The Worldwide C3S CORDEX Grand Ensemble: A Major Contribution to Assess Regional Climate Change in the IPCC AR6 Atlas

The collaboration between the Coordinated Regional Climate Downscaling Experiment
(CORDEX) and the Earth System Grid Federation (ESGF) provides open access to an unprecedented
ensemble of regional climate model (RCM) simulations, across the 14 CORDEX continental-scale
domains, with global coverage. These simulations have been used as a new line of evidence to as-
sess regional climate projections in the latest contribution of the Working Group I (WGI) to the IPCC
Sixth Assessment Report (AR6), particularly in the regional chapters and the Atlas. Here, we present
the work done in the framework of the Copernicus Climate Change Service (C3S) to assemble a
consistent worldwide CORDEX grand ensemble, aligned with the deadlines and activities of IPCC
AR6. This work addressed the uneven and heterogeneous availability of CORDEX ESGF data by
supporting publication in CORDEX domains with few archived simulations and performing quality
control. It also addressed the lack of comprehensive documentation by compiling information from
all contributing regional models, allowing for an informed use of data. In addition to presenting
the worldwide CORDEX dataset, we assess here its consistency for precipitation and temperature
by comparing climate change signals in regions with overlapping CORDEX domains, obtaining
overall coincident regional climate change signals. The C3S CORDEX dataset has been used for
the assessment of regional climate change in the IPCC AR6 (and for the interactive Atlas) and is
available through the Copernicus Climate Data Store (CDS).