Geographers classify natural resources as part of four interrelated systems. These four physical systems are classified as either biotic or abiotic. A biotic system is composed of living organisms. An abiotic system is composed of nonliving or inorganic matter. Three of Earth’s four systems are abiotic:

• The atmosphere: a thin layer of gases surrounding Earth.

• The hydrosphere: all the water on and near Earth’s surface.

• The lithosphere: Earth’s crust and a portion of upper mantle directly below the crust.

One of the four systems is biotic:

• The biosphere: all living organisms on Earth, including plants and animals as well as microorganisms.

The names of the four spheres are derived from the Greek words for “stone” (litho), “air” (atmo), “water” (hydro), and “life” (bio).

Climate Regions

Geographers frequently classify global climates according to a system developed by Vladimir Köppen. The modified Köppen system divides the world into five main climate regions, represented by the letters A, B, C, D, and E, plus H for highlands.

Humans have a limited tolerance for extreme temperature and precipitation levels and thus avoid living in places that are too hot, too cold, too wet, or too dry. Compare the map of global climate to the distribution of population. Relatively few people live in the Dry (B) and Polar (E) climate regions, or in the Highlands.

The modified Köppen system divides the five main climate regions into several subtypes. For all but the B climate, the basis for the subdivision is the amount of precipitation and the season in which it falls. For the B climate, subdivision is made on the basis of temperature and precipitation. In addition, “H” denotes highland areas in which climates are cold due to elevation, or local climate variations that are too great to show on the map.

The long-term average weather condition at a particular location is climate. Geographers frequently classify climates according to a system developed by German climatologist Vladimir Köppen. The modified Köppen system divides the world into five main climate regions that are identified by the letters A through E as well as by names:

• A: Tropical climates.

• B: Dry climates.

• C: Warm mid-latitude climates.

• D: Cold mid-latitude climates.

• E: Polar climates.

Atmosphere

A thin layer of gases surrounds Earth at an altitude up to 690 kilometers (429 miles). Pure dry air in the lower atmosphere contains approximately 78.08 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, 0.04 percent carbon dioxide, and trace amounts of a few other gases (measured by volume). As atmospheric gases are held to Earth by gravity, pressure is created. Variations in air pressure from one location to another are responsible for producing such weather features as wind blowing, storms brewing, and rain falling.

Hydrosphere

Water exists in liquid form in the oceans, lakes, and rivers as well as groundwater in soil and rock. It can also exist as water vapor in the atmosphere, and as ice in glaciers. Over 97 percent of the world’s water is in the oceans. The oceans supply the atmosphere with water vapor, which returns to Earth’s surface as precipitation, the most important source of freshwater. Consumption of water is essential for the survival of plants and animals, and a large quantity and variety of plants and animals live in it. Because water gains and loses heat relatively slowly, it also moderates seasonal temperature extremes over much of Earth’s surface.

The climate of a particular location influences human activities, especially production of the food needed to survive. People in parts of the A climate region, especially southwestern India, Bangladesh, and the Myanmar coast, anxiously await the annual monsoon winds bringing rain, which is essential for successful agriculture and provides nearly 90 percent of India’s water supply. For most of the year, the region receives dry, somewhat cool air from the northeast. In June, the wind direction suddenly shifts, bringing moist, warm, southwesterly air, known as the monsoon, from the Indian Ocean. The monsoon brings rain that lasts until September. In years when the rain is delayed or fails to arrive—in recent decades, at least one-fourth of the time—agricultural output falls and famine threatens in the countries of South Asia, where nearly 20 percent of the world’s people live. The seasonal rain is so important in India that the words for “year,” “rain,” and “rainy season” are identical in many local languages.

Lithosphere

Earth is composed of concentric spheres. The inner core is a dense, metallic sphere about 1,200 kilometers (760 miles) in radius. Surrounding the inner core is an outer core, about 2,400 kilometers (1,500 miles) thick, composed mostly of liquid iron and nickel. The next layer is the mantle about 2,900 kilometers (1,800 miles) thick. The outermost layer is the crust, a thin, brittle outer shell 5 to 70 kilometers (3 to 44 miles) thick. The lithosphere encompasses the crust and a portion of the mantle. Powerful forces deep within Earth bend and break the crust to form mountain chains and shape the crust to form continents and ocean basins.