1.) The Earth System consists of three inanimate components (or subsystems) and one animate, or living, component.  Describe the three inanimate components and give an example of how each is coupled to the other two
    The three inanimate components of the Earth system are the geosphere, hydrosphere, and atmosphere.  The geosphere is known as the "solid earth," or "earth beneath our feet."  It includes continental and oceanic crust, as well as the various layers of the Earth's interior - much of which is actually viscous liquid.  The hydrosphere is the fluid envelope that covers the Earthís surface and includes the oceans, which account for 70% of the Earth's surface.  The atmosphere is the thin gaseous mixture that surrounds the geosphere in a very thin envelope.  These three inanimate components are coupled in some of the following ways:
- The atmosphere extends into the porous soil that overlies much of the geosphere. Water is transferred between the hydrosphere and atmosphere by evaporation and precipitation.
- The hydrosphere includes water vapor in the atmosphere and also accounts for raindrops, cloud droplets, and ice crystals - all of which play significant roles in determining Earthís temperature.  The hydrosphere also consists of ground water in the top layers of the geosphere and "primal water," which is buried far below.
- Volcanoes in the geosphere spew significant amounts of gases (water vapor, carbon dioxide, sulfur dioxide, hydrogen, hydrogen sulfide, etc.) into the atmosphere, which can result in the cooling of a climate in the years immediate following the eruption. Running, surface water of the hydrosphere is also the main agent of chemical and mechanical erosion of the geosphere.

2.) According to the theory of plate tectonics, Earth's crust consists of a number of "plates" that float on the mantle and move about.  How does this view of the Earth's crust explain why most earthquakes and volcanoes are concentrated in a few regions of the Earth's surface?

3.) The chemical composition of Earth's atmosphere has changed dramatically over the history of the planet, from one dominated by CO2 to the oxygen-rich atmosphere we experience today.  Discuss why we can say that the parallel evolution of the composition of the Earth's atmosphere and the appearance and development of life on the planet is an example of "co-evolution."
    Co-evolution is evolution involving successive changes in two or more interdependent systems under mutual influence on one another that affect their interactions.   The evolution of the Earth's atmosphere to form the ozone shield between 0.6 and 0.8 billion years ago enabled life in the oceans to expand on land because the ozone layer protected living things from hard UV radiation.  At that point, CO2 started to be absorbed by terrestrial plants, which put O2 directly back into the atmosphere. This is an example of co-evolution because it involved the evolution of two interdependent subsystems of the Earth simultaneously, both of which involved successive changes ? evolution of life on land and increased levels of oxygen in the atmosphere.
http://www.esse.ou.edu/classes/geos2004/lectures/Lecture_17.htm
http://irina.colorado.edu/lectures/Lec9.html
 

4.) Consider the three different planets.  Let the first be in a perfectly circular orbit about a star, with its axis of rotation perpendicular to its orbital plane.  Let the second be in an elliptic orbit about its star, with its axis of rotation perpendicular to its orbital plane.  Finally, let the third planet be in an elliptic orbit about its star, with its axis of rotation inclined somewhat with respect to its orbital plane.  Compare and contrast the types of seasons experienced on these three planets.
    For these scenarios, the star would be the main source of energy for the planet, much like the Sun is for the Earth.  The planetís orbit would determine the intensity of the radiation received from the star at each point along the orbit.  The tilt of the planet's axis would determine how that solar energy would spread across the planetís surface.  However, it is important to keep in mind that the characteristics of a planet's orbit are completely independent of the tilt.
    If a planet revolved around a star in a perfectly circular orbit and rotated about an axis that was perpendicular to its orbital plane, then this planet would have no seasons because every day would be the same in terms of length and the solar radiation received.

5.) On Planet Earth, it is very difficult to find rocks that can be dated back to near the formation of Earth's crust four billion years ago.  Discuss why this is so.
    It is extremely difficult to find rocks that can be dated back to the formation of the Earth's crust four billion years ago because new crust is continually being pushed away from divergent boundaries, where seafloor spreading occurs.

6.) Earth's global ocean current system, sometimes called the global oceanic conveyor, is driven by winds in the atmosphere, primarily in the tropics, and by the "thermohaline circulation" at high latitudes.  Describe each of these driving processes.
    The global oceanic conveyor is an interconnected system of both surface and deep ocean currents and moves water, heat, and salt among all the oceans.
    Surface currents are powered by atmospheric winds that drive the circulation of the upper 500 meters of the ocean.  These winds exert stress on the surface of the ocean, which generates waves and surface currents.  The wave motion keeps the surface layer of the world ocean well-mixed.  The tropical easterly trade winds converge into the Inter-Tropical Convergence Zone and drag the ocean surface waters westward along the equator as equatorial surface currents.  The equatorial surface currents do not encircle the globe because of the location of continental land masses.  The blocked equatorial currents in the Atlantic and Pacific push water against the east sides of continents and raise the ocean surface, however, they pull it away from the west side of continents in the tropics causing a depressed ocean surface. When blocked by land masses, these currents are deflected north and south and give rise to warm surface currents called western boundary currents, which are the strongest of all currents in the oceans.  As they move into the mid-latitudes, these boundary currents are turned eastward by the prevailing westerly winds and cross the oceans at mid- to high latitudes.  They then turn equatorward down the west coasts of the continents. These cold surface currents tend to flow back towards the equator, which completes the cell.  Atmospheric winds, as well as the Earth's rotation drive oceanic surface currents.  A good example of a surface current is the Gulf Stream in the western North Atlantic and the Kuroshio Current in the western Pacific.

(Adapted with permission from a figure in Principles of Ocean Physics by John R. Apel, Academic Press, 1987).

    The deep ocean currents differ from surface currents because they are density-driven, move slower and in a predominantly north-south direction, and cross the equator. The deep ocean circulation is often referred to as a thermohaline circulation, because the circulation is controlled by differences in temperature and salinity. Varying combinations of temperature and salinity produce water of varying densities, and it is these density differences that produce the deep ocean circulation. Deep ocean currents are powered by an accumulation of cold dense bottom water of 4 C in source regions at high latitudes.  The primary source regions are in the North Atlantic between Greenland and far northern Europe and in the ocean waters around Antarctica.  Deep oceanic currents typically flow in the lowest 1000 meters of the ocean.  They flow towards the equator and are easily identified by temperature, oxygen content, and salinity.

    A great conveyor belt can simplistically define thermohaline circulation of the oceans, which is controlled by temperature and salinity density of the ocean water.  Warm, salty, poleward-flowing surface currents cool as they loose heat to the air and water evaporates.  At high latitudes, this water reaches 4 C and sinks into the deep ocean, where it then flows slowly toward the equator in the deep currents.  There are two regions in the North Atlantic where the sinking occurs ? both to the north and south of Iceland.  The resulting cold, salty water in the deep currents flows southward, around the southern tip of Africa, across the southern Indian Ocean, and ultimately to the North Pacific.  After upwelling primarily in the Pacific and Indian Oceans, the water returns as surface flow to the North Atlantic.  This surface water flows south, passes between Asia and Australia, and then westward around the southern tip of Africa.  It then crosses the South Atlantic and connects to the Gulf Stream.  Throughout its journey, the surface water is heated by the sun and collects fresh water from river outflow to become more buoyant until it reaches the Gulf Stream where it loses heat due to the air and evaporation.  This causes the water to increase density and become less buoyant, which enables the cycle to start again.  It can take up to a thousand years for water from the North Atlantic to find its way into the North Pacific.

Figure courtesy of Jim Kennett and Jeff Johnson, University of California Santa Barbara.

7.) Why was the formation of the ozone layer an important event in Earth's history?
    Approximately 0.6 to 0.8 billion years ago, oxygen levels in the atmosphere reached a level that enabled an ozone layer to form.  Once the ozone layer was established, it protected living things in the ocean from hard UV radiation.  As a result, living organisms moved out of the ocean and on to land.  At that point, CO2 started to be absorbed by terrestrial plants, which put O2 directly back into the atmosphere.  Thus, the evolution of the Earth's atmosphere to form the ozone shield is important because it enabled life in the oceans to evolve to allow the expansion of plants and animals on land.
http://www.esse.ou.edu/classes/geos2004/lectures/Lecture_17.htm
http://irina.colorado.edu/lectures/Lec9.html
 

8.) Consider a world like Earth with a large tropical ocean and an atmosphere containing clouds.  If you make a change (+/-) in the global mean temperature of the planet, all other things remaining the same, what change do you expect in the amount of area covered by clouds?  How does this subsequently change the global mean temperature?  What type of feedback is this?
    The amount of cloud formation in the Earth's atmosphere is directly proportional to the amount of water vapor in the atmosphere.  The amount of water vapor is directly related to the global mean temperature.  Clouds are white and reflect sunlight (energy) to space and do not contribute to warming the Earth system.  Therefore, a small increase in the global mean temperature would mean a small warming in the oceans.  This leads to more water being evaporated into the atmosphere, which also increases the cloud formation in the sky.  This means that more sunlight is reflected from the clouds and less solar radiation reaches the ocean surface to be absorbed.  The ocean now experiences a small cooling, which leads to less water evaporated into the atmosphere and less cloud formation.  This results in less sunlight reflection by clouds, which allows more solar radiation to be absorbed by the oceans and so on with the recycling of the process.   This example is a negative feedback loop because there is a damping of the initial disturbance in the system.
Next consider a world like Earth with ice caps in both polar regions.  If you make a change (+/-) in the global mean temperature of the planet, all other things remaining the same, what change do you expect in the amount of area covered by ice caps?  How does this subsequently change the global mean temperature?  What type of feedback is this?
    Ice caps reflect sunlight just as clouds do.  Therefore, a small decrease in the global mean temperature would equal a small increase in the area covered by ice and snow, which allows more sunlight to be reflected back to space and less solar radiation to be absorbed by the Earth.  This results in global cooling and a decrease in the global mean temperature, which allows the amount of area covered by the ice caps to expand.  This system eventually runs away and would lead to a snowball-covered Earth.  However, an increase in the global mean temperature would result in less area covered by snow and ice, which would allow less sunlight to be reflected back to space and more solar radiation to be absorbed by the earth.  Both scenarios are examples of positive feedback loops because there is an amplification of the initial disturbance.
Finally, consider a world with ice caps, a large tropical ocean, and an atmosphere containing clouds.  Again make a change (+/-) in the global mean temperature.  What happens?
    If the global mean temperature increased and started melting the ice caps in this scenario, then the planet would still be cooled even though the oceans would be warming.   More water would be evaporated into the atmosphere, which also would increase the cloud formation in the sky.  Thus, more sunlight would be reflected from the clouds and less solar radiation would reach the planet's surface.  This example shows how the Earth regulates its own temperature by using a combination of both a negative and positive feedback system working at the same time.
    If the global mean temperature decreased and the ice caps were able to cover more surface area, then the planet would still be able to heat itself.  The oceans would be cooling, which would mean less water being evaporated into the atmosphere and less cloud formation in the sky.  Thus, less sunlight would be reflected from the clouds and more solar radiation would reach the planet's surface to be absorbed.  This is another example of how the Earth regulates its own temperature using both a negative and positive feedback system working at the same time.
    However, there is always a danger that one of the positive feedback loops could dominate.  In this case, if the ice albedo loop became the dominant one, then it could result in the global warming of the Earth or a snow-balled Earth depending on the initial global temperature change.  Neither of these situations has occurred though because the Earth is a dynamic equilibrium system where both positive and negative feedback loops are operating at the same time.  The dynamic equilibrium system output continually changes, but always stays within some bounds as is evident above when the Earth eventually re-regulates its global temperature.
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