Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.5.2 Summary ERF Assessment ====

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Figure 7.6 shows the industrial-era ERF estimates for 1750 to 2019 for the concentration change in different forcing agents. The assessed uncertainty distributions for each individual component are combined with a 100,000-member Monte Carlo simulation that samples the different distributions, assuming they are independent, to obtain the overall assessment of total present-day ERF (Supplementary Material 7.SM.1). The corresponding emissions-based ERF figure is shown in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Figure 6.12).

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[[File:5b77ae447f35f1ef8b2afd934611d5d8 IPCC_AR6_WGI_Figure_7_6.png]]

'''Figure 7.6''' '''|''' '''Change in effective radiative forcing (ERF) from 1750 to 2019 by contributing forcing agents (carbon dioxide, other well-mixed greenhouse gases (WMGHGs), ozone, stratospheric water vapour, surface albedo, contrails and aviation-induced cirrus, aerosols, anthropogenic total, and solar).''' Solid bars represent best estimates, and ''very likely'' (5–95%) ranges are given by error bars. Non-CO <sub>2</sub> WMGHGs are further broken down into contributions from methane (CH <sub>4</sub> ), nitrous oxide (N <sub>2</sub> O) and halogenated compounds. Surface albedo is broken down into land-use changes and light-absorbing particles on snow and ice. Aerosols are broken down into contributions from aerosol–cloud interactions (ERFaci) and aerosol–radiation interactions (ERFari). For aerosols and solar, the 2019 single-year values are given (Table 7.8), which differ from the headline assessments in both cases. Volcanic forcing is not shown due to the episodic nature of volcanic eruptions. Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

The total anthropogenic ERF over the industrial era (1750–2019) is estimated as 2.72 [1.96 to 3.48] W m <sup>–2</sup> ( ''high confidence'' ) (Table 7.8 and Annex III) ''.'' This represents a 0.43 W m <sup>–2</sup> increase over the assessment made in AR5 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) for the period 1750–2011. This increase is a result of compensating effects. Atmospheric concentration increases of GHGs since 2011 and upwards revisions of their forcing estimates have led to a 0.59 W m <sup>–2</sup> increase in their ERF. However, the total aerosol ERF is assessed to be more negative compared to AR5, due to revised estimates rather than trends ( ''high confidence'' ) ''.''

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'''Table 7.8''' '''|''' '''Summary table of effective radiative forcing (ERF) estimates for AR6 and comparison with the four previous IPCC assessment reports.''' Prior to AR5 values are stratospheric-temperature-adjusted radiative forcing (SARF). For AR5 aerosol–radiation interactions (ari) and aerosol–cloud interactions (aci) are ERF; all other values assume ERF equals SARF. Ranges shown are 5–95%. Volcanic ERF is not added to the table due to the episodic nature of volcanic eruptions which makes it difficult to compare to the other forcing mechanisms. Solar ERF is based on total solar irradiance (TSI) and not spectral variation.

{| class="wikitable"
|-
| rowspan="2"| '''Driver'''

| colspan="6"| '''Global Mean Effective Radiative Forcing (W m''' <sup>–2</sup> ''')'''

|-
| SAR

(1750–1993)

| TAR

(1750–1998)

| AR4

(1750–2005)

| AR5

(1750–2011)

| AR6

(1750–2019)

| Comment

|-
| CO <sub>2</sub>

| 1.56 [1.33 to 1.79]

| 1.46 [1.31 to 1.61]

| 1.66 [1.49 to 1.83]

| 1.82 [1.63 to 2.01]

| 2.16 [1.90 to 2.41]

| rowspan="4"| Increases in concentrations. Changes to radiative efficiencies.

Inclusion of tropospheric adjustments.

|-
| CH <sub>4</sub>

| 0.47 [0.40 to 0.54

| 0.48 [0.41 to 0.55]

| 0.48 [0.43 to 0.53]

| 0.48 [0.43 to 0.53]

| 0.54 [0.43 to 0.65]

|-
| N <sub>2</sub> O

| 0.14 [0.12 to 0.16]

| 0.15 [0.14 to 0.16]

| 0.16 [0.14 to 0.18]

| 0.17 [0.14 to 0.20]

| 0.21 [0.18 to 0.24]

|-
| Halogenated species

| 0.26 [0.22 to 0.30]

| 0.36 [0.31 to 0.41]

| 0.33 [0.30 to 0.36]

| 0.36 [0.32 to 0.40]

| 0.41 [0.33 to 0.49]

|-
| Tropospheric ozone

| 0.4 [0.2 to 0.6]

| 0.35 [0.20 to 0.50]

| 0.35 [0.25 to 0.65]

| 0.40 [0.20 to 0.60]

| rowspan="2"| 0.47 [0.24 to 0.71]

| rowspan="2"| Revised precursor emissions. No tropospheric adjustment assessed. No troposphere–stratosphere separation.

|-
| Stratospheric ozone

| –0.1 [–0.2 to –0.05]

| –0.15 [–0.25 to –0.05]

| –0.05 [–0.15 to 0.05]

| –0.05 [–0.15 to 0.05]

|-
| Stratospheric water vapour

| Not estimated

| [0.01 to 0.03]

| 0.07 [0.02 to 0.1]

| 0.07 [0.02 to 0.12]

| 0.05 [0.00 to 0.10]

| Downward revision due to adjustments.

|-
| Aerosol–radiation interactions

| –0.5 [–0.25 to –1.0]

| Not estimated

| –0.50 [–0.90 to –0.10]

| –0.45 [–0.95 to 0.05]

| –0.22 [–0.47 to 0.04]

| ERFari magnitude reduced by about 50% compared to AR5, based on agreement between observation-based and modelling-based evidence.

|-
| Aerosol–cloud interactions

| [–1.5 to 0.0]

(sulphate only)

| [–2.0 to 0.0]

(all aerosols)

| –0.7 [–1.8 to –0.3]

(all aerosols)

| –0.45 [–1.2 to 0.0]

| –0.84 [–1.45 to –0.25]

| ERFaci magnitude increased by about 85% compared to AR5, based on agreement between observation-based and modelling-based lines of evidence.

|-
| Land use

| Not estimated

| –0.2 [–0.4 to 0.0]

| –0.2 [–0.4 to 0.0]

| –0.15 [–0.25 to –0.05]

| –0.20 [–0.30 to –0.10]

| Includes irrigation.

|-
| Surface albedo (black + organic carbon aerosol on snow and ice)

| Not estimated

| Not estimated

| 0.10 [0.00 to 0.20]

| 0.04 [0.02 to 0.09]

| 0.08 [0.00 to 0.18]

| Increased since AR5 to better account for temperature effects.

|-
| Combined contrails and aviation-induced cirrus

| Not estimated

| [0.00 to 0.04]

| Not estimated

| 0.05 [0.02 to 0.15]

| 0.06 [0.02 to 0.10]

| Narrower range since AR5.

|-
| Total anthropogenic

| Not estimated

| Not estimated

| 1.6 [0.6 to 2.4]

| 2.3 [1.1 to 3.3]

| '''2.72 [1.96 to 3.48]'''

| Increase due to GHGs, compensated slightly by aerosol ERFaci.

|-
| Solar irradiance

| 0.3 [0.1 to 0.5]

| 0.3 [0.1 to 0.5]

| 0.12 [0.06 to 0.30]

| 0.05 [0.0 to 0.10]

| 0.01 [–0.06 to 0.08]

| Revised historical TSI estimates and methodology.

|}

Greenhouse gases, including ozone and stratospheric water vapour from methane oxidation, are estimated to contribute an ERF of 3.84 [3.46 to 4.22] W m <sup>–2</sup> over 1750–2019. Carbon dioxide continues to contribute the largest part (56 ± 16%) of this GHG ERF ( ''high confidence'' ).

As discussed in ( [[#7.3.3|Section 7.3.3]] , aerosols have in total contributed an ERF of –1.1 [–1.7 to –0.4] W m <sup>–2</sup> over 1750–2019 ( ''medium confidence'' ). Aerosol–cloud interactions contribute approximately 75–80% of this ERF with the remainder due to aerosol–radiation interactions (Table 7.8).

For the purpose of comparing forcing changes with historical temperature change ( [[#7.5.2|Section 7.5.2]] ), longer averaging periods are useful. The change in ERF from the second half of the 19th century (1850–1900) compared with a recent period (2006–2019) is +2.20 [1.53 to 2.91] W m <sup>–2</sup> , of which 1.71 [1.51 to 1.92] W m <sup>–2</sup> is due to CO <sub>2</sub> .

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