Editing IPCC:AR6/WGI/Chapter-2 (section)

=== Changes in Climate System Drivers ===

<div id="h2-1-siblings" class="h2-siblings"></div>

Climate system drivers lead to climate change by altering the Earth’s energy balance. The influence of a climate driver is described in terms of its effective radiative forcing (ERF), measured in W m <sup>–2</sup> . Positive ERF values exert a warming influence and negative ERF values exert a cooling influence (Chapter 7).

'''Present-day global concentrations of atmospheric carbon dioxide (CO''' <sub>2</sub> ''') are at higher levels than at any time in at least the past two million years''' ( ''high confidence'' '''). Changes in ERF since the late 19th century are dominated by increases in concentrations of greenhouse gases and trends in aerosols; the net ERF is positive and changing at an increasing rate since the 1970s''' ( ''medium confidence'' ''').''' {2.2, 7.2, 7.3}

'''Change in ERF from natural factors since 1750 is negligible in comparison to anthropogenic drivers''' ( ''very high confidence'' ''').''' Solar activity since 1900 was high but not exceptional compared to the past 9000 years ( ''high confidence'' ). The average magnitude and variability of volcanic aerosol forcing since 1900 have not been unusual compared to the past 2500 years ( ''medium confidence'' ). {2.2.1, 2.2.2}

'''In 2019, concentrations of CO''' <sub>2</sub> ''', methane (CH''' <sub>4</sub> ''') and nitrous oxide (N''' <sub>2</sub> '''O) reached levels of 409.9 (±0.4) parts per million (ppm), 1866.3 (±3.3) parts per billion (ppb) and 332.1 (±0.4) ppb, respectively. Since 1850, these well-mixed greenhouse gases (GHGs) have increased at rates that have no precedent on centennial time scales in at least the past 800,000 years.''' Concentrations of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O increased from 1750 to 2019 by 131.6 ± 2.9 ppm (47.3%), 1137 ± 10 ppb (156%), and 62 ± 6 ppb (23.0%) respectively. These changes are larger than those between glacial and interglacial periods over the last 800,000 years for CO <sub>2</sub> and CH <sub>4</sub> and of comparable magnitude for N <sub>2</sub> O ( ''very high confidence'' ). The best estimate of the total ERF from CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O in 2019 relative to 1750 is 2.9 W m <sup>–2</sup> , an increase of 12.5% from 2011. ERF from halogenated components in 2019 was 0.4 W m <sup>–2</sup> , an increase of 3.5% since 2011. {2.2.3, 2.2.4, 7.3.2}

'''Tropospheric aerosol concentrations across the Northern Hemisphere mid-latitudes increased from 1700 to the last quarter of the 20th century, but have subsequently declined''' ( ''high confidence'' ''').''' Aerosol optical depth (AOD) has decreased since 2000 over Northern Hemisphere mid-latitudes and Southern Hemisphere mid-latitude continents, but increased over South Asia and East Africa ( ''high confidence'' ). These trends are even more pronounced in AOD from sub-micrometre aerosols for which the anthropogenic contribution is particularly large. The best-estimate of aerosol ERF in 2019 relative to 1750 is –1.1 W m <sup>–2</sup> . {2.2.6, 7.3.3}

'''Changes in other short-lived gases are associated with an overall positive ERF''' ( ''medium confidence'' ''').''' Stratospheric ozone has declined between 60°S and 60°N by 2.2% from the 1980s to 2014–2017 ( ''high confidence'' ). Since the mid-20th century, tropospheric ozone has increased by 30–70% across the Northern Hemisphere ( ''medium confidence'' ). Since the mid-1990s, free tropospheric ozone increases were 2–7% per decade in the northern mid-latitudes ( ''high confidence'' ), 2–12% per decade in the tropics ( ''high confidence'' ) and &lt;5% per decade in southern mid-latitudes ( ''medium confidence'' ). The best estimate of ozone column ERF (0.5 W m <sup>–2</sup> relative to 1750) is dominated by changes in tropospheric ozone. Due to discrepancies in satellite and in situ records, there is ''low confidence'' in estimates of stratospheric water vapour change. {2.2.5, 7.3.2}

'''Biophysical effects from historical changes in land use have an overall negative ERF''' ( ''medium confidence'' ''').''' The best-estimate ERF from the increase in global albedo is –0.15 W m <sup>–2</sup> since 1700 and –0.12 W m <sup>–2</sup> since 1850 ( ''medium confidence'' ). {2.2.7, 7.3.4}

<span id="changes-in-key-indicators-of-global-climate-change"></span>