Editing IPCC:AR6/WGI/TS (section)

=== TS.3.1 Radiative Forcing and Energy Budget ===

<div id="h2-22-siblings" class="h2-siblings"></div>

'''Since AR5, the accumulation of energy in the Earth system, quantified by observations of warming of the ocean, atmosphere, and land and melting of ice, has become established as a robust measure of the rate of global climate change on interannual-to-decadal time scales. Compared to changes in global surface temperature, the increase in the global energy inventory exhibits less variability, and thus better indicates underlying climate trends.''' '''The global energy inventory increased by 282 [177 to 387] zettajoules (ZJ, equal to 10 <sup>21</sup> joules) for the period 1971–2006 <sup></sup> and 152 [100 to 205] ZJ for the period 2006–2018 (Figure TS.13), with more than 90% accounted for by ocean warming. To put these numbers in context, the 2006–2018 average Earth energy imbalance is equivalent to approximately 20 times the annual rate of global energy consumption in 2018. The accumulation of energy is driven by a positive total anthropogenic effective radiative forcing (ERF) relative to 1750.''' '''The best estimate ERF of 2.72 W m <sup>−2</sup> has increased by 0.43 W m <sup>–2</sup> relative to that given in AR5 (for 1750–2014) due to an increase in the greenhouse gas ERF that is partly compensated by a more negative aerosol ERF compared to AR5. The greenhouse gas ERF has been revised due to changes in atmospheric concentrations and updates to forcing efficiencies, while the revision to aerosol ERF is due to increased understanding of aerosol–cloud interactions and is supported by improved agreement between different lines of evidence. Improved quantifications of ERF, the climate system radiative response, and the observed energy increase in the Earth system for the period 1971–2018 demonstrate improved closure of the global energy budget (i.e., the extent to which the sum of the integrated forcing and the integrated radiative response equals the energy gain of the Earth system) compared to AR5 ( ''high confidence'' ). (See FAQ 7.1). Links to chapters 7.2.2, 7.3.5, 7.5.2, Box 7.2, Table 7.1'''

<div id="_idContainer029"></div>

[[File:a856ffc53230d9d9e42f8e50136fe6d5 IPCC_AR6_WGI_TS_Figure_13.png]]
<div id="_idContainer028" class="_idGenObjectStyleOverride-2"></div>

'''Figure TS.13 |''' '''Estimates of the net cumulative energy change (ZJ = 10''' '''21''' '''Joules) for the period 1971–2018 associated with (a) observations of changes in the global energy inventory, (b) integrated radiative forcing, and (c) integrated radiative response.''' ''The intent is to show assessed changes in energy budget and effective radiative forcings (ERFs).'' Black dotted lines indicate the central estimate with ''likely'' and ''very likely'' ranges as indicated in the legend. The grey dotted lines indicate the energy change associated with an estimated 1850–1900 Earth energy imbalance of 0.2 W m <sup>–2</sup> (panel a) and an illustration of an assumed pattern effect of –0.5 W m <sup>–2</sup> °C <sup>–1</sup> (panel c). Background grey lines indicate equivalent heating rates in W m <sup>–2</sup> per unit area of Earth’s surface. Panels (d) and (e) show the breakdown of components, as indicated in the legend, for the global energy inventory and integrated radiative forcing, respectively. Panel (f) shows the global energy budget assessed for the period 1971–2018, that is, the consistency between the change in the global energy inventory relative to 1850–1900 and the implied energy change from integrated radiative forcing plus integrated radiative response under a number of different assumptions, as indicated in the figure legend, including assumptions of correlated and uncorrelated uncertainties in forcing plus response. Shading represents the ''very likely'' range for observed energy change relative to 1850–1900 and ''likely'' range for all other quantities. Forcing and response time series are expressed relative to a baseline period of 1850–1900. Links to chapters Box 7.2, Figure 1

The global energy inventory change for the period 1971–2006 corresponds to an Earth energy imbalance (Box TS.1) of 0.50 [0.32 to 0.69] W m <sup>–2</sup> , increasing to 0.79 [0.52 to 1.06] W m <sup>–2</sup> for the period 2006 '''–''' 2018. Ocean heat uptake is by far the largest contribution and accounts for 91% of the total energy change. Land warming, melting of ice and warming of the atmosphere account for about 5%, 3% and 1% of the total change, respectively. More comprehensive analysis of inventory components, cross-validation of satellite and in situ-based estimates of the global energy imbalance, and closure of the global sea level budget have led to a strengthened assessment relative to AR5. ( ''high confidence'' ) Links to chapters 7.2.2, 7.5.2.3, Box 7.2, Table 7.1, 9.6.1, Cross-Chapter Box 9.1, Table 9.5

As in AR5, the perturbations to Earth’s top-of-atmosphere energy budget are quantified using ERFs (see also Section TS.2.2). These include any consequent adjustments to the climate system (e.g., from changes in atmospheric temperatures, clouds and water vapour as shown in Figure TS.14), but exclude any surface temperature response. Since AR5, ERFs have been estimated for a larger number of forcing agents and shown to be more closely related to the temperature response than the stratospheric-temperature-adjusted radiative forcing. ( ''high confidence'' ) Links to chapters 7.3.1

Improved quantifications of ERF, the climate system radiative response, and the observed energy increase in the Earth system for the period 1971–2018 demonstrate improved closure of the global energy budget relative to AR5 (Figure TS.13). Combining the ''likely'' range of ERF over this period with the central estimate of radiative response gives an expected energy gain of 340 [47 to 662] ZJ. Both estimates are consistent with an independent observation-based assessment of the global energy increase of 284 [96 to 471] ZJ ( ''very likely'' ''range'' ), expressed relative to the estimated 1850–1900 Earth energy imbalance. ( ''high confidence'' ) Links to chapters 7.2.2, 7.3.5, Box 7.2

<div id="_idContainer032"></div>

[[File:80cdf6fd832e0f5d2590f285ff6ba479 IPCC_AR6_WGI_TS_Figure_14.png]]
<div id="_idContainer031" class="Basic-Text-Frame"></div>

'''Figure TS.14 |''' '''Schematic representation of changes in the top-of-atmosphere (TOA) radiation budget following a perturbation.''' ''The intent of this figure is to illustrate the concept of adjustments in the climate system following a perturbation in the radiation budget.'' The baseline TOA energy budget (a) responds instantaneously to perturbations (b) , leading to adjustments in the atmospheric meteorology and composition and land surface that are independent of changes in surface temperature (c) . Surface temperature changes (here using an increase as an example) lead to physical, biogeophysical and biogeochemical feedback processes (d) . Long-term feedback processes, such as those involving ice sheets, are not shown here. Links to chapters adapted from Figure 7.2; FAQ 7.2, Figure 1; and Figure 8.3

The assessed greenhouse gas ERF over the 1750–2019 period (Section TS.2.2) has increased by +0.59 W m <sup>−2</sup> over AR5 estimates for 1750–2011. This increase includes +0.34 W m <sup>–2</sup> from increases in atmospheric concentrations of well-mixed greenhouse gases (including halogenated species) since 2011, +0.15 W m <sup>–2</sup> from upwards revisions of their radiative efficiencies and +0.10 W m <sup>–2</sup> from re-evaluation of the ozone and stratospheric water vapour ERF. Links to chapters 7.3.2, 7.3.4, 7.3.5

For CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O, and chlorofluorocarbons, there is now evidence to quantify the effect on ERF of tropospheric adjustments. The assessed ERF for a doubling of CO <sub>2</sub> compared to 1750 levels (3.9 ± 0.5 Wm <sup>–2</sup> ) is larger than in AR5. For CO <sub>2</sub> , the adjustments include the physiological effects on vegetation. The reactive well-mixed greenhouse gases (CH <sub>4</sub> , N <sub>2</sub> O, and halocarbons) cause additional chemical adjustments to the atmosphere through changes in ozone and aerosols (Figure TS.15a). The ERF due to CH <sub>4</sub> emissions is 1.19 [0.81 to 1.58] W m <sup>–2</sup> , of which 0.35 [0.16 to 0.54] W m <sup>–2</sup> is attributed to chemical adjustments mainly via ozone. These chemical adjustments also affect the emissions metrics (Section TS.3.3.3). Changes in sulphur dioxide (SO <sub>2</sub> <sub>)</sub> emissions make the dominant contribution to the ERF from aerosol–cloud interactions ( ''high confidence'' ). Over the 1750–2019 period, the contributions from the emitted compounds to global surface temperature changes broadly match their contributions to the ERF ( ''high confidence'' ) (Figure TS.15b). Since a peak in emissions-induced SO <sub>2</sub> eRF has already occurred recently (Section TS.2.2) and since there is a delay in the full global surface temperature response owing to the thermal inertia in the climate system, changes in SO <sub>2</sub> emissions have a slightly larger contribution to global surface temperature change compared with changes in CO <sub>2</sub> emissions, relative to their respective contributions to ERF. Links to chapters 6.4.2, 7.3.2

<div id="_idContainer105" class="Basic-Text-Frame"></div>

[[File:762e2223513fd09b64a76afc9b8a44dc IPCC_AR6_WGI_TS_Figure_15.png]]

'''Figure TS.15 |''' '''Contribution to (a) effective radiative forcing (ERF) and (b) global surface temperature change from component emissions for''' '''1750–2019''' '''based on Coupled Model Intercomparison Project Phase 6 (CMIP6) models and (c) net aerosol ERF for 1750–2014 from different lines of evidence.''' ''The intent of this figure is to show advances since AR5 in the understanding of (a) emissions-based ERF, (b) global surface temperature response for'' ''short-lived'' ''climate forcers as estimated in Chapter 6, and (c) aerosol ERF from different lines of evidence as assessed in Chapter 7.'' In panel (a), ERFs for well-mixed greenhouse gases (WMGHGs) are from the analytical formulae. ERFs for other components are multi-model means based on Earth system model simulations that quantify the effect of individual components. The derived emissions-based ERFs are rescaled to match the concentration-based ERFs in Figure 7.6. Error bars are 5–95% and for the ERF account for uncertainty in radiative efficiencies and multi-model error in the means. In panel (b), the global mean temperature response is calculated from the ERF time series using an impulse response function. In panel (c), the AR6 assessment is based on energy balance constraints, observational evidence from satellite retrievals, and climate model-based evidence. For each line of evidence, the assessed best-estimate contributions from ERF due to aerosol–radiation interactions (ERFari) and aerosol–cloud interactions (ERFaci) are shown with darker and paler shading, respectively. Estimates from individual CMIP Phase 5 (CMIP5) and CMIP6 models are depicted by blue and red crosses, respectively. The observational assessment for ERFari is taken from the instantaneous forcing due to aerosol–radiation interactions (IRFari). Uncertainty ranges are given in black bars for the total aerosol ERF and depict ''very likely'' ranges. Links to chapters 6.4.2, Figure 6.12, 7.3.3, Cross-Chapter Box 7.1, Table 7.8, Figure 7.5

Aerosols contributed an ERF of –1.3 [–2.0 to –0.6] W m <sup>–2</sup> over the period 1750 to 2014 ( ''medium confidence'' ). The ERF due to aerosol–cloud interactions (ERFaci) contributes most to the magnitude of the total aerosol ERF ( ''high confidence'' ) and is assessed to be –1.0 [–1.7 to –0.3] W m <sup>–2</sup> ( ''medium confidence'' ), with the remainder due to aerosol–radiation interactions (ERFari), assessed to be –0.3 [–0.6 to 0.0] W m <sup>–2</sup> ( ''medium confidence'' ). There has been an increase in the estimated magnitude – but a reduction in the uncertainty – of the total aerosol ERF relative to AR5, supported by a combination of increased process-understanding and progress in modelling and observational analyses (Figure TS.15c). Effective radiative forcing estimates from these separate lines of evidence are now consistent with each other, in contrast to AR5, and support the assessment that it is ''virtually certain'' that the total aerosol ERF is negative. Compared to AR5, the assessed magnitude of ERFaci has increased, while that of ERFari has decreased ''.'' Links to chapters 7.3.3, 7.3.5

<div id="TS.3.2" class="h2-container"></div>

<span id="ts.3.2-climate-sensitivity-and-earth-system-feedbacks"></span>