Southeast Asia or South-eastern Asia is a subregion of Asia, bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. The region is the only part of Asia that lies partly within the Southern Hemisphere, although the majority of it is in the Northern Hemisphere. In contemporary definition, Southeast Asia consists of two geographic regions:
- Mainland Southeast Asia, comprising Myanmar, Thailand, Peninsular Malaysia, Laos, Cambodia, and Vietnam.
- Maritime Southeast Asia, comprising the Andaman and Nicobar Islands of India, Indonesia (except Western New Guinea, which is considered part of the Australian continent), East Malaysia, Brunei, Singapore, the Philippines, East Timor, Christmas Island, and the Cocos (Keeling) Islands.
The region lies near the intersection of geological plates, with both heavy seismic and volcanic activities. The Sunda Plate is the main plate of the region, featuring almost all Southeast Asian countries except Myanmar, northern Thailand, northern Laos, northern Vietnam, and northern Luzon in the Philippines. The mountain ranges in Myanmar, Thailand, and peninsular Malaysia are part of the Alpide belt, while the islands of the Philippines are part of the Pacific Ring of Fire. Both seismic belts meet in Indonesia, causing the region to have relatively high occurrences of earthquakes and volcanic eruptions.
Southeast Asia covers about 4.5 million km2 (1.7 million mi2), which is 10.5% of Asia or 3% of earth’s total land area. Its total population is more than 655 million, about 8.5% of the world’s population. It is the third most populous geographical region in the world after South Asia and East Asia. The region is culturally and ethnically diverse, with hundreds of languages spoken by different ethnic groups. Ten countries in the region are members of ASEAN, a regional organization established for economic, political, military, educational and cultural integration amongst its members.
The climate in Southeast Asia is mainly tropical–hot and humid all year round with plentiful rainfall. Northern Vietnam and the Myanmar Himalayas are the only regions in Southeast Asia that feature a subtropical climate, which has a cold winter with snow. The majority of Southeast Asia has a wet and dry season caused by seasonal shift in winds or monsoon. The tropical rain belt causes additional rainfall during the monsoon season. An exception to this type of climate and vegetation is the mountain areas in the northern region, where high altitudes lead to milder temperatures and drier landscape.
Southeast Asia is one of the most vulnerable regions to climate change in the world. Climate change will have a big effect on agriculture in Southeast Asia such as irrigation systems, which will be affected by changes in rainfall and runoff, and subsequently, water quality and supply. Climate change is also likely to pose a serious threat to the fisheries industry in Southeast Asia.