(17) Environmental Science

What If The Sun Disappeared?


What Keeps Us and Other Organisms Alive?

CONCEPT 3-2 Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.

The Earth’s Life-Support System Has Four Major Components

Scientific studies reveal that the earth’s life-support system consists of four main spherical systems that interact with one another-the atmosphere (air), the hydrosphere (water), the geosphere (rock, soil, sediment), and the biosphere (living things).

The atmosphere is a thin spherical envelope of gases surrounding the earth’s surface. Its inner layer, the troposphere, extends only about 17 kilometers (11 miles) above sea level. It contains the majority of the planet’s air that we breathe, consisting mostly of nitrogen (78% of the total volume) and oxygen (21%).

The remaining 1% of the air includes water vapor, carbon dioxide, and methane, all of which are called greenhouse gases, because they help trap heat and warm the atmosphere. Almost all of the earth’s weather occurs in this layer.

The atmospheric layer, stretching 17-48 kilometers (11-31 miles) above the earth’s surface, is the stratosphere Its lower portion contains enough ozone (O3) gas to filter out most of the sun’s harmful ultraviolet radiation. This global sunscreen allows life to exist on land and in the surface layers of bodies of water.

The hydrosphere consists of the earth’s water. It is found as liquid water (on the surface and underground), ice (polar ice, icebergs, and ice in frozen soil layers called permafrost), and water vapor in the atmosphere. Most of this water is in the oceans, which cover about 71% of the globe.

The geosphere consists of the earth’s intensely hot core, a thick mantle composed mostly of rock, and a thin outer crust. The lithosphere is the earth’s solid crust and upper mantle. It contains nonrenewable fossil fuels and minerals we use as well as renewable soil chemicals that organisms need to live, grow, and reproduce.

The biosphere occupies those parts of the atmosphere, hydrosphere, and geosphere where life is found. If the earth were an apple, the biosphere would be no thicker than the apple’s skin. The goal of ecology is to understand the interactions in this thin layer of air, water, soil, and organisms.

Life Exists on Land and in Water

Biologists have classified the terrestrial (land) portion of the biosphere into biomes-large regions such as forests, deserts, and grasslands, with distinct climates and certain species (especially vegetation) adapted to them. Scientists divide the watery parts of the biosphere into aquatic life zones, each containing numerous ecosystems.

There are freshwater life zones (such as lakes and streams) and ocean or marine life zones (such as coral reefs, coastal estuaries, and the deep ocean). The earth is mostly a water planet with saltwater and freshwater life zones covering almost three-fourths of its surface.

Three Factors Sustain Life on Earth

Life on the earth depends on three interconnected factors (Concept 3-2):

• The one-way flow of high-quality energy from the sun through living things in their feeding interactions, into the environment as low-quality energy (mostly heat dispersed into air or water at a low temperature), and eventually back into space as heat. No round-trips are allowed because energy cannot be recycled. The first law of thermodynamics and the second law of thermodynamics, govern this energy flow.

• The cycling of matter or nutrients (the atoms, ions, or compounds needed for survival by living organisms) through parts of the biosphere. Because the earth is closed to significant inputs of matter from space, its essentially fixed supply of nutrients must be continually recycled to support life. Nutrient movements in ecosystems and the biosphere are round-trips, which can take from seconds to centuries to complete. The law of conservation of matter governs this nutrient cycling process.

• Gravity, which allows the planet to hold on to its atmosphere and helps enable the movement and cycling of chemicals through the air, water, soil, and organisms.


Energy Flow and the First and Second Laws of Thermodynamics

Explain the relationship between energy flow through ecosystems and the biosphere and the first and second laws of thermodynamics.

What Happens to Solar Energy Reaching the Earth?

Millions of kilometers from the earth, in the immense nuclear fusion reactor that is the sun, nuclei of hydrogen fuse together to form larger helium nuclei, releasing tremendous amounts of energy into space. Only a very small amount of this output of energy reaches the earth-a tiny sphere in the vastness of space. This energy reaches the earth in the form of electromagnetic waves, mostly as visible light, ultraviolet (UV) radiation, and heat (infrared radiation). Much of this energy is absorbed or reflected back into space by the earth’s atmosphere, clouds, and surface.

Ozone gas (O3) in the lower stratosphere absorbs about 95% of the sun’s harmful incoming ultraviolet radiation. Without this ozone layer, life as we know it on the land and in the upper layer of water would not exist.

The UV, visible, and infrared energy that reaches the atmosphere lights the earth during daytime, warms the air, and evaporates and cycles water through the biosphere. Approximately 1% of this incoming energy generates winds. Green plants, algae, and some types of bacteria use less than 0.1% of it to produce the nutrients they need through photosynthesis, and in turn to feed animals that eat plants and flesh Of the total solar radiation intercepted by the earth, about 1% reaches the earth’s surface, and most of it is then reflected as longer-wavelength infrared radiation.

As this infrared radiation travels back up through the lower atmosphere toward space, it encounters the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. It causes these gaseous molecules to vibrate and release infrared radiation with even longer wavelengths. The vibrating gaseous molecules then have higher kinetic energy, which helps to warm the lower atmosphere and the earth’s surface. Without this natural greenhouse effect, the earth would be too cold to support the forms of life we find here today.

Human activities add greenhouse gases to the atmosphere. For example, burning carbon-containing fuels such as coal, gasoline, and natural gas releases huge amounts of carbon dioxide (CO2) into the atmosphere. Growing crops and raising livestock release large amounts of methane (CH4) and nitrous oxide (N2O).

There is considerable and growing evidence that these inputs are increasing the natural greenhouse effect and warming the earth’s atmosphere and surface-something often referred to as human-enhanced global warming.

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