Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.2.1.2 Changes in global land surface air temperature ====

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Based on analysis of several global and regional land surface air temperature (LSAT) datasets, AR5 concluded that the global LSAT had increased over the instrumental period of record, with the warming rate approximately double that reported over the oceans since 1979 and that ‘it is certain that globally averaged LSAT has risen since the late 19th century and that this warming has been particularly marked since the 1970s’. Warming found in the global land datasets is also in a broad agreement with station observations (Hartmann et al. 2013a <sup>[[#fn:r37|37]]</sup> ).

Since AR5, LSAT datasets have been improved and extended. The National Center for Environmental Information, which is a part of the US National Oceanic and Atmospheric Administration (NOAA), developed a new, fourth version of the Global Historical Climatology Network monthly dataset (GHCNm, v4). The dataset provides an expanded set of station temperature records with more than 25,000 total monthly temperature stations compared to 7200 in versions v2 and v3 (Menne et al. 2018 <sup>[[#fn:r38|38]]</sup> ). Goddard Institute for Space Studies, which is a part of the US National Aeronautics and Space Administration, (NASA/ GISS), provides estimate of land and ocean temperature anomalies (GISTEMP). The GISTEMP land temperature anomalies are based upon primarily NOAA/GHCN version 3 dataset (Lawrimore et al. 2011 <sup>[[#fn:r39|39]]</sup> ) and account for urban effects through nightlight adjustments (Hansen et al. 2010 <sup>[[#fn:r40|40]]</sup> ). The Climatic Research Unit (CRU) of the University of East Anglia, UK (CRUTEM) dataset, now version CRUTEM4.6,

incorporates additional stations (Jones et al. 2012 <sup>[[#fn:r41|41]]</sup> ). Finally, the Berkeley Earth Surface Temperature (BEST) dataset provides LSAT from 1750 to present based on almost 46,000 time series and has the longest temporal coverage of the four datasets (Rohde et al. 2013 <sup>[[#fn:r42|42]]</sup> ). This dataset was derived with methods distinct from those used for development of the NOAA GHCNm, NASA/GISS GISTEMP and the University of East Anglia CRUTEM datasets.

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'''Table 2.1'''

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'''Increases in land surface air temperature (LSAT) from preindustrial period and the late 19th century to present day.'''

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{| class="wikitable"
|-
|

Dataset of LSAT increase (°C)

|-

Time period

BEST

CRUTEM4.6

GHCNm, v4

GISTEMP

|-

''From'' 1850–1900 to 2006–2015

1.53<br />
1.38–1.68<br />
(95% confidence)

1.32*

|
|-

''From'' 1850–1900 to 1999–2018

1.52<br />
1.39–1.66<br />
(95% confidence)

1.31

NA

NA

|-

''From'' 1881–1900 to 1999–2018

1.51<br />
1.40–1.63<br />
(95% confidence)

1.31

1.37

1.45

|}
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\* CRUTEM4.6 LSAT increase is computed from 1856–1900 average.

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According to the available observations in the four datasets, the globally averaged LSAT increased by 1.44°C from the preindustrial period (1850–1900) to the present day (1999–2018).The warming from the late 19th century (1881–1900) to the present day (1999–2018) was 1.41°C (1.31°C–1.51°C) (Table 2.1). The 1.31°C–1.51°C range represents the spread in median estimates from the four available land datasets and does not reflect uncertainty in data coverage or methods used. Based on the BEST dataset (the longest dataset with the most extensive land coverage) the total observed increase in LSAT between the average of the 1850–1900 period and the 2006– 2015 period was 1.53°C, (1.38–1.68°C; 95% confidence), while the GMST increase for the same period was 0.87°C (0.75–0.99°C; 90% confidence) (IPCC, 2018: Summary for policymakers, Allen et al. 2018 <sup>[[#fn:r43|43]]</sup> ).

The extended and improved land datasets reaffirmed the AR5 conclusion that it is certain that globally averaged LSAT has risen since the preindustrial period and that this warming has been particularly marked since the 1970s (Figure 2.2).

Recent analyses of LSAT and sea surface temperature (SST) observations, as well as analyses of climate model simulations, have refined our understanding of underlying mechanisms responsible for a faster rate of warming over land than over oceans. Analyses of paleo records, historical observations, model simulations and underlying physical principles are all in agreement that the land is warming faster than the oceans as a result of differences in evaporation, land–climate feedbacks (Section 2.5) and changes in the aerosol forcing over land ( ''very high confidence'' ) (Braconnot et al. 2012 <sup>[[#fn:r44|44]]</sup> ; Joshi et al. 2013 <sup>[[#fn:r45|45]]</sup> ; Sejas et al. 2014 <sup>[[#fn:r46|46]]</sup> ; Byrne and O’Gorman 2013 <sup>[[#fn:r47|47]]</sup> , 2015 <sup>[[#fn:r48|48]]</sup> ; Wallace and Joshi 2018 <sup>[[#fn:r49|49]]</sup> ; Allen et al. 2019 <sup>[[#fn:r50|50]]</sup> ). There is also ''high confidence'' that difference in land and ocean heat capacity is not the primary reason for faster land than ocean warming. For the recent period, the land-to-ocean warming ratio is in close agreement between different observational records (about 1.6) and the CMIP5 climate model simulations (the ''likely'' range of 1.54°C to 1.81°C). Earlier studies analysing slab ocean models (models in which it is assumed that the deep ocean has equilibrated) produced a higher land temperature increases than sea surface temperature (Manabe et al. 1991 <sup>[[#fn:r51|51]]</sup> ; Sutton et al. 2007 <sup>[[#fn:r52|52]]</sup> ).

It is certain that globally averaged LSAT has risen faster than GMST from the preindustrial period (1850–1900) to the present day (1999–2018). This is because the warming rate of the land compared to the ocean is substantially higher over the historical period (by approximately 60%) and because the Earth’s surface is approximately one-third land and two-thirds ocean. This enhanced land warming impacts land processes with implications for desertification (Section 2.2.2 and Chapter 3), food security (Section 2.2.3 and Chapter 5), terrestrial ecosystems (Section 2.2.4), and GHG and non-GHG fluxes between the land and climate (Sections 2.3 and 2.4). Future changes in land characteristics through adaptation and mitigation processes and associated land–climate feedbacks can dampen warming in some regions and enhance warming in others (Section 2.5).

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'''Figure 2.2'''

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'''Evolution of land surface air temperature (LSAT) and global mean surface temperature (GMST) over the period of instrumental observations. The brown line shows annual mean LSAT in the BEST, CRUTEM4.6, GHCNmv4 and GISTEMP datasets, expressed as departures from global average LSAT in 1850–1900, with the brown line thickness indicating inter-dataset range. The blue line shows […]'''
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[[File:e3db74bab240f8ac71cfee37c41f7fd2 Figure-2.2-1024x670.jpg]]

Evolution of land surface air temperature (LSAT) and global mean surface temperature (GMST) over the period of instrumental observations. The brown line shows annual mean LSAT in the BEST, CRUTEM4.6, GHCNmv4 and GISTEMP datasets, expressed as departures from global average LSAT in 1850–1900, with the brown line thickness indicating inter-dataset range. The blue line shows annual mean GMST in the HadCRUT4, NOAAGlobal Temp, GISTEMP and Cowtan&amp;Way datasets (monthly values of which were reported in the Special Report on Global Warming of 1.5ºC; Allen et al. 2018 <sup>[[#fn:r53|53]]</sup> ).

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