PURDUE UNIVERSITY

EAS 105-THE PLANETS

Prof. Robert L. Nowack

 

Lecture 11

 

 

The Earth's Oceans and Atmosphere

 

The oceans constitute the great water reservoir on Earth having a mass of about 1018 metric tons of H2O.  But, ocean water is not just pure H2O, salts, like sodium chloride ( NaCL ), as well as other dissolved compounds, make up 3.5% of the ocean.  Also, gases are dissolved in the ocean (such as carbon dioxide (CO2)and oxygen (O2).

 

Carbon dioxide, CO2, combines with calcium in the shells of small creatures and ultimately settles to form carbonate limestone rocks ( CaCO3 ).  For example, there is 60 times more carbon dioxide dissolved in sea water than in the atmosphere (and 3000 times more CO2 buried in sedimentary rocks than in the oceans).  In addition, the oceans are a reservoir for heat with the temperature averaging 3.9°C (just above freezing).  However, surface water temperatures vary from 30°C at the equator to below freezing on the polar ice caps.

 

 

Note: Water has the odd property of being less dense as a solid than as a liquid (at surface pressure and 0°C).

 

 

Oceans are heated by the Sun and stirred by the air.  Above the ocean is the atmosphere, which is composed primarily of:

 

Nitrogen, N2

78.1%

Oxygen, O2

20.9%

Argon, Ar

0.93%

Water Vapor, H2O

0.1 to 3%

Carbon Dioxide, CO2
340 ppm

0.036% (and increasing)

Methane, CH4

0.0002% (increasing)

Neon, Ne

0.0018%

Helium, He

0.0005%

 

Water vapor varies from less than 1% where it is cold and dry to up to 3% where it is wet and warm.  N2 and Ar accumulate in the atmosphere and have relatively heavy atomic masses which prevents them from escaping into outer space.  Oxygen (O2) and carbon dioxide (CO2) are both very chemically reactive.  The major oxygen source in the atmosphere comes from photosynthesis by living plants.  If oxygen occupied more than 25% of the Earth’s atmosphere, trees would spontaneously combust!!

 

The atmosphere exerts 1 bar of pressure (1000 millibars) on the

surface.  This is the same pressure exerted by a 10 meter deep layer of water.  If the

Earth were heated to 100°C, the oceans would vaporize.  The oceans having an average

depth of 3 kilometers would result in a new atmospheric pressure of 300 Bars.  Water vapor would then become the dominant constituent of the atmosphere.  Carbon dioxide also

would increase, particularly if carbonate rocks broke down releasing much more CO2.

 

 

 

The Structure of the Present Atmosphere

 

The density of the gases in the atmosphere decreases rapidly with height.  At a height of 10 km, air density is about 1/3 of that near the surface. 

 

 

Vertical Pressure Profile of the Atmosphere

 

 

 

 

The lower 10-15 km of the atmosphere is called the troposphere (tropo means "turning" or convecting air).

 

 

 

 

The troposphere contains 90% of the mass of the atmosphere.  Most "weather" occurs here.  Convection occurs in the troposphere when the lower atmosphere is heated by sunlight causing the warmer air to expand, becoming less dense and rising, replaced by cooler downdrafts.  The top of the troposphere is the tropopause, where the temperature is about -60°C. 

 

Above this is the stratosphere.  In this layer ultraviolet (UV) radiation creates a layer of ozone, (O3), extending to heights from 20 to 50 km.  Above 90 km, the temperature rapidly increases causing some atoms to lose electrons (from the absorption of sunlight) creating the electrically charged ionosphere (important for radio communication).  Note:  The gas is very thin at these heights.

 

 

One of the most extraordinary aspects of the Earth is the existence of life.  So far, life on the Earth is the only known life in the solar system.  Life is a remarkable phenomenon through which complex genetic material (DNA) in the cells of living tissue, organizes chemical reactions that permit it to reproduce itself and maintain its existence.  As early as 3.5 billion years ago, fossils known as stromatolites (the remaining calcium carbonate structures) were formed by a blue-green algae which used photosynthesis.  It took another 2.7 billion years to develop more complicated multi-celled organisms.

 

This early atmosphere was lacking in oxygen (and hence "reducing").  Indeed, since oxygen is reactive, it would have altered or oxidized compounds necessary to form early life.  Rocks much older than 2.5 billion years seemed to have formed in the absence of free oxygen.  After initial transitions, carbon dioxide (CO2) and nitrogen (N2) dominated the early atmosphere of the Earth.  As life progressed, it began to modify the atmosphere.  Marine shell forming organisms used calcium and carbon dioxide (CO2) to form tiny shells.  These shells settled to form great deposits of carbonate sediments.  Today, huge amounts of carbon dioxide (CO2) are bound in sedimentary rocks.

 

Photosynthesis generates oxygen as a by-product.  A "pollution" catastrophe resulted in creating a new atmosphere with a major amount of free oxygen.

 

Evolution of the Atmosphere

 

 

 

 

One hypothesis of the evolution of oxygen in the atmosphere is its relation to the origin of life and the evolution of higher organisms.  However, there is as yet no general agreement on exactly when and to what levels oxygen accumulated in the Precambrian, nor on how close the levels of oxygen in Phanerozoic time came to the present-day levels.

 

 

James Lovelock and Lynn Margulis have suggested that life on Earth, to some extent, regulates the composition and conditions of the Earth's lower atmosphere.  This is called the Gaia Hypothesis.  Gaia (after the Greek goddess of the Earth) is the name for the air, ocean-organism system.  (This self-regulating feature might explain how life has lasted so long on the Earth.)

 

Ex):   If the average solar heat output decreased, organisms could release more CO2 trapping more thermal energy.  Thus, returning the Earth's surface to its original temperature. 

 

Nonetheless, this is only a hypothesis.  It would require a large diversity of plants and animals in order to regulate any change in climate.  Note that mankind is in the process of decreasing plant and animal diversity by cutting

down rainforests, etc…

 

 

 

Climate and Weather

 

The Sun heats the Earth's atmosphere and surface.

 

~ 30% is reflected back to space where clouds, snow and water are main reflectors.

~ 70% the Earth absorbs with an average power of 240 Watts/sq. meter of surface area.

 

For the Earth to be in temperature equilibrium, the 240 watts of heat energy ultimately has to be re-radiated back to space in the infrared.  The Earth's average surface temperature is about 10°C.  Without the blanketing effect of the atmosphere, it would be 25°C colder.

 

Carbon dioxide is a so-called greenhouse gas and so is water vapor and methane.  It absorbs infrared radiation emitted from the surface and acts like a blanket to retain heat.

 

 

Note:  carbon dioxide (CO2) is transparent to incoming light radiation.

 

 

An important observation is that CO2 has been increasing in the atmosphere, presumably due to the burning of fossil fuel and releasing carbon dioxide from the rocks.

 

 

 

 

This would increase the greenhouse effect, but could also make more clouds, thus reflecting more sunlight.  As yet, it is controversial how the climate will ultimately be affected, but there are strong arguments to suggest that this will lead to global warming.

 

Sunlight heats the Earth more at the equator than at the poles. However, the atmosphere and oceans moderate this heating variation.

 

Simplified Global Circulation of Earth’s Atmosphere

 

 

 

 

 

Air warmed at the equator tends to rise and move to the cooler regions setting up a global circulation of air.  Rising air near the equator gives rise to precipitation producing tropical rain forests within 10° latitude of the equator.  Between 15° to 35° latitude, deserts occur where dry air sinks and flows back to the equator.

 

Air circulation gets more involved in the higher latitudes of the northern and southern hemispheres because the Earth rotates.  This rotation complicates circulation.  Rotating circular weather systems called cyclones, hurricanes and typhoons are a consequence of a rotating Earth.

 

 

 

 

Coriolis Effect on air moving North or South on a rotating planet.  The same force can be encountered by trying to walk toward or away from the center of a moving carousel.

 

 

Because of the Earth's rotation, cyclones rotating counterclockwise will tend to form about low pressure regions in the northern hemisphere.  In the southern hemisphere, this will result in “anti-cyclones” swirling in the opposite direction.  These features have the potential of becoming major storms.

 

 

 

Long Term Climate Change

 

Earth has experienced great "ice ages" over the past million years.  About every 100,000 years, the Earth's average temperature drops by 2 or 3 degrees.  This is sufficient to produce vast ice sheets on land.  At present, only Greenland and Antarctica are covered year round by thick ice sheets.  We are currently in a relative warm phase, probably between major ice ages.  The last major ice age ended only 10,000 years ago when ice covered parts of North America and Europe year round.

 

Milan Milankovitch, in 1920, calculated that changes in the average solar heat ouput impinging on the Earth would change when the Earth's orbit changed.

 

 

 

 

The Earth's rotation axis "precesses" like a top completing a cycle every 26,000 years.  13,000 years ago, the Earth's North Pole pointed toward the star Vega and not Polaris as it is today.  The Earth's orbit also changes its eccentricity.  It is currently in a nearly circular orbit.

 

 

 

 

These so called Milankovitch cycles can be correlated with past ice ages. However, there are other possible factors which could affect climate, including large volcanic eruptions putting a large amount of dust into the atmosphere, as well as large impacts by an asteroid or comet (or even a nuclear war!).