Being one of the major factors influencing global heat energy content in the surface-atmosphere system, global air temperature is studied and the dominating periodicities therein obtained. It is established that since 1856, variations in global air temperature have been dominantly amplitude-modulated by an oscillation whose period is about 105 years. This oscillation, which is of solar origin, is apparently largely responsible for the ongoing warming trend at global level. Analysis of air temperature records from individual regions yields different dominant periodicities for the two hemispheres. While temperature variations in many parts of the northern hemisphere are dominantly amplitude-modulated by an oscillation at a period of approximate to 180 years, corresponding variations in many parts of the southern hemisphere are dominantly amplitude-modulated by an oscillation at a period of approximate to 55 years. Both of these oscillations are apparently of solar origin.