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

==== 1.2.1.2 Choice of reference period ====

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Any choice of reference period used to approximate ‘pre-industrial’ conditions is a compromise between data coverage and representativeness of typical pre-industrial solar and volcanic forcing conditions. This report adopts the 51-year reference period, 1850–1900 inclusive, assessed as an approximation of pre-industrial levels in AR5 (Box TS.5, Figure 1 of Field et al., 2014) <sup>[[#fn:r78|78]]</sup> . The years 1880–1900 are subject to strong but uncertain volcanic forcing, but in the HadCRUT4 dataset, average temperatures over 1850–1879, prior to the largest eruptions, are less than 0.01°C from the average for 1850–1900. Temperatures rose by 0.0°C–0.2°C from 1720–1800 to 1850–1900 (Hawkins et al., 2017) <sup>[[#fn:r79|79]]</sup> , but the anthropogenic contribution to this warming is uncertain (Abram et al., 2016; Schurer et al., 2017) <sup>[[#fn:r80|80]]</sup> . The 18th century represents a relatively cool period in the context of temperatures since the mid-Holocene (Marcott et al., 2013; Lüning and Vahrenholt, 2017; Marsicek et al., 2018) <sup>[[#fn:r81|81]]</sup> , which is indicated by the pink shaded region in Figure 1.2.

Projections of responses to emission scenarios, and associated impacts, may use a more recent reference period, offset by historical observations, to avoid conflating uncertainty in past and future changes (e.g., Hawkins et al., 2017; Millar et al., 2017b; Simmons et al., 2017) <sup>[[#fn:r82|82]]</sup> . Two recent reference periods are used in this report: 1986–2005 and 2006–2015. In the latter case, when using a single decade to represent a 30-year average centred on that decade, it is important to consider the potential impact of internal climate variability. The years 2008–2013 were characterised by persistent cool conditions in the Eastern Pacific (Kosaka and Xie, 2013; Medhaug et al., 2017) <sup>[[#fn:r83|83]]</sup> , related to both the El Niño-Southern Oscillation (ENSO) and, potentially, multi-decadal Pacific variability (e.g., England et al., 2014) <sup>[[#fn:r84|84]]</sup> , but these were partially compensated for by El Niño conditions in 2006 and 2015. Likewise, volcanic activity depressed temperatures in 1986–2005, partly offset by the very strong El Niño event in 1998. Figure 1.2 indicates that natural variability (internally generated and externally driven) had little net impact on average temperatures over 2006–2015, in that the average temperature of the decade is similar to the estimated externally driven warming. When solar, volcanic and ENSO-related variability is taken into account following the procedure of Foster and Rahmstorf (2011) <sup>[[#fn:r85|85]]</sup> , there is no indication of average temperatures in either 1986–2005 or 2006–2015 being substantially biased by short-term variability (see Supplementary Material 1.SM.2). The temperature difference between these two reference periods (0.21°C–0.27°C over 15 years across available datasets) is also consistent with the AR5 assessment of the current warming rate of 0.3°C–0.7°C over 30 years (Kirtman et al., 2013) <sup>[[#fn:r86|86]]</sup> .

On the definition of warming used here, warming to the decade 2006–2015 comprises an estimate of the 30-year average centred on this decade, or 1996–2025, assuming the current trend continues and that any volcanic eruptions that might occur over the final seven years are corrected for. Given this element of extrapolation, we use the AR5 near-term projection to provide a conservative uncertainty range. Combining the uncertainty in observed warming to 1986–2005 (±0.06°C) with the ''likely'' range in the current warming trend as assessed by AR5 (±0.2°C/30 years), assuming these are uncorrelated, and using observed warming relative to 1850–1900 to provide the central estimate (no evidence of bias from short-term variability), gives an assessed warming to the decade 2006–2015 of 0.87°C with a ±0.12°C ''likely''  range. This estimate has the advantage of traceability to the AR5, but more formal methods of quantifying externally driven warming (e.g., Bindoff et al., 2013; Jones et al., 2016; Haustein et al., 2017; Ribes et al., 2017) <sup>[[#fn:r87|87]]</sup> , which typically give smaller ranges of uncertainty, may be adopted in the future.

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<span id="table-1.1"></span>
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'''Table 1.1'''

<span id="observed-increase-in-global-average-surface-temperature-in-various-datasets.-numbers-in-square-brackets-correspond-to-595-uncertainty-ranges-from-individual-datasets-encompassing-known-sources-of-observational-uncertainty-only."></span>
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'''Observed increase in global average surface temperature in various datasets. Numbers in square brackets correspond to 5–95% uncertainty ranges from individual datasets, encompassing known sources of observational uncertainty only.'''
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{| class="wikitable"
|-
| '''Diagnostic / dataset'''
| '''1850–1900 to (1)<br />
''' '''2006–2015'''
| '''1850–1900 to (2)<br />
''' '''1986–2005'''
| '''1986–2005 to (3)<br />
''' '''2006–2015'''
| '''1850–1900 to (4)<br />
''' '''1981–2010'''
| '''1850–1900 to (5)<br />
''' '''1998–2017'''
| '''Trend (6)<br />
''' '''1880–2012'''
| '''Trend (6)<br />
''' '''1880–2015'''
|-
| '''HadCRUT4.6'''
| 0.84
[0.79–0.89]

| 0.60
[0.57–0.66]

| 0.22
[0.21–0.23]

| 0.62
[0.58–0.67]

| 0.83
[0.78–0.88]

| 0.83
[0.77–0.90]

| 0.88
[0.83–0.95]

|-
| '''NOAAGlobalTemp (7)'''
| 0.86
| 0.62
| 0.22
| 0.63
| 0.85
| 0.91
|-
| '''GISTEMP (7)'''
| 0.89
| 0.65
| 0.23
| 0.66
| 0.88
| 0.89
| 0.94
|-
| '''Cowtan-Way'''
| 0.91
[0.85–0.99]

| 0.65
[0.60–0.72]

| 0.26
[0.25–0.27]

| 0.65
[0.60–0.72]

| 0.88
[0.82–0.96]

| 0.88
[0.79–0.98]

| 0.93
[0.85–1.03]

|-
| '''Average (8)'''
| '''0.87'''
| 0.63
| 0.23
| 0.64
| 0.86
| 0.92
|-
| '''Berkeley (9)'''
| 0.98
| 0.73
| 0.25
| 0.73
| 0.97
| 1.02
|-
| '''JMA (9)'''
| 0.82
| 0.59
| 0.17
| 0.60
| 0.81
| 0.82
| 0.87
|-
| '''ERA-Interim'''
| N/A
| 0.26
| N/A
|-
| '''JRA-55'''
| N/A
| 0.23
| N/A
|-
| '''CMIP5 global SAT (10)'''
| 0.99
[0.65–1.37]

| 0.62
[0.38–0.94]

| 0.38
[0.24–0.62]

| 0.62
[0.34–0.93]

| 0.89
[0.62–1.29]

| 0.81
[0.58–1.31]

| 0.86
[0.63–1.39]

|-
| '''CMIP5 SAT/SST blend—masked'''
| 0.86
[0.54–1.18]

| 0.50
[0.31–0.79]

| 0.34
[0.19–0.54]

| 0.48
[0.26–0.79]

| 0.75
[0.52–1.11]

| 0.68
[0.45–1.08]

| 0.74
[0.51–1.14]

|}
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Notes:

# Most recent reference period used in this report.
# Most recent reference period used in AR5.
# Difference between recent reference periods.
# Current WMO standard reference periods.
# Most recent 20-year period.
# Linear trends estimated by a straight-line fit, expressed in degrees yr <sup>−1</sup> multiplied by 133 or 135 years respectively, with uncertainty ranges incorporating observational uncertainty only.
# To estimate changes in the NOAAGlobalTemp and GISTEMP datasets relative to the 1850–1900 reference period, warming is computed relative to 1850–1900 using the HadCRUT4.6 dataset and scaled by the ratio of the linear trend 1880–2015 in the NOAAGlobalTemp or GISTEMP dataset with the corresponding linear trend computed from HadCRUT4.
# Average of diagnostics derived – see (7) – from four peer-reviewed global datasets, HadCRUT4.6, NOAA, GISTEMP &amp; Cowtan-Way. Note that differences between averages may not coincide with average differences because of rounding.
# No peer-reviewed publication available for these global combined land–sea datasets.
# CMIP5 changes estimated relative to 1861–80 plus 0.02°C for the offset in HadCRUT4.6 from 1850–1900. CMIP5 values are the mean of the RCP8.5 ensemble, with 5–95% ensemble range. They are included to illustrate the difference between a complete global surface air temperature record (SAT) and a blended surface air and sea surface temperature (SST) record accounting for incomplete coverage (masked), following Richardson et al. (2016) <sup>[[#fn:r88|88]]</sup> . Note that 1986–2005 temperatures in CMIP5 appear to have been depressed more than observed temperatures by the eruption of Mount Pinatubo.

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