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Image:Vostok-ice-core-petit.png Variations in CO2, temperature and dust from the Vostok ice core over the last 400,000 years For current global climate change, see Global warming.
Climate change refers to the variation in the Earth's global climate or in regional climates over time. It describes changes in the variability or average state of the atmosphere over time scales ranging from decades to millions of years. These changes can be caused by processes internal to the Earth, external forces (e.g. variations in sunlight intensity) or, more recently, human activities. In recent usage, especially in the context of environmental policy, the term "climate change" often refers only to changes in modern climate, including the rise in average surface temperature known as global warming. In some cases, the term is also used with a presumption of human causation, as in the United Nations Framework Convention on Climate Change (UNFCCC). The UNFCCC uses "climate variability" for non-human caused variations.[1]
Climate change factorsClimate changes reflect variations within the Earth's atmosphere, processes in other parts of the Earth such as oceans and ice caps, and the impact of human activity. The external factors that can shape climate are often called climate forcings and include such processes as variations in solar radiation, the Earth's orbit, and greenhouse gas concentrations. Variations within the Earth's climateWeather is the day-to-day state of the atmosphere, and is a chaotic non-linear dynamical system. On the other hand, climate — the average state of weather — is fairly stable and predictable. Climate includes the average temperature, amount of precipitation, days of sunlight, and other variables that might be measured at any given site. However, there are also changes within the Earth's environment that can affect the climate. GlaciationImage:Glaciertermalp.JPG Percentage of advancing glaciers in the Alps in the last 80 years The most significant climate processes of the last several million years are the glacial and interglacial cycles of the present ice age. Though shaped by orbital variations, the internal responses involving continental ice sheets and 130 m sea-level change certainly played a key role in deciding what climate response would be observed in most regions. Other changes, including Heinrich events, Dansgaard–Oeschger events and the Younger Dryas show the potential for glacial variations to influence climate even in the absence of specific orbital changes. Ocean variabilityImage:Thermohaline circulation.png A schematic of modern thermohaline circulation
The memory of climateMore generally, most forms of internal variability in the climate system can be recognized as a form of hysteresis, meaning that the current state of climate reflects not only the inputs, but also the history of how it got there. For example, a decade of dry conditions may cause lakes to shrink, plains to dry up and deserts to expand. In turn, these conditions may lead to less rainfall in the following years. In short, climate change can be a self-perpetuating process because different aspects of the environment respond at different rates and in different ways to the fluctuations that inevitably occur. Non-climate factors driving climate changeGreenhouse gasesImage:Phanerozoic Carbon Dioxide.png Carbon dioxide variations during the last 500 million years Current studies indicate that radiative forcing by greenhouse gases is the primary cause of global warming. Greenhouse gases are also important in understanding Earth's climate history. According to these studies, the greenhouse effect, which is the warming produced as greenhouse gases trap heat, plays a key role in regulating Earth's temperature. Over the last 600 million years, carbon dioxide concentrations have varied from perhaps >5000 ppm to less than 200 ppm, due primarily to the impact of geological processes and biological innovations. It has been argued by Veizer et al., 1999, that variations in greenhouse gas concentrations over tens of millions of years have not been well correlated to climate change, with plate tectonics perhaps playing a more dominant role. More recently Royer et al. [1] have used the CO2-climate correlation to derive a value for the climate sensitivity. There are several examples of rapid changes in the concentrations of greenhouse gases in the Earth's atmosphere that do appear to correlate to strong warming, including the Paleocene–Eocene thermal maximum, the Permian–Triassic extinction event, and the end of the Varangian snowball earth event. During the modern era, rising carbon dioxide levels are implicated as the primary cause of global warming since 1950 by some, most notably Al Gore in his film An Inconvenient Truth. Other scientists disagree such as Dr. Arthur B. Robinson at the Oregon Institute of Science and Medicine.[2] While there may exist a correlation of atmospheric CO2 increases with global temperature increases, there is dispute as to whether CO2 is a cause of global warming, or an effect in response to rises in global temperatures.[3] Thermodynamics and Le Chatelier's principle explain the characteristics of the dynamic equilibrium of a gas in solution such as the vast amount of C02 held in solution in the worlds oceans moving into and returning from the atmosphere. These principals can be observed as bubbles which rise in a pot of water heated on a stove, or a in a glass of cold beer allowed to sit at room temperature; gases dissolved in liquids are released under certain circumstances. Plate tectonicsOn 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. 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. Solar variationThe sun is the ultimate source of essentially all heat in the climate system. The energy output of the sun, which is converted to heat at the Earth's surface, is an integral part of shaping the Earth's climate. On the longest time scales, the sun itself is getting brighter with higher energy output; as it continues its main sequence, this slow change or evolution affects the Earth's atmosphere. Early in Earth's history, it is thought to have been too cold to support liquid water at the Earth's surface, leading to what is known as the Faint young sun paradox. On more modern 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 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 occurring to the sun as it ages and evolves. Orbital variationsIn their impact 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. Such orbital variations, known as Milankovitch cycles, are a highly predictable consequence of basic physics due to the mutual interactions of the Earth, its moon, and the other planets. These variations are considered the driving factors underlying the glacial and interglacial cycles of the present ice age. Subtler variations are also present, such as the repeated advance and retreat of the Sahara desert in response to orbital precession. VolcanismA single eruption of the kind that occurs several times per century can impact climate, causing cooling for a period of a few years. For example, the eruption of Mount Pinatubo in 1991 is barely visible on the global temperature profile. Huge 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, scientists thought that the dust emitted into the atmosphere from large volcanic eruptions was responsible for the cooling by partially blocking the transmission of solar radiation to the Earth's surface. However, measurements indicate that most of the dust thrown in the atmosphere returns to the Earth's surface within six months. Volcanos 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 sinks. However, this contribution is insignificant compared to the current anthropogenic emissons. The US Geological Survey estimates that human activities generate 150 times the amount of carbon dioxide emitted by volcanoes. [2] Human influences on climate ChangeAnthropogenic factors are acts by humans that change the environment and influence climate. Various theories of human-induced climate change have been debated for many years. In the late 1800s, the Rain follows the plow theory had many adherants in the western United States. The biggest factor of present concern is the increase in CO2 levels due to emissions from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere) which exerts a cooling effect. Other factors, including land use, ozone depletion, animal agriculture [3] and deforestation also impact climate. Fossil fuelsImage:Carbon Dioxide 400kyr.png Carbon dioxide variations over the last 400,000 years, showing a rise since the industrial revolution. Beginning with the industrial revolution in the 1850s and accelerating ever since, the human consumption of fossil fuels has elevated CO2 levels from a concentration of ~280 ppm to more than 370 ppm today. These increases are projected to reach more than 560 ppm before the end of the 21st century. It is known that carbon dioxide levels are substantially higher now than at any time in the last 800,000 years [4] Along with rising methane levels, these changes are anticipated to cause an increase of 1.4–5.6 °C between 1990 and 2100 (see global warming). AerosolsAnthropogenic aerosols, particularly sulphate aerosols from fossil fuel combustion, are believed to exert a cooling influence; see graph.[4] This, together with natural variability, is believed to account for the relative "plateau" in the graph of 20th century temperatures in the middle of the century. Land usePrior to widespread fossil fuel use, humanity's largest impact on local climate is likely to have resulted from land use. Irrigation, deforestation, and agriculture fundamentally change the environment. For example, they change the amount of water going into and out of a given location. They also may change the local albedo by influencing the ground cover and altering the amount of sunlight that is absorbed. For example, there is evidence to suggest that the climate of Greece and other Mediterranean countries was permanently changed by widespread deforestation between 700 BC and 0 BC (the wood being used for ship-building, 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 ship-building in the ancient world can no longer be found in the area. A controversial hypothesis by William Ruddiman called the early anthropocene hypothesis [5] suggests that the rise of agriculture and the accompanying deforestation led to the increases in carbon dioxide and methane during the period 5000–8000 years ago. These increases, which reversed previous declines, may have been responsible for delaying the onset of the next glacial period, according to Ruddimann's overdue-glaciation hypothesis. LivestockAccording to a 2006 United Nations report, livestock is responsible for 18% of the world’s greenhouse gas emissions as measured in CO2 equivalents. This however includes land usage change, meaning deforestation in order to create grazing land. In the Amazon, 70% of deforestation is to make way for grazing land, so this is the major factor in the 2006 UN FAO report, which was the first agricultural report to include land usage change into the radiative forcing of livestock. In addition to CO2 emissions, livestock produces 65% of human-induced nitrous oxide (which has 296 times the global warming potential of CO2) and 37% of human-induced methane (which has 23 times the global warming potential of CO2)[6]. Interplay of factorsIf a certain forcing (for example, solar variation) acts to change the climate, then there may be mechanisms that act to amplify or reduce the effects. These are called positive and negative feedbacks. As far as is known, the climate system is generally stable with respect to these feedbacks: positive feedbacks do not "run away". Part of the reason for this is the existence of a powerful negative feedback between temperature and emitted radiation: radiation increases as the fourth power of absolute temperature. However, a number of important positive feedbacks do exist. The glacial and interglacial cycles of the present ice age provide an important example. It is believed that orbital variations provide the timing for the growth and retreat of ice sheets. However, the ice sheets themselves reflect sunlight back into space and hence promote cooling and their own growth, known as the ice-albedo feedback. Further, falling sea levels and expanding ice decrease plant growth and indirectly lead to declines in carbon dioxide and methane. This leads to further cooling. Similarly, rising temperatures caused, for example, by anthropogenic emissions of greenhouse gases could lead to retreating snow lines, revealing darker ground underneath, and consequently result in more absorption of sunlight. Water vapor, methane, and carbon dioxide can also act as significant positive feedbacks, their levels rising in response to a warming trend, thereby accelerating that trend. Water vapor acts strictly as a feedback (excepting small amounts in the stratosphere), unlike the other major greenhouse gases, which can also act as forcings. More complex feedbacks involve the possibility of changing circulation patterns in the ocean or atmosphere. For example, a significant concern in the modern case is that melting glacial ice from Greenland will interfere with sinking waters in the North Atlantic and inhibit thermohaline circulation. This could affect the Gulf Stream and the distribution of heat to Europe and the east coast of the United States. Other potential feedbacks are not well understood and may either inhibit or promote warming. For example, it is unclear whether rising temperatures promote or inhibit vegetative growth, which could in turn draw down either more or less carbon dioxide. Similarly, increasing temperatures may lead to either more or less cloud cover.[5] Since on balance cloud cover has a strong cooling effect, any change to the abundance of clouds also impacts climate.[6] In all, it seems likely that overall climate feedbacks are negative, as systems with overall positive feedback are highly unstable. Monitoring the current status of climateScientists use "Indicator time series" that represent the many aspects of climate and ecosystem status. The time history provides an historical context. Current status of the climate is also monitored with climate indices.[7][8][9][10] Evidence for climatic changeEvidence 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, dendrochronology, ice cores, sea level change, and glacial retreat. Pollen analysisSpecies have particular climatic requirements that influence their geographical distributions. Each plant species has a distinctively shaped pollen grain, and if these fall into oxygen-free environments, such as peat bogs, they resist decay. Changes in the pollen found in different levels of the bog indicate, by implication, changes in climate. One limitation of this method is the fact that pollen can be transported considerable distances by wind or sometimes by wildlife. BeetlesRemains of beetles are common in freshwater and land sediments. Different species of beetles tend to be found under different climatic conditions. Knowledge of the present climatic range of the different species, and the age of the sediments in which remains are found, allows past climatic conditions to be inferred. Glacial geologyAdvancing glaciers leave behind moraines and other features that often have datable material in them, recording the time when a glacier advanced and deposited a feature. Similarly, the lack of glacier cover can be identified by the presence of datable soil or volcanic tephra horizons. Glaciers are considered one of the most sensitive climate indicators by the IPCC, and their recent observed variations provide a global signal of climate change. See Retreat of glaciers since 1850. Historical recordsHistorical records include cave paintings, depth of grave digging in Greenland, diaries, documentary evidence of events (such as 'frost fairs' on the Thames) and evidence of areas of vine cultivation. Daily weather reports have been kept since 1873, and the Royal Society has encouraged the collection of data since the seventeenth century. Parish records are often a good source of climate data. Examples of climate changeClimate 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.
Climate change and economicsThere has been a debate about how climate change could affect the world economy. An October 29, 2006 report by former Chief Economist and Senior Vice-President of the World Bank Nicholas Stern states that climate change could affect growth, which could be cut by one-fifth unless drastic action is taken. (Report's stark warning on climate) Climate change in popular cultureThe issue of climate change has entered popular culture since the late 20th century. Science historian Naomi Oreskes has noted that "there's a huge disconnect between what professional scientists have studied and learned in the last 30 years, and what is out there in the popular culture".[7] An academic study contrasts the relatively rapid acceptance of ozone depletion as reflected in popular culture with the much slower acceptance of the scientific consensus on global warming.[8] Climate Change and Biodiversityclimate change and biodiversity Some of the most immediate effects of recent climate change are becoming apparent through impacts on biodiversity. The life cycles of many wild plants and animals are closely linked to the passing of the seasons; climatic changes can lead to interdependent pairs of species (e.g. a wild flower and its pollinating insect) losing synchronisation, if, for example, one has a cycle dependent on day length and the other on temperature or precipitation. In principle, at least, this could lead to extinctions or changes in the distribution and abundance of species. One phenomenon is the movement of species northwards in Europe. A recent study by Butterfly Conservation in the UK, [[9]], has shown that relative common species with a southerly distribution have moved north, whilst scarce upland species have become rarer and lost territory towards the south. This picture has been mirroed across several invertebrate groups. Drier summers could lead to more periods of drought, potentially affecting many species of animal and plant. For example, in the UK during the drought year of 2006 signficant numbers of trees dies or showed dieback on light sandy soils. Wetter, milder winters might impact on temperate mammals or insects by preventing them hibernating or entering torpor during periods when food is scarce. One predicted change is the ascendance of 'weedy' or opportunistic species at the expense of scarcer species with narrower or more specialised ecological requirements. One example could be the expanses of bluebell seen in many woodlands in the UK. These have an early growing and flowering season before competing weeds can develop and the tree canopy closes. Milder winters can allow weeds to overwinter as adult plants or germinate sooner, whilst trees leaf earlier, reducing the length of the window for bluebells to complete their life cycle. Organisations such as Wildlife Trust, World Wide Fund for Nature, Birdlife International and the Audobon Society are actively monitoring and research the effects of climate change on biodiversity. They also advance policies in areas such as landscape scale conservation to promote adaptation to climate change. A more detailed review of these issues can be found here [[10]] Humanitarian impact of climate changeThe events caused by climate change have three characteristics, which will have an impact in the humanitarian field They are extreme, in terms of intensity, which often cause extensive damage and substantial loss of life in a developing country They are repeated, and would therefore have an economic and social impact difficult to afford for poor countries They are difficult to foresee because they are not following the “traditional” climate cycle Consequences of these events in the humanitarian field a/ They are a threat to human life –Events of high intensity would have an impact in terms of human loss. The rise of the sea level will also endanger coastal cities, such as Lagos. b/ They impact on human health : the increase of temperatures is conducive to a rise of epidemics and doesn’t facilitate sanitary conditions. Therefore it is likely that epidemics such as cholera, malaria, dengue fever will increase in some areas. (In the Andes Mountain in Columbia for example, mosquitoes that can carry dengue and yellow fever viruses were previously limited to 3,300 feet (1,006 m) but recently appeared at 7,200 feet (2,195 m). In Mexico as well, dengue fever has spread above its former elevation limit of 3,300 feet (1,006 m) and has appeared at 5,600 feet (1,707 m). c/ They are a threat to food security and water supply. Climate change will increasingly put pressure on crop yields. Indeed, the decrease of agricultural productivity in tropical and sub-tropical zones particularly will endanger food security. As well as the warming of oceans, which will affect communities, who rely on fishing. d/ They are a threat to peace and security : scarcity of resources leads to competition and could exacerbate tensions already existing between different ethnic groups, countries, etc. e/ They will lead to environmental migrations and will be the source of a new category of refugees or internal displaced populations (IDPs) leaving lands at risk, voluntarily or forced by the governments. Rising sea levels damaging coastal regions through flooding and erosion, desertification and shrinking freshwater supplies displaced up to 10 million people in year 2006, and will create up to 50 million environmental refugees by the end of the decade. The Red Cross says environmental disasters already displace more people than war. Environment-related migration has been most acute in sub-Saharan Africa, but also affects millions of people in Asia and India. Europe and the US face increased pressure from people driven from North Africa and Latin America by deteriorating soil and water conditions. Small island states like Tuvalu are facing the immediate consequences of climate change. New Zealand has already agreed to accept the 11,600 inhabitants of the low-lying Pacific island state if rising sea levels swamp the country. Elsewhere, as many as 100 million people live in areas that are below sea level or liable to storm surge. A total of 213 communities in Alaska are threatened by tides that creep three metres further inland each year. Climate change exacerbate vulnerabilities and has a bigger impact in poor countries Climate change has a bigger impact on poor communities, which are the most vulnerable. Developing countries are more vulnerable, due to the economic importance of climatic sensitive sectors (agriculture, fisheries) and their limited human, institutional and financial capacity to anticipate and respond to the direct and indirect effects of climate change. It is likely that this vulnerability is higher for the least developed countries located in the tropical and sub-tropical areas. Often extreme weather events set back the development process for decades. See also
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