Climate change is a change in the statistical distribution of
weather over
periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average (for example, greater or fewer extreme weather events). Climate change may be limited to a specific
region, or may occur across the whole
Earth.
In recent usage, especially in the context of
environmental policy, climate change usually refers to changes in modern
climate. It may be qualified as
anthropogenic climate change, more generally known as "
global warming" or "
anthropogenic global warming" (
AGW).
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
Causes
Factors that can shape climate are
climate forcings. These include such processes as variations in
solar radiation, deviations in the Earth's
orbit,
mountain-building and
continental drift, and changes in
greenhouse gas concentrations. There are a variety of
climate change feedbacks that can either amplify or diminish the initial forcing. Some parts of the climate system, such as the oceans and ice caps, respond slowly in reaction to climate forcing because of their large mass. Therefore, the climate system can take centuries or longer to fully respond to new external forcings.
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.
[4]
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.
[5][6] During the
Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased
glaciation.
[7] 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.
[8]
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.
Solar output
Main article: Solar variation
Variations in solar activity during the last several centuries based on observations of
sunspots and
beryllium isotopes.
The
sun is the predominant source for
energy input to the Earth. Both long- and short-term variations in solar intensity are known to affect global climate.
Three to four billion years ago the sun emitted only 70% as much power as it does today. If the atmospheric composition had been the same as today, liquid water should not have existed on Earth. However, there is evidence for the presence of water on the early Earth, in the
Hadean[9][10] and
Archean[11][9] eons, leading to what is known as the
faint young sun paradox.
[12] Hypothesized solutions to this paradox include a vastly different atmosphere, with much higher concentrations of
greenhouse gases than currently exist
[13] Over the following approximately 4 billion years, the energy output of the sun increased and atmospheric composition changed, with the
oxygenation of the atmosphere around 2.4 billion years ago being the most notable alteration. These changes in luminosity, and 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 life on Earth.
Solar output also
varies on shorter time scales, including the 11-year
solar cycle[14] and longer-term
modulations.
[15] Solar intensity variations are considered to have been influential in triggering the
Little Ice Age,
[16] and some of the warming observed from 1900 to 1950. The cyclical nature of the sun's energy output is not yet fully understood; it differs from the very slow change that is happening within the sun as it ages and evolves. While most research indicates solar variability has induced a small cooling effect from 1750 to the present, a few studies point toward solar radiation increases from cyclical sunspot activity affecting
global warming.
[17] [18]
Orbital variations
Main article: Milankovitch cycles
Slight variations in Earth's
orbit 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 orbital variations are variations in Earth's
eccentricity, changes in
the tilt angle of Earth's axis of rotation, and
precession of Earth's axis. Combined together, these produce
Milankovitch cycles which have a large impact on climate and are notable for their correlation to
glacial and
interglacial periods,
[19] their correlation with the advance and retreat of the
Sahara,
[19] and for their
appearance in the
stratigraphic record.
[20]
Volcanism
Volcanism is a process of conveying material from the
crust and
mantle of the Earth to its surface. Volcanic eruptions,
geysers, and
hot springs, are examples of volcanic processes which release gases and/or
particulates into the atmosphere.
Eruptions large enough to affect climate occur on average several times per century, and cause cooling (by partially blocking the transmission of
solar radiation to the Earth's surface) for a period of a few years. The eruption of
Mount Pinatubo in 1991, the second largest terrestrial eruption of the 20th century
[21] (after the 1912 eruption of
Novarupta[22]) affected the climate substantially. Global temperatures decreased by about 0.5 °C (0.9 °F). The eruption of
Mount Tambora in 1815 caused the
Year Without a Summer.
[23] Much larger eruptions, known as
large igneous provinces, occur only a few times every hundred million years, but may cause global warming and
mass extinctions.
[24]
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. According to the
US Geological Survey, however, estimates are that human activities generate more than 130 times the amount of carbon dioxide emitted by volcanoes.
[25]
Ocean variability
Main article: Thermohaline circulation
A schematic of modern
thermohaline circulation
The ocean is a fundamental part of the climate system. Short-term fluctuations (years to a few decades) such as the
El Niño–Southern Oscillation, the
Pacific decadal oscillation, the
North Atlantic oscillation, and the
Arctic oscillation, represent climate variability rather than climate change. On longer time scales, alterations to ocean processes such as
thermohaline circulation play a key role in redistributing heat by carrying out a very slow and extremely deep movement of
water, and the long-term redistribution of
heat in the world's oceans.
Human influences
Main article: Global warming
Increase in Atmospheric CO2 Levels
Anthropogenic factors are human activities that change the environment. In some cases the chain of causality of human influence on the climate is direct and unambiguous (for example, the effects of irrigation on local humidity), while in other instances it is less clear. Various hypotheses for human-induced climate change have been argued for many years. Presently the
scientific consensus on climate change is that human activity is very likely the cause for the rapid increase in global average temperatures over the past several decades.
[26] Consequently, the debate has largely shifted onto ways to reduce further human impact and to find ways to adapt to change that has already occurred.
[27]
Of most concern in these anthropogenic factors is the increase in CO
2 levels due to emissions from
fossil fuel combustion, followed by
aerosols (particulate matter in the atmosphere) and
cement manufacture. Other factors, including land use,
ozone depletion, animal agriculture
[28] and
deforestation, are also of concern in the roles they play - both separately and in conjunction with other factors - in affecting climate,
microclimate, and measures of climate variables.
Physical evidence for climatic change
Evidence for climatic change is taken from a variety of sources that can be used to reconstruct past climates. Reasonably complete global records of surface temperature are available beginning from the mid-late 1800s. For earlier periods, most of the evidence is indirect—climatic changes are inferred from changes in
proxies, indicators that reflect climate, such as
vegetation,
ice cores,
[29] dendrochronology,
sea level change, and
glacial geology.
Historical and archaeological evidence
Main article: Historical impacts of climate change
Climate change in the recent past may be detected by corresponding changes in settlement and agricultural patterns.
[30] Archaeological evidence,
oral history and
historical documents can offer insights into past changes in the climate. Climate change effects have been linked to the collapse of various civilisations.
[30]
Glaciers
Variations in
CO2, temperature and dust from the
Vostok ice core over the last 450,000 years
Decline in thickness of glaciers worldwide
are considered among the most sensitive indicators of climate change,
[31] advancing when climate cools and retreating when climate warms. Glaciers grow and shrink, both contributing to natural variability and
amplifying externally forced changes. A world glacier inventory has been compiled since the 1970s, initially based mainly on aerial photographs and maps but now relying more on satellites. This compilation tracks more than 100,000 glaciers covering a total area of approximately 240,000 km
2, and preliminary estimates indicate that the remaining ice cover is around 445,000 km
2. The World Glacier Monitoring Service collects data annually on
glacier retreat and
glacier mass balance From this data, glaciers worldwide have been found to be shrinking significantly, with strong glacier retreats in the 1940s, stable or growing conditions during the 1920s and 1970s, and again retreating from the mid 1980s to present.
[32]
The most significant climate processes since the middle to late
Pliocene (approximately 3 million years ago) are the glacial and
interglacial cycles. The present interglacial period (the
Holocene) has lasted about 11,700 years.
[33] Shaped by
orbital variations, responses such as the rise and fall of
continental ice sheets and significant sea-level changes helped create the climate. Other changes, including
Heinrich events,
Dansgaard–Oeschger events and the
Younger Dryas, however, illustrate how glacial variations may also influence climate without the
orbital forcing.
Glaciers leave behind
moraines 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. Similarly, by
tephrochronological techniques, the lack of glacier cover can be identified by the presence of soil or volcanic
tephra horizons whose date of deposit may also be ascertained.
Vegetation
A change in the type, distribution and coverage of vegetation may occur given a change in the climate; this much is obvious. In any given scenario, a mild change in climate may result in increased precipitation and warmth, resulting in improved plant growth and the subsequent sequestration of airborne CO
2. Larger, faster or more radical changes, however, may well
[weasel words] result in vegetation stress, rapid plant loss and desertification in certain circumstances.
[34]
Ice cores
Analysis of ice in a core drilled from a
ice sheet such as the
Antarctic ice sheet, 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 CO
2 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 CO
2 over many millennia, and continues to provide valuable information about the differences between ancient and modern atmospheric conditions.
Dendroclimatology
Dendroclimatology is the analysis of tree ring growth patterns to determine past climate variations. Wide and thick rings indicate a fertile, well-watered growing period, whilst thin, narrow rings indicate a time of lower rainfall and less-than-ideal growing conditions.
Pollen analysis
Palynology is the study of contemporary and fossil
palynomorphs, including
pollen. Palynology is used to infer the geographical distribution of plant species, which vary under different climate conditions. Different groups of plants have
pollen with distinctive shapes and surface textures, and since the outer surface of pollen is composed of a very resilient material, they resist decay. Changes in the type of pollen found in different layers of sediment in lakes, bogs, or river deltas indicate changes in plant communities. These changes are often a sign of a changing climate.
[35][36] As an example, palynological studies have been used to track changing vegetation patterns throughout the
Quaternary glaciations[37] and especially since the
last glacial maximum.
[38]
Insects
Remains of
beetles are common in freshwater and land sediments. Different species of beetles tend to be found under different climatic conditions. Given the extensive lineage of beetles whose genetic makeup has not altered significantly over the 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.
[39]
Sea level change
Main articles: Sea level and Current sea level rise
Global sea level change for much of the last century has generally been estimated using
tide gauge measurements collated over long periods of time to give a long-term average. More recently,
altimeter measurements — in combination with accurately determined
satellite orbits — have provided an improved measurement of global sea level change.
[40] To measure sea levels prior to instrumental measurements, scientists have dated
coral reefs that grow near the surface of the ocean, coastal sediments,
marine terraces,
ooids in
limestones, and nearshore archaeological remains. The predominant dating methods used are
uranium series and
radiocarbon, with
cosmogenic radionuclides being sometimes used to date terraces that have experienced relative sea level fall.
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