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

=== 2.2.8 Effective Radiative Forcing (ERF) Exerted by the Assessed Climate Drivers ===

<div id="h2-12-siblings" class="h2-siblings"></div>

The AR5 concluded that changes in climate drivers over the industrial period corresponded to a positive ERF which increased more rapidly after 1970 than before. There was ''very high confidence'' in the positive ERF due to WMGHG, with CO <sub>2</sub> the single largest contributor. The AR5 concluded that there was ''high confidence'' that aerosols have offset a substantial portion of the WMGHG forcing.

This section reports the evolution in ERF with respect to 1750 as assessed in Section 7.3 and relies on the observed changes in climate drivers as assessed in [[#2.2|Section 2.2]] wherever possible, and models otherwise. The ERF is assessed using the methods and details described in Section 7.3.1 and includes, in addition to the radiative forcing, the rapid adjustments, especially implied by clouds. The time series are shown in Figure 2.10.

<div id="_idContainer028" class="Basic-Text-Frame"></div>

[[File:b59110924317078cfe585f9691ea0777 IPCC_AR6_WGI_Figure_2_10.png]]

'''Figure 2.1''' '''0 |''' '''Temporal evolution of effective radiative forcing (ERF) related to the drivers assessed in [[#2.2|Section 2.2]] .''' ERFs are based upon the calculations described in Chapter 7, of which the global annual mean, central assessment values are shown as lines and the 5 to 95% uncertainty range as shading (Section 7.3, see Figures 7.6 to 7.8 for more detail on uncertainties). The inset plot shows the rate of change (linear trend) in total anthropogenic ERF (total without TSI and volcanic ERF) for 30-year periods centred at each dot. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Increasing TSI ( [[#2.2.1|Section 2.2.1]] ) implies a small ERF of less than 0.1 W m <sup>–2</sup> between 1900 and 1980. TSI varies over the 11-year solar cycle with ERF of order ± 0.1 W m <sup>–2</sup> in the assessed period. Strong volcanic eruptions ( [[#2.2.2|Section 2.2.2]] ) with periods of strong negative ERF lasting 2–5 years in duration occurred in the late 19th and early 20th centuries. There followed a relatively quiescent period between about 1920 and 1960, and then three strong eruptions in 1963, 1982 and 1991, and only small-to-moderate eruptions thereafter ( [[#Schmidt--2018|Schmidt et al., 2018]] ).

The atmospheric concentrations of WMGHGs ( [[#2.2.3|Section 2.2.3]] ) have continuously increased since the early 19th century, with CO <sub>2</sub> contributing the largest share of the positive ERF. Compared to the last two decades of the 20th century, the growth rate of CO <sub>2</sub> in the atmosphere increased in the 21st century, showed strong fluctuations for CH <sub>4</sub> , and was about constant for N <sub>2</sub> O. Mixing ratios of the most abundant CFCs declined ( [[#2.2.4|Section 2.2.4]] ). Mixing ratios of HCFCs increased, but growth rates are starting to decelerate. Mixing ratios of HFCs and some other human-made components are increasing ( [[#2.2.4|Section 2.2.4]] ). The ERF for CO <sub>2</sub> alone is stronger than for all the other anthropogenic WMGHGs taken together throughout the industrial period, and its relative importance has increased in recent years (Figures 2.10 and 7.6).

Among the gaseous short-lived climate forcers ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] and Sections 2.2.5 and 7.3; excluding CH <sub>4</sub> here), ozone (O <sub>3</sub> ) is the component with the largest (positive) ERF. Concentrations from direct observations have increased since the mid-20th century and, mostly based on models, this extends to since 1750. Other gaseous short-lived climate forcers have small contributions to total ERF.

The net effect of aerosols (Sections 2.2.6 and 6.4) on the radiation budget, including their effect on clouds, and cloud adjustments, as well as the deposition of black carbon on snow (Section 7.3.4.3), was negative throughout the industrial period ( ''high confidence'' ). The net effect strengthened (becoming more negative) over most of the 20th century, but ''more likely than not'' weakened (becoming less negative) since the late 20th century. These trends are reflected in measurements of surface solar radiation (Section 7.2.2.3) and the Earth’s energy imbalance (Section 7.2.2.1). The relative importance of aerosol forcing compared to other forcing agents has decreased globally in the most recent 30 years ( ''medium confidence'' ) and the reduction of the negative forcing in the 21st century enhances the overall positive ERF.

Land use and land cover changes ( [[#2.2.7|Section 2.2.7]] ) over the industrial period introduce a negative radiative forcing by increasing the surface albedo. This effect increased since 1750, reaching current values of about –0.20 W m <sup>–2</sup> ( ''medium confidence'' ). This ERF value is taken from Section 7.3.4.1 and is different from the assessment in [[#2.2.7|Section 2.2.7]] in that it also includes the effect of irrigation. It also includes uncertain rapid adjustments and thus there is ''low confidence'' in its magnitude. Biogeochemical feedbacks can be substantial (Section 5.4) and are not included in ERF.

In conclusion, the net ERF due to all observed changes in climate drivers is positive, except for short periods (up to a few years in duration) following moderate to large volcanic eruptions, and has grown in magnitude since the late 19th century. The rate of change ''likely'' has increased in the last 30 years, since CO <sub>2</sub> concentrations increased at an increasing rate due to growing CO <sub>2</sub> emissions ( ''very likely'' ) , and since the aerosol forcing became less negative ( ''more likely than not'' ).

<div id="2.3" class="h1-container"></div>

<span id="changes-in-large-scale-climate"></span>