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History of the Earth Notes

The Earth Environment

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Page and figure numbers refer to the textbook: Stanley, Earth System History, 2nd Ed.

Composition of the Atmosphere (p. 85)

The Earth's atmosphere, the air that we breathe, is composed of several gases. Two gases make up about 99% of the atmosphere. Nitrogen accounts for about 78% and oxygen about 21%. Nitrogen is fixed in the soil by plant roots and is an important nutrient. Of course animals breathe oxygen. Plants "breathe" carbon dioxide to grow. Carbon dioxide is an important greenhouse gas, meaning that it holds heat. Without greenhouse gases the sun's energy would be reflected and radiated back into space and the Earth would be very cold. On the other hand, today we are concerned about adding too much carbon dioxide to the atmosphere by burning fossil fuels such as oil and coal. Too much carbon dioxide will cause the atmosphere to hold too much heat and the Earth's temperature will rise. This might have unwelcome consequences such as rising sea level and coastal flooding, changing rainfall patterns and crop losses, and increased heat stress on existing plant, crop, animal, and human populations. Carbon dioxide is also important in rock weathering. Recall that carbon dioxide dissolves in water and in turn forms carbonic acid. This carbonic acid is a primary agent of rock weathering. Later, we will see the relationship of this in controlling excessive changes in the climate.

Solar Insolation and Climate (Fig. 4-6)

When the sun's rays strike the Earth's surface some of the light is reflected back to space and some is transformed into infrared (heat) radiation. Some of this infrared radiation radiates out into space but some is also trapped by gases in the atmosphere, especially the small amounts of carbon dioxide, water vapor, and traces of methane. In this way the Earth retains some of the sun's heat.

In one year there is much more sunlight striking every square meter of the Earth's in the equatorial regions than at the poles. This is because the sun's rays are nearly perpendicular to the surface at the equator and nearly parallel to the surface at the poles. In the absence of other processes the equator would become much hotter and the poles would become much colder than they actually are.

Solar Insolation

It is commonly known that when materials (such as atmospheric gases) are heated they expand thereby becoming less dense or "lighter." In a room with a radiator or other such heater, as the air around the radiator becomes heated it rises to the ceiling and is replaced by cooler denser air. This is an example of convection. Convection occurs in the Earth's atmosphere; it helps redistribute the heat from the sun. Atmospheric circulation is driven by the uneven heating of the Earth and the pattern of circulation is modified by the Earth's rotation.

Climate Belts Simplified (Figs 4-8, 4-9)

equatorial: hot & wet
The equatorial regions are the most strongly heated areas on the Earth's surface. It is there that we find the most vigorous upward convection. Hot air is capable of holding much water vapor. Hot, humid air rises over the equator. As it rises to high altitude it expands because the air pressure decreases (there is less mass of air above it). As air expands due to this decreasing pressure, it also cools. Since cool air is able to hold less water vapor than warm air, condensation occurs. This is why the equatorial regions normally have very high rainfall. It is here that we find tropical rainforests such as those in the Amazon, Congo, and Indonesia. Areas of upward convection are dominated by low atmospheric pressure.

Atmospheric Convection and Rainfall

desert belts: hot & dry
The rising air at the equator is replaced by low-level air from higher latitudes north and south of the equator. To balance the air moving toward the equator at low altitude, the convecting air moves away from the equator, toward the north and south, at high altitude. It is now cool because of expansion and dry because it has dropped off excess moisture. To complete the convection loop, in the regions around 15 degrees north and 15 degrees south of the equator, this cool and dry air descends back to the surface. As it descends, the pressure increases. The increased pressure increases the temperature of the air and therefore increases the capacity of the air to hold water vapor. Now the air is very dry and has the capacity to soak up much evaporation. Consequently, these latitudes are very arid with high evaporation and low rainfall. These are the desert belts, including the Sahara, Mojave and Sonoran deserts of the U.S. southwest and Mexico, the Kalahari in southern Africa, the Australian desert, and the Atacama Desert on the west coast of South America. Areas of descending air are dominated by high atmospheric pressure.

midlatitude: temperate
In the midlatitudes there is another latitude belt of low pressure, upward convecting air masses. But here there is less solar energy and the convection is not as vigorous. Rainfall is not great as near the equator but evaporation is not as high either.

polar: cold & dry
The polar regions are the coldest on Earth. There, at high altitude, the air becomes extremely cold and descends. High pressure dominates. As the air descends it warms due to the increasing pressure. Thus its capacity to hold moisture is increased a bit (though cold air can hold much less moisture than warm desert air). The polar regions are extremely dry. The interior of Antarctica is a very dry region besides being very cold.

the coriolis effect (Fig. 4-7)
As air masses move north or south they are deflected due to the rotation of the Earth. As the Earth spins on its axis, a person standing on the equator moves from west to east at around 1000 mile per hour. At the poles, on the other hand, that person would not move at all, just spin around in place. So, the equator and anything on it moves west to east faster than any other place on Earth. The west to east motion decreases from the equator to the pole.

As air masses move away from the equator, their west to east momentum carries them to the east of a true poleward trajectory. In the northern hemisphere they are deflected to the right. In the southern hemisphere they are deflected to the left. For the opposite case, as air masses move toward the equator, their west to east momentum lags behind the west to east motion of the Earth at lower latitude and they curve to the west. In the northern hemisphere the are masses are deflected to the right. In the southern hemisphere they are deflected to the left.

prevailing winds (Fig. 4-9)
As air approaches the equator from the north and south they are deflected to the west. This is the prevailing winds called the "easterlies" (east to west wind) near the equator. Specifically, the "northeast trades" come from north of the equator, and the "southeast trades" blow from south of the equator. Winds moving toward the poles away from the desert high pressure belt are deflected to the east. This gives rise to the prevailing "westerlies" (west to east winds) in the midlatitudes of both the northern and southern hemispheres.

Ocean Circulation

surface currents (Fig. 4-21)
Wind drives both waves and surface currents in the oceans. Warm surface waters at the equator are driven westward by the easterly trade winds. When the equatorial currents reach the western edge of the ocean basin (east coast of some continent) they are diverted to the north and south along the continents. In the central Atlantic this northward flowing branch is called the Gulf Stream. It carries warm water from the equator and the Caribbean parallel to the east coast of North America, across the North Atlantic, and to northwester Europe. The westerlies help to divert the Gulf Stream toward Europe. The warmth of the Gulf Stream moderates the climate of northwester Europe. The waters in the North Atlantic cool. As the westerlies drive the currents to the east they are diverted southward along the coast of Europe and Africa. This southern current is called the Canary Current. It brings cool waters down to northwest Africa and keep the coast here relatively cool. Eventually this current approaches the equator where the waters warm again and the easterly winds drive the current across the Atlantic again to start another clockwise loop. Such large surface current loops (called gyres) are found in all the open ocean basins. They generally flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.

deep ocean currents (Fig. 4-23)
As water chills in the North Atlantic it becomes more dense. Also, when the cold water begins to freeze to form sea ice the ice that forms is from pure water; the salt is left behind in the remaining sea water. The sea water gets saltier. The saltier the water the denser it becomes. These cold, salty, dense surface waters sink down to the bottom of the Atlantic. The sinking waters are replaced by less dense surface waters from the south. The sinking waters flow southward along the bottom of the ocean as surface waters flow northward to replace them. The North Atlantic Deep Water continues south until they meet a northward flowing bottom current of even denser waters that formed off the coast of Antarctica. The North Atlantic Deep Water then rides up above the Antarctic Bottom Water and continues southward at intermediate depths until they eventually rise to the surface near Antarctica. From there they follow other currents that carry them throughout the oceans.

There is an ocean conveyor belt that mixes waters through all the ocean basins from the sea bottom to the sea surface, connecting the surface, intermediate, and bottom water currents. This mixing moves heat, dissolved gases, and nutrients through the oceans in one grand cycle. Breakdown of this conveyor belt may have been responsible for sudden changes in the Earth's climate in the past.

Climate Summary
The excess heat generated at the equator is distributed poleward by atmospheric and oceanic circulation. Another way of thinking about this is that atmospheric and oceanic circulation and the weather is the result of the uneven distribution of solar radiation on the Earth's surface.

 
Indicators of Past Climate

Plants and animals are adapted to live in certain climates, so fossils of organisms related to living plants and animals should tell us about the climate at the time the fossils were laid in the sediments. For example reptiles are cold blooded and do not live in severely cold climates. Dinosaurs, which were reptiles, lived in Antarctica during the Cretaceous. Therefore, Antarctica must have been relatively warm during the Cretaceous. This is confirmed from other lines of evidence such as the lack of glacial deposits of this age.

Certain types of sedimentary rock are also indicative of climate. Glacial tills are one obvious example. Limestones only accumulate in warm waters because most corals (today anyway) only survive in warm water and because limestone dissolves more easily in cold water. Although small, isolated pockets of coal form in bogs and marshes in temperate climates (like Irish peat bogs), massive, continuous coal layers indicate formation in a hot, ever-wet equatorial climate.