Earth's Interacting Systems

The science background section provides teachers with more in-depth information about the phenomena students explore in this unit. Below is an excerpt from the science background information on Earth’s interacting systems.  

Water interacts with the biosphere, which is made up of all living things on Earth. All living things need water to survive. Plants absorb water through their roots and release it through their leaves as water vapor back to the atmosphere. The process by which water moves through plants from roots to their leaves and then evaporates is called transpiration.

Plants can also access groundwater if their roots are deep enough to reach the water table—the highest point in an aquifer from which water can be obtained.

Water that doesn’t seep into the ground or get absorbed by plants and animals flows downhill because of gravity. Any area of land where all of the water that falls in it drains into a common outlet is called a watershed. The watershed is made up of all of the surface water, including lakes, streams, reservoirs, and wetlands, as well as all the underlying groundwater. Larger watersheds contain many smaller watersheds.

The circulation of water through the hydrosphere from Earth’s surface to the atmosphere and back is called the water cycle. The water cycle is complex, and its processes vary around the planet. For example, very deep groundwater can take more than a million years to complete the water cycle and return to the ocean. In contrast, in hot climates and seasons, precipitation sometimes evaporates just seconds after it falls to Earth’s surface.

The water cycle is a major contributor to weather and climate around the planet. Climate is the average weather over a span of 30 years. Weather refers to the conditions of the atmosphere (temperature, humidity, wind speed, air pressure, and precipitation) in a particular place at a particular time. Weather changes hour-to-hour, while climate changes over long periods of time.

A location’s climate is determined by its proximity to the ocean, its elevation, and its proximity to the equator. Regions of the United States that are at low elevations, are close to the equator, and close to the ocean tend to have a wetter and warmer climate. Regions that are at higher elevations, are farther away from the ocean, and farther from the equator tend to be cooler and drier.

The Everglades have a tropical climate because of their location near the equator. Remember that Earth is a sphere. Around the equator, the sun’s rays shine directly onto the planet. This equatorial region is the tropical climate zone. The sun’s rays are the most concentrated here, which causes temperatures to be high.

Moving north or south of the equator, the sun’s rays are spread over a larger area. This causes Earth’s surface north and south of the equator to receive less direct heat from the sun. As a result, these regions have lower temperatures.

Earth’s land, oceans, and atmosphere all absorb heat from the sun, but the oceans absorb the most. This means that the tropical waters around the equator absorb the most heat.

Ocean currents also transport warm water and precipitation from the equator toward the North and South poles, and cold water from the poles back to the tropical region. The Gulf Stream is one ocean current that flows eastward from the Gulf of Mexico, around the southern tip of Florida, and along the East Coast up to Iceland and Norway. Scientists first discovered the origin of the Gulf Stream in the mid-1800s by setting adrift bottles and monitoring where they traveled. Benjamin Franklin was the first to chart its path in the 1700s.

As water moves through the water cycle, it connects all of Earth’s systems. For example, the movement of water downhill on Earth’s surface is the largest source of weathering and erosion on the planet. It is also the reason that oceans are salty. Oceans are salty because of interactions between the hydrosphere, the geosphere (Earth’s landforms, including rocks and soil), and the atmosphere (the mixture of gasses, dust, water vapor, and other molecules above Earth’s crust).

One of the gasses in the atmosphere is carbon dioxide. When water falls to Earth’s surface as rain, it carries some of this carbon dioxide, making the water slightly acidic. This slightly acidic water causes chemical weathering of the rocks on Earth’s surface, an interaction between the geosphere and the hydrosphere. The chemical weathering breaks down the rocks, transforming the matter into new substances with different properties, including salt and other minerals.

All water that collects on Earth’s surface will eventually flow downhill because of the pull of Earth’s gravity. As water flows downhill, it erodes the particles of salt and other weathered matter. All water eventually ends up in the ocean, where it deposits the salt and other matter. Over millions of years, this salt has collected in the oceans, creating the salt water of the oceans.