Indonesia lies between latitudes 11°S and 6°N, and longitudes 95°E and 141°E. It is the largest archipelagic country in the world, extending 5,120 kilometres (3,181 mi) from east to west and 1,760 kilometres (1,094 mi) from north to south. According to a geospatial survey conducted between 2007 and 2010 by National Coordinating Agency for Survey and Mapping (Bakosurtanal), Indonesia has 13,466 islands, about 6,000 of which are inhabited. These are scattered over both sides of the equator. The largest are Java, Sumatra, Borneo (shared with Brunei and Malaysia), Sulawesi, and New Guinea (shared with Papua New Guinea). Indonesia shares land borders with Malaysia on Borneo, Papua New Guinea on the island of New Guinea, and East Timor on the island of Timor. Indonesia shares maritime borders across narrow straits with Singapore, Malaysia, Vietnam, the Philippines, and Palau to the north, and with Australia to the south. The capital, Jakarta, is on Java and is the country's largest city, followed by Surabaya, Bandung, Medan, and Semarang.
Indonesia average population density is 134 people per km2 (347 per sq mi), 79th in the world, although Java, the world's most populous island, has a population density of 940 people per km2 (2,435 per sq mi).
At 4,884 metres (16,024 ft), Puncak Jaya in Papua is Indonesia's highest peak, and Lake Toba in Sumatra its largest lake, with an area of 1,145 km2 (442 sq mi). Indonesia's largest rivers are in Kalimantan and New Guinea, and include the Kapuas, Barito, Mamberamo, Sepik and Mahakam; such rivers are communication and transport links between the island's river settlements.
Indonesia's location on the edges of the Pacific, Eurasian, and Australian tectonic plates makes it the site of numerous volcanoes and frequent earthquakes. Indonesia has at least 150 active volcanoes, including Krakatoa and Tambora, both famous for their devastating eruptions in the 19th century. The eruption of the Toba supervolcano, approximately 70,000 years ago, was one of the largest eruptions ever, and a global catastrophe. Recent disasters due to seismic activity include the 2004 tsunami that killed an estimated 167,736 in northern Sumatra, and the Yogyakarta earthquake in 2006. However, volcanic ash is a major contributor to the high agricultural fertility that has historically sustained the high population densities of Java and Bali.
Lying along the equator, Indonesia's climate tends to be relatively even year-round. Indonesia has two seasons—a wet season and a dry season—with no extremes of summer or winter. For most of Indonesia, the dry season falls between April and October with the wet season between November and March. Indonesia's climate is almost entirely tropical, dominated by the Tropical rainforest climate found in every major island of Indonesia, followed by the Tropical monsoon climate that predominantly lies along Java's coastal north, Sulawesi's coastal south and east, and Bali, and finally the tropical Savanna climate, found in isolated locations of Central Java, lowland East Java, coastal southern Papua and smaller islands to the east of Lombok. However, cooler climate types do exist in mountainous regions of Indonesia 1,300 to 1,500 metres (4,300 to 4,900 feet) above sea level. The oceanic climate (Köppen Cfb) prevail in highland areas with fairly uniform precipitation year-round, adjacent to rainforest climates, while the subtropical highland climate (Köppen Cwb) exist in highland areas with a more pronounced dry season, adjacent to tropical monsoon and savanna climates.
Some regions, such as Kalimantan and Sumatra, experience only slight differences in rainfall and temperature between the seasons, whereas others, such as Nusa Tenggara, experience far more pronounced differences with droughts in the dry season, and floods in the wet. Rainfall in Indonesia is plentiful, particularly in West Sumatra, West Kalimantan, West Java, and Papua. Parts of Sulawesi and some islands closer to Australia, such as Sumba is drier. The almost uniformly warm waters that make up 81% of Indonesia's area ensure that temperatures on land remain fairly constant. The coastal plains averaging 28 °C (82.4 °F), the inland and mountain areas averaging 26 °C (78.8 °F), and the higher mountain regions, 23 °C (73.4 °F). The area's relative humidity ranges between 70 and 90%.
Winds are moderate and generally predictable, with monsoons usually blowing in from the south and east in June through October and from the northwest in November through March. Typhoons and large scale storms pose little hazard to mariners in Indonesia waters; the major danger comes from swift currents in channels, such as the Lombok and Sape straits.
Tectonically, Indonesia is highly unstable. It lies on the Pacific Ring of Fire where the Indo-Australian Plate and the Pacific Plate are pushed under the Eurasian plate where they melt at about 100 kilometres (62 miles) deep. A string of volcanoes stretches from Sumatra to the Banda Sea. While the volcanic ash has resulted in fertile soils, it makes agricultural conditions unpredictable in some areas. The string of volcanoes runs through Sumatra, Java, Bali and Nusa Tenggara, and then to the Banda Islands of Maluku to northeastern Sulawesi. Of the 400 volcanoes, approximately 150 are active.
The most massive supervolcano eruption was the Toba eruption that took place at the present location of Lake Toba, about 75000 years Before Present. The supervolcano eruption is believed to have caused volcanic winter and cooling of the climate, and subsequently led to a genetic bottleneck in human evolution about 50,000 years ago.
Between 1972 and 1991, 29 volcanic eruptions were recorded, mostly on Java. The two most violent volcanic eruptions in modern times occurred in Indonesia; in 1815 Mount Tambora in Sumbawa erupted killing 92,000 people. Tambora produced the largest eruption known on the planet during the past 10,000 years. Also the eruption created an umbrella of volcanic ash which spread and blanketed Southeast Asia, plunging it into darkness for a week, and made a whole world without a summer in 1815. The 1883 eruption of Krakatoa was one of the deadliest and most destructive volcanic events in recorded history. Nearly 40,000 deaths are attributed to the eruption itself and the tsunamis it created. Significant additional effects were also felt around the world in the days and weeks after the volcano's destruction.
Indonesia's size, tropical climate, and archipelagic geography, support the world's third highest level of biodiversity after Brazil and Colombia. Its flora and fauna is a mixture of Asian and Australasian species. The islands of the Sunda Shelf (Sumatra, Java, Borneo, and Bali) were once linked to the Asian mainland, and have a wealth of Asian fauna. Large species such as the tiger, rhinoceros, orangutan, elephant, and leopard, were once abundant as far east as Bali, but numbers and distribution have dwindled drastically. Forests cover approximately 60% of the country. In Sumatra and Kalimantan, these are predominantly of Asian species. However, the forests of the smaller, and more densely populated Java, have largely been removed for human habitation and agriculture. Sulawesi, Nusa Tenggara, and Maluku – having been long separated from the continental landmasses—have developed their own unique flora and fauna. Papua was part of the Australian landmass, and is home to a unique fauna and flora closely related to that of Australia, including over 600 bird species.
Indonesia is second only to Australia in terms of total endemic species, with 36% of its 1,531 species of bird and 39% of its 515 species of mammal being endemic. Indonesia's 80,000 kilometres (50,000 miles) of coastline are surrounded by tropical seas that contribute to the country's high level of biodiversity. Indonesia has a range of sea and coastal ecosystems, including beaches, sand dunes, estuaries, mangroves, coral reefs, seagrass beds, coastal mudflats, tidal flats, algal beds, and small island ecosystems. Indonesia is one of Coral Triangle countries with the world's greatest diversity of coral reef fish with more than 1,650 species in eastern Indonesia only.
The British naturalist Alfred Russel Wallace described a dividing line between the distribution of Indonesia's Asian and Australasian species. Known as the Wallace Line, it runs roughly north–south along the edge of the Sunda Shelf, between Kalimantan and Sulawesi, and along the deep Lombok Strait, between Lombok and Bali. West of the line the flora and fauna are more Asian – moving east from Lombok they are increasingly Australian until the tipping point at the Weber Line. In his 1869 book, The Malay Archipelago, Wallace described numerous species unique to the area. The region of islands between his line and New Guinea is now termed Wallacea.
Indonesia's high population and rapid industrialisation present serious environmental issues, which are often given a lower priority due to high poverty levels and weak, under-resourced governance. Issues include destruction of peatlands, large-scale deforestation (much of it illegal) and related wildfires causing heavy smog over parts of western Indonesia, Malaysia and Singapore; over-exploitation of marine resources; and environmental problems associated with rapid urbanisation and economic development, including air pollution, traffic congestion, garbage management, and reliable water and waste water services. Indonesia has a below average but slightly improving performance in the global Environmental Performance Index (EPI) with an overall ranking of 107 out of 180 countries in 2016. This is also below average in the Asia-Pacific region, behind Thailand but slightly ahead of China.
Much of Indonesia's deforestation is caused by forest clearing for the palm oil industry, which has cleared 18 million hectares of forest for palm oil expansion. This requires land reallocation as well as changes to the local and natural ecosystems. Palm oil expansion can generate wealth for local communities, but it can also degrade ecosystems and cause social problems. This makes Indonesia the world's fourth largest emitter of greenhouse gases. Such activity also threatens the survival of indigenous and endemic species, including 140 species of mammals identified by the International Union for Conservation of Nature (IUCN) as threatened, and 15 identified as critically endangered, including the Bali starling, Sumatran orangutan, and Javan rhinoceros.