High elevations on mountains produce colder climates than at sea level. These colder climates strongly affect the ecosystems of mountains: different elevations have different plants and animals. Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction and recreation, such as mountain climbing.
The highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is 8,850 m (29,035 ft) above mean sea level. The highest known mountain on any planet in the Solar System is Olympus Mons on Mars at 21,171 m (69,459 ft).
There is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing and continuity have been used as criteria for defining a mountain. In the Oxford English Dictionary a mountain is defined as "a natural elevation of the earth surface rising more or less abruptly from the surrounding level and attaining an altitude which, relatively to the adjacent elevation, is impressive or notable."
Whether a landform is called a mountain may depend on local usage. Mount Scott outside Lawton, Oklahoma is only 251 m (823 ft) from its base to its highest point. Whittow's Dictionary of Physical Geography states "Some authorities regard eminences above 600 metres (2,000 ft) as mountains, those below being referred to as hills."
In the United Kingdom and the Republic of Ireland, a mountain is usually defined as any summit at least 2,000 feet (or 610 metres) high, whilst the official UK government's definition of a mountain, for the purposes of access, is a summit of 600 metres or higher. In addition, some definitions also include a topographical prominence requirement, typically 100 or 500 feet (30 or 152 m). For a while, the US defined a mountain as being 1,000 feet (300 m) or taller. Any similar landform lower than this height was considered a hill. However, today, the United States Geological Survey (USGS) concludes that these terms do not have technical definitions in the US.
The UN Environmental Programme's definition of "mountainous environment" includes any of the following:
Elevation of at least 2,500 m (8,200 ft);
Elevation of at least 1,500 m (4,900 ft), with a slope greater than 2 degrees;
Elevation of at least 1,000 m (3,300 ft), with a slope greater than 5 degrees;
Elevation of at least 300 m (980 ft), with a 300 m (980 ft) elevation range within 7 km (4.3 mi).
Using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa. As a whole, 24% of the Earth's land mass is mountainous.