A global comparison between station air temperatures and MODIS land surface temperatures reveals the cooling role of forests

Most global temperature analyses are based on station air temperatures. This study presents a global analysis of the relationship between remotely sensed annual maximum LST (LSTmax) from the Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) sensor and the corresponding site-based maximum air temperature (T-amax) for every World Meteorological Organization station on Earth. The relationship is analyzed for different land cover types. We observed a strong positive correlation between LSTmax and T-amax. As temperature increases, LSTmax increases faster than T-amax and captures additional information on the concentration of thermal energy at the Earth's surface, and biophysical controls on surface temperature, such as surface roughness and transpirational cooling. For hot conditions and in nonforested cover types, LST is more closely coupled to the radiative and thermodynamic characteristics of the Earth than the air temperature (T-air). Barren areas, shrublands, grasslands, savannas, and croplands have LSTmax values between 10 degrees C and 20 degrees C hotter than the corresponding T-amax at higher temperatures. Forest cover types are the exception with a near 1:1 relationship between LSTmax and T-amax across the temperature range and 38 degrees C as the approximate upper limit of LSTmax with the exception of subtropical deciduous forest types where LSTmax occurs after canopy senescence. The study shows a complex interaction between land cover and surface energy balances. This global, semiautomated annual analysis could provide a new, unique, monitoring metric for integrating land cover change and energy balance changes.