Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

=== 1.3.3 Lines of Evidence: Identifying Natural and Human Drivers ===

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The climate is a globally interconnected system driven by solar energy. Scientists in the 19th century established the main physical principles governing Earth’s temperature. By 1822, the principle of radiative equilibrium (the balance between absorbed solar radiation and the energy Earth re-radiates into space) had been articulated, and the atmosphere’s role in retaining heat had been likened to a greenhouse ( [[#Fourier--1822|Fourier, 1822]] ). The primary explanations for natural climate change – greenhouse gases, orbital factors, solar irradiance, continental position, volcanic outgassing, silicate rock weathering, and the formation of coal and carbonate rock – were all identified by the late 19th century ( [[#Fleming--1998|Fleming, 1998]] ; [[#Weart--2008|Weart, 2008]] ).

The natural and anthropogenic factors responsible for climate change are known today as radiative ‘drivers’ or ‘forcers’. The net change in the energy budget at the top of the atmosphere, resulting from a change in one or more such drivers, is termed ‘radiative forcing’ (RF; Glossary) and measured in watts per square metre (W m <sup>–2</sup> ). The total radiative forcing over a given time interval (often since 1750) represents the sum of positive drivers (inducing warming) and negative ones (inducing cooling). Past IPCC reports have assessed scientific knowledge of these drivers, quantified their range for the period since 1750, and presented the current understanding of how they interact in the climate system. Like all previous IPCC reports, AR5 assessed that total radiative forcing has been positive at least since 1850–1900, leading to an uptake of energy by the climate system, and that the largest single contribution to total radiative forcing is the rising atmospheric concentration of CO <sub>2</sub> since 1750 (Chapter 7, and Cross-Chapter Box 1.2; [[#IPCC--2013a|IPCC, 2013a]] ).

Natural drivers include changes in solar irradiance, ocean currents, naturally occurring aerosols, and natural sources and sinks of radiatively active gases such as water vapour, CO <sub>2</sub> , CH <sub>4</sub> , and sulphur dioxide (SO <sub>2</sub> ). Detailed global measurements of surface-level solar irradiance were first conducted during the 1957–1958 International Geophysical Year ( [[#Landsberg--1961|Landsberg, 1961]] ), while top-of-atmosphere irradiance has been measured by satellites since 1959 ( [[#House--1986|House et al., 1986]] ). Measured changes in solar irradiance have been small and slightly negative since about 1980 ( [[#Matthes--2017|Matthes et al., 2017]] ). Water vapour is the most abundant radiatively active gas, accounting for about 75% of the terrestrial greenhouse effect, but because its residence time in the atmosphere averages just 8–10 days, its atmospheric concentration is largely governed by temperature ( [[#van%20der%20Ent--2017|van der Ent and Tuinenburg, 2017]] ; [[#Nieto--2019|Nieto and Gimeno, 2019]] ). As a result, non-condensing GHGs with much longer residence times serve as ‘control knobs’, regulating planetary temperature, with water vapour concentrations as a feedback effect ( [[#Lacis--2010|Lacis et al., 2010]] , 2013). The most important of these non-condensing gases is CO <sub>2</sub> (a positive driver), released naturally by volcanism at about 637 MtCO <sub>2</sub> yr <sup>–1</sup> in recent decades, or roughly 1.6% of the 37 GtCO <sub>2</sub> emitted by human activities in 2018 ( [[#Burton--2013|Burton et al., 2013]] ; [[#Le%20Quéré--2018|Le Quéré et al., 2018]] ). Absorption by the ocean and uptake by plants and soils are the primary natural CO <sub>2</sub> sinks on decadal to centennial time scales (Section 5.1.2 and Figure 5.3).

Aerosols (tiny airborne particles) interact with climate in numerous ways, some direct (e.g., reflecting solar radiation back into space) and others indirect (e.g., cloud droplet nucleation); specific effects may cause either positive or negative radiative forcing. Major volcanic eruptions inject SO <sub>2</sub> (a negative driver) into the stratosphere, creating aerosols that can cool the planet for years at a time by reflecting some incoming solar radiation. The history and climatic effects of volcanic activity have been traced through historical records, geological traces, and observations of major eruptions by aircraft, satellites and other instruments ( [[#Dörries--2006|Dörries, 2006]] ). The negative RF of major volcanic eruptions was considered in the First Assessment Report (FAR; [[#IPCC--1990a|IPCC, 1990a]] ). In subsequent assessments, the negative RF of smaller eruptions has also been considered (e.g., Cross-Chapter Box 4.1 in [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] of this Report; [[IPCC:Wg1:Chapter:Chapter-2#2.4.3|Section 2.4.3]] in [[#IPCC--1996|IPCC, 1996]] ). Dust and other natural aerosols have been studied since the 1880s (e.g., [[#Aitken--1889|Aitken, 1889]] ; [[#Ångström--1929|Ångström, 1929]] , 1964; [[#Twomey--1959|Twomey, 1959]] ), particularly in relation to their role in cloud nucleation, an aerosol indirect effect whose RF may be either positive or negative depending on such factors as cloud altitude, depth and albedo ( [[#Stevens--2009|Stevens and Feingold, 2009]] ; [[#Boucher--2013|Boucher et al., 2013]] ).

Anthropogenic drivers of climatic change were hypothesized as early as the 17th century, with a primary focus on forest clearing and agriculture ( [[#Grove--1995|Grove, 1995]] ; [[#Fleming--1998|Fleming, 1998]] ). In the 1890s, Arrhenius was first to calculate the effects of increased or decreased CO <sub>2</sub> concentrations on planetary temperature, and Högbom estimated that worldwide coal combustion of about 500 Mt yr <sup>–1</sup> had already completely offset the natural absorption of CO <sub>2</sub> silicate rock weathering ( [[#Högbom--1894|Högbom, 1894]] ; [[#Arrhenius--1896|Arrhenius, 1896]] ; [[#Berner--1995|Berner, 1995]] ; [[#Crawford--1997|Crawford, 1997]] ). As coal consumption reached 900 Mt yr <sup>–1</sup> only a decade later, Arrhenius wrote that anthropogenic CO <sub>2</sub> from fossil fuel combustion might eventually warm the planet ( [[#Arrhenius--1908|Arrhenius, 1908]] ). In 1938, analysing records from 147 stations around the globe, Callendar calculated atmospheric warming over land at 0.3°C–0.4°C from 1880–1935 and attributed about half of this warming to anthropogenic CO <sub>2</sub> (Figure 1.8; [[#Callendar--1938|Callendar, 1938]] ; [[#Fleming--2007|Fleming, 2007]] ; [[#Hawkins--2013|Hawkins and Jones, 2013]] ).

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'''Figure 1.8 |''' '''G.S. Callendar’s estimates of global land temperature variations and their possible causes.''' '''(a)''' The original figure from [[#Callendar--1938|Callendar (1938)]] , using measurements from 147 surface stations for 1880–1935, showing: '''(top)''' ten-year moving departures from the mean of 1901–1930 (°C), with the dashed line representing his estimate of the ‘CO <sub>2</sub> effect’ on temperature rise, and '''(bottom)''' annual departures from the 1901–1930 mean (°C). '''(b)''' Comparing the estimates of global land (60°S–60°N) temperatures tabulated in Callendar (1938, 1961) with a modern reconstruction (CRUTEM5, [[#Osborn--2021|Osborn et al., 2021]] ) for the same period, following [[#Hawkins--2013|Hawkins and Jones (2013)]] . Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).
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Studiesof radiocarbon ( <sup>14</sup> C) in the 1950s established that increasing atmospheric CO <sub>2</sub> concentrations were due to fossil fuel combustion. Since all the <sup>14</sup> C once contained in fossil fuels long ago decayed into non-radioactive <sup>12</sup> C, the CO <sub>2</sub> produced by their combustion reduces the overall concentration of atmospheric <sup>14</sup> C ( [[#Suess--1955|Suess, 1955]] ). Related work demonstrated that while the ocean was absorbing around 30% of anthropogenic CO <sub>2</sub> , these emissions were also accumulating in the atmosphere and biosphere ( [[#1.3.1|Section 1.3.1]] and Chapter 5, Section 5.2.1.5). Further work later established that atmospheric oxygen levels were decreasing in inverse relation to the anthropogenic CO <sub>2</sub> increase, because combustion of carbon consumes oxygen to produce CO <sub>2</sub> (Chapters 2 and 6; [[#Keeling--1992|Keeling and Shertz, 1992]] ; [[#IPCC--2013a|IPCC, 2013a]] ). [[#Revelle--1957|Revelle and Suess (1957)]] famously described fossil fuel emissions as a ‘large scale geophysical experiment’, in which ‘within a few centuries we are returning to the atmosphere and ocean the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years.’ The 1960s saw increasing attention to other radiatively active gases, especially ozone (O <sub>3</sub> ; [[#Manabe--1961|Manabe and Möller, 1961]] ; [[#Plass--1961|Plass, 1961]] ). Methane and nitrous oxide (N <sub>2</sub> O) were not considered systematically until the 1970s, when anthropogenic increases in those gases were first noted ( [[#Wang--1976|Wang et al., 1976]] ). In the 1970s and 1980s, scientists established that synthetic halocarbons (see Glossary), including widely used refrigerants and propellants, were extremely potent greenhouse gases (Sections 2.2.4.3 and 6.2.2.9; [[#Ramanathan--1975|Ramanathan, 1975]] ). When these chemicals were also found to be depleting the stratospheric ozone layer, they were stringently and successfully regulated on a global basis by the 1987 Montreal Protocol on the Ozone Layer and successor agreements ( [[#Parson--2003|Parson, 2003]] ).

Radioactive fallout from atmospheric nuclear weapons testing (1940s–1950s) and urban smog (1950s–1960s) first provoked widespread attention to anthropogenic aerosols and ozone in the troposphere ( [[#Edwards--2012|Edwards, 2012]] ). Theory, measurement and modelling of these substances developed steadily from the 1950s ( [[#Hidy--2019|Hidy, 2019]] ). However, the radiative effects of anthropogenic aerosols did not receive sustained study until around 1970 ( [[#Bryson--1970|Bryson and Wendland, 1970]] ; [[#Rasool--1971|Rasool and Schneider, 1971]] ), when their potential as cooling agents was recognized ( [[#Peterson--2008|Peterson et al., 2008]] ). The US Climatic Impact Assessment Program (CIAP) found that proposed fleets of supersonic aircraft, flying in the stratosphere, might cause substantial aerosol cooling and depletion of the ozone layer, stimulating efforts to understand and model stratospheric circulation, atmospheric chemistry, and aerosol radiative effects ( [[#Mormino--1975|Mormino et al., 1975]] ; [[#Toon--1976|Toon and Pollack, 1976]] ). Since the 1980s, aerosols have increasingly been integrated into comprehensive modelling studies of transient climate evolution and anthropogenic influences, through treatment of volcanic forcing, links to global dimming and cloud brightening, and their influence on cloud nucleation and other properties (e.g., thickness, lifetime and extent), and precipitation (e.g., [[#Hansen--1981|Hansen et al., 1981]] ; [[#Charlson--1987|Charlson et al., 1987]] , 1992; [[#Albrecht--1989|Albrecht, 1989]] ; [[#Twomey--1991|Twomey, 1991]] ).

The FAR (1990) focused attention on human emissions of CO <sub>2</sub> , CH <sub>4</sub> , tropospheric O <sub>3</sub> , chlorofluorocarbons (CFCs), and N <sub>2</sub> O. Of these, at that time only the emissions of CO <sub>2</sub> and CFCs were well measured, with methane sources known only ‘semi-quantitatively’ ( [[#IPCC--1990a|IPCC, 1990a]] ). The FAR assessed that some other trace gases, especially CFCs, have global warming potentials hundreds to thousands of times greater than CO <sub>2</sub> and CH <sub>4</sub> , but are emitted in much smaller amounts. As a result, CO <sub>2</sub> remains by far the most important positive anthropogenic driver, with CH <sub>4</sub> next most significant ( [[#1.6.3|Section 1.6.3]] ); anthropogenic methane stems from such sources as fossil fuel extraction, natural gas pipeline leakage, agriculture and landfills. In 2001, increased greenhouse forcing attributable to CO <sub>2</sub> , CH <sub>4</sub> , O <sub>3</sub> , CFC-11 and CFC-12 was detected by comparing satellite measurements of outgoing longwave radiation measurements taken in 1970 and in 1997 ( [[#Harries--2001|Harries et al., 2001]] ). AR5 assessed that the 40% increase in atmospheric CO <sub>2</sub> contributed most to positive RF since 1750. Together, changes in atmospheric concentrations of CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O and halocarbons from 1750–2011 were assessed to contribute a positive RF of 2.83 [2.26 to 3.40] W m <sup>–2</sup> ( [[#IPCC--2013b|IPCC, 2013b]] ).

All IPCC reports have assessed the total RF as positive when considering all sources. However, due to the considerable variability of both natural and anthropogenic aerosol loads, FAR characterized total aerosol RF as ‘highly uncertain’ and was unable even to determine its sign (positive or negative). Major advances in quantification of aerosol loads and their effects have taken place since then, and IPCC reports since 1992 have consistently assessed total forcing by anthropogenic aerosols as negative ( [[#IPCC--1992|IPCC, 1992]] , 1995a, 1996). However, due to their complexity and the difficulty of obtaining precise measurements, aerosol effects have been consistently assessed as the largest single source of uncertainty in estimating total RF ( [[#Stevens--2009|Stevens and Feingold, 2009]] ; [[#IPCC--2013a|IPCC, 2013a]] ). Overall, AR5 assessed that total aerosol effects, including cloud adjustments, resulted in a negative RF of –0.9 [–1.9 to −0.1] W m <sup>−2</sup> ( ''medium confidence'' ), offsetting a substantial portion of the positive RF resulting from the increase in GHGs ( ''high confidence'' ) ( [[#IPCC--2013b|IPCC, 2013b]] ). [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] provides an updated assessment of the total and per-component RF for the WGI contribution to AR6.

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