Causes Of Climate Change

Causes Of Climate Change

It is less difficult to document the evidence of climate variability and past climate change than it is to determine their underlying mechanisms. Climate is influenced by a multitude of factors that operate at timescales ranging from hours to vast sums of years. A number of the causes of climate change are external to the Earth system. Others are part of the Earth system but external to the atmosphere. Still others involve interactions between the atmosphere and other components of the Earth system and they are collectively described as feedbacks within the Earth system. Feedbacks are among the most recently discovered and challenging causal factors to study. Nevertheless, these factors are increasingly recognized as playing fundamental roles in climate variation. The essential important mechanisms are described in this section.

Solar variability

The luminosity, or brightness, of the Sun has been increasing steadily since its formation. This occurrence is important to Earth’s climate, due to the fact Sun provides the energy to drive atmospheric circulation and constitutes the input for Earth’s heat budget. Low solar luminosity during Precambrian time underlies the faint young Sun paradox, described in the section Climates of early Earth.

Radiative energy from the Sun is variable at very small timescales, owing to solar storms and other disturbances, but variations in solar activity, particularly the frequency of sunspots, are also documented at decadal to millennial timescales and probably occur at longer timescales as well. The ‘Maunder minimum,’ a period of drastically reduced sunspot activity between AD 1645 and 1715, has been suggested as a contributing factor to the Little Ice Age. (See below Climatic variation and change since the emergence of civilization.)

The Sun as imaged in extreme ultraviolet light by the Earth-orbiting Solar and Heliospheric Observatory (SOHO) satellite. A massive loop-shaped eruptive prominence is visible at the lower left. Nearly white areas are the hottest; deeper reds indicate cooler temperatures.NASA

Volcanic activity

Volcanic activity can influence climate in a range ways at different timescales. Individual volcanic eruptions can release large quantities of sulfur dioxide and other aerosols into the stratosphere, reducing atmospheric transparency and thus the amount of solar radiation reaching Earth’s surface and troposphere. a recent example is the 1991 eruption in the Philippines of Mount Pinatubo, which had measurable influences on atmospheric circulation and heat budgets. The 1815 eruption of Mount Tambora on the island of Sumbawa had more dramatic consequences, as the spring and summer of the following year (1816, known as ‘the year without any summer’) were unusually cold over much of society. New England and Europe experienced snowfalls and frosts throughout the summer of 1816.

Mount PinatuboA column of gas and ash rising from Mount Pinatubo in the Philippines on June 12, 1991, just days before the volcano’s climactic explosion on June 15.David H. Harlow/U.S.Geological Survey

Volcanoes and related phenomena, such as ocean rifting and subduction, release carbon dioxide into both the oceans plus the atmosphere. Emissions are low; even a massive volcanic eruption such as Mount Pinatubo releases only a fraction of the carbon dioxide emitted by fossil-fuel combustion in a year. At geologic timescales, however, release of this greenhouse gas can have important effects. Variations in carbon dioxide release by volcanoes and ocean rifts over millions of years can alter the chemistry of the atmosphere. Such changeability in carbon dioxide concentrations probably accounts for much of the climatic variation that has taken place during the Phanerozoic Eon. (See below Phanerozoic climates.)

Tectonic activity

continental driftThe changing Earth through geologic time, from the late Cambrian Period (c. 500 million years ago) to the projected period of ‘Pangea Proxima’ (c. 250 million years from now). The locations over time of the present-day continents are shown in the inset.Adapted from C.R. Scotese, The University of Texas at ArlingtonSee all videos for this article

Tectonic movements of Earth’s crust have had profound effects on climate at timescales of millions to tens of years. These movements have changed the shape, size, position, and elevation of the continental masses as well as the bathymetry of the oceans. Topographic and bathymetric changes in turn have had strong effects on the circulation of both the atmosphere plus the oceans. For example, the uplift of the Tibetan Plateau during the Cenozoic Era affected atmospheric circulation patterns, creating the South Asian monsoon and influencing climate over much of the rest of Asia and neighbouring regions.

Tectonic activity also influences atmospheric chemistry, particularly carbon dioxide concentrations. Carbon dioxide is emitted from volcanoes and vents in rift zones and subduction zones. Variations in the rate of spreading in rift zones plus the degree of volcanic activity near plate margins have influenced atmospheric carbon dioxide concentrations throughout Earth’s history. Even the chemical weathering of rock constitutes a important sink for environmental changes essay carbon dioxide. (A carbon sink is any process that removes carbon dioxide from the atmosphere by the chemical conversion of CO2 to organic or inorganic carbon compounds.) Carbonic acid, formed from carbon dioxide and water, is a reactant in dissolution of silicates and other minerals. Weathering rates are pertaining to the mass, elevation, and exposure of bedrock. Tectonic uplift can increase all those factors and thus lead to increased weathering and carbon dioxide absorption. For example, the chemical weathering of the rising Tibetan Plateau may have played a important role in depleting the atmosphere of carbon dioxide during a global cooling period in the late Cenozoic Era. (See below Cenozoic climates.)

Orbital (Milankovich) variations

The orbital geometry of Earth is affected in predictable ways by the gravitational influences of other planets in the solar system. Three primary attributes of Earth’s orbit are affected, each in a cyclic, or regularly recurring, manner. First, the shape of Earth’s orbit around the Sun, varies from nearly circular to elliptical (eccentric), with periodicities of 100,000 and 413,000 years. Second, the tilt of Earth’s axis with respect to the Sun, that will be primarily responsible for Earth’s seasonal climates, varies between 22.1° and 24.5° from the plane of Earth’s rotation around the Sun. This variation occurs on a cycle of 41,000 years. In general, the greater the tilt, the greater the solar radiation received by hemispheres in summer plus the less received in winter. The third cyclic change to Earth’s orbital geometry results from two combined phenomena: (1) Earth’s axis of rotation wobbles, changing the direction of the axis with respect to the Sun, and (2) the orientation of Earth’s orbital ellipse rotates slowly. These two processes create a 26,000-year cycle, called precession of the equinoxes, in which the position of Earth at the equinoxes and solstices changes. Today Earth is closest to the Sun (perihelion) near the December solstice, whereas 9,000 years ago perihelion occurred near the June solstice.

These orbital variations cause changes in the latitudinal and seasonal distribution of solar radiation, which in turn drive a number of climate variations. Orbital variations play major roles in pacing glacial-interglacial and monsoonal patterns. Their influences have been identified in climatic changes over much of the Phanerozoic. For example, cyclothems—which are interbedded marine, fluvial, and coal beds characteristic regarding the Pennsylvanian Subperiod (318.1 million to 299 million years ago)—appear to represent Milankovitch-driven changes in mean sea level.

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Greenhouse gases

greenhouse effectThe greenhouse effect is caused by the atmospheric accumulation of gases eg carbon dioxide and methane, which contain some of the heat emitted from Earth’s surface.Created and produced by QA International. © QA International, 2010. All rights reserved. www.qa-international.comSee all videos for this article

Greenhouse gases are gas molecules that have the property of absorbing infrared radiation (net heat energy) emitted from Earth’s surface and reradiating it returning to Earth’s surface, thus contributing to the occurrence known as the greenhouse effect. Carbon dioxide, methane, and water vapour are the most important greenhouse gases, and they have a profound effect on the energy budget of the Earth system despite making up only a fraction of all atmospheric gases. Concentrations of greenhouse gases have varied substantially during Earth’s history, and these variations have driven substantial climate changes at a wide range of timescales. In general, greenhouse gas concentrations have been particularly high during warm periods and low during cold phases. A number of processes influence greenhouse gas concentrations. Some, such as tectonic activities, operate at timescales of years, whereas others, such as vegetation, soil, wetland, and ocean sources and sinks, operate at timescales of hundreds to thousands of years. Human activities—especially fossil-fuel combustion since the Industrial Revolution—are responsible for steady increases in atmospheric concentrations of various greenhouse gases, especially carbon dioxide, methane, ozone, and chlorofluorocarbons (CFCs).

greenhouse effect on EarthThe greenhouse effect on Earth. Some incoming sunlight is reflected by Earth’s atmosphere and surface, but most is absorbed by the surface, that will be warmed. Infrared (IR) radiation is then emitted from the surface. Some IR radiation escapes to space, but some is absorbed by the atmosphere’s greenhouse gases (especially water vapour, carbon dioxide, and methane) and reradiated in all directions, some to space and some back toward the surface, where it further warms the surface plus the lower atmosphere.Encyclopædia Britannica, Inc.
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As was explained earlier on, the oceans can moderate the climate of certain regions. Not only do they affect such geographic variations, but…

Feedback

Perhaps the most intensively discussed and researched topic in climate variability is the role of interactions and feedbacks among the various components of the Earth system. The feedbacks involve different components that operate at different rates and timescales. Ice sheets, sea ice, terrestrial vegetation, ocean temperatures, weathering rates, ocean circulation, and greenhouse gas concentrations are all influenced either directly or indirectly by the atmosphere; however, they also all feed back into the atmosphere, thereby influencing it in important ways. For example, different forms and densities of vegetation on the land surface influence the albedo, or reflectivity, of Earth’s surface, thus affecting the overall radiation budget at local summary of as you like it in short to regional scales. At the same time, the transfer of water molecules from soil to the atmosphere is mediated by vegetation, both directly (from transpiration through plant stomata) and indirectly (from shading and temperature influences on direct evaporation from soil). This regulation of latent heat flux by vegetation can influence climate at local to global scales. As a result, changes in vegetation, which are partially controlled by climate, can in turn influence the climate system. Vegetation also influences greenhouse gas concentrations; living plants constitute an important sink for atmospheric carbon dioxide, whereas they act as sources of carbon dioxide when they are burned by wildfires or undergo decomposition. These and other feedbacks among the various components of the Earth system are critical for both understanding past climate changes and predicting future ones.

Mixed evergreen and hardwood forest on the slopes of the Adirondack Mountains near Keene Valley, New York.Jerome Wyckoff
Surface reflectance (albedo) of solar energy under different patterns of land use. (Left) In a preagricultural landscape, large forest-covered areas of low surface albedo alternate with large open areas of high albedo. (Right) In a agricultural landscape, a patchwork of smaller forested and open areas exists, each along with its characteristic albedo.Encyclopædia Britannica, Inc.

Human activities

Recognition of global climate change as an environmental issue has drawn attention to the climatic impact of human activities. Most of this attention has focused on carbon dioxide emission via fossil-fuel combustion and deforestation. Human activities also yield releases of other greenhouse gases, such as methane (from rice cultivation, livestock, landfills, and other sources) and chlorofluorocarbons (from industrial sources). Discover little doubt among climatologists that these greenhouse gases affect the radiation budget of Earth; the nature and magnitude of the climatic response are a definite subject of intense research activity. Paleoclimate records from tree rings, coral, and ice cores indicate a clear warming trend spanning the entire 20th century plus the first decade of the 21st century. In fact, the 20th century was the warmest of the past 10 centuries, and the decade 2001–10 was the warmest decade since the beginning of modern instrumental record keeping. Many climatologists have pointed to this warming pattern as clear evidence of human-induced climate change resulting from the production of greenhouse gases.

The global average surface temperature range for each year from 1861 to 2000 is shown by solid red bars, with the confidence range in the data for each year shown by thin whisker bars. The average change over time is shown by the solid curve.Encyclopædia Britannica, Inc.

A second type of human impact, the conversion of vegetation by deforestation, afforestation, and agriculture, is receiving mounting attention as a further source of climate change. It is becoming increasingly clear that personal impacts on vegetation cover can have local, regional, and even global effects on climate, due to changes in the sensible and latent heat flux to the atmosphere plus the distribution of energy within the climate system. The extent to which these factors contribute to recent and ongoing climate change is an important, promising area of study.

Tropical forests and deforestationTropical forests and deforestation in the early 21st century.Encyclopædia Britannica, Inc.

Climate Change Within A Human Life Span

Regardless of their locations on the planet, all humans experience climate variability and change within their lifetimes. The essential familiar and predictable phenomena are the seasonal cycles, to which people adjust their clothing, outdoor activities, thermostats, and agricultural practices. However, no two summers or winters are exactly alike in the same place; some are warmer, wetter, or stormier than others. This interannual variation in climate is partly responsible for year-to-year variations in fuel prices, crop yields, road maintenance budgets, and wildfire hazards. Single-year, precipitation-driven floods can cause severe economic damage, such as those of the upper Mississippi River drainage basin during the summer of 1993, and loss of life, such as those that devastated much of Bangladesh in the summer of 1998. Similar damage and loss of life can also occur as the result of wildfires, severe storms, hurricanes, heat waves, and other climate-related events.

Climate variation and change may also occur over longer periods, such as decades. Some locations experience multiple years of drought, floods, or other harsh conditions. Such decadal variation of climate poses challenges to human activities and planning. For example, multiyear droughts can disrupt water supplies, induce crop failures, and cause economic and social dislocation, as in the case of the Dust Bowl droughts in the midcontinent of North America during the 1930s. Multiyear droughts may even cause widespread starvation, as in the Sahel drought that occurred in northern Africa during the 1970s and ’80s.

Abandoned farmstead showing the effects of wind erosion in the Dust Bowl, Texas county, Okla., 1937.USDA Photo

Seasonal variation

Every place on Earth experiences seasonal variation in climate ( though the shift are slight in some tropical regions). This cyclic variation is driven by seasonal changes in the supply of solar radiation to Earth’s atmosphere and surface. Earth’s orbit around the Sun is elliptical; it is closer to the Sun ( 147 million km [about 91 million miles]) near the winter season solstice and farther from the Sun (152 million km [about 94 million miles]) near the summer solstice in the Northern Hemisphere. Furthermore, Earth’s axis of rotation occurs at an oblique angle (23.5°) with respect to its orbit. Thus, each hemisphere is tilted away from the Sun during its winter period and toward the Sun in its summer period. When a hemisphere is tilted away from the Sun, it receives less solar radiation than the opposite hemisphere, which at that time is pointed toward the Sun. Thus, despite the closer proximity of the Sun at the winter solstice, the Northern Hemisphere receives less solar radiation during the winter than it does during the summer. Also as a consequence of the tilt, when the Northern Hemisphere experiences winter, the Southern Hemisphere experiences summer.

A diagram shows the position of Earth at the beginning of each season in the Northern Hemisphere.Encyclopædia Britannica, Inc.

Earth’s climate system is driven by solar radiation; seasonal differences in climate ultimately result from the seasonal changes in Earth’s orbit. The circulation of air in the atmosphere and water in the oceans responds to seasonal variations of available energy from the Sun. Specific seasonal changes in climate occurring at any given location on Earth’s surface largely result from the transfer of energy from atmospheric and oceanic circulation. Differences in surface heating taking place between summer and winter cause storm tracks and pressure centres to shift position and strength. These heating differences also drive seasonal changes in cloudiness, precipitation, and wind.

Seasonal responses of the biosphere (especially vegetation) and cryosphere (glaciers, sea ice, snowfields) also feed into atmospheric circulation and climate. Leaf fall by deciduous trees as they go into winter dormancy increases the albedo (reflectivity) of Earth’s surface and might induce greater local and regional cooling. Similarly, snow accumulation also increases the albedo of land surfaces and often amplifies winter’s effects.