occurred have generally been global in extent and have occurred globally synchronously on time scales ranging from decades to millions of years (Figure 1).
Earth History and Climate Change Geologists have subdivided the 4.5 billion years of Earth’s history into a series of eons, eras, and periods, which were first characterized broadly in terms of evolutionary changes in plants and animals (Figure 1). Many of these changes were influenced by changing global environments, and the periods may also be used to describe or classify palaeoenvironmental conditions. The most recent era is called the Cainozoic or Cenozoic. It is subdivided into two periods, the Tertiary and the Quaternary, and it comprises a 60-million-year period of generally cooling global mean temperatures, culminating in the ice age that characterizes the Quaternary. The periods are in turn divided into various series or epochs. The Pleistocene and Holocene series together constitute the Quaternary, which is sometimes simply termed the late Cenozoic. The Quaternary Ice Age Until recently, the date accepted for the onset of the Quaternary was fixed, based on the arrival of cold water marine organisms such as Arctica islandica and Hyalinea balthica to the sediments of the Mediterranean sea; their appearance heralded the onset of the cold conditions that characterize the Quaternary. This occurs stratigraphically close to a reversal in the Earth’s magnetic field, termed the Olduvian geomagnetic reversal, which has been dated in volcanic rocks to around 1.8 million years ago. Research over the past few decades on deep ocean sediments indicates that the true date for the onset of cold conditions and extensive northern hemisphere glaciation that characterize the Quaternary lies somewhat closer to 2.5 million years ago. The commencement of the relatively stable and warm interglacial Holocene epoch in which we presently live has been fixed arbitrarily at 10,000 years before present. The Quaternary ice age is by no means the only such cold period of Earth history, and at least five other extensive ice ages have been described. Such major phases of ice age activity appear to have been separated by periods of around 300–400 million years of nonglacial activity. The ice ages are thought to be controlled by numerous factors, the most important being the orientation, position, and fragmentation of continental plates, the intensity of mountain building, and ocean basin formation. Warm periods are thought to be associated with the formation of large supercontinental plates with ocean circulation taking place zonally, and confined principally to equatorial latitudes. This results in higher average global temperatures and small equator-to-polar temperature gradients. Ice ages are associated with periods of widespread mountain building and continental fragmentation, which result in the isolation of continental masses at high latitudes (thereby facilitating their refrigeration), meridionally oriented oceanic circulation patterns, increases in weathering rates, and large equator-to-polar temperature gradients. Given that colder ocean water temperatures facilitate the drawdown of atmospheric gases into the oceans, the ice age periods are also associated with reduced atmospheric greenhouse gas concentrations. Greenhouse gases increase planetary temperatures by absorbing outgoing longwave radiation, and their reduction during ice age periods causes a strong positive feedback which accentuates the global cooling. Oceanic drawdown of the greenhouse gas carbon dioxide is further increased by the higher rates of delivery of weathered continental material during mountain-building events, which causes further assimilation of atmospheric gases. Conversely, periods between the ice ages are associated with high levels of atmospheric greenhouse gases. This distinction has led some workers to consider the Earth’s history as representing a series of shifts between "icehouse" and "greenhouse" climatic megacycles. For the overwhelming majority of the Earth’s history, its climate has been in a greenhouse state. While the Quaternary ice age commenced 2.5 million years ago, a series of events over the preceding 100 million years caused substantial global climate changes that were critical preconditions for the ice age to develop. Although the overall trend observed is that of a global cooling, the changes frequently occurred as a step function in response to specific events. At around 100 million years ago, Australia separated from Antarctica and allowed the initiation of sea floor spreading, which would eventually lead to the formation of the South Indian Ocean. At much the same time, other plate tectonic reorganizations started the northward migration of India towards the Asian landmass. While these were significant events that were critical to subsequent climate changes, substantial global cooling did not occur until around 50 million years ago, in the early Cenozoic, when the flow of equatorial warm currents was permanently interrupted by the closure of the Tethys Sea as the African plate collided with Europe. [See Plate Tectonics.] During approximately the same period the Atlantic Ocean was undergoing a series of reorganizations that accentuated ocean circulation and allowed meridional flow from the equator to high northern and southern latitudes. After about 30 million years ago, Antarctica was fully isolated from both Australia and South America and the resulting development of the cold Circum-Antarctic Bottom Water current cooled deep or abyssal ocean waters and the surface waters of the southern latitudes. At this time Antarctica contained only limited land and sea ice, and pollen evidence indicates that abundant forests persisted until around 15 million years ago. When India collided with Asia around 8 million years ago and formed the uplifted Tibetan Plateau and Himalayan Ranges, a further series of important global climate changes occurred. This event caused the deflection of midlatitude Northern Hemisphere wind systems, particularly the trajectories of midlatitude jet streams; allowed the development of the Asian monsoonal circulation system; and accentuated continental-scale chemical weathering and denudation, which further reduced global atmospheric carbon dioxide levels. The formation of the Tibetan Plateau was accompanied by mountain building along the Pacific margin of South America, in western North America, and in the rift system of East Africa, which also contributed to the global cooling trend. The closure of the Isthmus of Panama around three million years ago caused significant reorganizations of North Atlantic ocean circulation, and the intensification of the Gulf Stream resulted in enhanced levels of atmospheric moisture export to the northern high latitudes. This led, around 2.5 million years ago, to the formation of continental ice sheets in the Northern Hemisphere and the commencement of the ice age proper. Interglacial Phases Over the past few decades it has been recognized that major climate changes were numerous during the Quaternary ice age, and involved dramatic shifts between colder glacial conditions and warmer interglacial conditions. The glacial phases are typically long lasting (many tens of thousands of years) in comparison with the interglacial phases, which typically last for roughly ten thousand years. Generally, cold glacial conditions in which polar and alpine glaciers have undergone dramatic expansion and covered large areas in North America (the Laurentide Ice Sheet) and Eurasia (the Fennoscandinavian Ice Sheet) have predominated. During glacial periods, midlatitude temperatures were depressed by around 7°–10°C and the accumulation of snow and ice at high latitudes resulted in the global lowering of sea levels by up to 120 meters. The intervening interglacials are defined as nonglacial climatic phases, and were sufficiently stable and benign to allow the development of deciduous forests similar to those that occurred in northwestern Europe. Another type of interruption to full glacial conditions was the interstadial, a period that was either too cold or too protracted to allow the development of temperate deciduous forest of the full interglacial type. (The term stadial refers to an ice advance.)The concept of multiple glacials, interglacials, stadials, and interstadials during the ice age was proposed in the nineteenth century and further confirmed in the first years of the twentieth century by Penck and Brückner (1909), working in the European Alps. They developed much of the terminology and interpretation of sequences used to this day, and their work is one of the great landmarks in the study of environmental change. They recognized four major glacial advances (Wurm, Riss, Mindel, and Gunz). Similar glacial–interglacial cycles were recognized and correlated elsewhere, as subsequently were stadial and interstadial events of shorter duration and intensity. In North America, the acknowledged glacial periods were also four in number (Nebraskan, Kansan, Illinoian, and Wisconsin) and it was natural to hypothesize contemporaneity and a common cause, namely, variation in the amount and distribution of insolation.
Stratigraphic Evidence The main basis for stratigraphic correlation and interpretation of Quaternary environmental records, whether on land or offshore, is via the numbered series of more than 20 glacial–interglacial stages defined principally on the basis of oxygen isotopic evidence from deep-sea cores. There are three stable isotopes of oxygen, of which oxygen-16 is the most abundant; oxygen-17 and oxygen-18 are much rarer. Of the two minority species, oxygen-18 is the more abundant, but even so its ratio with respect to oxygen-16 is only in the range 0.0019–0.0021 in natural materials. Although the isotopes are similar in chemical behavior, there are some processes that discriminate against the heavier isotopes and some that give preference to them, to a degree dependent on temperature. As a net result of such fractionation processes, the ratio of oxygen-18 to oxygen-16 in glaciers is slightly lower than in sea water (that is, the oxygen in the water of glaciers is isotopically lighter). During glacial times, because of the greater amount of water locked up in glaciers, sea water is isotopically heavier. Shells formed in this water are heavier still because there is further fractionation during the formation of shell carbonate, the lower temperature favoring incorporation of oxygen-18. A record of past oxygen and carbon isotope ratio values is available from calcareous skeletons (commonly called shells) of marine organisms (usually foraminifera) in deep-sea sediments. Samples are obtained by means of long coring tubes, of the order of 10 centimeters in diameter and up to 50 meters in length. The continuous sediment cores so obtained carry, in addition, a magnetic polarity record that allows correlation of the climatic variations with the magnetic polarity time scale. [See Dating Methods.] One of the first records of marine oxygen isotopes was obtained by Cesare Emiliani during the 1950s. This showed variations initially interpreted as reflecting primarily the temperature of the water in which the shells had been formed. However, it was later argued that the influence of glacier volume was dominant and that the isotope variations could be considered as a palaeoglaciation record. This core showed evidence of 13 warm and cold phases. These were numbered from the top down, with odd numbers corresponding to warm stages and limited global ice volumes and even numbers to cold stages with greater global ice volumes (Figure 1). They are now referred to as marine isotope stages (MIS). The stage numbers allocated by Emiliani continue to be used but have been developed so as to include substages. Commonly, these are designated by letters (for example, the warm substages of MIS 5 are named 5a, 5c, and 5e, and the intervening cool troughs are named 5b and 5d). A decimal system has also been developed for the naming of additional layers or horizons within the marine isotope stages in which subtle isotopic variations occur so as to give greater flexibility in dealing with the complexities of the isotope curve. In addition, boundaries between pronounced isotopic maxima (full glacials) and consecutive pronounced minima (peak interglacials) are called terminations. These are numbered by roman numerals in order of increasing age. The most recent period of full glacial conditions was at a maximum around 21,000 bp. Because of the slow sedimentation rate on the ocean floor, and sometimes because of bioturbation (the mixing of sediments on the sea floor by the actions of animals and plants), there is a tendency for any short-term changes to be smoothed out, so that, in most cores, changes persisting for less than a few thousand years are unlikely to be seen and the record is one of long-term changes. Astronomical Theory of Climate Change The astronomical theory of climate change is by far the most widely accepted theory proposed so far to explain the numerous large-scale glacial–interglacial variations that occurred during the Quaternary period. The theory, also known as the orbital theory or the Croll—Milankovitch theory of climate change, has only in the past two decades gained the widespread support of the scientific community. The theory was first systematically proposed by Meutin Milankovitch, who argued that if the Earth’s orbit around the Sun exhibits any degree of cyclical variability, then the subtle variations in incoming solar energy might be responsible for shifts from glacial to interglacial climatic states. Changes in orbital geometries occur as a result of the gravitational effects of the planetary bodies. There are three major variations in the Earth–Sun orbital configuration, with five primary periodicities that are considered to be significant (Figure 2). The Earth’s orbit around the Sun undergoes changes in its eccentricity from a near-circular to slightly elliptical at periodicities of about 100,000 and 400,000 years. The second major factor is the tilt or obliquity of the Earth’s axis relative to the plane in which the bodies of the solar system lie (the ecliptic), which varies between 22° and 25°, with a periodicity of about 41,000 years. The effect of the obliquity variations is to amplify the seasonal cycles at high latitudes; the variations have a relatively minor direct effect at lower latitudes. The third key variation, known as the precession of the equinoxes, actually consists of two components that change the distance between the Earth and the Sun at any given season. The axial precessional component relates to the wobble of the Earth’s axis of rotation and has a periodicity of 26,000 years. The elliptical precessional component has a 22,000-year periodicity and relates to the rotation of the Earth around one of the foci of the orbit. The combination of these two effects results in a shift of the equinoxes through the Earth’s elliptical orbit, providing scope for anomalous seasonal patterns that are opposite in direction in the opposing hemisphere. The precessional effects are most pronounced at low latitudes and exhibit periodicities of 19,000 and 23,000 years. It is only the eccentricity cycle that actually changes the total incoming insolation budget; the other cycles simply serve to redistribute the energy in different spatial and seasonal configurations. Milankovitch hypothesized that the onset of glacial conditions required a minimum in northern hemisphere summer insolation, which in turn would require a maximum of eccentricity and a minimum in obliquity, reducing seasonal contrasts and increasing latitudinal energy gradients. Given these conditions, Milankovitch inferred that the summers would remain sufficiently cool to prevent the thaw of winter snow and ice, and the relative mildness of the winter would allow substantial evaporation at intertropical latitudes, which would lead to abundant snowfall at middle and high latitudes. If snow and ice were preserved throughout the year, the land surface albedo would increase, resulting in a positive feedback process ultimately leading to the formation of persistent ice sheets. An additional important positive feedback in this process is the associated cooling of middle- and high-latitude ocean waters, significantly reducing atmospheric greenhouse gas concentrations and therefore reducing the absorption of outgoing longwave (heat) radiation and leading to further cooling. Spectral analysis of the oxygen isotopic variations in deep-sea sediments demonstrates that the main periodicities of variations in global climate changes closely match those predicted by the astronomical theory (Figure 3). In the absence of internal factors, the astronomical theory would predict simultaneous glacial and interglacial conditions for the northern and southern hemispheres. In fact, climate changes through glacials and interglacials are globally synchronous. Patterns of insolation changes at northern latitudes are most significant because it is here that much of the global surface is covered by land rather than ocean, and it is the land surface that is capable of accumulating snow and ice and dramatically changing the surface albedo. The climatic state as influenced by the northern insolation patterns is propagated throughout the globe principally via oceanic thermohaline currents, which transfer energy vertically and laterally in response to gradients of heat and salinity. Although the Milankovitch theory accounts for the periodicities observed in the deep-sea core records, it does not fully explain two important aspects of the record. First, there is a shift at around one million years ago from climatic variability dominated by precessional and obliquity-based (19–23,000-year and 41,000-year) cycles to a pattern dominated by variations occurring at periodicities corresponding to the eccentricity cycle. Secondly, the deep-sea record indicates a relatively progressive increase in the amplitude of climatic changes through the Quaternary period. Short-Term Climate Variations A continuous high-resolution multiproxy climatic record is available in long ice cores, some more than 3 kilometers in length, collected from polar and high-altitude locations. Oxygen isotopic variations, in this case as measured on snow and ice, are again useful for reconstructing past changes. The stable oxygen isotope ratio reflects the ambient temperature at formation, with higher oxygen-18/oxygen-16 values indicating higher temperatures. The long-term pattern of variation is similar to the marine pattern, but there is the possibility of seeing shorter-term (stadial and interstadial) fluctuations since the accretion rate is much greater and there is little opportunity for bioturbation or other disturbances to occur. In the upper part of cores, annual layers are discernible because of seasonal variation of dust and acidity. Further down, estimates have to be made on the basis of models of glacier flow and ice movement or of past accretion rates. The records collected from both Greenland and the Antarctic indicate rapid fluctuations of climate which must be related to changes in the polar atmospheres. These interstadials are referred to as Dansgaard–Oeschger events and are given a numbering system with the prefix IS. Their duration was only of the order of a thousand years, and the onset and termination sometimes occurred in a matter of decades. The pattern of cooling and warming during an interstadial–glacial cycle is not symmetrical; instead, cooling occurs gradually and the cycle is completed by rapid warming (Figure 4). A total of twenty-four interstadial intervals have been recognized, lasting from five hundred to two thousand years. The steadiness of climate indicated for the present interglacial—the Holocene, from about 10,000 bp to the present, is in marked contrast to the variability of climate indicated for the last glacial and earlier periods. The rapid fluctuations during the last glacial period recorded in the arctic ice cores are matched by global variations in mid- and high-latitude sea surface temperature and hence in air temperature. These fluctuations were initially deduced from the abundance of a temperature-sensitive planktonic (surface-dwelling) foraminifera (Neoglobigerina pachyderma) in two cores from the North Atlantic at 50°–55° north latitude, and are now recognized within other ocean basins including the Southern Hemisphere via isotopic and other sediment parameters. The fluctuations are grouped into Bond cycles (Figure 5). In each cycle, there is a gradual decrease in amplitude of the variations as well as a decrease in the temperature of the base level. Near the end of each cycle (that is, at the coldest part), the nature of the ocean-floor detritus indicates the occurrence of a Heinrich event, ascribed to a massive discharge of icebergs into the North Atlantic. Two leading theories have emerged to explain the observed sub-Milankovitch climatic excursions: one proposes catastrophic collapse of large Northern Hemisphere ice sheets caused by internal feedbacks and basal instability (termed binge–purge cycles), while the other argues that the global synchrony of the rapid shifts requires an atmospheric mechanism, probably relating to a water vapor feedback process. As well as recording isotopic variations, polar ice cores also provide a record of greenhouse gas variations. Analysis of the gas records in combination with estimates of past temperatures derived from the analysis of stable isotopes of hydrogen reveals a close correspondence between global temperature changes and greenhouse gas concentrations (Figure 6). Carbon dioxide exhibits variations that closely match the overall record of insolation variations, while methane abundance exhibits pronounced variations at precessional (19–23,000-year) time scales. Low-latitude environments did not experience cold conditions during glacial phases at higher latitudes. Since the climatic transitions at these latitudes are related more to changes in moisture balance than temperature, there has been a tendency to consider low-latitude climates to shift from pluvial (wet) to interpluvial (dry) conditions. The main sources of paleoclimatic evidence in these settings are records of past eolian (wind-blown) activity, lake level data, and analyses of terrestrial dust from marine cores. It is widely recognized that the period of the last glacial was characterized on land by cold, dry, and windy conditions, and that the transition into the Holocene period was accompanied by widespread increases in moisture, at least for the low-latitude regions of the Northern Hemisphere. Glacial-age interpluvial conditions relate to reduced evaporation from oceans, increased continentality caused by lower global sea levels (i.e., the transport distance required to move moist air to inland continental areas), and increased evaporation rates due to enhanced trade wind circulation patterns which are, in turn, related to increased equator-to-polar temperature gradients. Pluvial conditions such as those that occurred at the start of the Holocene period result in reduced eolian activity, development or expansion of lakes, and widespread reductions in surface albedo. A significant exception to this general pattern is Lake Bonneville in the southwestern United States, which was full during the last glacial period because of the southward deflection of the westerly midlatitude jet stream by the Laurentide Ice Sheet. One of the most contentious contemporary debates relates to the degree of cooling of tropical sea surface temperatures (SSTs) during glacial periods and its impacts on tropical and extratropical climate. The long-standing view that tropical SSTs have varied little has recently been challenge by a variety of data that collectively indicate that the tropical oceans cooled by about 4°–6°C. Influences on moisture balance in these low-latitude areas include the direct effect of changing monsoon intensity caused by variations in the precessional orbital cycle, and the effects of changing rates of evaporation in adjacent tropical oceans, in part related to sea surface temperatures, which are linked to higher-latitude climates and are more closely related to the eccentricity and obliquity cycles (about 100,000 and 41,000 years, respectively). It has also been noted that dramatic changes in low-latitude moisture may occur over shorter (sub-Milankovitch) time scales. The Holocene Climate changes leading to the present (Holocene) interglacial commenced around 13,000 bp with a series of rapid fluctuations in climate that culminated in the cold Younger Dryas interstadial. Recovery from the Younger Dryas has been estimated to represent a global warming of the order of 5°–8°C in a period as short as thirty years. Since that time, climate changes have been relatively subtle and related to factors other than the external climatic forcing caused by orbital changes. A range of data sources point to successive variations between warmer and colder periods causing minor retreats and advances of alpine glaciers. The warmest phase of the Holocene period recorded in ice cores was between 10,000 and 8,000 bp, although the traditionally recognized Holocene climatic optimum, termed the altithermal or hypsithermal, is usually described as occurring between 7,000 and 4,000 bp. Where this optimum of Holocene climate is recognized, it is thought to represent a warming of the order of 1°–2°C above the modern (preindustrial) levels. Post-optimal global cooling resulted in a neoglacial period of glacier readvance, but in early medieval times there was a return to more favorable conditions known as the Little Climatic Optimum or the Medieval Warm Period. This phase lasted from around 750 to 1300 and correlates with the climax of high medieval cultural development and energetic activity. Warmer summer temperatures at this time allowed the development of vineyards in the United Kingdom as far north as York. [See Medieval Climatic Optimum; and Younger Dryas.] The best known of the climate fluctuations of the Holocene is that of the Little Ice Age. Its occurrence is well documented by a range of archival materials (including annals, chronicles, and ship’s records) and dendrochronological records. It corresponds to a period of cold and highly variable climate and glacial readvance following the Medieval Warm Period, lasting from around 1300 to 1800. The Little Ice Age had widespread consequences for human populations, including high incidences of crop failure (particularly wheat in the United Kingdom), difficult navigation at higher latitudes, abandonment of settlement in marginal areas (including the final decline of the Anasazi culture in the southwestern United States and abandonment of areas of the Scottish uplands), and extreme winter storms. Causes of Short-Term Climate Change Variations in the climate during Holocene times have been linked with a number of possible causes. The sunspot cycle, which occurs with a 22-year periodicity, has frequently been cited as a potentially significant cause of Holocene and older climate changes. The overall amplitude of the cycles seems to increase slowly and then fall rapidly, with a period of 80–100 years. There also appear to be quasicyclic fluctuations on timescales ranging from 180 to 2,200 years. While correlations between solar activity and instrumental and proxy (tree ring and ice core) data have been described, no mechanistic link between sunspot activity and surface conditions on the Earth has so far been demonstrated. The Little Ice Age has been linked by many workers to the Maunder minimum in sunspot activity. Collisions of comets with the Earth and very large meteoritic impacts have also been proposed as causes of climatic fluctuations, the best documented being the impact event at the end of the Mesozoic period (about 63 million years ago), which may have resulted in the extinction of the dinosaurs. Many of the disturbances that such impacts would cause, such as an increase in stratospheric and tropospheric aerosols, are similar to disturbances internal to the system. Volcanoes influence climate by projecting large quantities of particulates and gases into the atmosphere. The effect of injected aerosol upon the radiation balance, and whether heating or cooling results, depends largely on the height of injection into the atmosphere. Most eruptions inject particulates into the troposphere at heights between five and eight kilometers. These tend to be removed rapidly either by fallout or by rain, and the effect on climate is minimal. More violent eruptions inject debris into the upper troposphere or even into the lower stratosphere (15–25 kilometers). The particulates have a long residence time in the stratosphere (up to years). Nonabsorbing aerosols increase the albedo of the atmosphere and reduce the amount of solar radiation that reaches the surface. Aerosols that absorb in the visible part of the spectrum result in energy transfer directly to the atmosphere. If the aerosol absorbs and emits in the infrared, the greenhouse effect is increased. Mount Pinatubo injected around 20 million metric tons of sulfur dioxide to heights of 25 kilometers, where it was photochemically transformed into sulfate aerosols. The aerosols generated by the eruption of Mount Pinatubo have been estimated to have resulted in a forcing on the climate system of about - 0.4 watts per square meter, with a resultant temporary global cooling of about 0.5°C. Eruptions such as Mount Pinatubo and their effects on the atmosphere are very short-lived compared with the time required to influence the heat storage of the oceans, and they are not likely to initiate significant long-term climatic changes. [See Mount Pinatubo.] The natural variability of the ocean circulation is an important factor for climate. The ocean circulation varies on glacial time scales, when the circulation is known to change markedly, and on interannual time scales, where the El Niño Southern–Oscillation (ENSO) phenomenon is important. [See El Niño–Southern Oscillation.] Climate Change in the Twentieth Century Climate change has also occurred during the twentieth century. It has been estimated that global (land and sea) surface temperatures have warmed by between 0.3° and 0.6°C, and by between 0.2° and 0.3°C over the last 40 years (Figure 7). These temperature levels are close to those of the Northern Hemisphere during the Medieval Warm Period. The observed climate changes are not uniformly spaced, with some areas exhibiting cooling, and the greatest extent of warming occurring over the continents between 40 and 70° north latitude. The warming is thought to be at least partly the result of human activity. Because populations, national economies, and the use of technology are all growing, the global average temperature is expected to continue increasing, by an additional 1.0°–3.5°C by the year 2100. In addition to the warming trend, there have also been changes in precipitation and other climatic factors. Precipitation has increased globally by around one percent over the past century. The increased frequency and intensity of ENSO events in the past two decades is also thought to relate to climatic changes relating to tropical ocean–atmosphere interactions. There are a number of human activities that are believed to be capable of inducing changes in global climate. The most substantial changes have occurred since the industrial revolution and relate to the burning of fossil fuels and increased industrialization and pollution. The Greenhouse Effect Increases in atmospheric greenhouse gas concentrations over the past few decades were first detected at the Mauna Loa Observatory in Hawaii. The trend has been confirmed by analyses of gas bubbles preserved in polar ice cores. Levels of carbon dioxide (CO2) have increased in the past century by more than 25 percent since the beginning of the industrial revolution. In addition to CO2, the levels of methane (CH4), nitrous oxides (NOx) and halocarbons (CFCs and HCFCs) have also increased. These gases have differing atmospheric concentrations, residence times, and potentials to induce greenhouse warming (Table 1). The relative radiative effects of the various greenhouse gases are measured by their global warming potential (GWP). This is defined as the cumulative radiative forcing caused by a unit mass of gas emitted, relative to a reference gas (usually CO2). Table 1 demonstrates that while CO2 is by far the most abundant of the anthropogenically influenced greenhouse gases, its efficiency as a greenhouse gas is considerably lower than many of the other atmospheric constituents, in some cases by many orders of magnitude. Increased levels of greenhouse gases will result in a warming of global climate. However, the magnitude of the warming, the time that will be required for the warming trend to reach an equilibrium, and the relative impacts on different regions of the world will depend on the nature of the feedbacks within the climate system. [See Greenhouse Effect.] Aerosols The influence of tropospheric aerosols associated with industrial pollution and fossil fuel and biomass burning has only recently been identified and, to some extent, quantified. Solid sulfate particles result from the oxidation of sulfur dioxide, emitted when fossil fuels are burned. Other industrial processes and natural and human-initiated biomass burning also contribute particulates, often termed aerosols, to the troposphere. These aerosols are localized, being confined mainly to Northern Hemisphere midlatitudes, and have two effects on the climate system. The direct effect of most aerosols is to reflect incoming solar radiation back into space and hence cause cooling. Some particulates, such as soot, are dark in color and have the opposite effect, causing warming. The magnitude of the cooling or warming depends on the nature of the aerosols and their distribution in the atmosphere. There is also an important indirect effect of tropospheric aerosols. They act as additional cloud condensation nuclei and cause more (and smaller) drops to form in clouds, increasing the reflectivity of the clouds and further cooling the planet. The effect of changes in cloud character can have complex repercussions, since the clouds also affect the amount of radiation that escapes from the Earth system. [See Aerosols.] Ozone Depletion The discovery of the Antarctic ozone hole in 1986 and, more recently, a similar but less intense ozone depletion over the Arctic has been a cause of much recent discussion relating to climate changes. The observed ozone destruction now appears to be due to the disturbance of the natural balance of destruction and production that previously existed in the stratosphere. The presence of free chlorine atoms in the stratosphere can now be traced to the photochemical disruption caused by halocarbons when these gases migrate from the troposphere. Chlorine is the principal cause of the disturbance in ozone chemistry that produces the ozone hole. Although the buildup of CFCs, at least, in the atmosphere is leveling off as a result of the Vienna Convention and the Montreal Protocol, the very long lifetimes of many of these gases mean that they will persist in the atmosphere for perhaps thousands of years. The particular reactions that act to accelerate the ozone destruction rely on the presence of free chlorine atoms and a solid surface, provided by stratospheric ice clouds. Since CFCs, HCFCs, and the hydrofluorocarbons (HFCs) that are replacing them are radiatively active (they are much more effective greenhouse gases than CO2), they also act to change the atmospheric temperature, and this alters the rate of the chemical reactions. CFCs that remain in the troposphere are effective absorbers of infrared radiation, which would otherwise escape to space. These gases therefore act to enhance the atmospheric greenhouse and to provide a warming influence for the planet. The radiative effect of the reduced stratospheric ozone is to cool the planet. The enhanced levels of tropospheric ozone that have been observed result in a warming. [See Ozone.] Land Surface Changes Regional changes to the character of the Earth’s surface caused by human activites may also cause regional and global changes in climate. These include desertification, changes in levels of forestation, and urbanization. Removal of vegetation and exposure of bare soil during the process of desertification decreases soil water storage because of increased runoff and increased albedo. Less moisture available at the surface means decreased latent heat flux, leading to an increase in surface temperature. On the other hand, the increased albedo produces a net radiative loss. At present, around 30 percent of the land surface of the Earth is forested and about a third as much again is cultivated. However, the amount of forest land, particularly in the tropics, is rapidly being reduced, while reforesting is prevalent in middle latitudes. As a consequence, the surface characteristics of large areas are being greatly modified. The change in surface character can be especially noticeable when forests are replaced by cropland. The important climatic change after deforestation is in the surface hydrological characteristics, since the evapotranspiration from a forested area can be many times greater than that from adjacent open ground. The largest impacts are the local and regional effects on the climate, which could exacerbate the effects of soil impoverishment and reduced biodiversity accompanying the deforestation. [See also Deforestation; Desertification; Earth History; Ecosystems; Glaciation; Global Warming; Sea Level; and the biographies of Emiliani, Lamb, and Milankovitch.] Bibliography Aitken M. J., and S. Stokes. "Climatostratigraphy." In Science-Based Dating in Archaeology, edited by M. J. Aitken and R. A. Taylor. New York: Plenum Press, 1998.
