Paul Nolan

The global climate simulations described in this report constitute Ireland’s contribution to the Coupled Model Intercomparison Project (CMIP) (phase 6) (CMIP6) and will be included for assessment in the United Nations Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6). Since 1995, CMIP has co-ordinated climate model experiments involving multiple international modelling teams. The CMIP project has led to a better understanding of past, present and future climate, and CMIP model experiments have routinely been the basis for future climate change assessments carried out by the IPCC. The CMIP phase 5 (CMIP5) simulations have demonstrated the added value of improved models and enhanced resolution when compared with outputs from the CMIP phase 3 (CMIP3) project. This improvement in skill is expected to continue with the CMIP6 simulations. The EC-Earth consortium participated in CMIP5 and is currently participating in CMIP6 using a model that includes biogeochemical cycles and atmospheric chemistry. The current version of EC-Earth is based on the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) atmospheric model, the Nucleus for European Modelling of the Ocean (NEMO) model, the Louvainla-Neuve sea ice model (LIM3), the atmospheric Tracer Model version 5 (TM5), the Lund-Potsdam-Jena General Ecosystem Simulator (LPJ-GUESS) vegetation model and the Pelagic Interactions Scheme for Carbon and Ecosystem Studies (PISCES) ocean biogeochemistry model. Coupling is provided by OASIS3-MCT (the Ocean Atmosphere Sea Ice Soil – OASIS – coupler interfaced with the Model Coupling Toolkit – MCT).
As part of the current project, the EC-Earth Atmosphere–Ocean General Circulation Model (AOGCM) configuration was employed. The atmosphere was simulated with ~79-km horizontal grid spacings (T255) and 91 vertical levels. The ocean was simulated with 1-degree horizontal resolution and 75 vertical levels. In total, five historical (1850–2014) and 20 Scenario Model Intercomparison Project (ScenarioMIP) simulations (2015–2100) were run. The future climate was simulated under the full range of ScenarioMIP “tier 1” shared socioeconomic pathways (SSPs); SSP1–2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5. For one ensemble member, all model levels were archived, allowing for regional downscaling using regional climate models and participation in the CMIP6 Coordinated Regional Downscaling Experiment (CORDEX) Model Intercomparison Project (MIP). All CMIP6 data were published on the ICHEC Earth System Grid Federation (ESGF) node.
Chapter 2 provides an overview of validations of 2-m temperature, precipitation, 10-m wind speed, mean sea level pressure (MSLP), total cloud cover, snowfall, sea surface temperature and sea ice fraction. The EC-Earth historical data were compared with Climatic Research Unit observational datasets and ERA5 global reanalysis data (ERA5 is the fifth generation of the ECMWF global climate reanalysis dataset). Results confirm the ability of the EC-Earth model to simulate the global climate with a high level of accuracy. Chapter 3 provides an overview of EC-Earth global climate projections. The future global climate was simulated to the year 2100 under each of the four SSPs (SSP1–2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5). This results in 20 future global climate experiments (five ensembles multiplied by four SSPs). Projections of climate change were assessed by comparing the two 30-year future periods 2041–2070 and 2071–2100 with the 30-year historical period 1981–2010. Climate projections are presented for the Northern Hemisphere winter (December, January and February), Northern Hemisphere summer (June, July and August) and over the full year. Results show large projected increases in temperature; the largest are noted over the land masses, in particular the northern-most regions and the Arctic. Projected temperature increases range from ~0.5°C over the Southern Hemisphere oceans for SSP1–2.6 (2041–2070) to ~18°C over the Arctic for SSP5–8.5 (2071–2100). By the year 2100, the global mean temperature is projected to increase by approximately 1.5°C, 2.8°C, 4.2°C and 5.5°C for SSP1–2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5, respectively. For precipitation, all ensemble members show a steady increase in mean global precipitation from around 2000, with a noticeable divergence between the SSPs around 2060. By the year 2100, global mean precipitation is projected to increase by approximately 4%, 6%, 8% and 10% for SSP1–2.6, SSP2–4.5, SSP3–7.0 and SSP5–8.5, respectively. Northern Hemisphere sea ice is projected to disappear in the September months by 2071–2100 under SSP2–4.5, SSP3–7.0 and SSP5–8.5. Projections of 10-m wind speed, MSLP, total cloud cover, snowfall and sea surface temperature are also presented in Chapter 3