Emergence time of CO2-forced European summer climate trends

This study investigates the time of emergence (ToE) of European summer climate trends in a warming world. We use a large ensemble of simulations performed with the Kiel Climate Model, in which atmospheric CO2 levels increase by 1% per year starting from pre-industrial concentration. The ToE for near-surface temperature, soil moisture, and the hydrological cycle highlights a relatively fast (20–40 years) emergence of near-surface temperature trends, while precipitation trends remain within the range of natural variability until the end of the 140-year-long simulation when CO2 levels quadruple. Soil moisture trends emerge after approximately 30 years in parts of the Mediterranean region, whereas in Western and Central Europe, they only emerge in the west after about 70 years when CO2 concentrations have doubled. Although many CO2-forced climate trends are not emerging over Europe, a comparison of 5,000 pre-industrial and 3,000 4xCO2 summers reveals statistically significant differences in the distribution of these summer variables: we find that the 1% driest summers are projected to be more extreme in a warmer 4xCO2 climate compared to the pre-industrial climate. This can have major implications for agriculture, as water shortages may become more severe during these extreme summers.