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Climate variability includes all the variations in the
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
that last longer than individual
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the t ...
events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. ''Climate change'' may refer to any time in Earth's history, but the term is now commonly used to describe contemporary climate change. Since the Industrial Revolution, the climate has increasingly been affected by
human activities Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, an ...
. The
climate system Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...
receives nearly all of its energy from the sun and radiates energy to outer space. The balance of incoming and outgoing energy and the passage of the energy through the climate system is
Earth's energy budget Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...
. When the incoming energy is greater than the outgoing energy, Earth's energy budget is positive and the climate system is warming. If more energy goes out, the energy budget is negative and Earth experiences cooling. The energy moving through Earth's climate system finds expression in
weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the t ...
, varying on geographic scales and time. Long-term averages and variability of weather in a region constitute the region's
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
. Such changes can be the result of "internal variability", when natural processes inherent to the various parts of the climate system alter the distribution of energy. Examples include variability in ocean basins such as the Pacific decadal oscillation and Atlantic multidecadal oscillation. Climate variability can also result from ''external forcing'', when events outside of the climate system's components produce changes within the system. Examples include changes in solar output and volcanism. Climate variability has consequences for sea level changes, plant life, and mass extinctions; it also affects human societies.


Terminology

''Climate variability'' is the term to describe variations in the mean state and other characteristics of
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
(such as chances or possibility of extreme weather, etc.) "on all spatial and temporal scales beyond that of individual weather events." Some of the variability does not appear to be caused by known systems and occurs at seemingly random times. Such variability is called ''random variability'' or ''
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
''. On the other hand, periodic variability occurs relatively regularly and in distinct modes of variability or climate patterns. The term ''climate change'' is often used to refer specifically to anthropogenic climate change (also known as global warming). Anthropogenic climate change is caused by human activity, as opposed to changes in climate that may have resulted as part of Earth's natural processes. In this sense, the term climate change has become synonymous with
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human im ...
global warming. Within scientific journals, global warming refers to surface temperature increases while climate change includes global warming and everything else that increasing
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
levels affect. A related term, ''climatic change'', was proposed by the
World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Internat ...
(WMO) in 1966 to encompass all forms of climatic variability on time-scales longer than 10 years, but regardless of cause. During the 1970s, the term climate change replaced climatic change to focus on anthropogenic causes, as it became clear that human activities had a potential to drastically alter the climate. Climate change was incorporated in the title of the Intergovernmental Panel on Climate Change (IPCC) and the UN Framework Convention on Climate Change (UNFCCC). Climate change is now used as both a technical description of the process, as well as a noun used to describe the problem.


Causes

On the broadest scale, the rate at which energy is received from the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and the rate at which it is lost to space determine the equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents, and other mechanisms to affect the climates of different regions. Factors that can shape climate are called
climate forcing Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...
s or "forcing mechanisms". These include processes such as variations in
solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...
, variations in the Earth's orbit, variations in the
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
or reflectivity of the continents, atmosphere, and oceans, mountain-building and
continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...
and changes in
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
concentrations. External forcing can be either anthropogenic (e.g. increased emissions of greenhouse gases and dust) or natural (e.g., changes in solar output, the Earth's orbit, volcano eruptions). There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. There are also key thresholds which when exceeded can produce rapid or irreversible change. Some parts of the climate system, such as the oceans and ice caps, respond more slowly in reaction to climate forcings, while others respond more quickly. An example of fast change is the atmospheric cooling after a volcanic eruption, when
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...
reflects sunlight. Thermal expansion of ocean water after atmospheric warming is slow, and can take thousands of years. A combination is also possible, e.g., sudden loss of
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
in the Arctic Ocean as sea ice melts, followed by more gradual thermal expansion of the water. Climate variability can also occur due to internal processes. Internal unforced processes often involve changes in the distribution of energy in the ocean and atmosphere, for instance, changes in the
thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to tempe ...
.


Internal variability

Climatic changes due to internal variability sometimes occur in cycles or oscillations. For other types of natural climatic change, we cannot predict when it happens; the change is called ''random'' or ''stochastic''. From a climate perspective, the weather can be considered random. If there are little clouds in a particular year, there is an energy imbalance and extra heat can be absorbed by the oceans. Due to
climate inertia Climate inertia is the phenomenon by which climate systems show resistance or slowness to changes in significant factors, such as greenhouse gas levels. In the context of climate change, this means that mitigation strategies, such as the stabiliz ...
, this signal can be 'stored' in the ocean and be expressed as variability on longer time scales than the original weather disturbances. If the weather disturbances are completely random, occurring as white noise, the inertia of glaciers or oceans can transform this into climate changes where longer-duration oscillations are also larger oscillations, a phenomenon called red noise. Many climate changes have a random aspect and a cyclical aspect. This behavior is dubbed '' stochastic resonance''. Half of the 2021 Nobel prize on physics was awarded for this work to
Klaus Hasselmann Klaus Ferdinand Hasselmann (, born 25 October 1931) is a German oceanographer and climate modeller. He is Professor Emeritus at the University of Hamburg and former Director of the Max Planck Institute for Meteorology. He was awarded the 202 ...
jointly with Syukuro Manabe for related work on
climate model Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the c ...
ling. While Giorgio Parisi who with collaborators introduced the concept of stochastic resonance was awarded the other half but mainly for work on theoretical physics.


Ocean-atmosphere variability

The ocean and atmosphere can work together to spontaneously generate internal climate variability that can persist for years to decades at a time. These variations can affect global average surface temperature by redistributing heat between the deep ocean and the atmosphere and/or by altering the cloud/water vapor/sea ice distribution which can affect the total energy budget of the Earth.


Oscillations and cycles

A ''climate oscillation'' or ''climate cycle'' is any recurring cyclical oscillation within global or regional
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
. They are
quasiperiodic Quasiperiodicity is the property of a system that displays irregular periodicity. Periodic behavior is defined as recurring at regular intervals, such as "every 24 hours". Quasiperiodic behavior is a pattern of recurrence with a component of unpre ...
(not perfectly periodic), so a Fourier analysis of the data does not have sharp peaks in the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
. Many oscillations on different time-scales have been found or hypothesized: * the
El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the Tropics, tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of t ...
(ENSO) – A large scale pattern of warmer ( El Niño) and colder ( La Niña) tropical sea surface temperatures in the Pacific Ocean with worldwide effects. It is a self-sustaining oscillation, whose mechanisms are well-studied. ENSO is the most prominent known source of inter-annual variability in weather and climate around the world. The cycle occurs every two to seven years, with El Niño lasting nine months to two years within the longer term cycle. The cold tongue of the equatorial Pacific Ocean is not warming as fast as the rest of the ocean, due to increased
upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nutr ...
of cold waters off the west coast of South America. * the
Madden–Julian oscillation The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal (30- to 90-day) variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric ...
(MJO) – An eastward moving pattern of increased rainfall over the tropics with a period of 30 to 60 days, observed mainly over the Indian and Pacific Oceans. * the North Atlantic oscillation (NAO) – Indices of the NAO are based on the difference of normalized
sea-level pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
(SLP) between Ponta Delgada, Azores and Stykkishólmur/ Reykjavík, Iceland. Positive values of the index indicate stronger-than-average westerlies over the middle latitudes.National Center for Atmospheric Research
Climate Analysis Section.
Retrieved on 7 June 2007.
* the
Quasi-biennial oscillation The quasi-biennial oscillation (QBO) is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the to ...
– a well-understood oscillation in wind patterns in the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
around the equator. Over a period of 28 months the dominant wind changes from easterly to westerly and back. * Pacific Centennial Oscillation - a
climate oscillation Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more ...
predicted by some
climate model Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the c ...
s * the Pacific decadal oscillation – The dominant pattern of sea surface variability in the North Pacific on a decadal scale. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. It is thought not as a single phenomenon, but instead a combination of different physical processes. * the Interdecadal Pacific oscillation (IPO) – Basin wide variability in the Pacific Ocean with a period between 20 and 30 years. * the Atlantic multidecadal oscillation – A pattern of variability in the North Atlantic of about 55 to 70 years, with effects on rainfall, droughts and hurricane frequency and intensity. * North African climate cycles – climate variation driven by the
North African Monsoon North African climate cycles have a unique history that can be traced back millions of years. The cyclic climate pattern of the Sahara is characterized by significant shifts in the strength of the North African Monsoon. When the North African Monsoo ...
, with a period of tens of thousands of years. * the
Arctic oscillation The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The s ...
(AO) and Antarctic oscillation (AAO) – The annular modes are naturally occurring, hemispheric-wide patterns of climate variability. On timescales of weeks to months they explain 20-30% of the variability in their respective hemispheres. The Northern Annular Mode or
Arctic oscillation The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The s ...
(AO) in the Northern Hemisphere, and the Southern Annular Mode or Antarctic oscillation (AAO) in the southern hemisphere. The annular modes have a strong influence on the temperature and precipitation of mid-to-high latitude land masses, such as Europe and Australia, by altering the average paths of storms. The NAO can be considered a regional index of the AO/NAM. They are defined as the first EOF of sea level pressure or geopotential height from 20°N to 90°N (NAM) or 20°S to 90°S (SAM). * Dansgaard–Oeschger cycles – occurring on roughly 1,500-year cycles during the Last Glacial Maximum


Ocean current changes

The oceanic aspects of climate variability can generate variability on centennial timescales due to the ocean having hundreds of times more mass than in the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
, and thus very high thermal inertia. For example, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat in the world's oceans. Ocean currents transport a lot of energy from the warm tropical regions to the colder polar regions. Changes occurring around the last ice age (in technical terms, the last
glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
) show that the circulation is the North Atlantic can change suddenly and substantially, leading to global climate changes, even though the total amount of energy coming into the climate system didn't change much. These large changes may have come from so called
Heinrich events A Heinrich event is a natural phenomenon in which large groups of icebergs break off from glaciers and traverse the North Atlantic. First described by marine geologist Hartmut Heinrich (Heinrich, H., 1988), they occurred during five of the last s ...
where internal instability of ice sheets caused huge ice bergs to be released into the ocean. When the ice sheet melts, the resulting water is very low in salt and cold, driving changes in circulation.


Life

Life affects climate through its role in the carbon and water cycles and through such mechanisms as
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
,
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpira ...
,
cloud formation In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compo ...
, and weathering. Examples of how life may have affected past climate include: *
glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
2.3 billion years ago triggered by the evolution of oxygenic photosynthesis, which depleted the atmosphere of the greenhouse gas carbon dioxide and introduced free oxygen * another glaciation 300 million years ago ushered in by long-term burial of decomposition-resistant detritus of vascular land-plants (creating a
carbon sink A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide () from the atmosphere. Globally, the two most important carbon ...
and forming coal) * termination of the Paleocene–Eocene Thermal Maximum 55 million years ago by flourishing marine phytoplankton * reversal of global warming 49 million years ago by 800,000 years of arctic azolla blooms * global cooling over the past 40 million years driven by the expansion of grass-grazer ecosystems


External climate forcing


Greenhouse gases

Whereas
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es released by the biosphere is often seen as a feedback or internal climate process, greenhouse gases emitted from volcanoes are typically classified as external by climatologists. Greenhouse gases, such as , methane and nitrous oxide, heat the climate system by trapping infrared light. Volcanoes are also part of the extended
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
. Over very long (geological) time periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological
carbon dioxide sink A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide () from the atmosphere. Globally, the two most important carbon ...
s. Since the industrial revolution, humanity has been adding to greenhouse gases by emitting CO2 from fossil fuel combustion, changing land use through deforestation, and has further altered the climate with
aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthropogen ...
(particulate matter in the atmosphere), release of trace gases (e.g. nitrogen oxides, carbon monoxide, or methane). Other factors, including land use, ozone depletion, animal husbandry ( ruminant animals such as
cattle Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus ''Bos''. Adult females are referred to as cows and adult ma ...
produce methane), and deforestation, also play a role. The US Geological Survey estimates are that volcanic emissions are at a much lower level than the effects of current human activities, which generate 100–300 times the amount of carbon dioxide emitted by volcanoes. The annual amount put out by human activities may be greater than the amount released by supereruptions, the most recent of which was the
Toba eruption The Youngest Toba eruption was a supervolcano eruption that occurred around 74,000 years ago at the site of present-day Lake Toba in Sumatra, Indonesia. It is one of the Earth's largest known explosive eruptions. The Toba catastrophe theory hold ...
in Indonesia 74,000 years ago.


Orbital variations

Slight variations in Earth's motion lead to changes in the seasonal distribution of sunlight reaching the Earth's surface and how it is distributed across the globe. There is very little change to the area-averaged annually averaged sunshine; but there can be strong changes in the geographical and seasonal distribution. The three types of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined, these produce Milankovitch cycles which affect climate and are notable for their correlation to
glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
and
interglacial period An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene i ...
s, their correlation with the advance and retreat of the Sahara, and for their
appearance Appearance may refer to: * Visual appearance, the way in which objects reflect and transmit light * Human physical appearance, what someone looks like * ''Appearances'' (film), a 1921 film directed by Donald Crisp * Appearance (philosophy), or p ...
in the
stratigraphic record Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostr ...
. During the glacial cycles, there was a high correlation between concentrations and temperatures. Early studies indicated that concentrations lagged temperatures, but it has become clear that this isn't always the case. When ocean temperatures increase, the solubility of decreases so that it is released from the ocean. The exchange of between the air and the ocean can also be impacted by further aspects of climatic change. These and other self-reinforcing processes allow small changes in Earth's motion to have a large effect on climate.


Solar output

The
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
is the predominant source of energy input to the Earth's
climate system Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...
. Other sources include geothermal energy from the Earth's core, tidal energy from the Moon and heat from the decay of radioactive compounds. Both long term variations in solar intensity are known to affect global climate. Solar output varies on shorter time scales, including the 11-year solar cycle and longer-term modulations. Correlation between sunspots and climate and tenuous at best. Three to four billion years ago, the Sun emitted only 75% as much power as it does today. If the atmospheric composition had been the same as today, liquid water should not have existed on the Earth's surface. However, there is evidence for the presence of water on the early Earth, in the Hadean and
Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Archea ...
eons, leading to what is known as the
faint young Sun paradox The faint young Sun paradox or faint young Sun problem describes the apparent contradiction between observations of liquid water early in Earth's history and the astrophysical expectation that the Sun's output would be only 70 percent as intense ...
. Hypothesized solutions to this paradox include a vastly different atmosphere, with much higher concentrations of greenhouse gases than currently exist. Over the following approximately 4 billion years, the energy output of the Sun increased. Over the next five billion years, the Sun's ultimate death as it becomes a
red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
and then a
white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes f ...
will have large effects on climate, with the red giant phase possibly ending any life on Earth that survives until that time.


Volcanism

The
eruptions Several types of volcanic eruptions—during which lava, tephra ( ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are of ...
considered to be large enough to affect the Earth's climate on a scale of more than 1 year are the ones that inject over 100,000
ton Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses. Mainly it describes units of weight. Confusion can arise because ''ton'' can mean * the long ton, which is 2,240 pounds ...
s of SO2 into the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
. This is due to the optical properties of SO2 and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze. On average, such eruptions occur several times per century, and cause cooling (by partially blocking the transmission of solar radiation to the Earth's surface) for a period of several years. Although volcanoes are technically part of the lithosphere, which itself is part of the climate system, the IPCC explicitly defines volcanism as an external forcing agent. Notable eruptions in the historical records are the
1991 eruption of Mount Pinatubo The 1991 eruption of Mount Pinatubo in the Philippines' Luzon Volcanic Arc was the second-largest volcanic eruption of the 20th century, behind only the 1912 eruption of Novarupta in Alaska. Eruptive activity began on April 2 as a series of ...
which lowered global temperatures by about 0.5 °C (0.9 °F) for up to three years, and the 1815 eruption of Mount Tambora causing the Year Without a Summer. At a larger scale—a few times every 50 million to 100 million years—the eruption of
large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...
s brings large quantities of igneous rock from the
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
and lithosphere to the Earth's surface. Carbon dioxide in the rock is then released into the atmosphere. Small eruptions, with injections of less than 0.1 Mt of sulfur dioxide into the stratosphere, affect the atmosphere only subtly, as temperature changes are comparable with natural variability. However, because smaller eruptions occur at a much higher frequency, they too significantly affect Earth's atmosphere.


Plate tectonics

Over the course of millions of years, the motion of tectonic plates reconfigures global land and ocean areas and generates topography. This can affect both global and local patterns of climate and atmosphere-ocean circulation. The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate. A recent example of tectonic control on ocean circulation is the formation of the
Isthmus of Panama The Isthmus of Panama ( es, Istmo de Panamá), also historically known as the Isthmus of Darien (), is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the countr ...
about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans. This strongly affected the ocean dynamics of what is now the
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida and up the eastern coastline of the Unit ...
and may have led to Northern Hemisphere ice cover. During the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased
glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
. Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinent
Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons. The size of continents is also important. Because of the stabilizing effect of the oceans on temperature, yearly temperature variations are generally lower in coastal areas than they are inland. A larger supercontinent will therefore have more area in which climate is strongly seasonal than will several smaller continents or islands.


Other mechanisms

It has been postulated that
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
ized particles known as cosmic rays could impact cloud cover and thereby the climate. As the sun shields the Earth from these particles, changes in solar activity were hypothesized to influence climate indirectly as well. To test the hypothesis,
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
designed the
CLOUD experiment Cosmics Leaving Outdoor Droplets (CLOUD) is an experiment being run at CERN by a group of researchers led by Jasper Kirkby to investigate the microphysics between galactic cosmic rays (GCRs) and aerosols under controlled conditions. This is a f ...
, which showed the effect of cosmic rays is too weak to influence climate noticeably. Evidence exists that the Chicxulub asteroid impact some 66 million years ago had severely affected the Earth's climate. Large quantities of sulfate aerosols were kicked up into the atmosphere, decreasing global temperatures by up to 26 °C and producing sub-freezing temperatures for a period of 3–16 years. The recovery time for this event took more than 30 years. The large-scale use of nuclear weapons has also been investigated for its impact on the climate. The hypothesis is that soot released by large-scale fires blocks a significant fraction of sunlight for as much as a year, leading to a sharp drop in temperatures for a few years. This possible event is described as nuclear winter. Humans' use of land impact how much sunlight the surface reflects and the concentration of dust. Cloud formation is not only influenced by how much water is in the air and the temperature, but also by the amount of
aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthropogen ...
in the air such as dust. Globally, more dust is available if there are many regions with dry soils, little vegetation and strong winds.


Evidence and measurement of climate changes

Paleoclimatology is the study of changes in climate through the entire history of Earth. It uses a variety of
proxy Proxy may refer to: * Proxy or agent (law), a substitute authorized to act for another entity or a document which authorizes the agent so to act * Proxy (climate), a measured variable used to infer the value of a variable of interest in climate re ...
methods from the Earth and life sciences to obtain data preserved within things such as rocks, sediments, ice sheets, tree rings, corals, shells, and microfossils. It then uses the records to determine the past states of the Earth's various climate regions and its atmospheric system. Direct measurements give a more complete overview of climate variability.


Direct measurements

Climate changes that occurred after the widespread deployment of measuring devices can be observed directly. Reasonably complete global records of surface temperature are available beginning from the mid-late 19th century. Further observations are derived indirectly from historical documents. Satellite cloud and precipitation data has been available since the 1970s.
Historical climatology Historical climatology is the study of historical changes in climate and their effect on civilization from the emergence of homininis to the present day. This differs from paleoclimatology which encompasses climate change over the entire history ...
is the study of historical changes in climate and their effect on human history and development. The primary sources include written records such as
sagas is a series of science fantasy role-playing video games by Square Enix. The series originated on the Game Boy in 1989 as the creation of Akitoshi Kawazu at Square. It has since continued across multiple platforms, from the Super NES to the Play ...
,
chronicle A chronicle ( la, chronica, from Greek ''chroniká'', from , ''chrónos'' – "time") is a historical account of events arranged in chronological order, as in a timeline. Typically, equal weight is given for historically important events and l ...
s, maps and
local history Local history is the study of history in a geographically local context, often concentrating on a relatively small local community. It incorporates cultural and social aspects of history. Local history is not merely national history writ small b ...
literature as well as pictorial representations such as paintings, drawings and even
rock art In archaeology, rock art is human-made markings placed on natural surfaces, typically vertical stone surfaces. A high proportion of surviving historic and prehistoric rock art is found in caves or partly enclosed rock shelters; this type also m ...
. Climate variability in the recent past may be derived from changes in settlement and agricultural patterns.
Archaeological Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
evidence, oral history and
historical documents Historical documents are original documents that contain important historical information about a person, place, or event and can thus serve as primary sources as important ingredients of the historical methodology. Significant historical documen ...
can offer insights into past changes in the climate. Changes in climate have been linked to the rise and the collapse of various civilizations.


Proxy measurements

Various archives of past climate are present in rocks, trees and fossils. From these archives, indirect measures of climate, so-called proxies, can be derived. Quantification of climatological variation of precipitation in prior centuries and epochs is less complete but approximated using proxies such as marine sediments, ice cores, cave stalagmites, and tree rings. Stress, too little precipitation or unsuitable temperatures, can alter the growth rate of trees, which allows scientists to infer climate trends by analyzing the growth rate of tree rings. This branch of science studying this called
dendroclimatology Dendroclimatology is the science of determining past climates from trees (primarily properties of the annual tree rings). Tree rings are wider when conditions favor growth, narrower when times are difficult. Other properties of the annual rings, s ...
. Glaciers leave behind
moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice she ...
s that contain a wealth of material—including organic matter, quartz, and potassium that may be dated—recording the periods in which a glacier advanced and retreated. Analysis of ice in cores drilled from an
ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at L ...
such as the
Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...
, 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 continues to provide valuable information about the differences between ancient and modern atmospheric conditions. The 18O/16O ratio in calcite and ice core samples used to deduce ocean temperature in the distant past is an example of a temperature proxy method. The remnants of plants, and specifically pollen, are also used to study climatic change. Plant distributions 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 layers of sediment indicate changes in plant communities. These changes are often a sign of a changing climate. As an example, pollen studies have been used to track changing vegetation patterns throughout the Quaternary glaciations and especially since the last glacial maximum. Remains of
beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s 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 millennia, 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.


Analysis and uncertainties

One difficulty in detecting climate cycles is that the Earth's climate has been changing in non-cyclic ways over most paleoclimatological timescales. Currently we are in a period of
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human im ...
global warming. In a larger timeframe, the Earth is emerging from the latest ice age, cooling from the
Holocene climatic optimum The Holocene Climate Optimum (HCO) was a warm period that occurred in the interval roughly 9,000 to 5,000 years ago BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimu ...
and warming from the "
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Ma ...
", which means that climate has been constantly changing over the last 15,000 years or so. During warm periods, temperature fluctuations are often of a lesser amplitude. The Pleistocene period, dominated by repeated
glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
s, developed out of more stable conditions in the Miocene and Pliocene climate. Holocene climate has been relatively stable. All of these changes complicate the task of looking for cyclical behavior in the climate. Positive feedback,
negative feedback Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...
, and ecological inertia from the land-ocean-atmosphere system often attenuate or reverse smaller effects, whether from orbital forcings, solar variations or changes in concentrations of greenhouse gases. Certain feedbacks involving processes such as clouds are also uncertain; for
contrail Contrails (; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails ar ...
s, natural
cirrus Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal * Cirrus (botany), a tendril *Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a typ ...
clouds, oceanic dimethyl sulfide and a land-based equivalent, competing theories exist concerning effects on climatic temperatures, for example contrasting the Iris hypothesis and
CLAW hypothesis The CLAW hypothesis proposes a negative feedback, negative feedback loop that operates between ocean ecosystems and the Earth's climate. The hypothesis specifically proposes that particular phytoplankton that produce dimethyl sulfide are respon ...
.


Impacts


Life


Vegetation

A change in the type, distribution and coverage of vegetation may occur given a change in the climate. Some changes in climate may result in increased precipitation and warmth, resulting in improved plant growth and the subsequent sequestration of airborne CO2. The effects are expected to affect the rate of many natural cycles like plant litter decomposition rates. A gradual increase in warmth in a region will lead to earlier flowering and fruiting times, driving a change in the timing of life cycles of dependent organisms. Conversely, cold will cause plant bio-cycles to lag. Larger, faster or more radical changes, however, may result in vegetation stress, rapid plant loss and
desertification Desertification is a type of land degradation in drylands in which biological productivity is lost due to natural processes or induced by human activities whereby fertile areas become increasingly arid. It is the spread of arid areas caused b ...
in certain circumstances. An example of this occurred during the Carboniferous Rainforest Collapse (CRC), an extinction event 300 million years ago. At this time vast rainforests covered the equatorial region of Europe and America. Climate change devastated these tropical rainforests, abruptly fragmenting the habitat into isolated 'islands' and causing the extinction of many plant and animal species.


Wildlife

One of the most important ways animals can deal with climatic change is migration to warmer or colder regions. On a longer timescale, evolution makes ecosystems including animals better adapted to a new climate. Rapid or large climate change can cause
mass extinctions An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It ...
when creatures are stretched too far to be able to adapt.


Humanity

Collapses of past civilizations such as the Maya may be related to cycles of precipitation, especially drought, that in this example also correlates to the Western Hemisphere Warm Pool. Around 70 000 years ago the Toba supervolcano eruption created an especially cold period during the ice age, leading to a possible
genetic bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as specicide, widespread violen ...
in human populations.


Changes in the cryosphere


Glaciers and ice sheets

Glaciers are considered among the most sensitive indicators of a changing climate. Their size is determined by a mass balance between snow input and melt output. As temperatures increase, glaciers retreat unless snow precipitation increases to make up for the additional melt. Glaciers grow and shrink due both to natural variability and external forcings. Variability in temperature, precipitation and hydrology can strongly determine the evolution of a glacier in a particular season. The most significant climate processes since the middle to late Pliocene (approximately 3 million years ago) are the glacial and
interglacial An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene i ...
cycles. The present interglacial period (the Holocene) has lasted about 11,700 years. Shaped by orbital variations, responses such as the rise and fall of
continental Continental may refer to: Places * Continent, the major landmasses of Earth * Continental, Arizona, a small community in Pima County, Arizona, US * Continental, Ohio, a small town in Putnam County, US Arts and entertainment * ''Continental'' ( ...
ice sheets and significant sea-level changes helped create the climate. Other changes, including Heinrich events,
Dansgaard–Oeschger event Dansgaard–Oeschger events (often abbreviated D–O events), named after palaeoclimatologists Willi Dansgaard and Hans Oeschger, are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists say that th ...
s and the
Younger Dryas The Younger Dryas (c. 12,900 to 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM, c. 27,000 to 20,000 years BP). The Younger Dryas was the last stage ...
, however, illustrate how glacial variations may also influence climate without the orbital forcing.


Sea level change

During the Last Glacial Maximum, some 25,000 years ago, sea levels were roughly 130 m lower than today. The deglaciation afterwards was characterized by rapid sea level change. In the early Pliocene, global temperatures were 1–2˚C warmer than the present temperature, yet sea level was 15–25 meters higher than today.


Sea ice

Sea ice plays an important role in Earth's climate as it affects the total amount of sunlight that is reflected away from the Earth. In the past, the Earth's oceans have been almost entirely covered by sea ice on a number of occasions, when the Earth was in a so-called Snowball Earth state, and completely ice-free in periods of warm climate. When there is a lot of sea ice present globally, especially in the tropics and subtropics, the climate is more sensitive to forcings as the
ice–albedo feedback Ice–albedo feedback is a positive feedback climate process where a change in the area of ice caps, glaciers, and sea ice alters the albedo and surface temperature of a planet. Ice is very reflective, therefore it reflects far more solar energy ba ...
is very strong.


Climate History

Various
climate forcing Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...
s are typically in flux throughout
geologic time The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochron ...
, and some processes of the Earth's temperature may be self-regulating. For example, during the Snowball Earth period, large glacial ice sheets spanned to Earth's equator, covering nearly its entire surface, and very high
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
created extremely low temperatures, while the accumulation of snow and ice likely removed carbon dioxide through atmospheric deposition. However, the absence of
plant cover The abundances of plant species are often measured by plant cover, which is the relative area covered by different plant species in a small plot. Plant cover is not biased by the size and distributions of individuals, and is an important and oft ...
to absorb atmospheric CO2 emitted by volcanoes meant that the greenhouse gas could accumulate in the atmosphere. There was also an absence of exposed silicate rocks, which use CO2 when they undergo weathering. This created a warming that later melted the ice and brought Earth's temperature back up.


Paleo-Eocene Thermal maximum

The Paleocene–Eocene Thermal Maximum (PETM) was a time period with more than 5–8 °C global average temperature rise across the event. This climate event occurred at the time boundary of the Paleocene and Eocene geological
epochs In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...
. During the event large amounts of methane was released, a potent greenhouse gas. The PETM represents a "case study" for modern climate change as in the greenhouse gases were released in a geologically relatively short amount of time. During the PETM, a mass extinction of organisms in the deep ocean took place.


The Cenozoic

Throughout the
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configurat ...
, multiple climate forcings led to warming and cooling of the atmosphere, which led to the early formation of the
Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...
, subsequent melting, and its later reglaciation. The temperature changes occurred somewhat suddenly, at carbon dioxide concentrations of about 600–760 ppm and temperatures approximately 4 °C warmer than today. During the Pleistocene, cycles of glaciations and interglacials occurred on cycles of roughly 100,000 years, but may stay longer within an interglacial when
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values bet ...
approaches zero, as during the current interglacial. Previous interglacials such as the Eemian phase created temperatures higher than today, higher sea levels, and some partial melting of the West Antarctic ice sheet. Climatological temperatures substantially affect cloud cover and precipitation. At lower temperatures, air can hold less water vapour, which can lead to decreased precipitation. During the Last Glacial Maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto continental landmasses was low, causing large areas of extreme desert, including polar deserts (cold but with low rates of cloud cover and precipitation). In contrast, the world's climate was cloudier and wetter than today near the start of the warm
Atlantic Period The Atlantic in palaeoclimatology was the warmest and moistest Blytt–Sernander period, pollen zone and chronozone of Holocene northern Europe. The climate was generally warmer than today. It was preceded by the Boreal, with a climate simil ...
of 8000 years ago.


The Holocene

The Holocene is characterized by a long-term cooling starting after the Holocene Optimum, when temperatures were probably only just below current temperatures (second decade of the 21st century), and a strong
African Monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscill ...
created grassland conditions in the Sahara during the
Neolithic Subpluvial The African humid period (AHP) (also known by other names) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, ...
. Since that time, several cooling events have occurred, including: *the
Piora Oscillation The Piora Oscillation was an abrupt cold and wet period in the climate history of the Holocene Epoch; it is roughly dated to c. 3900-3000 BC. Some researchers associate the Piora Oscillation with the end of the Atlantic climate regime, and the sta ...
*the Middle Bronze Age Cold Epoch *the Iron Age Cold Epoch *the
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Ma ...
*the phase of cooling c. 1940–1970, which led to
global cooling Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing. Some press reports in the 1970s specula ...
hypothesis In contrast, several warm periods have also taken place, and they include but are not limited to: *a warm period during the apex of the Minoan civilization *the Roman Warm Period *the Medieval Warm Period * Modern warming during the 20th century Certain effects have occurred during these cycles. For example, during the Medieval Warm Period, the
American Midwest The Midwestern United States, also referred to as the Midwest or the American Midwest, is one of four census regions of the United States Census Bureau (also known as "Region 2"). It occupies the northern central part of the United States. I ...
was in drought, including the Sand Hills of Nebraska which were active
sand dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
s. The
black death The Black Death (also known as the Pestilence, the Great Mortality or the Plague) was a bubonic plague pandemic occurring in Western Eurasia and North Africa from 1346 to 1353. It is the most fatal pandemic recorded in human history, causing ...
plague of '' Yersinia pestis'' also occurred during Medieval temperature fluctuations, and may be related to changing climates. Solar activity may have contributed to part of the modern warming that peaked in the 1930s. However, solar cycles fail to account for warming observed since the 1980s to the present day . Events such as the opening of the Northwest Passage and recent record low ice minima of the modern
Arctic shrinkage Arctic sea ice decline has occurred in recent decades due to the effects of climate change on oceans, with declines in sea ice area, extent, and volume. Sea ice in the Arctic Ocean has been melting more in summer than it refreezes in the winter. ...
have not taken place for at least several centuries, as early explorers were all unable to make an Arctic crossing, even in summer. Shifts in
biome A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
s and habitat ranges are also unprecedented, occurring at rates that do not coincide with known climate oscillations .


Modern climate change and global warming

As a consequence of humans emitting
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es, global surface temperatures have started rising. Global warming is an aspect of modern climate change, a term that also includes the observed changes in precipitation, storm tracks and cloudiness. As a consequence, glaciers worldwide have been found to be shrinking significantly. Land ice sheets in both
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
and Greenland have been losing mass since 2002 and have seen an acceleration of ice mass loss since 2009. Global sea levels have been rising as a consequence of thermal expansion and ice melt. The decline in Arctic sea ice, both in extent and thickness, over the last several decades is further evidence for rapid climate change.


Variability between regions

In addition to global climate variability and global climate change over time, numerous climatic variations occur contemporaneously across different physical regions. The oceans' absorption of about 90% of excess heat has helped to cause land surface temperatures to grow more rapidly than sea surface temperatures. The Northern Hemisphere, having a larger landmass-to-ocean ratio than the Southern Hemisphere, shows greater average temperature increases. Variations across different latitude bands also reflect this divergence in average temperature increase, with the temperature increase of northern extratropics exceeding that of the tropics, which in turn exceeds that of the southern extratropics. Upper regions of the atmosphere have been cooling contemporaneously with a warming in the lower atmosphere, confirming the action of the greenhouse effect and ozone depletion. Observed regional climatic variations confirm predictions concerning ongoing changes, for example, by contrasting (smoother) year-to-year global variations with (more volatile) year-to-year variations in localized regions. Conversely, comparing different regions' warming patterns to their respective historical variabilities, allows the raw magnitudes of temperature changes to be placed in the perspective of what is normal variability for each region. Regional variability observations permit study of regionalized climate tipping points such as rainforest loss, ice sheet and sea ice melt, and permafrost thawing. Correction dated 9 April 2020 Such distinctions underlie research into a possible global cascade of tipping points.


See also

*
Climatological normal Climatological normal or climate normal (CN) is a 30-year average of a weather variable for a given time of year.Anthropocene The Anthropocene ( ) is a proposed geological epoch dating from the commencement of significant human impact on Earth's geology and ecosystems, including, but not limited to, anthropogenic climate change. , neither the International Commissio ...


Notes


References

* * (pb: ). * . * * * *


External links


Global Climate Change
from NASA (US)
Intergovernmental Panel on Climate Change (IPCC)

Climate Variability
– NASA Science
Climate Change and Variability, National Centers for Environmental Information
{{DEFAULTSORT:Climate Change Climate and weather statistics History of climate variability and change Articles containing video clips Climatology Natural environment