Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.3.1 Global Changes in Climate ===

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There is ''high confidence'' that the increase in global mean surface temperature (GMST) has reached 0.87°C (±0.10°C ''likely'' range) above pre-industrial values in the 2006–2015 decade (Chapter 1). AR5 assessed that the globally averaged temperature (combined over land and ocean) displayed a warming of about 0.85°C [0.65°C to 1.06°C] during the period 1880–2012, with a large fraction of the detected global warming being attributed to anthropogenic forcing (Bindoff et al., 2013a; Hartmann et al., 2013; Stocker et al., 2013) <sup>[[#fn:r45|45]]</sup> . While new evidence has highlighted that sampling biases and the choice of approaches used to estimate GMST (e.g., using water versus air temperature over oceans and using model simulations versus observations-based estimates) can affect estimates of GMST increase (Richardson et al., 2016; <sup>[[#fn:r46|46]]</sup> see also Supplementary Material 3.SM.2), the present assessment is consistent with that of AR5 regarding a detectable and dominant effect of anthropogenic forcing on observed trends in global temperature (also confirmed in Ribes et al., 2017) <sup>[[#fn:r47|47]]</sup> . As highlighted in Chapter 1, human-induced warming reached approximately 1°C (±0.2°C ''likely'' range) in 2017. More background on recent observed trends in global climate is provided in the Supplementary Material 3.SM.2.

A global warming of 1.5°C implies higher mean temperatures compared to during pre-industrial times in almost all locations, both on land and in oceans ( ''high confidence'' ) (Figure 3.3). In addition, a global warming of 2°C versus 1.5°C results in robust differences in the mean temperatures in almost all locations, both on land and in the ocean ( ''high confidence'' ). The land–sea contrast in warming is important and implies particularly large changes in temperature over land, with mean warming of more than 1.5°C in most land regions ( ''high confidence'' ; see Section 3.3.2 for more details). The largest increase in mean temperature is found in the high latitudes of the Northern Hemisphere ( ''high confidence'' ; Figure 3.3, see Section 3.3.2 for more details). Projections for precipitation are more uncertain, but they highlight robust increases in mean precipitation in the Northern Hemisphere high latitudes at 1.5ºC global warming versus pre-industrial conditions, as well as at 2ºC global warming versus pre-industrial conditions ( ''high confidence)'' (Figure 3.3). There are consistent but less robust signals when comparing changes in mean precipitation at 2ºC versus 1.5°C of global warming. Hence, it is assessed that there is ''medium confidence'' in an increase of mean precipitation in high-latitudes at 2ºC versus 1.5ºC of global warming (Figure 3.3). For droughts, changes in evapotranspiration and precipitation timing are also relevant (see Section 3.3.4). Figure 3.4 displays changes in temperature extremes (the hottest daytime temperature of the year, TXx, and the coldest night-time temperature of the year, TNn) and heavy precipitation (the annual maximum 5-day precipitation, Rx5day). These analyses reveal distinct patterns of changes, with the largest changes in TXx occurring on mid-latitude land and the largest changes in TNn occurring at high latitudes (both on land and in oceans). Differences in TXx and TNn compared to pre-industrial climate are robust at both global warming levels. Differences in TXx and TNn at 2°C versus 1.5°C of global warming are robust across most of the globe. Changes in heavy precipitation are less robust, but particularly strong increases are apparent at high latitudes as well as in the tropics at both 1.5°C and 2°C of global warming compared to pre-industrial conditions. The differences in heavy precipitation at 2ºC versus 1.5ºC global warming are generally not robust at grid-cell scale, but they display consistent increases in most locations (Figure 3.4). However, as addressed in Section 3.3.3, statistically significant differences are found in several large regions and when aggregated over the global land area. We thus assess that there is ''high confidence'' regarding global-scale differences in temperature means and extremes at 2°C versus 1.5°C global warming, and ''medium confidence'' regarding global-scale differences in precipitation means and extremes. Further analyses, including differences at 1.5°C and 2°C global warming versus 1°C (i.e., present-day) conditions are provided in the Supplementary Material 3.SM.2.

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'''Figure 3.3'''

<span id="projected-changes-in-mean-temperature-top-and-mean-precipitation-bottom-at-1.5c-left-and-2c-middle-of-global-warming-compared-to-the-pre-industrial-period-18611880-and-the-difference-between-1.5c-and-2c-of-global-warming-right."></span>
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'''Projected changes in mean temperature (top) and mean precipitation (bottom) at 1.5°C (left) and 2°C (middle) of global warming compared to the pre-industrial period (1861–1880), and the difference between 1.5°C and 2°C of global warming (right).'''
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[[File:428f98820d0ddabd99c0c17501450cc4 Figure-3.3-1-1024x528.jpg]]

Cross-hatching highlights areas where at least two-thirds of the models agree on the sign of change as a measure of robustness (18 or more out of 26). Values were assessed from the transient response over a 10-year period at a given warming level, based on Representative Concentration Pathway (RCP)8.5 Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations (adapted from Seneviratne et al., 2016 <sup>[[#fn:r48|48]]</sup> and Wartenburger et al., 2017 <sup>[[#fn:r49|49]]</sup> , see Supplementary Material 3.SM.2 for more details). Note that the responses at 1.5°C of global warming are similar for RCP2.6 simulations (see Supplementary Material 3.SM.2). Differences compared to 1°C of global warming are provided in the Supplementary Material 3.SM.2.

Original Creation for this Report using CMIP5 multi-model ensemble output data.

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'''Figure 3.4'''

<span id="projected-changes-in-extremes-at-1.5c-left-and-2c-middle-of-global-warming-compared-to-the-pre-industrial-period-18611880-and-the-difference-between-1.5c-and-2c-of-global-warming-right."></span>
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'''Projected changes in extremes at 1.5°C (left) and 2°C (middle) of global warming compared to the pre-industrial period (1861–1880), and the difference between 1.5°C and 2°C of global warming (right).'''
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[[File:30449f014aa3614940a5105effb5397a figure-3.4-2-1024x746.jpg]]

Cross-hatching highlights areas where at least two-thirds of the models agree on the sign of change as a measure of robustness (18 or more out of 26): T: temperature of annual hottest day (maximum temperature), TXx (top), and temperature of annual coldest night (minimum temperature), TNn (middle), and annual maximum 5-day precipitation, Rx5day (bottom). The underlying methodology and data basis are the same as for Figure 3.3 (see Supplementary Material 3.SM.2 for more details). Note that the responses at 1.5°C of global warming are similar for Representative Concentration Pathway (RCP) 2.6 simulations (see Supplementary Material 3.SM.2). Differences compared to 1°C of global warming are provided in the Supplementary Material 3.SM.2.

Original Creation for this Report using CMIP5 multi-model ensemble output data.

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These projected changes at 1.5°C and 2°C of global warming are consistent with the attribution of observed historical global trends in temperature and precipitation means and extremes (Bindoff et al., 2013a) <sup>[[#fn:r50|50]]</sup> , as well as with some observed changes under the recent global warming of 0.5°C (Schleussner et al., 2017) <sup>[[#fn:r51|51]]</sup> . These comparisons are addressed in more detail in Sections 3.3.2 and 3.3.3. Attribution studies have shown that there is ''high confidence'' that anthropogenic forcing has had a detectable influence on trends in global warming ( ''virtually certain'' since the mid-20th century), in land warming on all continents except Antarctica ( ''likely'' since the mid-20th century), in ocean warming since 1970 ( ''very likely'' ), and in increases in hot extremes and decreases in cold extremes since the mid-20th century ( ''very likely'' ) (Bindoff et al., 2013a) <sup>[[#fn:r52|52]]</sup> . In addition, there is ''medium confidence'' that anthropogenic forcing has contributed to increases in mean precipitation at high latitudes in the Northern Hemisphere since the mid-20th century and to global-scale increases in heavy precipitation in land regions with sufficient observations over the same period (Bindoff et al., 2013a) <sup>[[#fn:r53|53]]</sup> . Schleussner et al. (2017) <sup>[[#fn:r54|54]]</sup> showed, through analyses of recent observed tendencies, that changes in temperature extremes and heavy precipitation indices are detectable in observations for the 1991–2010 period compared with those for 1960–1979, with a global warming of approximately 0.5°C occurring between these two periods ( ''high confidence'' ). The observed tendencies over that time frame are thus consistent with attributed changes since the mid-20th century ( ''high confidence'' ).

The next sections assess changes in several different types of climate-related hazards. It should be noted that the different types of hazards are considered in isolation but some regions are projected to be affected by collocated and/or concomitant changes in several types of hazards ( ''high confidence'' ). Two examples are sea level rise and heavy precipitation in some regions, possibly leading together to more flooding, and droughts and heatwaves, which can together increase the risk of fire occurrence. Such events, also called compound events, may substantially increase risks in some regions (e.g., AghaKouchak et al., 2014; Van Den Hurk et al., 2015; Martius et al., 2016; Zscheischler et al., 2018) <sup>[[#fn:r55|55]]</sup> . A detailed assessment of physically-defined compound events was not possible as part of this report, but aspects related to overlapping multi-sector risks are highlighted in Sections 3.4 and 3.5.

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