Projection of future changes in the summer thermal stress index in Romania using statistical downscaling and associated uncertainties

The main goal of this paper is to infer local-scale information on future changes in a complex index quantifying the direct discomfort felt by the human body under global warming. A statistical downscaling model based on the canonical correlation analysis (CCA-SDM) is used to project future changes in heat thermal stress (THI) at high spatial resolution across Romania and the frequency of extremely high THI values (FRTHI) over southern regions. The uncertainty associated with the statistically downscaled changes and validity of the CCA-SDM under future changed climate is assessed. The 2021-2050 and 2071-2100 time horizons are considered. Large-scale predictors (temperature at 850 hPa, specific humidity at 700 hPa) simulated by 6 CMIP5 (RCP4.5 and RCP8.5 scenarios) and 17 CMIP6 (SSP2-4.5 and SSP5-4.5 scenarios) global climate models (GCMs) are considered to estimate the GCM uncertainty. The uncertainty related to the downscaling technique is estimated by comparing the THI changes (derived directly through dynamical downscaling of 3 EURO-CORDEX regional climate models [RCM5J) with those obtained through the CCA-SDM (driven by the same GCM). The CCA-SDM is skilful in capturing the observed THI magnitude and temporal variability over periods with different climate regimes and less skilful in capturing the FRTHI magnitude. Future statistically downscaled THI changes exhibit an increase over the entire country, with magnitudes depending on the GCM driver and time horizon, the highest values being revealed over mountainous regions. Increases are also revealed for FRTHI changes. THI changes derived from RCMs show similar behaviours to those obtained through CCA-SDM driven by the same GCM. The 2 climate signals are closest for those RCMs that are more skilful in capturing the observed characteristics of THI variability, providing an increased confidence in the CCA-SDM transferability in future changed climate.