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

==== 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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