Köppen-Geiger climate classification of Iran and investigation of its changes during 20th century

Climate classification has a long history dating back to the ancient Greek scientists and philosophers, but the first quantitative classification of world climates was presented by the German scientist Wladimir Köppen (1846–1940) in 1900 (Kottek et al, 2006). Being trained as a plant physiologist and realizing that plants are indicators for many climatic elements; Köppen (1900) established a climate classification system which uses monthly temperature and precipitation to define boundaries of different climate types around the world, i.e., linking climate and natural vegetation. This system has been further developed (e.g. Köppen and Geiger, 1930; Stern et al., 2000) and widely used by geographers and climatologists around the world. Although there have been many efforts to find alternative ways to classify the climate, the Köppen system remains one of the most widely used climate classification systems. In this research, monthly precipitation and temperature of 155 Iranian synoptic weather stations with relatively regular distribution over the country were used to provide an updated map of climate classification for Iran which is one of the largest countries with diverse climates in the world. Missing values of the used data were estimated and replaced using inverse distance weighed method. Monthly averages of precipitation and temperature for the considered time period (1990-2014) were then interpolated at a network of grids with 0.1 spatial resolutions using ordinary Kriging method. Subsequently, the climate types of the used stations as well as of the predefined grid points were determined using Köppen-Geiger classification method (Kottek et al, 2006; Chen and Chen, 2013). Additionally, following Rubel and Kottek (2010), monthly mean temperature of the Climatic Research Unit (CRU) of the University of East Anglia and monthly total precipitation of the Global Precipitation Climatology Centre (GPCC), both covering 1901-2014 time period and having 0.5 spatial resolution were used for computing Köppen-Geiger climate classification for different time sections of present time, in order to examine if the Iranian climate types have experienced any shift due to global climate change.
Based on the observational data for 1990-2014 time section Iran composes of 9 climate types out of 31 possible Köppen-Geiger climate types. Most parts of central, eastern and southern Iran is characterized with BWh and BWk climate types. The coastal areas of the Caspian Sea and most parts of mountainous areas of Zagros and Alborz in west and north of Iran have moderate climate type (Csa). However, the eastern slope of Zagros and southern slope of Alborz that are connected to the central arid and semi-arid climate of central Iran are distinguished with BSk climate. The southern parts of Zagros region is mostly dominated by BSh climate. Dsa and Dsb climate types are found in some parts of mountainous areas of Zagros and Alborz, while Csb and Cfa are the localized climate types that can be found in coastal areas of the Caspian Sea. Using CRU and GPCC datasets for 1951-2000 time section the same climate types were found for Iran although the sources of the data and its spatial and temporal resolution differs from that of observational data. The identified climate types in this study using observational data are in agreement with those of Kottek et al. (2006) and Chen and Chen (2013) for Iran. The identified climate types for different time sections of 1901-1925, 1926-1950, 1951-1975, 1976-2000 and 1990-2014 revealed that some Iranian climate types were not stable during these five time periods. Comparison of climate classification using observational data for 1990-2014 with those of gridded datasets for 1901-1925, 1926-1950, 1951-1975, 1976-2000 and 1990-2014 revealed that Dfa and Dfb climate types have disappeared from Iran in the map of 1990-2014 climate classification, suggesting that the number of Iranian climate types have decreased from 11 to 9 in most recent years. It was found that the area of BWk climate in central-eastern Iran is continuously retreated by time and it replaced by BWh climate. The Ds climate types were found to be very vulnerable to change and shift. It was also found that the Dsb climate type tends to shift into Dsa climate types in recent years. Most importantly, it was observed that the Ds climate types in western Iran tend to be replaced by Csa climate type. However, the obvious shift from Ds or Csa climate types into BSk climate type is observed in northwestern Iran. This result indicates a rapid and widespread desertification in northwestern Iran due to global climate change.