Methodology:

The simulations were carried out using the HadGEM3 and HadCM3 climate models. These model simulations were run on the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) and processed on the JASMIN data analysis platform (http://jasmin.ac.uk/). They are Tier 1 LIG simulations, on the basis of the standard CMIP6-PMIP4 LIG experimental protocol. The prescribed LIG (127,000 yr) protocol differs from the CMIP6 PI simulation protocol in astronomical parameters and the atmospheric trace GHG concentrations. LIG astronomical parameters are prescribed according to orbital constants43, and atmospheric trace GHG concentrations are based on ice-core measurements. other boundary conditions, including solar activity, ice sheets and aerosol emissions, are identical to the PI simulation. We run two LIG simulations, one using the UK CMIP6 HadGEM3 model and the other using the CMIP3 HadCM3 model. HadGEM3 is the lowest-resolution version of the UK CMIP6 physical climate model. It is a global coupled atmosphere-land-ocean-ice model that comprises the Unified Model atmosphere model, the JULES land surface model, the NEMO ocean model and the CICE sea-ice model.

Data collection:

HadGEM3 (HadGEM3-GC3.1 or HadGEM3-GC3.1-N96ORCA1).

HadCM3.

Simulations were run on the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) and processed on the JASMIN data analysis platform (http://jasmin.ac.uk/), between 2018 and 2020.

Data quality:

The Unified Model atmospheric component of HadGEM3 utilizes a horizontal grid spacing of approximately 135 km on a regular latitude-longitude grid. The NEMO ocean model of HadGEM3 employs an orthogonal curvilinear grid with a 1° resolution everywhere but near the equator, where it decreases to 0.33°. The atmosphere and ocean models use 85 and 75 vertical (pressure) levels, respectively. HadCM3 is the UK CMIP3 coupled atmosphere-ocean general circulation model30. Our version of HadCM3 includes the dynamic vegetation model TRIFFID (ref. 47) and the land surface model MOSES 2.1 (ref. 48). The horizontal resolution of the atmosphere model is 3.75° longitude by 2.5° latitude. The ocean model uses a resolution of 1.25° longitude by 1.25° latitude. The atmospheric and oceanic components use 19 and 20 vertical levels, respectively.