This dataset consists of 5x5 historical simulations (1850-2014) with HadGEM3-GC3.1 (Met Office Hadley Centre Global Coupled model General Circulation 3.1). This is an ensemble dataset, part of the Securing Multidisciplinary UndeRstanding and Prediction of Hiatus and Surge events (SMURPHS) project. The model version used here is a development version of the UK's submission to Coupled Model Intercomparison Project Phase 6 (CMIP6) and differs from the CMIP6 version in its treatment of prescribed ozone. This ensemble was designed to sample a range in plausible historical aerosol forcing, with the present-day aerosol effective radiative forcing ranging from -0.38 W/m2 to -1.5 W/m2, which spans a large range of the total aerosol effective radiative forcing presented in Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5).

The targeted aerosol forcings were achieved by applying a constant scaling factor in space and time to the standard historical CMIP6 historical AA and precursor emissions, namely organic and black carbon (fossil and biofuel) and sulfur dioxide (SO2) emissions. All other forcing agents follow historical CMIP6 emissions. The scalings were chosen such that the targeted aerosol forcings are approximately equally spaced:
- 0.2x scaling to give -0.38 W/m2
- 0.4x scaling to give -0.60 W/m2
- 0.7x scaling to give -0.93 W/m2
- 1.0x scaling to give -1.17 W/m2
- 1.5x scaling to give -1.50 W/m2.

The scalings required to reach the intended forcing values were determined from 5 10-year atmosphere-only time-slice runs for the year 2014 with pre-industrial sea-surface temperatures. Note that the 1x scaling is not strictly a ‘scaling’ but corresponds to the standard emissions. The configuration for the scaled aerosol simulations was derived from the 1x scaling simulations, therefore these are directly comparable with the only difference being the scaled aerosol emissions. Five simulations were performed for each scaling and the simulations cover the period 1850-2014. The same five initial conditions were used for each scaling sub-ensemble, and the first four correspond to the initial conditions selected for the four CMIP6 historical simulations. These were well spaced in a pre-industrial control simulation and sample different phases of internal variability in both the Pacific and Atlantic. We recommend only analysing data from 1900, as introducing the scaled aerosols in 1850 produces a small initial drift in the climate system. We estimate that most of this drift has been removed by 1900.

The reference paper for this dataset is Dittus et al., 2020. Please cite this paper when using the dataset. Dittus, A. J., Hawkins, E., Wilcox, L. J., Sutton, R. T., Smith, C. J., Andrews, M. B. and P. M. Forster, 2020: Sensitivity of historical climate simulations to uncertain aerosol forcing. In press, Geophysical Research Letters