Water can be found almost everywhere on Earth, as it covers 71% of Earth’s surface. It is in the atmosphere, in both fresh and saline bodies of water, in living organisms, and in the upper layers of Earth’s crust (its solid surface layer). Of the water on Earth, 97.5% is salt water; only 2.5% is the fresh water that organisms that do not live in the oceans can use to survive. Most of the fresh water is in the form of glaciers and ice caps (68.7%), leaving about 30% as groundwater. Fresh water found in lakes and rivers is available for nonoceanic organisms to use to meet their needs.
More than two-thirds of the fresh water on Earth is frozen in glaciers or polar ice caps and is, therefore, unavailable for use by organisms. A glacier is a huge, slowly flowing mass of snow and ice that accumulates on land in areas with cold climates if precipitation builds up faster than it melts. Glaciers are often found in alpine or polar regions. A polar ice cap is an immense sheet of ice found at the North Pole or the South Pole. Polar ice caps can float on top of the ocean and can develop into layers of ice between 1 and 4 meters thick.
Of Earth’s fresh water, 30% is groundwater. Groundwater is water that collects underground. After it precipitates, some water infiltrates the ground. It then slowly percolates through porous rock and eventually ends up back in the ocean as the water cycle continues. An aquifer is an underground area where water collects. The water table is the upper layer of underground water.
Usable surface water can be found in rivers, lakes, and wetlands, and it makes up only about .3% of all water on Earth. A river is large body of water that flows and empties into a lake, an ocean, or another river. A freshwater lake is a large body of water surrounded by land that is usually fed and drained by rivers. A wetland is an area where water covers and saturates the soil for most of the year. Fresh water in rivers, lakes, and wetlands is available to both terrestrial and aquatic organisms that live outside of oceans.
Water is in constant motion on our planet. The movement of water under, above, and on the planet is called the hydrologic cycle. Throughout this cycle, water is constantly changing states: gas to liquid (condensation), gas to solid (crystallization as snowflakes), liquid to gas (evaporation), liquid to solid (freezing), solid to liquid (melting), and solid to gas (sublimation). Through the processes of sublimation and evaporation, water from the oceans and freshwater bodies enters the atmosphere. There it changes states (condensation and crystallization) and returns to Earth’s surface and bodies of water during precipitation (rain, sleet, hail, and snow). Precipitation creates runoff, moving water over the ground’s surface and refilling bodies of water. It also infiltrates openings in the ground to replenish aquifers and moisten soil. During infiltration, seepage, and runoff, water is also returned to freshwater and saltwater sources, where the cycle begins again.
Water is a natural solvent and mixes easily with many pollutants. Therefore, not all of Earth’s water is safe to drink. Because of this, it must be treated to make it drinkable. While people can live for approximately 2 weeks without food, they can survive only a few days without water. The average human body is 53% water. Water makes up over 80% of our blood, 70% of our brains, and 90% of our lungs. Water is also a major percentage of the food living things eat. An apple, an ear of corn, and a pineapple are more than 80% water. Most plants are at least 75% water.
As the world’s population continues to grow, several problems may impact the availability and use of water. Fresh water is used for drinking, transportation, heating and cooling, industry, and other purposes. Water shortage, pollution, and quality as well as the infrastructure to secure and supply water are all potential water concerns today and in the future.
Earth is the only planet in our solar system that has the perfect conditions for life. Earth has not only the right amount of sunlight but also air, water, and land that promote life. Earth has various systems working together to sustain life for the organisms that occupy it. Earth’s major systems are the geosphere (solid and molten rock, soil, and sediments), the hydrosphere (water and ice), the atmosphere (air), and the biosphere (living organisms, including humans). These systems interact in multiple ways to affect Earth’s surface materials and processes that support life. There have been constant changes on Earth since the time it was formed approximately 4.6 billion years ago. These changes have occurred over both short and long periods of time. The systems on Earth are responsible for these changes.
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Earth’s major systems are the geosphere, hydrosphere, atmosphere, and biosphere.
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The geosphere is made of the crust, the mantle, and the core. The crust layer is the cold, hard rock that makes up Earth’s surface. The mantle is the hot, semihard rock layer just under the crust. The hot, liquid rock and solid iron center beneath the mantle is the core. Earth’s surface includes many formations, such as continents, bodies of water, mountains, valleys, canyons, and caves.
The formation of these structures is the result of both gradual events, such as precipitation, weathering, erosion, deposition, and freezing, and sudden events, such as hurricanes, tornados, earthquakes, volcanic eruptions, tsunamis, and floods. Ice, wind, and moving water weather and erode Earth’s surface, breaking down rock and soil and moving it to different locations. Weathering and erosion are both responsible for creating new soil deposits (deposition or sedimentation). Erosion is also responsible for washing away fertile soil. In addition, living organisms aid in the processes of weathering and erosion.
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| Volcanic activity creates new land in the geosphere. |
The hydrosphere contains all of Earth’s water—liquid, solid, and gaseous. It includes both fresh water (rivers, lakes, and glaciers) and salt water (oceans). The majority of water on Earth (97%) is salt water found in oceans. Oceans support a variety of ecosystems and organisms. The hydrologic cycle, or water cycle, is central to connecting Earth’s systems. It is powered by energy from the Sun. The cycle of evaporation, condensation, precipitation, infiltration, percolation, and surface runoff as well as freezing and melting moves water between the clouds, land, and oceans (and other bodies of water) and back to the clouds. Energy is exchanged as water moves through the cycle, causing natural variation in global climates. Climate is the long-term pattern of temperature and weather and is impacted by the absorption and release of heat. When water evaporates, it removes heat from the surrounding environment. Condensation causes the release of heat and the warming of the environment. Oceans release and absorb heat more slowly than land. During winter, oceans are warmer than surrounding lands, and during summer, oceans are cooler than surrounding lands.
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| The water cycle connects Earth’s systems. |
The atmosphere is all the air on Earth. The air on our planet is 78% nitrogen and just under 21% oxygen. The small component remaining is composed of carbon dioxide and other gases. Weather and climate are both determined by the energy provided by the Sun. On any given day, different parts of Earth’s surfaces are heated differently because the Sun rises at different times, climbs to different heights in the sky, and shines for different amounts of time at different latitudes. The amount of heating at any given place varies seasonally because Earth’s axis is tilted 23.5 degrees compared to a line perpendicular to the plane of its orbit around the Sun. A common misconception is that seasons result from Earth’s distance from the Sun during its orbit around the Sun. However, seasons are a direct result of the tilt of Earth on its axis. If Earth’s axis were not tilted, there would be no seasons.
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| Seasons are caused by the tilt of Earth on its axis, not by Earth’s distance from the Sun. |
The intensity of energy from the Sun on Earth’s surface is also impacted by cloud cover and proximity to mountains and oceans. Because the Sun heats Earth unevenly, areas of high and low pressure occur. Wind is caused by the movement of air from areas of high pressure to areas of low pressure.
Climate also results from the interaction between the atmosphere, landforms, and bodies of water. Weather is different from climate and is the short-term condition that occurs in specific regions when energy and water are moved into and out of the atmosphere. Winds and clouds in the atmosphere interact with landforms and bodies of water to create weather patterns.
The atmosphere includes wind, weather, and climate.
The biosphere is a system containing all of the planet’s living things. The biosphere is made up of smaller parts called biomes that contain different living organisms. Biomes may be terrestrial (land) or aquatic (water). Terrestrial biomes include rain forests, temperate forests, deserts, grasslands, taiga, and tundra. Aquatic biomes include marine, estuarine, and freshwater. Organisms in the biosphere include plants, animals, fungi, and microscopic organisms. Plants depend on each system to perform photosynthesis: soil (geosphere) for nutrients, air (atmosphere) for carbon dioxide, and water (hydrosphere). Every biome depends upon landforms (geosphere), available water (hydrosphere), and climate (atmosphere) to determine what type of life-forms will flourish there. In addition, living organisms affect other systems. For example, plants take in carbon dioxide and give off oxygen during photosynthesis.
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| Photosynthesis demonstrates how the biosphere, atmosphere, hydrosphere, and geosphere all interact. |
Soil contains living and nonliving components. It contains all four systems: broken-down rock, water, air, and living organisms. Soil supports life on Earth. It provides the resources for food, building materials, and energy that living things need for survival. Soil also plays a role in the planet’s climate through the exchange of water and energy between the biosphere and the atmosphere. Soil supports the breakdown and synthesis of organic materials by supplying organic and inorganic building blocks, such as carbon, nitrogen, and other trace minerals. Soil is formed by actions of the atmosphere, hydrosphere, and biosphere on the geosphere, such as weathering, erosion, and deposition. All four systems are necessary to create soil, which supports life on Earth.
A system is a group of interdependent parts within a set boundary. The geosphere, hydrosphere, atmosphere, and biosphere all work together to support life on Earth. Earth is a set of complex systems in which chemical elements and the building blocks of life are recycled on Earth’s surface among the oceans and landforms and the atmosphere. These systems are interconnected. A change to one system will most likely result in a change to at least one of the other systems. The change within a system may be the result of nature or human activities—an earthquake versus an oil spill. The change may be local, national, or worldwide. Flooding, El Niño, and ozone depletion are examples of these types of changes.
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| The geosphere, the hydrosphere, the atmosphere, and the biosphere all interact in various ways, helping shape the planet we have today. |