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Climate change is any long-term significant change in the expected patterns of average weather of a specific region (or, more relevantly to contemporary socio-political concerns, of the Earth as a whole) over an appropriately significant period of time. Climate change reflects abnormal variations to the expected climate within the Earth's atmosphere and subsequent effects on other parts of the Earth, such as in the ice caps over durations ranging from decades to millions of years.

In recent usage, especially in the context of environmental policy, climate change usually refers to changes in modern climate (see global warming). For information on temperature measurements over various periods, and the data sources available, see temperature record. For attribution of climate change over the past century, see attribution of recent climate change.

Contents 1 Climate change factors 1.1 Variations within the Earth's climate 1.1.1 Glaciation 1.1.2 Ocean variability 1.1.3 Hysteresis 1.1.4 Effects of CO2 on climate change 1.2 Other factors driving climate change 1.2.1 Plate tectonics 1.2.2 Solar variation 1.2.3 Orbital variations 1.2.4 Volcanism 1.3 Human influences 1.3.1 Fossil fuels 1.3.2 Aerosols 1.3.3 Cement manufacture 1.3.4 Land use 1.3.5 Livestock 2 Interplay of factors 3 Monitoring the current status of climate 4 Physical Evidence for Climatic Change 4.1 Vegetation 4.2 Ice Cores 4.3 Dendrochronology 4.4 Pollen analysis 4.5 Insects 4.6 Sea Level Change 4.7 Glacial geology 5 Historical impacts of climate change 6 Examples of climate change 7 See also 8 References 9 Further reading 10 External links

Climate change factors

Climate Change is the result of a great many factors including the dynamic processes of the Earth itself, external forces including variations in sunlight intensity, and more recently by human activities, which might in future be deliberate geoengineering. External factors that can shape climate are often called climate forcings and include such processes as variations in solar radiation, deviations in the Earth's orbit, and the level of greenhouse gas concentrations.

Variations within the Earth's climate

Glaciation

Glaciers are recognized as being among the most sensitive indicators of climate change ^[1], advancing during climate cooling (for example, during the period known as the Little Ice Age) and retreating during climate warming on moderate time scales. Glaciers grow and shrink, both contributing to natural variability and amplifying externally forced changes. A world glacier inventory has been compiled since the 1970s. Initially based mainly on aerial photographs and maps, this compilation has resulted in a detailed inventory of more than 100,000 glaciers covering a total area of approximately 240,000 km² and, in preliminary estimates, for the recording of the remaining ice cover estimated to be around 445,000 km². The World Glacier Monitoring Service collects data annually on glacier retreat and glacier mass balance From this data, glaciers worldwide have been shown to be shrinking significantly, with strong glacier retreats in the 1940s, stable or growing conditions during the 1920s and 1970s, and again increasing rates of ice loss from the mid 1980s to present.^[2]. Mass balance data indicate 17 consecutive years of negative glacier mass balance.

The most significant climate processes of the last several million years are the glacial and interglacial cycles of the present age. The present interglaciation (often termed the Holocene) has lasted about 10,000 years.^[3] Shaped by orbital variations, earth-based responses such as the rise and fall of continental ice sheets and significant sea-level changes helped create the climate. Other changes, including Heinrich events, Dansgaard–Oeschger events and the Younger Dryas, however, illustrate how glacial variations may also influence climate without the forcing effect of orbital changes.

Ocean variability

On a timescale often measured in decades or more, climate changes can also result from the interaction between the atmosphere and the oceans. Many climate fluctuations, including the El Niño Southern oscillation, the Pacific decadal oscillation, the North Atlantic oscillation, and the Arctic oscillation, owe their existence at least in part to the different ways that heat may be stored in the oceans and also to the way it moves between various 'reservoirs'. On longer time scales (with a complete cycle often taking up to a thousand years to complete), ocean processes such as thermohaline circulation also play a key role in redistributing heat by carrying out a very slow and extremely deep movement of water, and the long-term redistribution of heat in the oceans.

Hysteresis

More generally, most forms of internal variability in the climate system can be recognized as a form of hysteresis, where the current state of climate does not immediately reflect the inputs. Because the Earth's climate system is so large, it moves slowly and has time-lags in its reaction to inputs. For example, a year of dry conditions may do no more than to cause lakes to shrink slightly or plains to dry marginally. In the following year however, these conditions may result in less rainfall, possibly leading to a drier year the next. When a critical point is reached after "x" number of years, the entire system may be altered inexorably. In this case, resulting in no rainfall at all. It is this hysteresis that has been mooted to be the possible progenitor of rapid and irreversible climate change ^[4]

Effects of CO₂ on climate change

Increased carbon dioxide levels are thought to exacerbate the heating effects of the Greenhouse Effect by reducing the re-radiation of heat from the sun and, therefore, increasing the temperature contained in the atmosphere. As the ability of the atmosphere to capture and recycle energy emitted by the Earth's surface is essential to a stable climate, this heightened temperature may introduce a de-stabilising influence and potentially affect global weather patterns and, eventually, long-term climate change.

Other factors driving climate change

Plate tectonics

On the longest time scales, plate tectonics will reposition continents, shape oceans, build and tear down mountains and generally serve to define the stage upon which climate exists. During the Carboniferous period, plate tectonics may have triggered the large-scale storage of Carbon and increased glaciation.^[5] More recently, plate motions have been implicated in the intensification of the present ice age when, approximately 3 million years ago, the North and South American plates collided to form the Isthmus of Panama and shut off direct mixing between the Atlantic and Pacific Oceans.^[6]

Solar variation

The sun is the source of a large percentage of the heat energy input to the climate system. Lesser amounts of energy is provided by the gravitational pull of the Moon (manifested as tidal power), and geothermal energy. The energy output of the sun, which is converted to heat at the Earth's surface, is an integral part of the Earth's climate. Early in Earth's history, according to one theory, the sun was too cold to support liquid water at the Earth's surface, leading to what is known as the Faint young sun paradox.^[7] Over the coming millenia, the sun will continue to brighten and produce a correspondingly higher energy output; as it continues through what is known as its "main sequence", and the Earth's atmosphere will be affected accordingly.

On more contemporary time scales, there are also a variety of forms of solar variation, including the 11-year solar cycle and longer-term modulations. However, the 11-year sunspot cycle does not appear to manifest itself clearly in the climatological data. Solar intensity variations are considered to have been influential in triggering the Little Ice Age, and for some of the warming observed from 1900 to 1950. The cyclical nature of the sun's energy output is not yet fully understood; it differs from the very slow change that is happening within the sun as it ages and evolves, with some studies pointing toward solar radiation increases from cyclical sunspot activity affecting Global Warming^[8]

Orbital variations

In their effect on climate, orbital variations are in some sense an extension of solar variability, because slight variations in the Earth's orbit lead to changes in the distribution and abundance of sunlight reaching the Earth's surface. These orbital variations, known as Milankovitch cycles, directly affect glacial activity. Eccentricity, axial tilt, and precession comprise the three dominant cycles that make up the variations in Earth's orbit. The combined effect of the variations in these three cycles creates changes in the seasonal reception of solar radiation on the Earth's surface. As such, Milankovitch Cycles affecting the increase or decrease of received solar radiation directly influence the Earth's climate system, and influence the advance and retreat of Earth's glaciers. Subtler variations are also present, such as the repeated advance and retreat of the Sahara desert in response to orbital precession.^[9]

Volcanism

Volcanism is the process of conveying material from the depths of the Earth to the surface, as part of the process by which the planet removes excess heat and pressure from its interior. Volcanic eruptions, geysers and hot springs are all part of the volcanic process and all release varying levels of particulates into the atmosphere.

A single eruption of the kind that occurs several times per century can affect climate, causing cooling for a period of a few years or more. The eruption of Mount Pinatubo in 1991, for example, produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta)and affected the climate substantially, with global temperatures dropping by about 0.5 °C (0.9 °F), and ozone depletion being temporarily substantially increased. Much larger eruptions, known as large igneous provinces, occur only a few times every hundred million years, but can reshape climate for millions of years and cause mass extinctions. Initially, it was thought that the dust ejected into the atmosphere from large volcanic eruptions was responsible for longer-term cooling by partially blocking the transmission of solar radiation to the Earth's surface. However, measurements indicate that most of the dust hurled into the atmosphere may return to the Earth's surface within as little as six months, given the right conditions.^[10]

Volcanoes are also part of the extended carbon cycle. Over very long (geological) time periods, they release carbon dioxide from the earth's interior, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks. According to the US Geological Survey, however, estimates are that human activities generate more than 130 times the amount of carbon dioxide emitted by volcanoes.^[11]

Human influences

Anthropogenic factors are human activities that change the environment. In some cases the chain of causality of human influence on the climate is direct and unambiguous (for example, the effects of irrigation on local humidity), whilst in other instances it is less clear. Various hypotheses for human-induced climate change have been argued for many years though, generally, the scientific debate has moved on from scepticism to a scientific consensus on climate change that human activity is the probable cause for the rapid changes in world climate in the past several decades.^[12] Consequently, the debate has largely shifted onto ways to reduce further human impact and to find ways to adapt to change that has already occurred. ^[13]

Of most concern in these anthropogenic factors is the increase in CO₂ levels due to emissions from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere)and cement manufacture. Other factors, including land use, ozone depletion, animal agriculture ^[14] and deforestation, are also of concern in the roles they play - both separately and in conjunction with other factors - in affecting climate.

Fossil fuels

Carbon dioxide levels are substantially higher now than at any time in the last 750,000 years.^[15] Beginning with the industrial revolution in the 19th Century and accelerating since, the human consumption of fossil fuels has elevated CO₂ levels from a concentration of approximately 280 ppm in pre-industrial times ^[16] to around 387 ppm today.^[17] The concentrations are increasing at a rate of about 2-3 ppm/year. ^[18] If current rates of emission continue, these increasing concentrations are projected to reach a range of between 535 to 983 ppm by the end of the 21st century.^[19] Along with rising methane levels, it is suggested that these changes may possibly cause an increase of 1.4–5.6°C between 1990 and 2100 (see global warming). Proposals by some scientists and international coalitions, aimed at attempting to prevent drastic climate change, have suggested setting goals to try to limit concentrations to 450 or 500 ppm.^[20]

Aerosols

Anthropogenic aerosols, particularly sulphate aerosols from fossil fuel combustion, exert a cooling influence^[21]. This, together with natural variability (such as Orbital Precession), may account for the relative "plateau" in the temperature of the middle part of the 20th-century ^[22].

Cement manufacture

Cement manufacture contributes CO₂ to the atmosphere when calcium carbonate is heated, producing lime and carbon dioxide, and also as a result of burning fossil fuels in the process. It is estimated that the cement industry produces around 5% of global man-made CO₂ emissions, of which 50% is produced from the chemical process itself, and 40% from burning fuel to power that process. The amount of CO₂ emitted by the cement industry is more than 900 kg of CO₂ for every 1000 kg of cement produced. ^[23]

Land use

Prior to widespread fossil fuel use, humanity's largest effect on local climate was land use; irrigation, deforestation, and agriculture - on large scales - may fundamentally change the environment. For example, through the redirection of natural water courses or the destruction of animal habitats. Land use may also alter the local albedo by reducing ground cover and, therefore, altering the way sunlight is absorbed or reflected. There is evidence to suggest that the climate of Greece and other Mediterranean countries was permanently changed by widespread deforestation between 700 BC and 1 AD (the wood being used for shipbuilding, construction and fuel), with the result that the modern climate in the region is significantly hotter and drier, and the species of trees that were used for shipbuilding in the ancient world can no longer be found in the area.^[24] Similarly, large tracts of land in Australia were permanently altered shortly after humans arrived some 40,000+ years ago when vast areas of temperate rainforest were burned down to produce grasslands that favoured game that the new inhabitants preferred to eat. ^[25] In more modern times, an assessment of conterminous U.S. biomass burning speculated that the approximate 8 fold reduction in Wildland Fire Emissions (aerosols) from the pre-industrial era to present caused by land use changes and land management decisions may have had a regional warming affect if not for fossil fuel burning emission increases occurring concurrently ^[26].

A controversial hypothesis by William Ruddiman - the early anthropocene hypothesis ^[27] - suggests that the rise of agriculture and its accompanying deforestation may have led to significant increases in carbon dioxide and methane levels during the period 5000–8000 years ago. These increases, which apparently reversed previous declines, may have been responsible for delaying the onset of the next Ice Age.

More recently, a 2007 Jet Propulsion Laboratory study ^[28] found that the average mean temperature of California has risen approximately 2 degrees over the past 50 years. The change has been attributed mostly to extensive human development of the landscape. ^[29]

Livestock

According to a 2006 United Nations report, Livestock's Long Shadow, livestock is responsible for some 18% of the world’s greenhouse gas emissions as measured in CO₂ equivalents (this, however, also includes the net effect of deforestation in order to create grazing land, as well as livestock natural methane gas emissions), as well as 65% of human-induced nitrous oxide (which has 296 times the global warming potential of CO₂) and 37% of all human-induced methane (which has 23 times the global warming potential of CO₂).^[14] In the Amazon Rainforest, 70% of deforestation is specifically carried out to make way for grazing land, and so is a major factor in the 2006 UN FAO report; the first agricultural report to factor in land usage change and radiative forcing in regard to the influence of livestock production.

Interplay of factors

In a cybernetic system (as is the climate of the Earth), there exist feedback mechanisms that act to amplify or reduce the effects of positive or negative forces acting upon it. In the case of the climate, these positive and negative feedback mechanisms maintain the stasis of the climate system. Without these mechanisms, the climate system would tend one way or another - too hot or too cold. Too much additional energy fed into a system with over-stressed feedback mechanisms may result in that system breaking down and a disastrous climate change occurring through thermal runaway. Ordinarily, a large part of the reason that this does not occur is the existence of a powerful negative feedback between generated temperature and emitted radiation: with radiation increasing as the fourth power of absolute temperature.

However, a number of important positive feedback mechanisms do exist; the glacial and interglacial cycles of the recent epoch being an important example. It is mooted that orbital variations directly influence the timing for the retreat of ice sheets in proportion to the radiant heat (or insolation) arriving on the Earth's surface. However, as the ice sheets themselves reflect sunlight back into space the increased insolation may actually result in a cooling effect and the growth in the ice through what is known as the albedo feedback effect. Similarly, falling sea levels and expanding ice may decrease plant growth and indirectly lead to declines in carbon dioxide and methane, which may then lead to further cooling. Conversely, rising temperatures caused, for example, by anthropogenic emissions of greenhouse gases could lead to decreased snow and ice cover, revealing darker ground underneath and, consequently, result in more absorption of sunlight and a retreat of glacial ice.^[30] Either way, it is feared that these changes may overload the system sufficiently to produce the runaway feedback described and lead to sudden and disastrous climate change.

Water vapor, methane, and carbon dioxide can also act as significant influences on positive feedback, with their levels rising in response to a warming trend and, as a result, possibly accelerating that trend. Water vapor acts strictly as a feedback mechanism (excepting small amounts in the stratosphere), unlike the other major greenhouse gases, which may also act as forcings. More complex climate feedback influences include heat movement from the equatorial regions to the northern latitudes and involve the possibility of altered water currents with in the oceans or air currents within the atmosphere. A significant concern is that melting glacial ice from Greenland may interfere with (and possibly change) the thermohaline circulation of water in the North Atlantic, affecting the Gulf Stream that conveys warmer water in to replace sinking colder water. Alterations in these flows may affect the distribution of heat to the coast of Europe and the east coast of the United States, with a possible resulting change in climate.

Monitoring the current status of climate

Testing for spatial dependence between independently measured values in an ordered set is based on applying Fisher’s F-test to the variance of a set and the first variance term of the ordered set. Charting statistically significant variance terms gives a sampling variogram that shows where spatial dependence in our sample space of time dissipates into randomness. The lag of a sampling variogram is a statistically robust measure for a change in a climate statistic.

Scientists use "Indicator time series" that represent the many aspects of climate and ecosystem status. The time history provides a historical context. Current status of the climate is also monitored with climate indices.^{[31][32][33][34]}

Physical Evidence for Climatic Change

Evidence for climatic change is taken from a variety of sources that can be used to reconstruct past climates. Most of the evidence is indirect—climatic changes are inferred from changes in indicators that reflect climate, such as vegetation, ice cores^[35], dendrochronology, sea level change, and glacial geology.

Atmospheric sciences [cat.]
Meteorology [cat.] weather [cat.] tropical cyclones [cat.]
Climatology [cat.] climate [cat.] climate change [cat.]
Portal Atmospheric Sciences Portal Weather

Vegetation

A change in the type, distribution and coverage of vegetation may occur given a change in the climate; this much is obvious. However, to what extent particular plant life changes, dies or thrives, depends largely on the model of prediction used. In any given scenario, a mild change in climate may result in increased precipitation and warmth, resulting in improved plant growth and the subsequent sequestration of airborne CO₂. Larger, faster or more radical changes, however, may well^{[weasel words]} result in vegetation stress, rapid plant loss and desertification in certain circumstances.^[36]

Ice Cores

Analysis of ice in a core drilled from a permafrost area, such as the Antarctic, can be used to show a link between temperature and global sea level variations. The air trapped in bubbles in the ice can also reveal the CO₂ variations of the atmosphere from the distant past, well before modern environmental influences. The study of these ice cores has been a significant indicator of the changes in CO₂ over many millennia, and continue to provide valuable information about the differences between ancient and modern atmospheric conditions.

Dendrochronology

Basically, Dendochronology is the analysis of tree ring growth patterns to determine the age of a tree. From a climate change viewpoint, however, Dendochronology can also indicate the climatic conditions for a given number of years. Wide and thick rings indicate a fertile, well-watered growing period, whilst thin, narrow rings indicate a time of lower rainfall and less-than-ideal growing conditions.

Pollen analysis

Palynology is the science that studies contemporary and fossil palynomorphs, including pollen. Palynology is used to infer the geographical distribution of plant species, which vary under different climate conditions. Different groups of plants have pollen with distinctive shapes and surface textures, and since the outer surface of pollen is composed of a very resilient material, they resist decay. Changes in the type of pollen found in different sedimentation levels in lakes, bogs or river deltas indicate changes in plant communities; which are dependent on climate conditions^[37][38].

Insects

Remains of beetles are common in freshwater and land sediments. Different species of beetles tend to be found under different climatic conditions. Given the extensive lineage of beetles whose genetic makeup has not altered significantly over the millenia, knowledge of the present climatic range of the different species, and the age of the sediments in which remains are found, past climatic conditions may be inferred.^[39]

Sea Level Change

Climate models for the substantiation of theories regarding global warming rely heavily on the measurement of long-term changes in global average sea level. Global sea level change for much of the last century has generally been estimated using tide gauge measurements collated over long periods of time to give a long-term average. More recently, altimeter measurements — in combination with accurately determined satellite orbits — have provided an improved measurement of global sea level change.^[40]

Glacial geology

Advancing glaciers leave behind moraines that contain a wealth of material - including organic matter that may be accurately dated - recording the periods in which a glacier advanced and retreated. Similarly, by tephrochronological techniques, the lack of glacier cover can be identified by the presence of soil or volcanic tephra horizons whose date of deposit may also be precisely ascertained. Glaciers are considered one of the most sensitive climate indicators by the IPCC, and their recent observed variations are considered a prominent indicator of impending climate change. See also Retreat of glaciers since 1850.^{[citation needed]}

Historical impacts of climate change

Climate change is frequently associated with the collapse of civilisations, cities and dynasties. Notable collapses for which climate change has been suggested as a cause include Harappa, the Hittites and the civilisation in Ancient Egypt.^[41] Other, smaller communities such as the Danish settlement of Greenland have also suffered collapse^[42] with climate change being a suggested contributory factor.

Examples of climate change

Climate change has continued throughout the entire history of Earth. The field of paleoclimatology has provided information of climate change in the ancient past, supplementing modern observations of climate.

1. Climate of the deep past
   • Faint young sun paradox
   • Snowball earth
   • Oxygen Catastrophe
2. Climate of the last 500 million years
   • Phanerozoic overview
   • Paleocene–Eocene Thermal Maximum
   • Cretaceous Thermal Maximum
   • Permo–Carboniferous Glaciation
   • Ice ages
3. Climate of recent glaciations
   • Dansgaard–Oeschger event
   • Bond event
   • Younger Dryas
   • Ice age temperatures
4. Recent climate
   • Holocene Climatic Optimum
   • Medieval Warm Period
   • Little Ice Age
   • Year Without a Summer
   • Temperature record of the past 1000 years
   • Global warming
   • Hardiness Zone Migration

See also

Wikinews has related news: Climate change

• Atmospheric physics
• Global Warming
• Glossary of climate change
• List of climate change topics

References

Further reading

External links

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