An Investigation of Changes in the New Thornthwaite Climate Classification Based on Temperature, Rainfall, and Evapotranspiration over Thailand, Using CMIP 5 for the Mid-21st Century Period

This study aims (1) to study the trend and characteristics of monthly air temperature, monthly rainfall, and potential evapotranspiration (PET) in Thailand over the mid-21st century (2022–2060) period, and (2) to create a climate pattern map using the New Thornthwaite Climate Classification in Thailand over the same period under RCP4.5 and RCP8.5 scenarios using CSIRO-Mk3 in the CMIP5 dataset with Empirical Quantile Mapping (EQM) statistical downscaling. Spatial analyses of temperature and PET reveal significant warming trends, with temperatures rising by approximately 0.033 °C/year and PET rising by about 10 mm/year, especially in the Bangkok Metropolitan Region due to the urban heat island effect, with temperature values under RCP8.5 remaining consistently higher than those under RCP4.5. Rainfall projections show relatively stable spatial patterns across both scenarios, with higher concentrations along the Andaman coast, the eastern peninsula, and northeastern Thailand; these are areas influenced by the southwest monsoon and tropical cyclones. Central Thailand, however, exhibits persistently low rainfall, likely due to rain-shadow effects. PET patterns mirror early 21st-century observations, with the highest values projected in central Thailand and increasing trends under both scenarios, suggesting heightened drought risks. By 2060, The New Thornthwaite Climate Classification indicates that Moist climate zones are projected to disappear nationwide, with Semi-arid and Dry climates dominating under both RCP4.5 and RCP8.5 scenarios. Annual mean temperature will rise by 0.033 °C/year and PET by ~10 mm/year, while rainfall trends remain nearly stable. The classification’s reliance on minimal parameters—temperature, precipitation, and PET—provides a practical tool for climate monitoring and policy development.