Editing IPCC:AR6/WGI/Chapter-12 (section)

=== 12.5.1 A Global Synthesis ===

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( [[#12.4|Section 12.4]] assessed changes in climatic impact-drivers by region, primarily based on a large number of local- and regional-scale studies (even though global studies are also used). This section presents an assessment of changes in CIDs at the global scale. It is based on both a bottom-up synthesis of the results in [[#12.4|Section 12.4]] , and a top-down assessment from global-scale studies undertaken here. Cross-Chapter Box 12.1 summarizes global-scale CIDs with levels of warming.

Global-scale studies use similar indices of climatic impact-drivers across space, although these indices may not always be those used at the local or regional scale. Most published global-scale studies concentrate on single sectors or climatic impact-drivers, but some take a multi-sectoral perspective (e.g., [[#Warszawski--2014|Warszawski et al., 2014]] ; [[#Arnell--2016|Arnell et al., 2016]] , 2019; [[#Schleussner--2016|Schleussner et al., 2016]] ; [[#Mitchell--2017|Mitchell et al., 2017]] ; [[#Betts--2018|Betts et al., 2018]] ; [[#Byers--2018|Byers et al., 2018]] ; [[#Mora--2018|Mora et al., 2018]] ; [[#O’Neill--2018|O’Neill et al., 2018]] ; [[#Zscheischler--2018|Zscheischler et al., 2018]] ). Only a few published global-scale studies (e.g., Coppola et al., 2021; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ) have used CMIP6 scenarios to date.

All regions will experience, before 2050, increased warming, an increase of extreme heat and a decrease in cold spells, regardless of the emissions trajectory ( ''high confidence'' ). Tropical regions, but also mid-latitude regions to a lesser extent, will experience an increasing number of days with heat indices crossing dangerous thresholds used to characterize heat stress, such as HI &gt; 41°C (Figure 12.4). The increase, by the end of century, exceeds 100 days per year in most tropical areas under SSP5-8.5 but remains much more limited (almost half) under SSP1-2.6. Several global-scale studies have shown that high temperature extremes will increase everywhere ( ''high confidence'' ) ( [[#Gourdji--2013|Gourdji et al., 2013]] ; [[#Perkins-Kirkpatrick--2017|Perkins-Kirkpatrick and Gibson, 2017]] ; [[#Harrington--2018|Harrington et al., 2018]] ; [[#Jones--2018|Jones et al., 2018]] ; [[#Lehner--2018|Lehner et al., 2018]] ; [[#Shi--2018|Shi et al., 2018]] ; [[#Tebaldi--2018|Tebaldi and Wehner, 2018]] ; [[#Arnell--2019|Arnell et al., 2019]] ; [[#Russo--2019|Russo et al., 2019]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ), although the change depends on the indicator (see also Chapter 11). For example, by 2080, at least 80% of the land surface is expected to experience average summer temperatures greater than the historical (1920–2014) maximum with high (RCP8.5) emissions ( [[#Lehner--2018|Lehner et al., 2018]] ). The areas of rice and maize cropland with damaging extreme temperatures during the reproductive season will increase by a factor of three under RCP8.5 ( [[#Gourdji--2013|Gourdji et al., 2013]] ). Under a high emissions scenario, heatwaves that are currently considered rare are projected to become the norm almost everywhere by the end of the century ( [[#Russo--2014|Russo et al., 2014]] ). Heat stress as a combined function of temperature and humidity also increases at the global scale, especially with high emissions (e.g., [[#Matthews--2017|Matthews et al., 2017]] ). Growing degree-days and cooling degree-days also increase everywhere ( [[#Arnell--2019|Arnell et al., 2019]] ) with the absolute and proportional changes depending on temperature threshold. Increases in temperatures will result in reductions in heating degree days ( [[#Arnell--2019|Arnell et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] ) and a widespread reduced frequency of cold extremes ( ''high confidence'' ).

Integrating the results of the regional assessments in [[#12.4|Section 12.4]] shows that changes in CIDs linked with the water cycle or atmospheric dynamics (e.g., storms) vary more among regions, largely due to the spatial pattern of changes in atmospheric circulation and changes in precipitation and evaporation (Chapters 8 and 11). There is ''high confidence'' that heavy precipitation and pluvial floods will be increasing in a majority of land regions, primarily due to the well-understood Clausius–Clapeyron relationship describing the increase in moisture content with air temperature (Chapters 8 and 11), but there is a large spatial variability in fluvial flood hazards. Top-down global-scale studies show that although fluvial flood hazards are projected to decrease in regions where there are large reductions in seasonal rainfall totals or where warmer temperatures mean less accumulated snow, at the global scale, fluvial flood hazard (characterized as the area affected, size of peak or likelihood of an event) is projected to increase substantially through the century ( [[#Giuntoli--2015|Giuntoli et al., 2015]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; [[#Winsemius--2016|Winsemius et al., 2016]] ; [[#Alfieri--2017|Alfieri et al., 2017]] ; [[#Dottori--2018|Dottori et al., 2018]] ; [[#Arnell--2019|Arnell et al., 2019]] ). Projected changes in agricultural and hydrological drought characteristics are dependent on the indicator used to define drought (Sections 11.6 and [[#12.3.2|Section 12.3.2]] ), but there is at least ''medium confidence'' of an increase in the drought hazard in many parts of the world. This is also reflected in global-scale studies, with [[#Naumann--2018|Naumann et al. (2018)]] , for example, showing that the global mean average drought duration (based on the SPEI index which is calculated from the difference between precipitation and potential evaporation) increased from 7 months with the current climate to 18.5 months for a global warming level of 3°C. The apparent global increase in drought occurrence is greater when evaporation is captured in the drought indicator (e.g., SPEI) than when the indicator is based on precipitation alone (as in SPI; [[#Carrão--2018|Carrão et al., 2018]] ). There is evidence that the likelihood of simultaneous events in several locations will increase: [[#Trnka--2019|Trnka et al. (2019)]] found that the proportion of wheat-growing areas experiencing simultaneous severe water stress events (based on SPEI) in a year increased from 15% under current conditions to up to 60% at the end of the 21st century under high emissions.

The regional assessment in [[#12.4|Section 12.4]] shows that fire weather is projected to increase with ''medium'' or ''high confidence'' in every continent of the world, including Arctic polar regions. Globally, fire weather is projected to increase in future, primarily due to higher temperatures and exacerbated where precipitation reduces. By 2050, 60% of the global land area would see a significant increase in fire weather under RCP8.5 ( [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ). There is less confidence in the projected distribution of change in fire weather across regions in global-scale studies. For example, Moritz et al., (2012) projected an increase in fire weather in mid- and high latitudes but a reduction in the tropics, whilst [[#Yu--2019|Yu et al. (2019)]] and [[#Bedia--2015|Bedia et al. (2015)]] projected an increase in the tropics. These differences reflect differences in methodologies and fire weather indices adopted in different studies.

Integration of the results of ( [[#12.4|Section 12.4]] shows that the total number of tropical cyclones is projected to decrease through the 21st century, particularly with high emissions, but the number of very intense tropical cyclones is projected to increase in most areas (at least ''medium confidence'' ) (e.g., [[#Bacmeister--2018|Bacmeister et al., 2018]] ; see [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] ). Furthermore, regions with glaciers will lose glacier mass and regions concerned with snow cover will see a reduction in snow depth, the duration, or extent of cover ( ''medium confidence'' in polar regions '', high confidence'' elsewhere).

Relative sea level rise (RSLR) is projected in all regions (except for a few Arctic polar regions) with likelihoods varying from ''very likely'' to ''virtually certain'' depending on the region. This will increase the frequency of extreme sea levels and, depending on the level of coastal flood protection, coastal flooding ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). In terms of globally averaged extreme total water level (ETWL) frequency changes, the present-day 1-in-100-year event is projected to become 1-in-30-year and 1-in-20-year events by 2050 under RCP4.5 and RCP8.5, respectively. The present day 1-in-100-year ETWL is projected to become a 1-in-5-year event by 2100 under RCP4.5, while under RCP8.5, such events are projected to occur more than once a year ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ).

There is ''high confidence'' that most of the world’s sandy coasts will experience shoreline retreat, in the absence of terrestrial or offshore sediment sources. Median projections presented by ( [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ) indicate that 13.6% (36,097 km) and 15.2% (40,511 km) of the world’s sandy beaches could retreat by more than 100 m by 2050 (relative to 2010) under RCP4.5 and RCP8.5 respectively, implying a 12% increase in severely threatened shoreline length under RCP8.5, relative to RCP4.5. These median projections increase to 35.7%–49.5% (RCP4.5 and RCP8.5, respectively; or 95,061 km–131,745 km) by the end of the century, implying a 38% increase in severely threatened shoreline length under RCP8.5, relative to RCP4.5.

Figure 12.11 highlights that each region will, with ''high confidence'' , experience changes in multiple CIDs, challenging the vulnerability of the region and its adaptation and mitigation capacity. All non-polar regions with a coastline will see an increase in relative sea level, extreme sea level and coastal erosion, and will also see an increase in hot extremes, a decrease in cold extremes, and many will experience an increase in heavy precipitation. One cluster of regions – East Southern and West Southern Africa regions, the Mediterranean, Northern Central America, Western North America, several regions in South America and Australia – will experience, in addition to the aforementioned globally changing CIDs, increases in either drought/aridity or fire weather ( ''high confidence'' ). This will impact upon agricultural resources, infrastructure and health and ecosystems. A second cluster of regions including mountainous areas or regions with seasonal snow cover will experience (in addition to increases in heat extremes, more intense short-duration rainfall, and increases in coastal hazards where coasts exist) reductions in snow and ice cover and/or increases in river flooding in many cases (Western, North-Western, Central and Eastern North America, Arctic regions, Andes regions, Europe, Siberia, Central and East Asia, Southern Australia and New Zealand) ( ''high confidence'' ). These are places where energy production, ski tourism, river transportation, and infrastructure could in particular face increased risks.

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[[File:cf4b28c478912fa18db35405a6ec8e7e IPCC_AR6_WGI_Figure_12_11.png]]

'''Figure 12.11''' '''|''' '''Synthesis of the climatic impact-driver (CID) changes projected by 2050 (204''' '''1–2''' '''060) with''' ''high confidence'' ''', relative to reference period (199''' '''5–2''' '''014), together with the sign (direction) of change.''' Information is taken from the CID tables in [[#12.4|Section 12.4]] . Some CIDs are grouped in order to streamline the information in order to fit in all information in the figure. Mean temperature, extreme heat, cold spells and frost are grouped under a single ‘heat’ icon, as they are projected to change simultaneously, albeit heat and cold are changing in opposite directions. Coastal CIDs (relative sea level, coastal flooding and coastal erosion at sandy beaches) are also grouped. In the figure, the ‘coastal’ icon indicates regions where at least two of the three individual coastal CIDs are projected to change with ''high confidence'' . Cases where only two of the three CIDs increase with high confidence are in Arctic Northern Europe, Russian Arctic and Arctic North-Western North America. A single icon is used for aridity, hydrological drought, and agricultural and ecological drought, and only the number of drought types that change is indicated. For the ‘Snow, ice’ icon, information is taken from the evolution of the ‘Snow, glacier and ice sheet’ CID; most regions also have similar changes for ‘permafrost’ and ‘lake, river and sea ice’. Exceptions are for North-Eastern North America, Russian Arctic and Arctic North-Western North America where snow is decreasing with ''medium confidence'' (thus not appearing in the figure), while permafrost and lake, river and sea ice is decreasing with ''high confidence'' . The location of the icons within the regions is arbitrary. Icon sources: https://www.flaticon.com/authors/freepik .

In a few other regions, only a small number of CIDs are projected to change with ''high confidence'' (e.g., Sahara, Central Africa, Western Africa, Madagascar, Arabian Peninsula, South-Eeastern South America, New Zealand, Small Islands). The lower confidence levels associated with changes in CIDs in these regions can be due either to weaker change signals compared to natural variability, or due to ''limited evidence'' and model uncertainties leading to ''low agreement'' , and does not mean that climate change will affect these regions any less than in other regions.

'''In summary, there is''' high confidence '''that all regions of the world will experience changes in several climatic impact-drivers by mid-century, albeit at region-specific rates of change and confidence levels for each CID. Consequently, changing CIDs have the potential to affect climate-related risks in all regions of the world.'''

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