Meteorological observations from 1904 to 2000 reveal that annual mean outdoor temperature in South Korea has increased by about 1.5 °C since the beginning of the twenty-first century because of from global warming and urbanization. Owing to this phenomenon, the cooling degree-days of major cities in South Korea might be increased, and thus, it is necessary to determine the insulation of buildings. In this study, the regional cooling energy requirement was analyzed by reexamining the cooling degree-days of major cities in South Korea. Daily mean temperatures for the month of August for the years 1980 to 2012 were provided by the meteorological agency. This study proposed reduction of cooling energy consumption in order to prevent blackouts during summer and to strengthen design standards in the buildings sector. We compared the period before the effects of climate change were recognized (from 1980 to 2005) and the period after that (from 2006 to 2012) and concluded that the cooling degree-days of major cities in South Korea have increased (Seoul 56%, Gangneung 25%, Daejeon 32%, Daegu 53%, Gwangju 44%, Busan 48%, and Jeju 28%) to a level equal to 65% of the cooling energy requirement in South Korea. 40% of this energy could be saved by strengthening design standards in the buildings sector.