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

==== 7.2.2.1 Changes in Earth’s Top-of-atmosphere Energy Budget ====

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Since 2000, changes in top-of-atmosphere (TOA) energy fluxes can be tracked from space using CERES satellite observations (Figure 7.3). The variations in TOA energy fluxes reflect the influence of internal climate variability, particularly that of El Niño–Southern Oscillation (ENSO), in addition to radiative forcing of the climate system and climate feedbacks ( [[#Allan--2014|Allan et al., 2014]] ; [[#Loeb--2018b|Loeb et al., 2018b]] ). For example, globally, the reduction in both outgoing thermal and reflected solar radiation during La Niña conditions in 2008/2009 led to an energy gain for the climate system, whereas enhanced outgoing thermal and reflected solar radiation caused an energy loss during the El Niños of 2002/2003 and 2009/2010 (Figure 7.3; [[#Loeb--2018b|Loeb et al., 2018b]] ). An ensemble of CMIP6 models is able to track the variability in global mean TOA fluxes observed by CERES, when driven with prescribed sea surface temperatures (SSTs) and sea ice concentrations (Figure 7.3; [[#Loeb--2020|Loeb et al., 2020]] ). Under cloud-free conditions, the CERES record shows a near zero trend in outgoing thermal radiation ( [[#Loeb--2018b|Loeb et al., 2018b]] ), which – combined with an increasing surface upwelling thermal flux – implies an increasing clear-sky greenhouse effect ( [[#Raghuraman--2019|Raghuraman et al., 2019]] ). Conversely, clear-sky solar reflected TOA radiation in the CERES record covering March 2000 to September 2017 shows a decrease due to reductions in aerosol optical depth in the Northern Hemisphere and sea ice fraction ( [[#Loeb--2018a|Loeb et al., 2018a]] ; [[#Paulot--2018|Paulot et al., 2018]] ).

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'''Figure 7.3''' '''|''' '''Anomalies in global mean all-sky top-of-atmosphere (TOA) fluxes from CERES-EBAF Ed4.0 (solid black lines) and various CMIP6 climate models (coloured lines) in terms of (a) reflected solar, (b) emitted thermal and (c) net TOA fluxes.''' The multi-model means are additionally depicted as solid red lines. Model fluxes stem from simulations driven with prescribed sea surface temperatures (SSTs) and all known anthropogenic and natural forcings. Shown are anomalies of 12-month running means. All flux anomalies are defined as positive downwards, consistent with the sign convention used throughout this chapter. The correlations between the multi-model means (solid red lines) and the CERES records (solid black lines) for 12-month running means are: 0.85 for the global mean reflected solar; 0.73 for outgoing thermal radiation; and 0.81 for net TOA radiation. Figure adapted from [[#Loeb--2020|Loeb et al. (2020)]] . Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

An effort to reconstruct variations in net TOA fluxes back to 1985, based on a combination of satellite data, atmospheric reanalysis and high-resolution climate model simulations ( [[#Allan--2014|Allan et al., 2014]] ; [[#Liu--2020|Liu et al., 2020]] ), exhibits strong interannual variability associated with the volcanic eruption of Mount Pinatubo in 1991 and the ENSO events before 2000. The same reconstruction suggests that Earth’s energy imbalance increased by several tenths of a W m <sup>–2</sup> between the periods 1985–1999 and 2000–2016, in agreement with the assessment of changes in the global energy inventory ( [[#7.2.2.2|Section 7.2.2.2]] , and Box 7.2, Figure 1). Comparisons of year-to-year variations in Earth’s energy imbalance estimated from CERES and independent estimates based on ocean heat content change are significantly correlated with similar phase and magnitude ( [[#Johnson--2016|Johnson et al., 2016]] ; [[#Meyssignac--2019|Meyssignac et al., 2019]] ), promoting confidence in both satellite and in situ-based estimates ( [[#7.2.2.2|Section 7.2.2.2]] ).

In summary, variations in the energy exchange between Earth and space can be accurately tracked since the advent of improved observations since the year 2000 ( ''high confidence'' ), while reconstructions indicate that the Earth’s energy imbalance was larger in the 2000s than in the 1985–1999 period ( ''high confidence'' ).

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