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Climate change

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

                         Atmospheric sciences [cat.]
                                                 Meteorology [cat.]

           weather [cat.]
           tropical cyclones [cat.]

                                                 Climatology [cat.]

           climate [cat.]
           climate change [cat.]
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             Portal Atmospheric Sciences
             Portal Weather

   Variations in CO2, temperature and dust from the Vostok ice core over
   the last 400 000 years
   Enlarge
   Variations in CO[2], temperature and dust from the Vostok ice core over
   the last 400 000 years

   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—or average weather—over time scales
   ranging from decades to millions of years. These changes may come from
   processes internal to the Earth, be driven by external forces (e.g.
   variations in sunlight intensity) or, most recently, be caused by human
   activities.

   In recent usage, especially in the context of environmental policy, the
   term "climate change" often refers only to the ongoing 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.

   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.

Climate change factors

   Climate changes reflect variations within the Earth's environment,
   natural processes going on around it, and the impact of human activity.
   The external factors which 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 climate

   Weather, in and of itself, is a chaotic non-linear dynamical system,
   but in many cases, it is observed that the climate (i.e., the average
   state of weather) is fairly stable and predictable. This includes the
   average temperature, amount of precipitation, days of sunlight, and
   many 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.

Glaciation

   Percentage of advancing glaciers in the Alps in the last 80 years
   Enlarge
   Percentage of advancing glaciers in the Alps in the last 80 years

   Glaciers are recognized as one of the most sensitive indicators of
   climate change, advancing substantially during climate cooling (e.g.,
   the Little Ice Age) and retreating during climate warming on moderate
   time scales. Glaciers grow and collapse, both contributing to natural
   variability and greatly amplifying external forces. For the last
   century, however, glaciers have been unable to regenerate enough ice
   during the winters to make up for the ice lost during the summer months
   (see glacier retreat).

   The most important 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 variability

   A schematic of modern thermohaline circulation
   Enlarge
   A schematic of modern thermohaline circulation

   On the scale of mere decades, climate changes can also result from
   changes within the ocean/atmosphere systems. Many climate states, most
   obviously El Niño Southern oscillation, but also including the Pacific
   decadal oscillation, the North Atlantic oscillation, and the Arctic
   oscillation, have been recognized as modes within the climate system,
   owing their existence at least in part to different ways that heat can
   be stored in the oceans and move between different reservoirs. On
   longer time scales, ocean processes such as thermohaline circulation
   play a key role in redistributing heat, and could, if changed,
   dramatically impact climate.

The memory of climate

   More 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 change

Greenhouse gases

   Carbon dioxide variations during the last 500 million years
   Enlarge
   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.
   Curiously, it has been argued (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. However, 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 to global warming since 1950.

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. 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 variation

   Variations in solar activity during the last several centuries based on
   observations of sunspots and beryllium isotopes.
   Enlarge
   Variations in solar activity during the last several centuries based on
   observations of sunspots and beryllium isotopes.

   The sun, as the ultimate source of nearly all energy in the climate
   system, is an integral part of shaping the Earth's climate. On the
   longest time scales, the sun itself is getting brighter as it continues
   its main sequence evolution. 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. These variations are
   considered to be influential in triggering the Little Ice Age and for
   some of the warming observed from 1900 to 1950.

Orbital variations

   In 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.

Volcanism

   A 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.
   Attribution of recent climate change
   Enlarge
   Attribution of recent climate change

Human influences on climate

   Anthropogenic factors are acts by humans that change the environment
   and influence climate. The biggest factor of present concern is the
   increase in CO[2] 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, and deforestation also impact climate.

Fossil fuels

   Carbon dioxide variations over the last 400,000 years, showing a rise
   since the industrial revolution.
   Enlarge
   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 CO[2]
   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. 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).

Aerosols

   Anthropogenic aerosols, particularly sulphate aerosols from fossil fuel
   combustion, are believed to exert a cooling influence; see graph. 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 use

   Prior 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 locale. They also may change the local albedo by influencing the
   ground cover and altering the amount of sunlight which 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 purposes), with the result that
   the modern climate in the region is significantly hotter and drier and
   the species of trees which were used for ship-building in the ancient
   world can no longer be found in the area.

   A controversial hypothesis by William Ruddiman 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 hypothesis.

Interplay of factors

   If a certain forcing (for example, solar variation) acts to change the
   climate, then there may be mechanisms which 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. Since on
   balance cloud cover has a strong cooling effect, any change to the
   abundance of clouds also impacts climate.

Monitoring the current status of climate

   Scientists 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.

Evidence for Climatic Change

   Evidence for climatic change is taken from a variety of sources which
   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, glacial retreat.

Pollen Analysis

   Species have particular climatic requirements which 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.

Coleoptera

   Remains of Coleoptera beetles are common in freshwater and land
   sediments. Different species of this beetle 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 worked out.

Glacial Geology

   Advancing glaciers leave behind moraines and other features which often
   have datable material in them, recording the time when a glacier
   advance 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 Records

   Historical 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. Since
   1873 daily weather reports have been documented, 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 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. Obviously, most of these prehistoric changes are solely the
   result of natural factors.
    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
          + Younger Dryas
          + Ice age temperatures
    4. Recent climate
          + Holocene Climatic Optimum
          + Medieval Warm Period
          + Little Ice Age
          + Temperature record of the past 1000 years
          + Global warming

Climate change and economics

   There has been a debate about how climate change could affect the world
   economy. In an October 29, 2006 report by the former Chief Economist
   and Senior Vice-President of the World Bank Nicholas Stern, he 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)

   Retrieved from " http://en.wikipedia.org/wiki/Climate_change"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
