Drivers of climate change from 1850–1900 to 2010–2019. There was no significant contribution from internal variability or solar and volcanic drivers.

The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times."[1]: 3  This consensus is supported by around 200 scientific organizations worldwide,[2] The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.[1]

Observed temperature from NASA[3] vs the 1850–1900 average used by the IPCC as a pre-industrial baseline.[4] The primary driver for increased global temperatures in the industrial era is human activity, with natural forces adding variability.[5]

The warming from the greenhouse effect has a logarithmic relationship with the concentration of greenhouse gases. This means that every additional fraction of CO2 and the other greenhouse gases in the atmosphere has a slightly smaller warming effect than the fractions before it as the total concentration increases. However, only around half of CO2 emissions continually reside in the atmosphere in the first place, as the other half is quickly absorbed by carbon sinks in the land and oceans.[6]: 450  Further, the warming per unit of greenhouse gases is also affected by feedbacks, such as the changes in water vapor concentrations or Earth's albedo (reflectivity).[7]: 2233 

As the warming from CO2 increases, carbon sinks absorb a smaller fraction of total emissions, while the "fast" climate change feedbacks amplify greenhouse gas warming. Thus, both effects are considered to each other out, and the warming from each unit of CO2 emitted by humans increases temperature in linear proportion to the total amount of emissions.[8]: 746  Further, some fraction of the greenhouse warming has been "masked" by the human-caused emissions of sulfur dioxide, which forms aerosols that have a cooling effect. However, this masking has been receding in the recent years, due to measures to combat acid rain and air pollution caused by sulfates.[9][10]

Factors affecting Earth's climate

A diagram which shows where the extra heat retained on Earth due to the energy imbalance is going.

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.[11] 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.[12] 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).[13]

Climate feedbacks can either amplify or dampen the response of the climate to a given forcing.[14]: 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.[15] The climate system will show internal variability both in the presence and absence of forcings imposed on it. This internal variability is a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean.[16] An example of internal variability is the El Niño–Southern Oscillation.

Human-caused influences

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 can be broken down into forcings, feedbacks and internal variations.[14]: 7  Four main lines of evidence support the dominant role of human activities in recent climate change:[17]

  1. A physical understanding of the climate system: greenhouse gas concentrations have increased and their warming properties are well-established.
  2. There are historical estimates of past climate changes suggest that the recent changes in global surface temperature are unusual.
  3. Advanced climate models are unable to replicate the observed warming unless human greenhouse gas emissions are included.
  4. Observations of natural forces, such as solar and volcanic activity) show that cannot explain the observed warming. For example, an increase in solar activity would have warmed the entire atmosphere, yet only the lower atmosphere has warmed.[18]

Greenhouse gases

Warming influence of atmospheric greenhouse gases has nearly doubled since 1979, with carbon dioxide and methane being the dominant drivers.[19]

Main articles: Greenhouse gas, Greenhouse gas emissions, and Greenhouse effect

Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth's surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth's surface and warming it over time.[20] While water vapour and clouds are the biggest contributors to the greenhouse effect, they primarily change as a function of temperature. Therefore, they are considered to be feedbacks that change climate sensitivity. On the other hand, gases such as CO2, tropospheric ozone,[21] CFCs and nitrous oxide are added or removed independently from temperature. Hence, they are considered to be external forcings that change global temperatures.[22][23]: 742 

CO2 concentrations over the last 800,000 years as measured from ice cores[24][25][26][27] (blue/green) and directly[28] (black)

Human activity since the Industrial Revolution (about 1750), mainly extracting and burning fossil fuels (coal, oil, and natural gas), has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. By 2019, the concentrations of CO2 and methane had increased by about 48% and 160%, respectively, since 1750.[29] These CO2 levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years.[30]

This has led to increases in mean global temperature, or global warming. 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. 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.[31]: 3, 443 

Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of CO2. Of these emissions, 75% was CO2, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases.[32]: 7 

Carbon dioxide

Main article: Carbon dioxide in Earth's atmosphere
The Global Carbon Project shows how additions to CO2 since 1880 have been caused by different sources ramping up one after another.

CO2 emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity.[33] Additional CO2 emissions come from deforestation and industrial processes, which include the CO2 released by the chemical reactions for making cement, steel, aluminum, and fertiliser.[34]

CO2 is absorbed and emitted naturally as part of the carbon cycle, through animal and plant respiration, volcanic eruptions, and ocean-atmosphere exchange.[35] 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.[35][36]

The Keeling Curve shows the long-term increase of atmospheric carbon dioxide (CO2) concentrations from 1958–2018.

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.[37] 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.[37]

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.[38]

Methane and nitrous oxide

Main sources of global methane emissions (2008-2017) according to the Global Carbon Project[39]

Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction.[40] Nitrous oxide emissions largely come from the microbial decomposition of fertiliser.[41]

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),[42] 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).[43]

Aerosols

Air pollution has substantially increased the presence of aerosols in the atmosphere when compared to the preindustrial background levels. Different types of particles have different effects, but overall, cooling from aerosols formed by sulfur dioxide emissions has the overwhelming impact. However, the complexity of aerosol interactions in atmospheric layers makes the exact strength of cooling very difficult to estimate.[44]

Air pollution, in the form of aerosols, affects the climate on a large scale.[45][46] Aerosols scatter and absorb solar radiation. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed. This phenomenon is popularly known as global dimming,[47] and is primarily attributed to sulfate aerosols produced by the combustion of fossil fuels with heavy sulfur concentrations like coal and bunker fuel.[9] Smaller contributions come from black carbon, organic carbon from combustion of fossil fuels and biofuels, and from anthropogenic dust.[48][49][50][51][52] Globally, aerosols have been declining since 1990 due to pollution controls, meaning that they no longer mask greenhouse gas warming as much.[53][9]

Aerosols also have indirect effects on the Earth's energy budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[54] They also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight.[55] Indirect effects of aerosols are the largest uncertainty in radiative forcing.[56]

While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise.[57] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.[58]

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.[59]

According to Food and Agriculture Organization, around 30% of Earth's land area is largely unusable for humans (glaciers, deserts, etc.), 26% is forests, 10% is shrubland and 34% is agricultural land.[60] Deforestation is the main land use change contributor to global warming,[61] Between 1750 and 2007, about one-third of anthropogenic CO2 emissions were from changes in land use - primarily from the decline in forest area and the growth in agricultural land.[62] primarily deforestation.[63] as the destroyed trees release CO2, and are not replaced by new trees, removing that carbon sink.[64] Between 2001 and 2018, 27% of deforestation was from permanent clearing to enable agricultural expansion for crops and livestock. Another 24% has been lost to temporary clearing under the shifting cultivation agricultural systems. 26% was due to logging for wood and derived products, and wildfires have accounted for the remaining 23%.[65] Some forests have not been fully cleared, but were already degraded by these impacts. Restoring these forests also recovers their potential as a carbon sink.[66]

Cumulative land-use change contributions to CO2 emissions, by region.[32]: Figure SPM.2b 

Local vegetation cover impacts how much of the sunlight gets reflected back into space (albedo), and how much heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also modify the release of chemical compounds that influence clouds, and by changing wind patterns.[67] In tropic and temperate areas the net effect is to produce significant warming, and forest restoration can make local temperatures cooler.[66] At latitudes closer to the poles, there is a cooling effect as forest is replaced by snow-covered (and more reflective) plains.[67] Globally, these increases in surface albedo have been the dominant direct influence on temperature from land use change. Thus, land use change to date is estimated to have a slight cooling effect.[68]

Livestock-associated emissions

See also: Greenhouse gas emissions from agriculture
Meat from cattle and sheep have the highest emissions intensity of any agricultural commodity.

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.[69] Specific attributions to the livestock sector include:

Ripple effects

Carbon sinks

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.[70]

The Earth's surface absorbs CO2 as part of the carbon cycle. Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for CO2. Land-surface sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global CO2 emissions.[71] The ocean also serves as a significant carbon sink via a two-step process. First, CO2 dissolves in the surface water. Afterwards, the ocean's overturning circulation distributes it deep into the ocean's interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted CO2.[6]: 450  Thus, around half of human-caused CO2 emissions have been absorbed by land plants and by the oceans.[72]

This fraction of absorbed emissions is not static. If future CO2 emissions decrease, the Earth will be able to absorb up to around 70%. If they increase substantially, it'll still absorb more carbon than now, but the overall fraction will decrease to below 40%.[73] This is because climate change increases droughts and heat waves that eventually inhibit plant growth on land, and soils will release more carbon from dead plants when they are warmer.[74][75] The rate at which oceans absorb atmospheric carbon will be lowered as they become more acidic and experience changes in thermohaline circulation and phytoplankton distribution.[76][77][78]

Climate change feedbacks

Main articles: Climate change feedback and Climate sensitivity
Sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. As an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. This is a positive feedback process.[79]

The response of the climate system to an initial forcing is modified by feedbacks: increased by "self-reinforcing" or "positive" feedbacks and reduced by "balancing" or "negative" feedbacks.[80] The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds.[81][82] The primary balancing mechanism is radiative cooling, as Earth's surface gives off more heat to space in response to rising temperature.[83] In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO2 on plant growth.[84]

Uncertainty over feedbacks, particularly cloud cover,[85] is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.[86] As air warms, it can hold more moisture. Water vapour, as a potent greenhouse gas, holds heat in the atmosphere.[81] If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet.[87]

Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth's surface.[88] More of the Sun's energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes.[89] Arctic amplification is also thawing permafrost, which releases methane and CO2 into the atmosphere.[90] Climate change can also cause methane releases from wetlands, marine systems, and freshwater systems.[91] Overall, climate feedbacks are expected to become increasingly positive.[92]

Natural variability

The Fourth National Climate Assessment ("NCA4", USGCRP, 2017) includes charts illustrating that neither solar nor volcanic activity can explain the observed warming.[93] [94]

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

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."[95] Solar irradiance has been measured directly by satellites,[96] and indirect measurements are available from the early 1600s onwards.[56] Yet, since 1880, there has been no upward trend in the amount of the Sun's energy reaching the Earth, in contrast to the warming of the lower atmosphere (the troposphere).[97] Similarly, volcanic activity has the single largest natural impact (forcing) on temperature, yet it is equivalent to less than 1% of current human-caused CO2 emissions.[98] Volcanic activity as a whole has had negligible impacts on global temperature trends since the Industrial Revolution.[99]

Between 1750 and 2007, solar radiation may have at most increased by 0.12 W/m2, compared to 1.6 W/m2 for the net anthropogenic forcing.[100]: 3  Consequently, the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability."[101] Further, the upper atmosphere (the stratosphere) would also be warming if the Sun was sending more energy to Earth, but instead, it has been cooling.[102] This is consistent with greenhouse gases preventing heat from leaving the Earth's atmosphere.[103]

Explosive volcanic eruptions can release gases, dust and ash that partially block sunlight and reduce temperatures, or they can send water vapor into the atmosphere, which adds to greenhouse gases and increases temperatures.[104] Because both water vapor and volcanic material have low persistence in the atmosphere, even the largest eruptions only have an effect for several years.[105]

See also

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