How Do Natural Systems Operate?
EARTH HAS A NUMBER OF SYSTEMS in which matter and energy are moved or transformed. These involve processes of the solid Earth, water in all its forms, the structure and motion of the atmosphere, and how these three domains (Earth, water, and air) influence life. Such systems are dynamic, responding to any changes in conditions, whether those changes arise internally within the system or are imposed externally, from outside the system.
What Are the Four Spheres of Earth?
Earth consists of four overlapping spheres — the atmosphere, biosphere, hydrosphere, and lithosphere — each of which interacts with the other three spheres. The atmosphere is mostly gas, but includes liquids (e.g., water drops) and solids (e.g., ice and dust). The hydrosphere represents Earth's water, and the lithosphere is the solid Earth. The biosphere includes all the places where there is life — in the atmosphere, on and beneath the land, and on and within the oceans.
The atmosphere is a mix of mostly nitrogen and oxygen gas that surrounds Earth's surface, gradually diminishing in concentration out to a distance of approximately 100 km, the approximate edge of outer space. In addition to gas, the atmosphere includes clouds, precipitation, and particles such as dust and volcanic ash. The atmosphere is approximately 78% nitrogen, 21% oxygen, less than 1% argon, and smaller amounts of carbon dioxide and other gases. It has a variable amount of water vapor, averaging about 1%.
The biosphere includes all types of life, including humans, and all of the places it can exist on, above, and below Earth's surface. In addition to the abundant life on Earth's surface, the biosphere extends about 10 km up into the atmosphere, to the bottom of the deepest oceans, and downward into the cracks and tiny spaces in the subsurface. In addition to visible plants and animals, Earth has a large population of diverse microorganisms.
The hydrosphere is water in oceans, glaciers, lakes, streams, wetlands, groundwater, moisture in soil, and clouds. Over 96% of water on Earth is saltwater in the oceans, and most fresh water is in ice caps, glaciers, and groundwater, not in lakes and rivers.
The lithosphere refers generally to the solid upper part of the Earth, including Earth's crust and uppermost mantle. Water, air, and life extend down into the lithosphere, so the boundary between the solid Earth and other spheres is not distinct, and the four spheres overlap.
What Are Open and Closed Systems?
Many aspects of Earth can be thought of as a system — a collection of matter, energy, and processes that are somehow related and interconnected. For example, an air-conditioning system consists of some mechanical apparatus to cool the air, ducts to carry the cool air from one place to another, a fan to move the air, and a power source. There are two main types of systems: open systems and closed systems.
An open system allows matter and energy to move into and out of the system. A tree, like these aspen, is an open system, taking in water and nutrients from the soil, extracting carbon dioxide from the air to make the carbon-rich wood and leaves, and expelling oxygen as a by-product of photosynthesis, fueled by externally derived energy from the Sun.
A closed system does not exchange matter, or perhaps even energy, with its surroundings. The Earth as a whole is fundamentally a closed system with regard to matter, except for the escape of some light gases into space, the arrival of occasional meteorites, and the exit and return of spacecraft and astronauts.
How Do Earth Systems Operate?
Systems consist of matter and energy, and they respond to internally or externally caused changes in matter and energy, as a tree responds to a decrease in rain (matter) or colder temperatures during the winter (energy). Systems can respond to such changes in various ways, either reinforcing the change or counteracting the change.
System Inputs and Responses
1. One of Earth's critical systems involves the interactions between ice, surface water, and atmospheric water. This complex system, greatly simplified here, remains one of the main challenges for computer models attempting to analyze the causes and possible consequences of climate change.
2. Liquid water on the surface evaporates (represented by the upward-directed blue arrows), becoming water vapor in the atmosphere. If there is enough water vapor, small airborne droplets of water accumulate, forming these low-level clouds.
3. Under the right conditions, the water freezes, becoming snowflakes or hail, which can fall to the ground. Over the centuries, if snow accumulates faster than it melts, the snow becomes thick and compressed into ice, as in glaciers.
4. The water molecules in snow and ice can return directly to the atmosphere via several processes.
5. If temperatures are warm enough, snow and ice can melt, releasing liquid water that can accumulate in streams and flow into the ocean or other bodies of surface water. Alternatively, the meltwater can evaporate back into the atmosphere. Melting also occurs when icebergs break off from the glacier.
6. The movement of matter and energy carried in the various forms of water is an example of a dynamic system — a system in which matter, energy, or both, are constantly changing their position, amounts, or form.
7. The system can respond to changes in various ways, which can either reinforce the effect, causing the overall changes to be more severe, or partially or completely counter act the effect, causing changes to be less severe. Such reinforcements or inhibitors are called feedbacks.
8. In our example, sunlight shines on the ice and water. The ice is relatively smooth and light-colored, reflecting much of the Sun's energy upward, into the atmosphere or into space. In contrast, the water is darker and absorbs more of the Sun's energy, which warms the water.
9. If the amount of solar energy reaching the surface, or trapped near the surface, increases, for whatever reason, this may cause more melting of the ice. As the front of the ice melts back, it exposes more dark water, which absorbs more heat and causes even more warming of the region. In this way, an initial change (warming) triggers a response that causes even more of that change (more warming). Such a reinforcing result is called a positive feedback.
10. The warming of the water results in more evaporation, moving water from the surface to the atmosphere, which in turn may result in more clouds. Low-level clouds are highly reflective, so as cloud cover increases they intercept more sunlight, leading to less warming. This type of response does not reinforce the change but instead dampens it and diminishes its overall effect. This dampening and resultant counteraction is called a negative feedback.
11. As this overly simplified example illustrates, a change in a system can be reinforced by positive feedbacks or stifled by negative ones. Both types of feedbacks are likely and often occur at the same time, each nudging the system toward opposite behaviors (e.g., overall warming or overall cooling). Feedbacks can leave the system largely unchanged, or the combined impact of positive and negative feedbacks can lead to a stable but gradually changing state, a condition called dynamic equilibrium.