  Estimating climate effects on future ocean storm severity is plagued by large
uncertainties, yet for safe design and operation of offshore structures, best
possible estimates of climate effects are required given available data. We
explore the variability in estimates of 100-year return value of significant
wave height (Hs) over time, for output of WAVEWATCHIII models from 7
representative CMIP5 GCMs, and the FIO-ESM v2.0 CMIP6 GCM, for neighbourhoods
of locations east of Madagascar and south of Australia. Non-stationary extreme
value analysis of peaks-over-threshold and block maxima using Bayesian
inference provide posterior estimates of return values as a function of time;
MATLAB software is provided. There is large variation between return value
estimates from different GCMs, and with longitude and latitude within each
neighbourhood. These sources of uncertainty tend to be larger than that due to
typical modelling choices (such as choice of threshold for POT, or block length
for BM). However, careful threshold and block length are critical east of
Madagascar, because of the presence of a mixed population of storms there. The
long 700-year pre-industrial control (piControl) output of the CMIP6 GCM allows
quantification of the apparent inherent variability in return value as a
function of time.
