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

=== Atlas.1.3 Defining Temporal and Spatial Scales and Regions ===

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Over the past decades scientists have engaged in a wide array of investigations aimed at quantifying and understanding the state of the components of the land surface-ocean-atmosphere system, the complex nature of their interactions and impacts over different temporal and spatial scales. As a result, a great deal has been learned about the importance of an appropriate choice of these scales when estimating changes due to internal climate variability, trends, characterization of the spatio-temporal variability, and quantifying the range of and establishing confidence in climate projections. It is therefore important to be able to explore a whole range of spatial and temporal scales and this section presents the basic definitions of those, and the domains of analysis, used by the Atlas accounting for potential synergies between WGI and WGII.

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==== Atlas.1.3.1 Baselines and Temporal Scales of Analysis for Projections Across Scenarios ====

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[[IPCC:Wg1:Chapter:Chapter-1|Chapter 1]] has extensively explored this topic in [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] and Cross-Chapter Box 1.2. A summary of the main points relevant to the Atlas chapter and the Interactive Atlas are provided here.

There is no standard baseline in the literature although the World Meteorological Organization (WMO) recommends a 30-year baseline approach such as the climate-normal period 1981–2010. However, it retains the 1961–1990 period as the historical baseline for the sake of supporting long-term climate change assessments ( [[#WMO--2017|WMO, 2017]] ). Using the WMO standards also provides sample sizes relevant to calculating changes in statistics other than the mean. The AR6 WGI has established the 1995–2014 period as the recent-past baseline period – for similar reasons to the 1986–2005 period used in AR5 WGI ( [[#IPCC--2013b|IPCC, 2013b]] ) – since 2014 (2005) is the final year of the historical simulations of the models (more details in Cross-Chapter Box 1.2).

The choice of a baseline can significantly influence the analysis results for future changes in mean climate (Cross-Chapter Box 1.2; [[#Hawkins--2016|Hawkins and Sutton, 2016]] ) as well as its variability and extremes. Thus, assessing the sensitivity of results to the baseline period is important. The Interactive Atlas ( [[#Atlas.2|Atlas.2]] ) allows users to explore and investigate a wide range of different baseline periods when analysing changes for future time slices or global warming levels:

* 1995–2014 (AR6 20-year baseline);
* 1986–2005 (AR5 20-year baseline);
* 1981–2010 (WMO 30-year climate normal);
* 1961–1990 (WMO 30-year long-term climate normal);
* 1850–1900 (baseline used in the calculation of global warming levels).

This promotes cross-Working Group consistency and facilitates comparability with previous reports and across datasets. For instance, the AR5 and long-term WMO baselines facilitate the intercomparison of CMIP5, CORDEX and CMIP6 projections since all have historical simulations in these periods. Using more recent baselines introduces discontinuity for the CMIP5 and CORDEX models, since historical simulations end in 2005. A pragmatic approximation to deal with this issue is to use scenario data to fill the missing segment, for example for 2006–2014 use the first years of RCP8.5-driven transient projections in which the emissions are close to those observed. This approach is used in the Atlas chapter and Chapter 12.

When assessing changes over the recent past, many studies analyse datasets using a range of climatologically significant periods (i.e., 30 years or more) with precise start and end dates depending on data availability and the year of the study. To account for this, when generating assessments from this literature the term ‘recent decades’ is used to refer to a period of approximately 30 to 40 years which ends within the period 2010–2020. An equivalent approximate description using specific years would be ‘since the 1980s’.

Regarding the future reference periods, the Interactive Atlas presents projected global and regional climate changes at near-, mid- and long-term periods, respectively 2021–2040, 2041–2060 and 2081–2100, for a range of emissions scenarios ( [[#Atlas.1.4.3|Atlas.1.4.3]] and Cross-Chapter Box 1.4).

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==== Atlas.1.3.2 Global Warming Levels ====

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Notingthe approach taken in the recent IPCC Special Report on Global Warming of 1.5°C (SR1.5) above 1850–1900 levels ( [[#IPCC--2018b|IPCC, 2018b]] ), the Atlas also presents global and regional climate change information at different global warming levels (GWLs, see Cross-Chapter Box 11.1). In particular, to provide policy-relevant climate information and represent the range of outcomes from the emissions scenario and time periods considered, GWLs of 1.5°C, 2°C, 3°C and 4°C are considered. The information is computed from all available scenarios (e.g., only 1.5°C and 2°C GWL information can be computed from projections under the SSP1-2.6 scenario). The Interactive Atlas allows comparison of timings for global warming across the different scenarios and of spatial patterns of change, for example information at 2°C GWL is calculated from SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5 projections ( [[IPCC:Wg1:Chapter:Chapter-4#4.2.4|Section 4.2.4]] ).

To calculate GWL information for the datasets used in the Atlas (CMIP6 and CMIP5; see [[#Atlas.1.4|Atlas.1.4]] ), this chapter adopted the procedure used in Cross-Chapter Box 11.1. A model future climate simulation reaches the defined GWL of 1.5°C, 2°C, 3°C or 4°C when its global near-surface air temperature change averaged over successive 20-year periods first attains that level of warming relative to its simulation of the 1851–1900 climate (1851–1900 defines the pre-industrial baseline period for calculating the required global surface temperature baseline, Cross-Chapter Box 1.2). Note that this process is different from the one used in the SR1.5 report which used 30-year future periods. If a projection stabilizes before reaching the required threshold it is unable to simulate climate at that GWL and is thus discarded. For CORDEX simulations, the periods of the driving GCM are used, as in [[#Nikulin--2018|Nikulin et al. (2018)]] . Detailed reproducible information on the GWLs used in the Atlas is provided in the Atlas repository ( [[#Iturbide--2021|Iturbide et al., 2021]] ).

Climateinformation at many temporal scales and over a wide range of temporal averaging periods is required for the assessment of climate change and its implications. These range from annual to multi-decadal averages required to characterize low-frequency variability and trends in climate to hourly or instantaneous maximum or minimum values of impactful climate variables. In between, information on, for example, seasonal rainfall is important and implies the need to include averaging periods whose relevance are geographically dependent. As a result, the Atlas chapter presents results over a wide range of time scales, from daily to decadal, and averaging periods with the Interactive Atlas allowing a choice of user-defined seasons and a range of predefined daily to multi-day climate indices.

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==== Atlas.1.3.3 Spatial Scales and Reference Regions ====

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Many factors influence the spatial scales and regions over which climate information is required and can be reliably generated. Despite all efforts in researching, analysing and understanding climate and climate change, a key factor in determining spatial scales at which analysis can be undertaken is the availability and reliability of data, both observational and from model simulations. In addition, information is required over a wide range of spatial domains defined either from a climatological or geographical perspective (e.g., a region affected by monsoon rainfall or a river basin) or from a socio-economic or political perspective (e.g., least-developed countries or nation states). [[IPCC:Wg1:Chapter:Chapter-1|Chapter 1]] provides an overview of these topics ( [[IPCC:Wg1:Chapter:Chapter-1#1.5.2|Section 1.5.2]] ). This subsection discusses some relevant issues, summarizes recent advances in defining domains and spatial scales used by AR6 analyses and how these can be explored with the Interactive Atlas.

Recent IPCC reports – AR5 Chapter 14 ( [[#Christensen--2013|Christensen et al., 2013]] ) and SR1.5 [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) – have summarized information on projected future climate changes over sub-continental regions defined in the SREX report ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ) and later extended in AR5 from the 26 regions in SREX to include the polar, Caribbean, two Indian Ocean, and three Pacific Ocean regions (hereafter known as AR5 WGI reference regions; Figure Atlas.2a). In recent literature, new sub-regions have been used, for example for North and South America, Africa and Central America, together with the new definition of reference oceanic regions. [[#Iturbide--2020|Iturbide et al. (2020)]] describes an updated version of the reference regions which is used in this report (hereafter known as AR6 WGI reference regions) and is shown in Figure Atlas.2b. The goal of these subsequent revisions was to ensure that they represented sub-continental areas of greater climatic coherency.

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[[File:f3463ff5a39b129b57a3c398f277892f IPCC_AR6_WGI_Atlas_Figure_2.png]]

'''Figure Atlas.2''' '''|''' '''WGI reference regions used in the (a) AR5 and (b) AR6 reports''' ( [[#Iturbide--2020|Iturbide et al., 2020]] ). Asterisks indicate regions that extend across both sides of the map. The latter includes both land and ocean regions and it is used as the standard for the regional analysis of atmospheric variables in the Atlas chapter and the Interactive Atlas. The codes used in the Interactive Atlas are included in the figure. The full description of the regions (grouped by continents) is as follows. North America: NWN (North-Western North America), NEN (North-Eastern North America), WNA (Western North America), CNA (Central North America), ENA (Eastern North America); Central America: NCA (Northern Central America), SCA (Southern Central America), CAR (Caribbean); South America: NWS (North-Western South America), NSA (Northern South America), NES (North-Eastern South America), SAM (South American Monsoon), SWS (South-Western South America), SES (South-Eastern South America), SSA (Southern South America); Europe: GIC (Greenland/Iceland), NEU (Northern Europe), WCE (Western and Central Europe), EEU (Eastern Europe), MED (Mediterranean); Africa: MED (Mediterranean), SAH (Sahara), WAF (Western Africa), CAF (Central Africa), NEAF (North Eastern Africa), SEAF (South Eastern Africa), WSAF (West Southern Africa), ESAF (East Southern Africa), MDG (Madagascar); Asia: RAR (Russian Arctic), WSB (West Siberia), ESB (East Siberia), RFE (Russian Far East), WCA (West Central Asia), ECA (East Central Asia), TIB (Tibetan Plateau), EAS (East Asia), ARP (Arabian Peninsula), SAS (South Asia), SEA (South East Asia); Australasia: NAU (Northern Australia), CAU (Central Australia), EAU (Eastern Australia), SAU (Southern Australia), NZ (New Zealand); Antarctica: WAN (Western Antarctica), EAS (Eastern Antarctica). The definition of the regions and companion notebooks and scripts are available at the Atlas repository ( [[#Iturbide--2021|Iturbide et al., 2021]] ). Figure from [[#Iturbide--2020|Iturbide et al. (2020)]] .

The rationale followed for the definition of the reference regions was guided by two basic principles: 1) climatic consistency and better representation of regional climate features, and 2) representativeness of model results (i.e., sufficient number of model grid boxes). The finer resolution of CMIP6 models (as compared, on average, to CMIP5) yields better model representation of the reference regions allowing them to be revised for better climatic consistency (e.g., dividing heterogeneous regions) while preserving the model representation. Figure Atlas.3 illustrates this issue, displaying the number of grid boxes (over land for land regions) in the AR6 reference regions for two Interactive Atlas reference grids of horizontal resolutions of 1° and 2°, representative of the typical resolution of CMIP6 and CMIP5 models respectively. This figure shows that the new reference regions are well suited for the assessment of model results, with poorest model coverage for the New Zealand (NZ), Caribbean (CAR) and Madagascar (MAD) regions.

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[[File:4ff1c658b5d44e45a83818da9a5ea2e1 IPCC_AR6_WGI_Atlas_Figure_3.png]]

'''Figure Atlas.3''' '''|''' '''Number of land grid boxes for each AR6 WGI reference region for the reference grids representative of (a) CMIP6 and (b) CMIP5, at 1° and 2° resolution respectively.''' Colour shading indicates regions with fewer than 250 grid boxes (the darkest shading is for regions with fewer than 20 grid boxes). The polygons show the AR6 WGI reference regions of Figure Atlas.2. Detailed information on the grids used is provided at the Atlas repository ( [[#Iturbide--2021|Iturbide et al., 2021]] ).

The AR6 WGI (land and open ocean) reference regions are used in the Interactive Atlas as the default regionalization for atmospheric variables. However, these regions are not optimum for the analysis of oceanic variables since, for instance, the five upwelling regions (Canary, California, Peru, Benguela and Somali) are mostly included in ‘land’ regions. Therefore, the alternative set of oceanic regions defined by their biological activity (Figure Atlas.4) is used in the Interactive Atlas for the regional analysis of oceanic variables (see [[#Fay--2014|Fay and McKinley, 2014]] ; [[#Gregor--2019|Gregor et al., 2019]] ). Due to the many potential definitions of the regions relevant for WGI and WGII, some additional typological and socio-economic regions have also been included in the Interactive Atlas.

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[[File:0ac0c25e16d8520c0959e0aac9b6f406 IPCC_AR6_WGI_Atlas_Figure_4.png]]

'''Figure Atlas.4''' '''|''' '''Typological and socio-economic regions used in the Interactive Atlas. (a)''' Eleven ocean regions defined by their biological activity and used for the regional analysis of oceanic variables; '''(b)''' ocean regions for Small Islands, including the Caribbean (CAR) and the north Indian Ocean (ARS and BOB); '''(c)''' land monsoon regions of North America, South America, Africa, Asia and Australasia; '''(d)''' major river basins; '''(e)''' mountain regions; '''(f)''' WGII continental regions. These regions can be used alternatively to the reference regions for the regional analysis of climatic variables in the Interactive Atlas. The definition of the regions and companion notebooks and scripts are available at the Atlas repository ( [[#Iturbide--2021|Iturbide et al., 2021]] ).

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==== Atlas.1.3.4 Typological and Socio-economic Regions ====

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In addition to contiguous spatial domains discussed in the previous section, some domains are defined by specific climatological, geographical, ecological or socio-economic properties where climate is an important determinant or influencer. In these cases the domains are subject to particular physical processes that are important for its climatology or that involve systems affected by the climate in a way that observations and climate model simulations can be used to understand. Many of these are the basis of the chapters and cross-chapter papers of the AR6 WGII report, namely river basins, biodiversity hotspots, tropical forests, cities, coastal settlements, deserts and semi-arid areas, the Mediterranean, mountains and polar regions. It is therefore important to generate climate information relevant to these typological domains and some examples of these used in the Interactive Atlas are shown in Figure Atlas.4.

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