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

==== 7.2.2.3 Changes in Earth’s Surface Energy Budget ====

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The AR5 ( [[#Hartmann--2013|Hartmann et al., 2013]] ) reported pronounced changes in multi-decadal records of in situ observations of surface solar radiation, including a widespread decline between the 1950s and 1980s, known as ‘global dimming’, and a partial recovery thereafter, termed ‘brightening’ [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] ). These changes have interacted with closely related elements of climate change, such as global and regional warming rates (Z. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Wild--2016|Wild, 2016]] ; [[#Du--2017|Du et al., 2017]] ; [[#Zhou--2018a|Zhou et al., 2018a]] ), glacier melt ( [[#Ohmura--2007|Ohmura et al., 2007]] ; [[#Huss--2009|Huss et al., 2009]] ), the intensity of the global water cycle ( [[#Wild--2012|Wild, 2012]] ) and terrestrial carbon uptake ( [[#Mercado--2009|Mercado et al., 2009]] ). These observed changes have also been used as emergent constraints to quantify aerosol effective radiative forcing ( [[#7.3.3.3|Section 7.3.3.3]] ).

Since AR5, additional evidence for dimming and/or subsequent brightening up to several percent per decade, based on direct surface observations, has been documented in previously less-studied areas of the globe, such as Iran, Bahrain, Tenerife, Hawaii, the Taklaman Desert and the Tibetan Plateau ( [[#Elagib--2013|Elagib and Alvi, 2013]] ; [[#You--2013|You et al., 2013]] ; [[#Garcia--2014|Garcia et al., 2014]] ; [[#Longman--2014|Longman et al., 2014]] ; [[#Rahimzadeh--2015|Rahimzadeh et al., 2015]] ). Strong decadal trends in surface solar radiation remain evident after careful data quality assessment and homogenization of long-term records ( [[#Sanchez-Lorenzo--2013|Sanchez-Lorenzo et al., 2013]] , 2015; [[#Manara--2015|Manara et al., 2015]] , 2016; [[#Wang--2015|Wang et al., 2015]] ; Z. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Wang--2016|Wang and]] [[#Wild--2016|Wild, 2016]] ; Y. [[#He--2018|]] [[#He--2018|He et al., 2018]] ; [[#Yang--2018|Yang et al., 2018]] ). Since AR5, new studies on the potential effects of urbanization on solar radiation trends indicate that these effects are generally small, with the exception of some specific sites in Russia and China ( [[#Wang--2014|Wang et al., 2014]] ; [[#Imamovic--2016|Imamovic et al., 2016]] ; [[#Tanaka--2016|Tanaka et al., 2016]] ). Also, surface-based solar radiation observations have been shown to be representative over large spatial domains of up to several degrees latitude/longitude on monthly and longer time scales ( [[#Hakuba--2014|Hakuba et al., 2014]] ; [[#Schwarz--2018|Schwarz et al., 2018]] ). Thus, there is ''high confidence'' that the observed dimming between the 1950s and 1980s and the subsequent brightening are robust and do not arise from measurement artefacts or localized phenomena.

As noted in AR5 ( [[#Hartmann--2013|Hartmann et al., 2013]] ) and supported by recent studies, the trends in surface solar radiation are less spatially coherent since the beginning of the 21st century, with evidence for continued brightening in parts of Europe and the USA, some stabilization in China and India, and dimming in other areas ( [[#Augustine--2013|Augustine and Dutton, 2013]] ; [[#Sanchez-Lorenzo--2015|Sanchez-Lorenzo et al., 2015]] ; [[#Manara--2016|Manara et al., 2016]] ; [[#Soni--2016|Soni et al., 2016]] ; [[#Wang--2016|Wang and]] [[#Wild--2016|Wild, 2016]] ; [[#Jahani--2018|Jahani et al., 2018]] ; [[#Pfeifroth--2018|Pfeifroth et al., 2018]] ; [[#Yang--2018|Yang et al., 2018]] ; [[#Schwarz--2020|Schwarz et al., 2020]] ). The CERES-EBAF satellite-derived dataset of surface solar radiation ( [[#Kato--2018|Kato et al., 2018]] ) does not indicate a globally significant trend over the short period 2001–2012 ( [[#Zhang--2015|Zhang et al., 2015]] ), whereas a statistically significant increase in surface solar radiation of +3.4 W m <sup>−2</sup> per decade over the period 1996–2010 has been found in the Satellite Application Facility on Climate Monitoring (CM SAF) record of the geostationary satellite Meteosat, which views Europe, Africa and adjacent ocean ( [[#Posselt--2014|Posselt et al., 2014]] ).

Since AR5, there is additional evidence that strong decadal changes in surface solar radiation have occurred under cloud-free conditions, as shown for long-term observational records in Europe, USA, China, India and Japan ( [[#Xu--2011|Xu et al., 2011]] ; [[#Gan--2014|Gan et al., 2014]] ; [[#Manara--2016|Manara et al., 2016]] ; [[#Soni--2016|Soni et al., 2016]] ; [[#Tanaka--2016|Tanaka et al., 2016]] ; [[#Kazadzis--2018|Kazadzis et al., 2018]] ; J. [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ; [[#Yang--2019|Yang et al., 2019]] ; [[#Wild--2021|Wild et al., 2021]] ). This suggests that changes in the composition of the cloud-free atmosphere, primarily in aerosols, contributed to these variations, particularly since the second half of the 20th century ( [[#Wild--2016|Wild, 2016]] ). Water vapour and other radiatively active gases seem to have played a minor role ( [[#Wild--2009|Wild, 2009]] ; [[#Mateos--2013|Mateos et al., 2013]] ; [[#Posselt--2014|Posselt et al., 2014]] ; [[#Yang--2019|Yang et al., 2019]] ). For Europe and East Asia, modelling studies also point to aerosols as an important factor for dimming and brightening by comparing simulations that include or exclude variations in anthropogenic aerosol and aerosol-precursor emissions ( [[#Golaz--2013|Golaz et al., 2013]] ; [[#Nabat--2014|Nabat et al., 2014]] ; [[#Persad--2014|Persad et al., 2014]] ; [[#Folini--2015|Folini and Wild, 2015]] ; [[#Turnock--2015|Turnock et al., 2015]] ; [[#Moseid--2020|Moseid et al., 2020]] ). Moreover, decadal changes in surface solar radiation have often occurred in line with changes in anthropogenic aerosol emissions and associated aerosol optical depth ( [[#Streets--2006|Streets et al., 2006]] ; [[#Wang--2014|Wang and Yang, 2014]] ; [[#Storelvmo--2016|Storelvmo et al., 2016]] ; [[#Wild--2016|Wild, 2016]] ; [[#Kinne--2019|Kinne, 2019]] ). However, further evidence for the influence of changes in cloudiness on dimming and brightening is emphasized in some studies ( [[#Augustine--2013|Augustine and Dutton, 2013]] ; [[#Parding--2014|Parding et al., 2014]] ; [[#Stanhill--2014|Stanhill et al., 2014]] ; [[#Pfeifroth--2018|Pfeifroth et al., 2018]] ; [[#Antuña-Marrero--2019|Antuña-Marrero et al., 2019]] ). Thus, the contribution of aerosol and clouds to dimming and brightening is still debated. The relative influence of cloud-mediated aerosol effects versus direct aerosol radiative effects on dimming and brightening in a specific region may depend on the prevailing pollution levels ( [[#7.3.3|Section 7.3.3]] ; [[#Wild--2016|Wild, 2016]] ).

ESMs and reanalyses often do not reproduce the full extent of observed dimming and brightening ( [[#Wild--2011|Wild and Schmucki, 2011]] ; [[#Allen--2013|Allen et al., 2013]] ; [[#Zhou--2017a|Zhou et al., 2017a]] ; [[#Storelvmo--2018|Storelvmo et al., 2018]] ; [[#Moseid--2020|Moseid et al., 2020]] ; [[#Wohland--2020|Wohland et al., 2020]] ), potentially pointing to inadequacies in the representation of aerosol mediated effects or related emissions data. The inclusion of assimilated aerosol optical depth inferred from satellite retrievals in the MERRA2 reanalysis ( [[#Buchard--2017|Buchard et al., 2017]] ; [[#Randles--2017|Randles et al., 2017]] ) helps to improve the accuracy of the simulated surface solar radiation changes in China ( [[#Feng--2019|Feng and Wang, 2019]] ). However, non-aerosol-related deficiencies in model representations of clouds and circulation, and/or an underestimation of natural variability, could further contribute to the lack of dimming and brightening in ESMs ( [[#Wild--2016|Wild, 2016]] ; [[#Storelvmo--2018|Storelvmo et al., 2018]] ).

The AR5 reported evidence for an increase in surface downward thermal radiation based on different studies covering 1964 to 2008, in line with what would be expected from an increased radiative forcing from GHGs and the warming and moistening of the atmosphere. Updates of the longest observational records from the Baseline Surface Radiation Network continue to show an increase at the majority of sites, in line with an overall increase predicted by ESMs of the order of 2 W m <sup>–2</sup> per decade ( [[#Wild--2016|Wild, 2016]] ). Upward longwave radiation at the surface is rarely measured but is expected to have increased over the same period due to rising surface temperatures.

Turbulent fluxes of latent and sensible heat are also an important part of the surface energy budget (Figure 7.2). Large uncertainties in measurements of surface turbulent fluxes continue to prevent the determination of their decadal changes. Nevertheless, over the ocean, reanalysis-based estimates of linear trends from 1948–2008 indicate high spatial variability and seasonality. Increases in magnitudes of 4 to 7 W m <sup>–2</sup> per decade for latent heat and 2 to 3 W m <sup>–2</sup> per decade for sensible heat in the western boundary current regions are mostly balanced by decreasing trends in other regions ( [[#Gulev--2012|Gulev and Belyaev, 2012]] ). Over land, the terrestrial latent heat flux is estimated to have increased in magnitude by 0.09 W m <sup>–2</sup> per decade from 1989–1997, and subsequently decreased by 0.13 W m <sup>–2</sup> per decade from 1998–2005 due to soil-moisture limitation mainly in the Southern Hemisphere (derived from [[#Mueller--2013|Mueller et al., 2013]] ). These trends are small in comparison to the uncertainty associated with satellite-derived and in situ observations, as well as from land-surface models forced by observations and atmospheric reanalyses. Ongoing advances in remote sensing of evapotranspiration from space ( [[#Mallick--2016|Mallick et al., 2016]] ; [[#Fisher--2017|Fisher et al., 2017]] ; [[#McCabe--2017a|McCabe et al., 2017a]] , b), as well as terrestrial water storage ( [[#Rodell--2018|Rodell et al., 2018]] ) may contribute to future constraints on changes in latent heat flux.

In summary, since AR5, multi-decadal decreasing and increasing trends in surface solar radiation of up to several percent per decade have been detected at many more locations, even in remote areas. There is ''high confidence'' that these trends are widespread, and not localized phenomena or measurement artefacts. The origin of these trends is not fully understood, although there is evidence that anthropogenic aerosols have made a substantial contribution ( ''medium confidence'' ). There is ''medium confidence'' that downward and upward thermal radiation has increased since the 1970s, while there remains ''low confidence'' in the trends in surface sensible and latent heat.

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'''Box 7.2 | The Global Energy Budget'''

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This box assesses the present knowledge of the global energy budget for the period 1971–2018, that is, the balance between radiative forcing, the climate system radiative response and observations of the changes in the global energy inventory (Box 7.2, Figure 1a,d).

The net effective radiative forcing (ERF) of the Earth system since 1971 has been positive ( [[#7.3|Section 7.3]] and Box 7.2, Figure 1b,e), mainly as a result of increases in atmospheric greenhouse gas concentrations (Sections 2.2.8 and 7.3.2). The ERF of these positive forcing agents have been partly offset by that of negative forcing agents, primarily due to anthropogenic aerosols ( [[#7.3.3|Section 7.3.3]] ), which dominate the overall uncertainty. The net energy inflow to the Earth system from ERF for the period 1971–2018 is estimated to be 937 ZJ (1 ZJ = 10 <sup>21</sup> J) with a ''likely'' range of 644 to 1259 ZJ (Box 7.2, Figure 1b).

Box 7.2

The ERF-induced heating of the climate system results in increased thermal radiation to space via the Planck response, but the picture is complicated by a variety of climate feedbacks ( [[#7.4.2|Section 7.4.2]] and Box 7.1) that also influence the climate system radiative response (Box 7.2, Figure 1c). The total radiative response is estimated by multiplying the assessed net feedback parameter, α , from process-based evidence ( [[#7.4.2|Section 7.4.2]] and Table 7.10) with the observed GSAT change for the period (Cross Chapter Box 2.3) and time-integrating (Box 7.2, Figure 1c). The net energy outflow from the Earth system associated with the integrated radiative response for the period 1971–2018 is estimated to be 621 ZJ with a ''likely'' range of 419 to 823 ZJ. Assuming a pattern effect ( [[#7.4.4|Section 7.4.4]] ) on α of –0.5 W m <sup>–2</sup> °C <sup>–1</sup> would lead to a systematically larger energy outflow by about 250 ZJ.

[[File:ebbad856065050657d92f215b2f625b9 IPCC_AR6_WGI_Box_7_2_Figure_1.png]]

'''Box 7.2, Figure'''

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'''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.''' 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 pre-industrial Earth energy imbalance of 0.2 W m <sup>–2</sup> (a), and an illustration of an assumed pattern effect of –0.5 W m <sup>–2</sup> °C <sup>–1</sup> (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 pre-industrial and the implied energy change from integrated radiative forcing plus integrated radiative response under a number of different assumptions, as indicated in the legend, including assumptions of correlated and uncorrelated uncertainties in forcing plus response. Shading represents the ''very likely'' range for observed energy change relative to pre-industrial levels and ''likely'' range for all other quantities. Forcing and response time series are expressed relative to a baseline period of 1850–1900. Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

Combining the ''likely'' range of integrated radiative forcing (Box 7.2, Figure 1b) with the central estimate of integrated radiative response (Box 7.2, Figure 1c) gives a central estimate and ''likely'' range of 340 [47 to 662] ZJ (Box 7.2, Figure 1f). Combining the ''likely'' range of integrated radiative response with the central estimate of integrated radiative forcing gives a ''likely'' range of 340 [147 to 527] ZJ (Box 7.2, Figure 1f). Both calculations yield an implied energy gain in the climate system that is consistent with an independent observation-based assessment of the increase in the global energy inventory expressed relative to the estimated 1850–1900 Earth energy imbalance ( [[#7.5.2|Section 7.5.2]] and Box 7.2, Figure 1a) with a central estimate and ''very likely'' range of 284 [96 to 471] ZJ ( ''high confidence'' ) (Box 7.2, Figure 1d; Table 7.1). Estimating the total uncertainty associated with radiative forcing and radiative response remains a scientific challenge and depends on the degree of correlation between the two (Box 7.2, Figure 1f). However, the central estimate of observed energy change falls well with the estimated ''likely'' range, assuming either correlated or uncorrelated uncertainties. Furthermore, the energy budget assessment would accommodate a substantial pattern effect ( [[#7.4.4.3|Section 7.4.4.3]] ) during 1971–2018 associated with systematically larger values of radiative response (Box 7.2, Figure 1c), and potentially improved closure of the global energy budget. For the period 1970–2011, AR5 reported that the global energy budget was closed within uncertainties ( ''high confidence'' ) and consistent with the ''likely'' range of assessed climate sensitivity ( [[#Church--2013|Church et al., 2013]] ). This Report provides a more robust quantitative assessment based on additional evidence and improved scientific understanding.

In addition to new and extended observations ( [[#7.2.2|Section 7.2.2]] ), confidence in the observed accumulation of energy in the Earth system is strengthened by cross-validation of heating rates based on satellite and in situ observations ( [[#7.2.2.1|Section 7.2.2.1]] ) and closure of the global sea level budget using consistent datasets (Cross-Chapter Box 9.1 and Table 9.5). Overall, there is ''high confidence'' that the global energy budget is closed for 1971–2018 with improved consistency compared to AR5.

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