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

==== 7.2.2.2 Changes in the Global Energy Inventory ====

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The global energy inventory quantifies the integrated energy gain of the climate system associated with global ocean heat uptake, warming of the atmosphere, warming of the land, and melting of ice. Due to energy conservation, the rate of accumulation of energy in the Earth system ( [[#7.1|Section 7.1]] ) is equivalent to the Earth energy imbalance (Δ ''N'' in Box 7.1, Equation 7.1). On annual and longer time scales, changes in the global energy inventory are dominated by changes in global ocean heat content (OHC; [[#Rhein--2013|Rhein et al., 2013]] ; [[#Palmer--2014|Palmer and McNeall, 2014]] ; [[#Johnson--2016|Johnson et al., 2016]] ). Thus, observational estimates and climate model simulations of OHC change are critical to the understanding of both past and future climate change (Sections 2.3.3.1, 3.5.1.3, 4.5.2.1 and 9.2.2.1).

Since AR5, both modelling and observation-based studies have established Earth’s energy imbalance (characterized by OHC change) as a more robust metric of the rate of global climate change than GSAT on interannual-to-decadal time scales ( [[#Palmer--2014|Palmer and McNeall, 2014]] ; [[#von%20Schuckmann--2016|von Schuckmann et al., 2016]] ; [[#Wijffels--2016|Wijffels et al., 2016]] ; [[#Cheng--2018|Cheng et al., 2018]] ; [[#Allison--2020|Allison et al., 2020]] ). This is because GSAT is influenced by large unforced variations, for example linked to ENSO and Pacific Decadal Variability ( [[#Roberts--2015|Roberts et al., 2015]] ; [[#Yan--2016|Yan et al., 2016]] ; [[#Cheng--2018|Cheng et al., 2018]] ). Measuring OHC change more comprehensively over the full ocean depth results in a higher signal-to-noise ratio and a time series that increases steadily over time (Box 7.2, Figure 1; [[#Allison--2020|Allison et al., 2020]] ). In addition, understanding of the potential effects of historical ocean sampling on estimated global ocean heating rates has improved ( [[#Durack--2014|Durack et al., 2014]] ; [[#Good--2017|Good, 2017]] ; [[#Allison--2019|Allison et al., 2019]] ) and there are now more estimates of OHC change available that aim to mitigate the effect of limited observational sampling in the Southern Hemisphere ( [[#Lyman--2008|Lyman and Johnson, 2008]] ; [[#Cheng--2017|Cheng et al., 2017]] ; [[#Ishii--2017|Ishii et al., 2017]] ).

The assessment of changes in the global energy inventory for the periods 1971–2018, 1993–2018 and 2006–2018 draws upon the latest observational time series and the assessments presented in other chapters of this report. The estimates of OHC change come directly from the assessment presented in ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] ). The assessment of land and atmospheric heating comes from [[#von%20Schuckmann--2020|von Schuckmann et al. (2020)]] , based on the estimates of [[#Cuesta-Valero--2021|Cuesta-Valero et al. (2021)]] and [[#Steiner--2020|Steiner et al. (2020)]] , respectively. Heating of inland waters, including lakes, reservoirs and rivers, is estimated to account for &lt;0.1% of the total energy change, and is therefore omitted from this assessment ( [[#Vanderkelen--2020|Vanderkelen et al., 2020]] ). The cryosphere contribution from the melting of grounded ice is based on the mass-loss assessments presented in Chapter 9, [[IPCC:Wg1:Chapter:Chapter-9#9.4.1|Section 9.4.1]] (Greenland Ice Sheet), [[IPCC:Wg1:Chapter:Chapter-9#9.4.2|Section 9.4.2]] (Antarctic Ice Sheet) and ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.1%20|Section 9.5.1]] (glaciers). Following AR5, the estimate of heating associated with loss of Arctic sea ice is based on a reanalysis ( [[#Schweiger--2011|Schweiger et al., 2011]] ), following the methods described by [[#Slater--2021|Slater et al. (2021)]] . [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] [[IPCC:Wg1:Chapter:Chapter-9#9.3.2|Section 9.3.2]] ) finds no significant trend in Antarctic sea ice area over the observational record, so a zero contribution is assumed. Ice melt associated with the calving and thinning of floating ice shelves is based on the decadal rates presented in [[#Slater--2021|Slater et al. (2021)]] . For all cryospheric components, mass loss is converted to heat input using a latent heat of fusion of 3.34 × 10 <sup>5</sup> J Kg <sup>–1</sup> °C <sup>–1</sup> with the second-order contributions from variations associated with ice type and warming of ice from sub-freezing temperatures disregarded, as in AR5. The net change in energy, quantified in Zettajoules (1 ZJ = 10 <sup>21</sup> Joules), is computed for each component as the difference between the first and last year of each period (Table 7.1). The uncertainties in the depth-interval contributions to OHC are summed to get the uncertainty in global OHC change. All other uncertainties are assumed to be independent and added in quadrature.

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'''Table''' '''7.1 |''' '''Contributions of the different components of the global energy inventory for the periods 1971–2018, 1993–2018 and 2006–2018 (Box 7.2 and Cross-Chapter Box 9.1).''' Energy changes are computed as the difference between annual mean values or year mid-points. The total heating rates correspond to Earth’s energy imbalance and are expressed per unit area of Earth’s surface.

{| class="wikitable"
|-
| rowspan="2"| Component

| colspan="2"| 1971–2018

| colspan="2"| 1993–2018

| colspan="2"| 2006–2018

|-
| Energy Gain (ZJ)

| %

| Energy Gain (ZJ)

| %

| Energy Gain (ZJ)

| %

|-
| Ocean

0–700 m

700–2000 m

&gt;2000 m

| 396.0 [285.7 to 506.2]

241.6 [162.7 to 320.5]

123.3 [96.0 to 150.5]

31.0 [15.7 to 46.4]

| 91.0

55.6

28.3

7.1

| 263.0 [194.1 to 331.9]

151.5 [114.1 to 188.9]

82.8 [59.9 to 105.6]

28.7 [14.5 to 43.0]

| 91.0

52.4

28.6

10.0

| 138.8 [86.4 to 191.3]

75.4 [48.7 to 102.0]

49.7 [29.0 to 70.4]

13.8 [7.0 to 20.6]

| 91.1

49.5

32.6

9.0

|-
| Land

| 21.8 [18.6 to 25.0]

| 5.0

| 13.7 [12.4 to 14.9]

| 4.7

| 7.2 [6.6 to 7.8]

| 4.7

|-
| Cryosphere

| 11.5 [9.0 to 14.0]

| 2.7

| 8.8 [7.0 to 10.5]

| 3.0

| 4.7 [3.3 to 6.2]

| 3.1

|-
| Atmosphere

| 5.6 [4.6 to 6.7]

| 1.3

| 3.8 [3.2 to 4.3]

| 1.3

| 1.6 [1.2 to 2.1]

| 1.1

|-
| '''TOTAL'''

| colspan="2"| '''434.9 [324.5 to 545.3] ZJ'''

| colspan="2"| '''289.2 [220.3 to 358.1] ZJ'''

| colspan="2"| '''152.4 [100.0 to 204.9] ZJ'''

|-
| '''Heating Rate'''

| colspan="2"| '''0.57 [0.43 to 0.72] W m''' <sup>–2</sup>

| colspan="2"| '''0.72 [0.55 to 0.89] W m''' <sup>–2</sup>

| colspan="2"| '''0.79 [0.52 to 1.06] W m''' <sup>–2</sup>

|}

For the period 1971–2010, AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ) found an increase in the global energy inventory of 274 [196 to 351] ZJ with a 93% contribution from total OHC change, approximately 3% for both ice melt and land heating, and approximately 1% for warming of the atmosphere. For the same period, this Report finds an upwards revision of OHC change for the upper (&lt;700 m depth) and deep (&gt;700 m depth) ocean of approximately 8% and 20%, respectively, compared to AR5 and a modest increase in the estimated uncertainties associated with the ensemble approach of [[#Palmer--2021|Palmer et al. (2021)]] . The other substantive change compared to AR5 is the updated assessment of land heating, with values approximately double those assessed previously, based on a more comprehensive analysis of the available observations ( [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ; [[#Cuesta-Valero--2021|Cuesta-Valero et al., 2021]] ). The result of these changes is an assessed energy gain of 329 [224 to 434] ZJ for the period 1971–2010, which is consistent with AR5 within the estimated uncertainties, despite the systematic increase.

The assessed changes in the global energy inventory (Box 7.2, Figure 1, and Table 7.1) yields an average value for Earth’s energy imbalance ( ''N'' in Box 7.1, Equation 7.1) of 0.57 [0.43 to 0.72] W m <sup>–2</sup> for the period 1971–2018, expressed relative to Earth’s surface area ( ''high confidence'' ). The estimates for the periods 1993–2018 and 2006–2018 yield substantially larger values of 0.72 [0.55 to 0.89] W m <sup>–2</sup> and 0.79 [0.52 to 1.06] W m <sup>–2</sup> , respectively, consistent with the increased radiative forcing from GHGs ( ''high confidence'' ). For the period 1971–2006, the total energy gain was 282 [177 to 387] ZJ, with an equivalent Earth energy imbalance of 0.50 [0.32 to 0.69] W m <sup>–2</sup> . To put these numbers in context, the 2006–2018 average Earth system heating is equivalent to approximately 20 times the annual rate of global energy consumption in 2018. <sup>[[#footnote-001|1]]</sup>

Consistent with AR5 ( [[#Rhein--2013|Rhein et al., 2013]] ), this Report finds that ocean warming dominates the changes in the global energy inventory ( ''high confidence'' ), accounting for 91% of the observed change for all periods considered (Table 7.1). The contributions from the other components across all periods are approximately 5% from land heating, 3% for cryosphere heating and 1% associated with warming of the atmosphere ( ''high confidence'' ). The assessed percentage contributions are similar to the recent study by [[#von%20Schuckmann--2020|von Schuckmann et al. (2020)]] and the total heating rates are consistent within the assessed uncertainties. 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), strengthen scientific confidence in the assessed changes in the global energy inventory relative to AR5.

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