Natural environment

The natural environment encompasses all biotic and abiotic things that occur naturally. Biotic refers to living species, while abiotic refers to non-living elements like climate, weather, and natural resources. These components interact to support human survival and economic activity. Scientists distinguish the natural environment from the built environment. The built environment includes landscapes fundamentally transformed by humans, such as urban settings or agricultural land. Even small human actions, like building a mud hut or a solar system in a desert, create an artificial environment.

Interestingly, works created by animals are still considered natural. For example, beaver dams and termite mounds are part of the natural world.

On Earth, naturalness exists on a continuum. It can range from 100% natural to 0% natural. There are no absolutely natural environments left on our planet. In the current era known as the Anthropocene, human activity has caused massive changes. These include climate change, pollution from chemicals and plastics, and biodiversity loss. Even in places that seem undisturbed, the degree of naturalness varies. An agricultural field might have soil with a similar mineral composition to a forest, but its physical structure is quite different.

Earth science uses several disciplines to study the planet. These include geography, geology, geophysics, and geodesy. Scientists recognize four major spheres that make up the Earth. The lithosphere consists of rocks. The hydrosphere includes all water. The atmosphere is the layer of air. The biosphere contains all life.

Some scientists also include the cryosphere, which is the ice, as part of the hydrosphere. Others include the pedosphere, or the soil, as an active sphere. These systems are all interconnected.

The lithosphere is the Earth's outermost solid surface. It is chemically and mechanically different from the mantle below it. Much of the crust is formed through igneous processes. This happens when magma cools and solidifies into rock. Beneath the lithosphere, the mantle is heated by the decay of radioactive elements. This heat creates a process called rheic convection. This convection causes the lithospheric plates to move slowly. This movement is known as plate tectonics.

This process can result in volcanoes at mid-ocean ridges or mantle plumes.

Water is a massive component of the hydrosphere. Approximately 71% of the Earth's surface is covered by oceans. This covers an area of about 362 million square kilometers. More than half of this area is deeper than 3,000 meters. The average salinity of the ocean is about 35 parts per thousand.

While we name several oceans, they are actually one interconnected body of salt water. The five major divisions are the Pacific, Atlantic, Indian, Southern, and Arctic oceans. The deep seabeds are among the least-modified natural environments on Earth.

Freshwater moves through rivers, lakes, and ponds. Rivers are natural watercourses that flow toward oceans, lakes, or seas. They are part of the hydrological cycle. Water enters rivers through precipitation, surface runoff, and groundwater recharge.

Rivers often have a channel with a bed and banks. Larger rivers may have wide floodplains. Lakes are bodies of water located in basins. They are larger and deeper than ponds.

Natural lakes are often found in mountains or areas with recent glaciation. All lakes are temporary over geologic time because they eventually fill with sediment.

The atmosphere is a thin layer of gases held by gravity. Dry air is mostly nitrogen at 78% and oxygen at 21%. The rest includes argon, carbon dioxide, and trace gases.

The atmosphere has five main layers. The exosphere is the outermost layer, made of hydrogen and helium. Below is the thermosphere, where the International Space Station orbits. The mesosphere is where most meteors burn up. The stratosphere contains the ozone layer. This layer is vital because it reduces ultraviolet radiation.

Finally, the troposphere is the layer closest to the surface. This atmosphere helps regulate temperature and protects life.