A request that this article title be changed to Causes of climate change is under discussion. Please do not move this article until the discussion is closed.

Observed temperature from NASA[1] vs the 1850–1900 average used by the IPCC as a pre-industrial baseline.[2] The primary driver for increased global temperatures in the industrial era is human activity, with natural forces adding variability.[3]

Scientific studies have investigated the causes of climate change. They have found that the main cause and driver of recent climate change is elevated levels of greenhouse gases produced by human activities. Natural forces add climate variability as well. Based on many scientific studies, it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times."[4]: 3  Studies on attribution have focused on changes observed during the period of instrumental temperature record, particularly in the last 50 years. This is the period when human activity has grown fastest and observations of the atmosphere above the surface have become available.[5] Some of the main human activities that contribute to global warming are:[6] (a) increasing atmospheric concentrations of greenhouse gases (mainly carbon dioxide), for a warming effect; (b) global changes to land surface, such as deforestation, for a warming effect; and (c) increasing atmospheric concentrations of aerosols, mainly for a cooling effect.

In addition to human activities, some natural mechanisms can also cause climate change, including for example, climate oscillations (for example El Niño–Southern Oscillation (ENSO)), changes in solar activity, and volcanic activity.

The IPCC's attribution of recent global warming to human activities reflects the view of the scientific community,[7][8][9] and is also supported by 196 other scientific organizations worldwide.[10]

Four main lines of evidence support attribution of recent climate change to human activities:[11] Firstly, a physical understanding of the climate system: greenhouse gas concentrations have increased and their warming properties are well-established. Secondly, there are historical estimates of past climate changes suggest that the recent changes in global surface temperature are unusual. Thirdly, computer-based climate models are unable to replicate the observed warming unless human greenhouse gas emissions are included. And finally, natural forces alone (such as solar and volcanic activity) cannot explain the observed warming.

Energy flows between space, the atmosphere, and Earth's surface. Rising greenhouse gas levels are contributing to an energy imbalance.

Factors affecting Earth's climate

CO2 sources and sinks since 1880. While there is little debate that excess carbon dioxide in the industrial era has mostly come from burning fossil fuels, the future strength of land and ocean carbon sinks is an area of study.[12]

Factors affecting Earth's climate can be broken down into forcings, feedbacks and internal variations.[8]: 7  A forcing is something that is imposed externally on the climate system. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output.[13] Human activities can also impose forcings, for example, through changing the composition of Earth's atmosphere.

Radiative forcing is a measure of how various factors alter the energy balance of planet Earth.[14] A positive radiative forcing will lead towards a warming of the surface and, over time, the climate system. Between the start of the Industrial Revolution in 1750, and the year 2005, the increase in the atmospheric concentration of carbon dioxide (chemical formula: CO2) led to a positive radiative forcing, averaged over the Earth's surface area, of about 1.66 watts per square metre (abbreviated W m−2).[15]

Climate feedbacks can either amplify or dampen the response of the climate to a given forcing.[8]: 7  There are many feedback mechanisms in the climate system that can either amplify (a positive feedback) or diminish (a negative feedback) the effects of a change in climate forcing.

The climate system will vary in response to changes in forcings.[16] The climate system will show internal variability both in the presence and absence of forcings imposed on it, (see images opposite). This internal variability is a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean (see also the later section on Internal climate variability and global warming).[17] An example of internal variability is the El Niño–Southern Oscillation.

Main causes

Further information: Climate change § Causes of recent global temperature rise

Greenhouse gases

See also: Climate change § Greenhouse gases, Greenhouse gas emissions, and Greenhouse effect
The warming influence (called radiative forcing) of different contributors to climate change in 2011, as reported in the fifth IPCC assessment report.[18]
Warming influence of atmospheric greenhouse gases has nearly doubled since 1979, with carbon dioxide and methane being the dominant drivers.[19]

CO2 is absorbed and emitted naturally as part of the carbon cycle, through animal and plant respiration, volcanic eruptions, and ocean-atmosphere exchange.[18] Human activities, such as the burning of fossil fuels and changes in land use (see below), release large amounts of carbon to the atmosphere, causing CO2 concentrations in the atmosphere to rise.[18][20]

The high-accuracy measurements of atmospheric CO2 concentration, initiated by Charles David Keeling in 1958, constitute the master time series documenting the changing composition of the atmosphere.[21] These data, known as the Keeling Curve, have iconic status in climate change science as evidence of the effect of human activities on the chemical composition of the global atmosphere.[21]

The Keeling Curve shows the long-term increase of atmospheric carbon dioxide (CO2) concentrations from 1958–2018. Monthly CO2 measurements display seasonal oscillations in an upward trend. Each year's maximum occurs during the Northern Hemisphere's late spring.

Keeling's initial 1958 measurements showed 313 parts per million by volume (ppm). Atmospheric CO2 concentrations, commonly written "ppm", are measured in parts-per-million by volume (ppmv). In May 2019, the concentration of CO2 in the atmosphere reached 415 ppm. The last time when it reached this level was 2.6–5.3 million years ago. Without human intervention, it would be 280 ppm.[22]

Along with CO2, methane and to a lesser extent nitrous oxide are also major forcing contributors to the greenhouse effect. The Kyoto Protocol lists these together with hydrofluorocarbon (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6),[23] which are entirely artificial gases, as contributors to radiative forcing. The chart at right attributes anthropogenic greenhouse gas emissions to eight main economic sectors, of which the largest contributors are power stations (many of which burn coal or other fossil fuels), industrial processes, transportation fuels (generally fossil fuels), and agricultural by-products (mainly methane from enteric fermentation and nitrous oxide from fertilizer use).[24]

Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. This has led to increases in mean global temperature, or global warming. Other human effects are relevant—for example, sulphate aerosols are believed to have a cooling effect. Natural factors also contribute. The likely range of human-induced surface-level air warming by 2010–2019 compared to levels in 1850–1900 is 0.8 °C to 1.3 °C, with a best estimate of 1.07 °C. This is close to the observed overall warming during that time of 0.9 °C to 1.2 °C, while temperature changes during that time were likely only ±0.1 °C due to natural forcings and ±0.2 °C due to variability in the climate.[25]: 3, 443 

Water vapor

Water vapor is the most abundant greenhouse gas and is the largest contributor to the natural greenhouse effect, despite having a short atmospheric lifetime[18] (about 10 days).[26] Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation.[18] Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.[18]

Land surface changes

Further information: Climate change § Land surface changes
The rate of global tree cover loss has approximately doubled since 2001, to an annual loss approaching an area the size of Italy.[27]
Cumulative land-use change contributions to emissions, by region.[28]

Climate change is attributed to land use for two main reasons. Between 1750 and 2007, about two-thirds of anthropogenic CO2 emissions were produced from burning fossil fuels, and about one-third of emissions from changes in land use,[29] primarily deforestation.[30] Deforestation reduces the amount of carbon dioxide absorbed by ecosystems and releases greenhouse gases directly, which together with aerosols, exacerbate climate change.[31]

Land use drivers of climate change are sometimes indirect effects of human activity. For example, elephants in Africa are generally protective of slow-growing trees that are good for carbon dioxide sequestration. The sharply declining elephant population, due to human predation, indirectly leads to a tree population which absorbs less carbon dioxide.[32]

A second reason that climate change has been attributed to land use is that the terrestrial albedo is often altered by use, which leads to radiative forcing. This effect is more significant locally than globally.[30]

Livestock and land use

More than 18% of anthropogenic greenhouse gas emissions are attributed to livestock and livestock-related activities such as deforestation and increasingly fuel-intensive farming practices.[33] Specific attributions to the livestock sector include:

Aerosols

Further information: Climate change § Aerosols and clouds, and Particulates § Climate effects

With virtual certainty, scientific consensus has attributed various forms of climate change, chiefly cooling effects, to aerosols, which are small particles or droplets suspended in the atmosphere.[34][obsolete source] Key sources to which anthropogenic aerosols are attributed[35] include:

Analytical methods

In detection and attribution, natural factors include changes in the Sun's output and volcanic eruptions, as well as natural modes of variability such as El Niño and La Niña. Human factors include the emissions of heat-trapping "greenhouse" gases and particulates as well as clearing of forests and other land-use changes. Figure source: NOAA NCDC.[36]

Detection and attribution

Refer to caption
This image shows three examples of internal climate variability measured between 1950 and 2012: the El Niño–Southern oscillation, the Arctic oscillation, and the North Atlantic oscillation.[37]

Detection and attribution of climate signals, as well as its common-sense meaning, has a more precise definition within the climate change literature, as expressed by the IPCC.[38] Detection of a climate signal does not always imply significant attribution. The IPCC's Fourth Assessment Report says "it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750", where "extremely likely" indicates a probability greater than 95%.[39] Detection of a signal requires demonstrating that an observed change is statistically significantly different from that which can be explained by natural internal variability.

Attribution requires demonstrating that a signal is:

The IPCC Fourth Assessment Report (2007), concluded that attribution was possible for a number of observed changes in the climate (see effects of global warming). However, attribution was found to be more difficult when assessing changes over smaller regions (less than continental scale) and over short time periods (less than 50 years).[40] Over larger regions, averaging reduces natural variability of the climate, making detection and attribution easier.

Lines of evidence

Among the possible factors that could produce changes in global mean temperature are internal variability of the climate system, external forcing, an increase in concentration of greenhouse gases, or any combination of these. Current studies indicate that the increase in greenhouse gases, most notably CO2, is mostly responsible for the observed warming. Evidence for this conclusion includes:

Recent scientific assessments find that most of the warming of the Earth's surface over the past 50 years has been caused by human activities. This conclusion rests on multiple lines of evidence. Like the warming "signal" that has gradually emerged from the "noise" of natural climate variability, the scientific evidence for a human influence on global climate has accumulated over the past several decades, from many hundreds of studies. [44]

The first line of evidence is based on a physical understanding of how greenhouse gases trap heat, how the climate system responds to increases in greenhouse gases, and how other human and natural factors influence climate. The second line of evidence is from indirect estimates of climate changes over the last 1,000 to 2,000 years. These records are obtained from living things and their remains (like tree rings and corals) and from physical quantities (like the ratio between lighter and heavier isotopes of oxygen in ice cores), which change in measurable ways as climate changes. [44]

The third line of evidence is based on the broad, qualitative consistency between observed changes in climate and the computer model simulations of how climate would be expected to change in response to human activities. For example, when climate models are run with historical increases in greenhouse gases, they show gradual warming of the Earth and ocean surface, increases in ocean heat content and the temperature of the lower atmosphere, a rise in global sea level, retreat of sea ice and snow cover, cooling of the stratosphere, an increase in the amount of atmospheric water vapor, and changes in large-scale precipitation and pressure patterns. These and other aspects of modelled climate change are in agreement with observations.[44]

"Fingerprint" studies

Human fingerprints for global warming (summary of observational evidence that human carbon dioxide emissions are causing the climate to warm).[45]
Top panel: Observed global average temperature change (1870— ).
Bottom panel: Data from the Fourth National Climate Assessment[46] is merged for display on the same scale to emphasize relative strengths of forces affecting temperature change. Human-caused forces have increasingly dominated.

To determine the human contribution to climate change, unique "fingerprints" for all potential causes are developed and compared with both observed patterns and known internal climate variability.[47][48]: 875–876  For example, solar forcing—whose fingerprint involves warming the entire atmosphere—is ruled out because only the lower atmosphere has warmed.[49]: 20  Atmospheric aerosols produce a smaller, cooling effect. Other drivers, such as changes in albedo, are less impactful.[50]: 7 

Fingerprint studies exploit these unique signatures, and allow detailed comparisons of modelled and observed climate change patterns. Scientists rely on such studies to attribute observed changes in climate to a particular cause or set of causes. In the real world, the climate changes that have occurred since the start of the Industrial Revolution are due to a complex mixture of human and natural causes. The importance of each individual influence in this mixture changes over time. Therefore, climate models are used to study how individual factors affect climate. For example, a single factor (like greenhouse gases) or a set of factors can be varied, and the response of the modelled climate system to these individual or combined changes can thus be studied.[44]

These projections have been confirmed by observations (shown above).[51] For example, when climate model simulations of the last century include all of the major influences on climate, both human-induced and natural, they can reproduce many important features of observed climate change patterns. When human influences are removed from the model experiments, results suggest that the surface of the Earth would actually have cooled slightly over the last 50 years. The clear message from fingerprint studies is that the observed warming over the last half-century cannot be explained by natural factors, and is instead caused primarily by human factors.[44]

Atmospheric fingerprints

Another fingerprint of human effects on climate has been identified by looking at a slice through the layers of the atmosphere, and studying the pattern of temperature changes from the surface up through the stratosphere (see the section on solar activity). The earliest fingerprint work focused on changes in surface and atmospheric temperature. Scientists then applied fingerprint methods to a whole range of climate variables, identifying human-caused climate signals in the heat content of the oceans, the height of the tropopause (the boundary between the troposphere and stratosphere, which has shifted upward by hundreds of feet in recent decades), the geographical patterns of precipitation, drought, surface pressure, and the runoff from major river basins.[52]

Studies published after the appearance of the IPCC Fourth Assessment Report in 2007 have also found human fingerprints in the increased levels of atmospheric moisture (both close to the surface and over the full extent of the atmosphere), in the decline of Arctic sea ice extent, and in the patterns of changes in Arctic and Antarctic surface temperatures.[52]

Extreme event attribution

Extreme event attribution, also known as attribution science, is a relatively new field of study in meteorology and climate science that tries to measure how ongoing climate change directly affects extreme events (rare events), for example extreme weather events.[53][54] Attribution science aims to determine which such recent events can be explained by or linked to a warming atmosphere and are not simply due to natural variations.[55]
refer to caption
Frequency of occurrence (vertical axis) of local June–July–August temperature anomalies (relative to 1951–1980 mean) for Northern Hemisphere land in units of local standard deviation (horizontal axis).[56] According to Hansen et al. (2012),[56] the distribution of anomalies has shifted to the right as a consequence of global warming, meaning that unusually hot summers have become more common. This is analogous to the rolling of a dice: cool summers now cover only half of one side of a six-sided die, white covers one side, red covers four sides, and an extremely hot (red-brown) anomaly covers half of one side.[56]

Potential causes that have been ruled out

See also: Climate change denial and History of climate change science § Discredited theories and reconciled apparent discrepancies

Scientists have investigated other possible causes of recent climate change but these have been ruled out:

With regards to solar variation, scientists reject the notion that the warming observed in the global mean surface temperature record since about 1850 is the result of solar variations: "The observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified."[57]

The consensus position is that solar radiation may have increased by 0.12 W/m2 since 1750, compared to 1.6 W/m2 for the net anthropogenic forcing.[58]: 3  Already in 2001, the IPCC Third Assessment Report had found that, "The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades."[59]

See also

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Sources

Public-domain sources

External links

Climate change
Categories