On the broadest scale, the rate at which energy is received from the
Sun 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 forcings or "forcing mechanisms". These include processes such as variations in
solar radiation, variations in the Earth's orbit, variations in the
albedo or reflectivity of the continents, atmosphere, and oceans,
mountain-building and
continental drift and changes in
greenhouse gas 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 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 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.
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, 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 jointly with
Syukuro Manabe for related work on
climate modelling. 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 . The
El Niño–Southern Oscillation has been linked to variability in longer-term global average temperature increase, with El Niño years usually corresponding to annual global temperature increases. A
climate oscillation or
climate cycle is any recurring cyclical
oscillation within global or regional
climate. They are
quasiperiodic (not perfectly periodic), so a
Fourier analysis of the data does not have sharp peaks in the
spectrum. Many oscillations on different time-scales have been found or hypothesized: • the
El Niño–Southern Oscillation (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 of cold waters off the west coast of South America. • the
Madden–Julian oscillation (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 (SLP) between
Ponta Delgada, Azores and
Stykkishólmur/
Reykjavík, Iceland. Positive values of the index indicate stronger-than-average westerlies over the middle latitudes. • the
Quasi-biennial oscillation – a well-understood oscillation in wind patterns in the
stratosphere 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 predicted by some
climate models • 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, with a period of tens of thousands of years. • the
Arctic oscillation (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 (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 . Tens of millions of years ago, continental-plate movement formed a land-free gap around Antarctica, allowing the formation of the
ACC, which keeps warm waters away from Antarctica. 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, 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 period) show that the circulation in 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 did not change much. These large changes may have come from so called
Heinrich events 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,
evapotranspiration,
cloud formation, and
weathering. Examples of how life may have affected past climate include: •
glaciation 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 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 gases 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. 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 sinks. 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 (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 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 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 and
interglacial periods, their correlation with the advance and retreat of the
Sahara, 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 is not 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. 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 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 and then a
white dwarf will have large effects on climate, with the red giant phase possibly ending any life on Earth that survives until that time.
Volcanism NASA satellites, effects appear from
aerosols released by major volcanic eruptions (
El Chichón and
Pinatubo).
El Niño is a separate event, from ocean variability. The
volcanic eruptions 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
tons of
SO2 into the
stratosphere. 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 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 provinces brings large quantities of
igneous rock from the
mantle 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 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 and may have led to Northern Hemisphere ice cover. During the
Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased
glaciation. Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the
supercontinent Pangaea, 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
ionized 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 designed the
CLOUD experiment, 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 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 ==