Different sets of climatic reference regions have been proposed for the regional synthesis of historical trends and future climate change projections, and have been subsequently used in the different Assessment Reports of the IPCC WGI (we refer to these sets as **IPCC-WGI reference regions**). The Giorgi reference regions (originally 23 rectangular regions proposed in [Giorgi and Francisco, 2000](https://doi.org/10.1007/PL00013733), denoted here as **version 1**) were used in the third (AR3) and fourth (AR4) IPCC Assessment reports. These regions were later modified using more flexible polygons in the IPCC SREX special report ([Seneviratne et al., 2012](https://www.ipcc.ch/site/assets/uploads/2018/03/SREX-Chap3_FINAL-1.pdf), **version 2**) and then slightly modified and extended to 33 regions (by including island states, the Arctic and Antarctica) for the fifth Assessment Report (AR5, [van Oldenborgh et al., 2013](https://www.ipcc.ch/report/ar5/wg1/atlas-of-global-and-regional-climate-projections), **version 3**).
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