An outlook on the Iranian Köppen-Geiger climate zones in the 21st century

The impacts of potential climate change on the surface climate variables can be appraised through the projections of the global circulation onto the target variables, considering that the observed climate shifts are commonly associated with the changing patterns of the general circulations (Fraedrich et al, 2001). Since a combination of climate variables are utilized to develop climate classification schemes, they are useful for validating the outputs of the general circulation models (GCMs). The Köppen climate classification (Köppen, 1936), as the widely used climate classification method, is well suited for simultaneously validating the temperature and precipitation model outputs, considering that it takes into account both precipitation and the near-surface air temperature as the major input variables, as well as their annual cycles and linkage with the natural vegetation patterns (Kalvova et al, 2003). Therefore, such a climate classification allows for an outlook on the possible future shifts in the climate zones under a changing climate. Many researchers applied the Köppen climate classification to the general circulation model outputs in order to assess the shifts in the climate zones caused by the foreseen climate changes, represented by the GCMs (Fraedrich et al, 2001; Diaz and Eischeid, 2007; De Castro et al, 2007; Ruble and Kottek, 2010; Chen and Chen, 2013; Chan et al, 2016; Engelbrecht and Engelbrecht, 2016). The present work aimed at investigating the shifts in the Iranian climate zones induced by the possible climate changes in the 21st century. Monthly total precipitation of the Global Precipitation Climatology Centre (GPCC) and the mean monthly temperature of the Climatic Research Unit (CRU) of the University of East Anglia, both having 0.5 degree spatial resolution, were used for computing present time (1951-2000 time period) Köppen-Geiger climate classification for Iran. Following Rubel and Kottek (2010), the global temperature and precipitation projections corresponding to the period 2001 to 2100 were also taken from the Tyndall Centre for Climate Change Research dataset, TYNSC2.03 (Mitchell et al., 2004), to compute Köppen-Geiger climate classification for the 21st century. The TYNSC2.03 consists of a total of 20 GCM runs, combining 4 possible future worlds of emission scenarios (A2, B1, B2, A1F1) described by SRES (ARNELL et al., 2004) with 5 state-of-the-art climate models, namely the Hadley Centre Coupled Model Version 3 (HadCM3), the National Center for Atmospheric Research-Parallel Climate Model (NCARPCM), the Second Generation Coupled Global Climate Model (CGCM2), the Industrial Research Organization-Climate Model Version 2 (CSIRO2) and the European Centre Model Hamburg Version 4 (ECHam4). The Köppen-Geiger climate types of the present time were computed on the basis of the GPCC precipitation and CRU temperature datasets for 1951-2000 and 1976-2000 time periods. The 21st century Köppen-Geiger climate types were further computed for 1901-1925, 1926-1950, 1951-975 and 1976-2100 time sections of the TYNSC2.03 datasets.
The comparison of the climate classifications of 1951-2000 and 1976-2000 time periods highlighted certain signals of change in the Iranian climate zones in the latter half of the 20th century. The most obvious changes were the tendency of northwestern Iran to a drier climate and the extensive retreatment of BWk climate type in the central and eastern Iran in favor of BWh climate type in the last quartile of the 20th century. Shifts and changes in the climate zones of Iran were more profoundly observed in the climate classification maps of the 21st century, particularly in the final quarter of the century. Except for A2 and A1F1 pessimistic scenarios which showed the maximum climate shifts in Iran, all the scenarios considered in this study more or less agree in displaying moderate changes in the climate zones of Iran. In general, based on the pessimistic scenarios, northwestern Iran is extremely susceptible to an extensive climate shift in the future. Nevertheless, all the scenarios indicate that northwestern Iran tends to have a much drier and warmer climate in the second half of the 21st century. Moreover, the Csb climate type, currently the main climate type of most parts of the Zagros mountainous areas of western Iran, will be replaced by Bsk and Bsh climate types at the end of the 21st century. Obviously, the BWk climate type will disappear from the country at the end of the century due to the widespread invasion of the BWh climate type in the central and eastern Iran, indicating an anticipated widespread desertification in almost all parts of the country, particularly in the northwest of Iran, under a changing climate.