ARIDITY INDEX FOR PALEOCLIMATE QUANTIFICATION

The water availability in the ecosystem is one of the most crucial climatic factors that determine the global terrestrial biome distribution. However, the condition concerning deep time aridity/humidity has hardly been quantitatively explored, mainly owing to the lack of practicable parameter. By using modern global climatic data, we here examine five selected aridity indices (AI), in which the climatic variables used are simple and likely available in the study of paleoclimatology and paleoecology, including both precipitation and temperature. The results show that AI_Köppen, expressed as mean annual precipitation divided by mean annual temperature plus a constant of 33, is the most reliable index among all these indices, supported by the highest correlation coefficient to the Thornthwaite Humidity Index (HI), Soil Moisture Index (SMI), and near-ground atmospheric Relative Humidity (RH), three widely-employed major indicators sensitive to hydrological dynamics in Climatology and Meteorology. Our results also distinctly reveal that the precipitation alone is inadequate to measure the hydrological condition, because both temperature and evapotranspiration are two other critical factors that strongly influence water balance in the ecosystem, meanwhile the evapotranspiration is mainly affected by temperature and some other subordinate factors, such as vegetation types. AI_Köppen is further justified by comparing with the corresponding HI along four representative transects around the world, which cover the typical arid and humid climates and span the main terrestrial biome types. Based on the validated AI_Köppen, we briefly discuss the aridity/humidity condition in China during the middle Miocene. The results demonstrate that the aridity did increase westward, but it is also clear that western China in the middle Miocene appears not to be as arid as previously thought, indicated by the AI_Köppen values representing sub-humid to humid climate.