Editing IPCC:AR6/WGI/Chapter-1 (section)

==== 1.5.4.3 The Sixth Phase of the Coupled Model Intercomparison Project (CMIP6) ====

<div id="h3-35-siblings" class="h3-siblings"></div>

The Coupled Model Intercomparison Project (CMIP) provides a framework to compare the results of different GCMs or ESMs performing similar experiments. Since its creation in the mid-1990s, it has evolved in different phases, involving all major climate modelling centres in the world (Figure 1.20). The results of these phases have played a key role in previous IPCC reports, and the present Report assesses a range of results from CMIP5 that were not published until after the AR5, as well as the first results of the 6th phase of CMIP (CMIP6; [[#Eyring--2016|Eyring et al., 2016]] ). The CMIP6 experiment design is somewhat different from previous phases. It now consists of a limited set of DECK (Diagnostic, Evaluation and Characterization of Klima) simulations and an historical simulation that must be performed by all participating models, as well as a wide range of CMIP6-Endorsed model intercomparison projects (MIPs) covering specialized topics (Figure 1.22; [[#Eyring--2016|Eyring et al., 2016]] ). Each MIP activity consists of a series of model experiments, documented in the literature (Table 1.3) and in an online database ( [http://es-doc.org es-doc.org] ; Annex II; [[#Pascoe--2020|Pascoe et al., 2020]] ).

<div id="_idContainer063" class="_idGenObjectStyleOverride-1"></div>

[[File:794fa33a3216f334817859f0cf2ff96b IPCC_AR6_WGI_Figure_1_22.png]]

'''Figure 1.22 |''' '''Structure of CMIP6, the 6th phase of the Coupled Model Intercomparison Project''' . The centre shows the common DECK (Diagnostic, Evaluation and Characterization of Klima) and historical experiments that all participating models must perform. The outer circles show the topics covered by the endorsed (red) and other MIPs (orange). See Table 1.3 for explanation of the MIP acronyms. Figure is adapted from [[#Eyring--2016|Eyring et al. (2016)]] .

'''Table 1.3 | CMIP6-Endorsed MIPs, their key references, and where they are used or referenced throughout this Report.'''

[[File:80356ae2da8a03e6aeb0d297efbb29ab IPCC_AR6_WGI_Chapter_1_Table_1_3.png]]
The CMIP DECK simulations form the basis for a range of assessments and projections in the following chapters. As in CMIP5, they consist of: a ‘pre-industrial’ control simulation (piControl, where ‘pre-industrial’ is taken as fixed 1850 conditions in these experiments); an idealized, abrupt quadrupling of CO <sub>2</sub> concentrations relative to piControl (to estimate equilibrium climate sensitivity); a 1% per year increase in CO <sub>2</sub> concentrations relative to piControl (to estimate the transient climate response); and a transient simulation with prescribed sea-surface temperatures for the period 1979–2014 (termed ‘AMIP’ for historical reasons). In addition, all participating models perform a historical simulation for the period 1850–2014. For the latter, common CMIP6 forcings are prescribed (Cross-Chapter Box 1.4, Table 2). Depending on the model setup, these include emissions and concentrations of short-lived species ( [[#Hoesly--2018|Hoesly et al., 2018]] ; [[#Gidden--2019|Gidden et al., 2019]] ), long-lived GHGs ( [[#Meinshausen--2017|Meinshausen et al., 2017]] ), biomass burning emissions ( [[#van%20Marle--2017|van Marle et al., 2017]] ), global gridded land-use forcing data ( [[#Ma--2020|Ma et al., 2020]] ), solar forcing ( [[#Matthes--2017|Matthes et al., 2017]] ), and stratospheric aerosol data from volcanoes ( [[#Zanchettin--2016|Zanchettin et al., 2016]] ). The methods for generating gridded datasets are described in [[#Feng--2020|Feng et al. (2020)]] . For AMIP simulations, common sea surface temperatures (SSTs) and sea ice concentrations (SICs) are prescribed. For simulations with prescribed aerosol abundances (i.e., not calculated from emissions), optical properties and fractional changes in cloud droplet effective radius are generally prescribed in order to provide a more consistent representation of aerosol forcing relative to earlier CMIP phases ( [[#Fiedler--2017|Fiedler et al., 2017]] ; [[#Stevens--2017|Stevens et al., 2017]] ). For models without ozone chemistry, time-varying gridded ozone concentrations and nitrogen deposition are also provided ( [[#Checa-Garcia--2018|Checa-Garcia et al., 2018]] ).

Beyond the DECK and the historical simulations, the CMIP6-Endorsed MIPs aim to investigate how models respond to specific forcings, their potential systematic biases, their variability, and their responses to detailed future scenarios such as the Shared Socio-economic Pathways (SSPs; [[#1.6|Section 1.6]] ). Table 1.3 lists the 23 CMIP6-Endorsed MIPs and key references. Results from a range of these MIPs, and many others outside of the most recent CMIP6 cycle, will be assessed in the following chapters (also shown in Table 1.3). References to all the CMIP6 datasets used in the report are found in Annex II, Table AII.10.

<div id="1.5.4.4" class="h3-container"></div>

<span id="coordinated-regional-downscaling-experiment-cordex"></span>