Questions related to forcing scenarios

Are future forcing conditions considered similarly in regional climate models as in global climate models?

Consistency facilitates interpretation of the results if forcing is the same in global and regional climate models and if it is treated in a similar way. In practice, however, details in the models differ and thereby forcing is often treated differently. This applies also for CORDEX regional climate model projections provided by the C3S.
Examples of differences include different radiation schemes that result in different treatment of both greenhouse gases and aerosols. For instance, some regional climate models have simplified radiation schemes where several greenhouse gases are lumped together and treated as CO₂-equivalents instead of separately calculating the impacts of the different greenhouse gases (e.g. CO₂, CH₄, N₂O, etc.). Further, many CORDEX regional climate models have a more simplified treatment of aerosols compared to their driving global models and thereby changing aerosol concentrations are not always included in the RCMs. Consequently, this may lead to different results as shown for Europe by e.g. Boé et al (2020). Land-use is another feature that may differ between GCMs and RCMs and in most of the EURO-CORDEX RCM projections land-use is not evolving with time. Further information about the details of some of the RCMs can be found at https://search.es-doc.org/?project=cordexp.
It is suggested that users of regional climate information carefully evaluate whether such differences in forcing are of relevance for their specific questions.

Are the scenarios from SRES similar to the RCP scenarios? And what about RCP scenarios, how do they differ from the SSP-RCPs?

Scenarios from different generations differ and cannot always easily be compared. The RCP scenarios used in CORDEX and CMIP5 differ both from the older SRES emission scenarios and from the newer SSP-RCP scenarios used in CMIP6.
Emission scenarios, such as those provided by SRES (Special Report on Emission Scenarios), or forcing scenarios such as the RCP (Representation Concentration Pathways) scenarios provide forcing data for the climate models. Such data includes long-term evolution of concentrations of a number of greenhouse gases (e.g. CO₂, CH₄, N₂O, CFCs, SF₆), different aerosol components (e.g. sulfate, nitrate, soot, dust, sea salt) and changes in land-use. For a more elaborate description of the SRES and RCP scenarios see the Appendix of the EURO-CORDEX User Guidance document https://www.euro-cordex.net/imperia/md/content/csc/cordex/guidance_for_euro-cordex_climate_projections_data_use_2021-02_1.pdf.
More details about the three generation of scenarios are available in the following reports and scientific papers

• SRES: https://www.ipcc.ch/site/assets/uploads/2018/03/sres-en.pdf
• RCP: https://link.springer.com/article/10.1007/s10584-011-0148-z,
• SSP: https://www.sciencedirect.com/science/article/pii/S0959378016300681

As a way of facilitating comparison of climate model projections with forcing scenarios from different generations, comparisons can be done at certain warming levels such as +2°C warming relative to pre-industrial conditions (Kjellström et al., 2018). By such an approach the specifics of the individual scenarios are of less importance. A caveat though is that regional forcing and response to changes may be different at different time periods despite a similar global mean response (Bärring and Strandberg, 2018).
It is suggested that users of regional climate information carefully evaluate to what extent the forcing scenarios are of relevance for their specific questions.

Which scenarios have been used in EURO-CORDEX?

EURO-CORDEX has focused on the RCP scenarios. RCP2.6, RCP4.5 and RCP8.5, where the numbers relate to the forcing in W/m² at the end of the 21^st century relative to that of pre-industrial conditions.
For a detailed picture of which forcing scenarios for which GCM-RCM data exist in the CDS, see CDS documentation and catalogue entries.
Documentations:
CMIP5: https://confluence.ecmwf.int/display/CKB/CMIP%3A+Global+climate+projections
CMIP6: https://confluence.ecmwf.int/display/CKB/CMIP6%3A+Global+climate+projections
CORDEX: https://confluence.ecmwf.int/display/CKB/CORDEX%3A+Regional+climate+projections
CDS catalogue entries:
CMIP5:
https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cmip5-monthly-single-levels
https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cmip5-monthly-pressure-levels
https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cmip5-daily-single-levels
https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cmip5-daily-pressure-levels
CMIP6: https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cmip6
CORDEX: https://cds.climate.copernicus.eu/cdsapp#!/dataset/projections-cordex-domains-single-levels
For future application when EURO-CORDEX may have generated CMIP6-based scenarios it is noted that the new SSP-RCP scenarios differ from the RCPs even if the nominal forcing levels at the end of the century (indicated by the numbers, e.g. 8.5 (W/m^-2) in SSP5-8.5 vs RCP8.5) are the same. This results from the fact that the specific pathways to 2100 differ in terms of how the different greenhouse gases evolve over time. For more information on differences between RCPs and SSP-RCPs see for instance Tebaldi et al. (2021).

References

Boé J, Somot S, Corre L and Nabat P (2020) Large discrepancies in summer climate change over Europe as projected by global and regional climate models: causes and consequences. Clim Dyn 54, 2981–3002. https://doi.org/10.1007/s00382-020-05153-1

Bärring L and Strandberg G (2018) Does the projected pathway to global warming targets matter? Environmental Research Letters, 13 (2018), 10.1088/1748-9326/aa9f72

Kjellström E, Nikulin G, Strandberg G, Christensen OB, Jacob D, Keuler K, Lenderink G, van Meijgaard E, Schär C, Somot S, Sørland SL, Teichmann C, and Vautard R (2018) European climate change at global mean temperature increases of 1.5 and 2 °C above pre-industrial conditions as simulated by the EURO-CORDEX regional climate models, Earth Syst. Dynam., 9, 459-478, DOI: 10.5194/esd-9-459-2018.

Tebaldi C, Debeire K, Eyring V, Fischer E, Fyfe J, Friedlingstein P, Knutti R, Lowe J, O'Neill B, Sanderson B, van Vuuren D, Riahi K, Meinshausen M, Nicholls Z, Tokarska KB, Hurtt G, Kriegler E, Lamarque J-F, Meehl G, Moss R, Bauer SE, Boucher O, Brovkin V, Byun Y-H, Dix M, Gualdi S, Guo H, John JG, Kharin S, Kim Y, Koshiro T, Ma L, Olivié D, Panickal S, Qiao F, Rong X, Rosenbloom N, Schupfner M, Séférian R, Sellar A, Semmler T, Shi X, Song Z, Steger C, Stouffer R, Swart N, Tachiiri K, Tang Q, Tatebe H, Voldoire A, Volodin E, Wyser K, Xin X, Yang S, Yu Y and Ziehn T, (2021) Climate model projections from the Scenario Model Intercomparison Project (ScenarioMIP) of CMIP6, Earth Syst. Dynam., 12, 253–293, https://doi.org/10.5194/esd-12-253-2021.