Editing IPCC:AR6/SR15/Chapter-1 (section)

==== 1.2.1.1 Definition of global average temperature ====

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The IPCC has traditionally defined changes in observed GMST as a weighted average of near-surface air temperature (SAT) changes over land and sea surface temperature (SST) changes over the oceans (Morice et al., 2012; Hartmann et al., 2013) <sup>[[#fn:r58|58]]</sup> , while modelling studies have typically used a simple global average SAT. For ambitious mitigation goals, and under conditions of rapid warming or declining sea ice (Berger et al., 2017) <sup>[[#fn:r59|59]]</sup> , the difference can be significant. Cowtan et al. (2015) <sup>[[#fn:r60|60]]</sup> and Richardson et al. (2016) <sup>[[#fn:r61|61]]</sup> show that the use of blended SAT/SST data and incomplete coverage together can give approximately 0.2°C less warming from the 19th century to the present relative to the use of complete global-average SAT (Stocker et al., 2013 <sup>[[#fn:r62|62]]</sup> , Figure TFE8.1 and Figure 1.2). However, Richardson et al. (2018) <sup>[[#fn:r63|63]]</sup>  show that this is primarily an issue for the interpretation of the historical record to date, with less absolute impact on projections of future changes, or estimated emissions budgets, under ambitious mitigation scenarios.

The three GMST reconstructions used in AR5 differ in their treatment of missing data. GISTEMP (Hansen et al., 2010) <sup>[[#fn:r64|64]]</sup> uses interpolation to infer trends in poorly observed regions like the Arctic (although even this product is spatially incomplete in the early record), while NOAAGlobalTemp (Vose et al., 2012) <sup>[[#fn:r65|65]]</sup> and HadCRUT (Morice et al., 2012) <sup>[[#fn:r66|66]]</sup> are progressively closer to a simple average of available observations. Since the AR5, considerable effort has been devoted to more sophisticated statistical modelling to account for the impact of incomplete observation coverage (Rohde et al., 2013; Cowtan and Way, 2014; Jones, 2016) <sup>[[#fn:r67|67]]</sup> . The main impact of statistical infilling is to increase estimated warming to date by about 0.1°C (Richardson et al., 2018 <sup>[[#fn:r68|68]]</sup> and Table 1.1).

We adopt a working definition of warming over the historical period based on an average of the four available global datasets that are supported by peer-reviewed publications: the three datasets used in the AR5, updated (Karl et al., 2015) <sup>[[#fn:r69|69]]</sup> , together with the Cowtan-Way infilled dataset (Cowtan and Way, 2014) <sup>[[#fn:r70|70]]</sup> . A further two datasets, Berkeley Earth (Rohde et al., 2013) <sup>[[#fn:r71|71]]</sup> and that of the Japan Meteorological Agency (JMA), are provided in Table 1.1. This working definition provides an updated estimate of 0.86°C for the warming over the period 1880–2012 based on a linear trend. This quantity was quoted as 0.85°C in the AR5. Hence the inclusion of the Cowtan-Way dataset does not introduce any inconsistency with the AR5, whereas redefining GMST to represent global SAT could increase this figure by up to 20% (Table 1.1, blue lines in Figure 1.2 and Richardson et al., 2016) <sup>[[#fn:r72|72]]</sup> .

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'''Figure 1.2'''

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'''Evolution of global mean surface temperature (GMST) over the period of instrumental observations.'''
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[[File:c7a573f15451c4f486ebc4cc479db4c0 figure-1.2-1024x626.png]]

Grey shaded line shows monthly mean GMST in the HadCRUT4, NOAAGlobalTemp, GISTEMP and Cowtan-Way datasets, expressed as departures from 1850–1900, with varying grey line thickness indicating inter-dataset range. All observational datasets shown represent GMST as a weighted average of near surface air temperature over land and sea surface temperature over oceans. Human-induced (yellow) and total (human- and naturally-forced, orange) contributions to these GMST changes are shown calculated following Otto et al. (2015) <sup>[[#fn:r73|73]]</sup> and Haustein et al. (2017) <sup>[[#fn:r74|74]]</sup> . Fractional uncertainty in the level of human-induced warming in 2017 is set equal to ±20% based on multiple lines of evidence. Thin blue lines show the modelled global mean surface air temperature (dashed) and blended surface air and sea surface temperature accounting for observational coverage (solid) from the CMIP5 historical ensemble average extended with RCP8.5 forcing (Cowtan et al., 2015; Richardson et al., 2018) <sup>[[#fn:r75|75]]</sup> . The pink shading indicates a range for temperature fluctuations over the Holocene (Marcott et al., 2013) <sup>[[#fn:r76|76]]</sup> . Light green plume shows the AR5 prediction for average GMST over 2016–2035 (Kirtman et al., 2013) <sup>[[#fn:r77|77]]</sup> . See Supplementary Material 1.SM for further details.

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