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

==== 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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