Evolution of extreme temperatures over Portugal: recent changes and future scenarios

Changes in surface air temperature extremes over mainland Portugal since the early 1940s were investigated on the basis of daily maximum and minimum temperatures available from time series from 23 weather stations. The maximum (minimum) temperature decreased by 0.17 degrees C decade(-1) (0.19 degrees C decade(-1)) for 1941-1975 followed by an increase of 0.49 degrees C decade(-1) (0.54 degrees C decade(-1)) for 1976-2006, significantly higher than similar trends computed at the global and European scales. A large set of climatic indices was analysed to detect the presence of trends and quantify the variations of different indices for different periods. In the 1976-2006 period, many stations revealed statistically significant positive trends in the annual number of tropical nights, summer days, warm spells, warm nights and warm days. At the seasonal level, we detected statistically significant increments of extreme heat events for spring and summer, and a decrease of cold extremes in winter. We then used the HadRM3 output to study changes in the maximum and minimum temperature distributions and associated changes in the likelihood of extreme events in the future (2071-2100) under 2 change scenarios. Changes obtained for the future are consistent with those found since the mid-1970s in Portugal with an increase in maximum temperature of 3.2 degrees C (4.7 degrees C) for the B2 (A2) scenario in summer and similar to 3.4 degrees C in both scenarios for spring. For minimum temperature, the results were similar, with increases for summer (spring) ranging from 2.7 degrees C (2.5 degrees C) in the B2 scenario to 4.1 degrees C (2.9 degrees C) in the A2 scenario.