Editing IPCC:AR6/WGI/TS (section)

=== TS.2.2 Changes in the Drivers of the Climate System ===

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'''Since 1750, changes in the drivers of the climate system are dominated by the warming influence of increases in atmospheric GHG concentrations and a cooling influence from aerosols, both resulting from human activities. In comparison there has been negligible long-term influence from solar activity and volcanoes. Concentrations of CO 2 , methane (CH 4 ), and nitrous oxide (N 2 O) have increased to levels unprecedented in at least 800,000 years, and there is high confidence that current CO 2 concentrations have not been experienced for at least 2 million years. Global mean concentrations of anthropogenic aerosols peaked in the late 20th century and have slowly declined since in northern mid-latitudes, although they continue to increase in South Asia and East Africa ( high confidence ).''' '''The total anthropogenic effective radiative forcing (ERF) in 2019, relative to 1750, was 2.72 [1.96 to 3.48] W m <sup>–2</sup> ( ''medium confidence'' ) and has ''likely'' been growing at an increasing rate since the 1970s. Links to chapters 2.2, 6.4, 7.2, 7.3'''
Solar activity since 1900 was high but not exceptional compared to the past 9000 years ( ''high confidence'' ). The average magnitude and variability of volcanic aerosols since 1900 has not been unusual compared to at least the past 2500 years ( ''medium confidence'' ). However, sporadic strong volcanic eruptions can lead to temporary drops in global surface temperature lasting 2–5 years. Links to chapters 2.2.1, 2.2.2, 2.2.8, Cross-Chapter Box 4.1

Atmospheric CO <sub>2</sub> concentrations have changed substantially over millions of years (Figure TS.1). Current levels of atmospheric CO <sub>2</sub> have not been experienced for at least 2 million years ( ''high confidence'' , Figure TS.9a). Over 1750–2019, CO <sub>2</sub> increased by 131.6 ± 2.9 ppm (47.3%). The centennial rate of change of CO <sub>2</sub> since 1850 has no precedent in at least the past 800,000 years (Figure TS.9), and the fastest rates of change over the last 56 million years were at least a factor of four lower ( ''low confidence'' ) than over 1900–2019. Several networks of high-accuracy surface observations show that concentrations of CO <sub>2</sub> have exceeded 400 ppm, reaching 409.9 (± 0.3) ppm in 2019 (Figure TS.9c). The ERF from CO <sub>2</sub> in 2019 (relative to 1750) was 2.16 Wm <sup>–2</sup> . Links to chapters 2.2.3, 5.1.2, 5.2.1, 7.3

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'''Figure TS.9 |''' '''Changes in well-mixed greenhouse gas (WMGHG) concentrations and effective radiative forcing (EFR).''' ''The intent of this figure is to show that the changes of the main drivers of climate system over the industrial period are exceptional in a long-term context.'' (a) Changes in carbon dioxide (CO <sub>2</sub> ) from proxy records over the past 3.5 million years. (b) Changes in all three WMGHGs from ice core records over the Common Era. (c) Directly observed WMGHG changes since the mid-20th century. (d) Evolution of ERF and components since 1750. Further details on data sources and processing are available in the associated FAIR data table. Links to chapters 2.2, Figures 2.3, 2.4 and 2.10

By 2019, concentrations of CH <sub>4</sub> reached 1866.3 (± 3.3) ppb (Figure TS.9c). The increase since 1750 of 1137 ± 10 ppb (157.8%) far exceeds the range over multiple glacial–interglacial transitions of the past 800,000 years ( ''high confidence'' ). In the 1990s, CH <sub>4</sub> concentrations plateaued, but started to increase again around 2007 at an average rate of 7.6 ± 2.7 ppb yr <sup>–1</sup> (2010–2019; ''high confidence'' ). There is ''high confidence'' that this recent growth is largely driven by emissions from fossil fuel exploitation, livestock, and waste, with ENSO driving multi-annual variability of wetland and biomass burning emissions. In 2019, ERF from CH <sub>4</sub> was 0.54 Wm <sup>–2</sup> . Links to chapters 2.2.3, 5.2.2, 7.3

Since 1750, N <sub>2</sub> O increased by 62.0 ± 6.0 ppb, reaching a level of 332.1 (± 0.4) ppb in 2019. The increase since 1750 is of comparable magnitude to glacial–interglacial fluctuations of the past 800,000 years (Figure TS.9c). N <sub>2</sub> O concentration trends since 1980 are largely driven by a 30% increase in emissions from the expansion and intensification of global agriculture ( ''high confidence'' ). By 2019 its ERF was 0.21 W m <sup>–2</sup> . Links to chapters 2.2.3, 5.2.3

Halogenated gases consist of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and other gases, many of which can deplete stratospheric ozone and warm the atmosphere. In response to controls on production and consumption mandated by the Montreal Protocol on Substances that Deplete the Ozone Layer and its amendments, the atmospheric abundances of most CFCs have continued to decline since AR5. Abundances of HFCs, which are replacements for CFCs and HCFCs, are increasing ( ''high confidence'' ), though increases of the major HCFCs have slowed in recent years. The ERF from halogenated components in 2019 was 0.41 Wm <sup>–2</sup> . Links to chapters 2.2.4, 6.3.4, 7.3.2

Tropospheric aerosols mainly act to cool the climate system, directly by reflecting solar radiation, and indirectly by enhancing cloud reflectance. Ice cores show increases in aerosols across the Northern Hemisphere mid-latitudes since 1700 and reductions since the late 20th century ( ''high confidence'' ). Aerosol optical depth (AOD), derived from satellite- and ground-based radiometers, has decreased since 2000 over the mid-latitude continents of both hemispheres, but increased over South Asia and East Africa ( ''high confidence'' ). Trends in AOD are more pronounced from sub-micrometre aerosols for which the anthropogenic contribution is particularly large. Global carbonaceous aerosol budgets and trends remain poorly characterized due to limited observations, but black carbon (BC), a warming aerosol component, is declining in several regions of the Northern Hemisphere ( ''low confidence'' ). Total aerosol ERF in 2019, relative to 1750, is −1.1 [−1.7 to −0.4] W m <sup>−2</sup> ( ''medium confidence'' ) and ''more likely than not'' became less negative since the late 20th century, with ''low confidence'' in the magnitude of post-2014 changes due to conflicting evidence (Section TS.3.1). Links to chapters 2.2.6, 6.2.1, 6.3.5, 6.4.1, 7.3.3

There is ''high confidence'' that tropospheric ozone has been increasing from 1750 in response to anthropogenic changes in ozone precursor emissions (nitrogen oxides, carbon monoxide, non-methane volatile organic compounds, and methane), but with ''medium confidenc'' e in the magnitude of this change, due to limited observational evidence and knowledge gaps. Since the mid-20th century, tropospheric ozone surface concentrations have increased by 30–70% across the Northern Hemisphere ( ''medium confidence'' ); since the mid-1990s, free tropospheric ozone has increased by 2–7% per decade in most northern mid-latitude regions and 2–12% per decade in sampled tropical regions. Future changes in surface ozone concentrations will be primarily driven by changes in precursor emissions rather than climate change ( ''high confidence'' ). Stratospheric ozone has declined between 60°S–60°N by 2.2% from 1964–1980 to 2014–2017 ( ''high confidence'' ), with the largest declines during 1980–1995. The strongest loss of stratospheric ozone continues to occur in austral spring over Antarctica (ozone hole), with emergent signs of recovery after 2000. The 1750–2019 ERF for total (stratospheric and tropospheric) ozone is 0.47 [0.24 to 0.71] W m <sup>−2</sup> , which is dominated by tropospheric ozone changes. Links to chapters 2.2.5, 6.3.2, 7.3.2, 7.3.5

The global mean abundance of hydroxyl (OH) radical, or ‘oxidizing capacity’, chemically regulates the lifetimes of many SLCFs, and therefore the radiative forcing of CH <sub>4</sub> , ozone, secondary aerosols and many halogenated species. Model estimates suggest no significant change in oxidizing capacity from 1850 to 1980 ( ''low confidence'' ). Increases of about 9% over 1980–2014 computed by ESMs and carbon cycle models are not confirmed by observationally constrained inverse models, rendering an overall ''medium confidence'' in stable OH or positive trends since the 1980s, and implying that OH is not the primary driver of recent observed growth in CH <sub>4</sub> . Links to chapters 6.3.6, Cross-Chapter Box 5.2

Land use and land-cover change exert biophysical and biogeochemical effects. There is ''medium confidence'' that the biophysical effects of land-use change since 1750, most notably the increase in global albedo, have had an overall cooling on climate, whereas biogeochemical effects (i.e., changes in GHG and volatile organic compound emissions or sinks) led to net warming. Overall land-use and land-cover ERF is estimated at –0.2 [–0.3 to –0.1] W m <sup>−2</sup> . Links to chapters 2.2.7, 7.3.4, SRCCL [[IPCC:Wg1:Chapter:Chapter-2#2.5|Section 2.5]]

The total anthropogenic ERF in 2019 relative to 1750 was 2.72 [1.96 to 3.48] W m <sup>−2</sup> (Figure TS.9), dominated by GHGs (positive ERF) and partially offset by aerosols (negative ERF). The rate of change of ERF ''likely'' has increased since the 1970s, mainly due to growing CO <sub>2</sub> concentrations and less negative aerosol ERF (Section TS.3.1). Links to chapters 2.2.8, 7.3

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