adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
The Earth is a complex system. Changes that take place in one part of the
system have effects on other parts. The tectonic
is driven by the heat in the Earth. This drives the rock
cycle, which is also affected by the atmosphere and biosphere. The
atmosphere is in chemical equilibrium with the oceans and exchanges matter
with the biosphere. All process act on a variety of time scales from
hundreds of millions of years to microseconds. We as human beings are only
now realizing that, as part of the biosphere, we have an effect on the
One of the reasons life exists on Earth is that the surface has a
controlled temperature in the range between the freezing and boiling
points of water. The Earth is the only planet in the solar system where
this is true. Part of the reason for this results from the distance from
the Sun. But, the
reason that temperature remains fixed is controlled by the atmosphere.
Solar Radiation and the Atmosphere
Radiation reaching the Earth from the Sun is electromagnetic radiation.
Electromagnetic radiation can be divided into different regions depending
on wavelength. Note that visible light is the part of the electromagnetic
spectrum to which human eyes are sensitive.
Earth receives all wavelengths of solar radiation. But certain gases and other
contaminants in the atmosphere have different effects on different
wavelengths of radiation.
Dry air is composed of about 79% Nitrogen, 20% oxygen, and 1% Argon. It
also contains water, 4% at saturation, but saturation depends on
In addition trace gases have an effect. Among the trace gases are:
Ozone is produced in the upper atmosphere (30 - 35 km above surface) by
incoming ultraviolet radiation. Ultraviolet radiation causes O2 to go to O
+ O. Some of the O then recombines with O2 to make O3. This ozone then
absorbs more ultraviolet radiation and breaks down to O2 + O. The process
is self regulating and results in less ultraviolet radiation reaching the
Earth's surface. Ultraviolet radiation is harmful to organisms because it
is high energy radiation that damages cells. In humans, excessive exposure
to ultraviolet light causes sunburns and skin cancer. The
Effect of Chlorofluorocarbons (CFCs)(animation) in the atmosphere.
CFCs are produced to make refrigerants and styrofoam. Chlorine from these
human made products enters the atmosphere and catalyzes the breakdown of
ozone. Cl combines with ozone to make ClO and O2. Ultraviolet radiation
then causes ClO to react with O to make Cl and O2. This Cl can then react
with Ozone, and the process repeats. It is estimated that for every Cl
molecule in atmosphere, 100,000 ozone molecules can be destroyed. It has
been observed that the protecting ozone layer in the upper atmosphere has
deteriorated over the last 50 years, a result thought to be produced by
human introduction of CFCs into the atmosphere.
Infrared radiation carries most of the heat from the Sun to the Earth.
Greenhouse gases scatter both the incoming infrared radiation as well as
infrared radiation reflected from the Earth's surface. Shorter wave length
radiation that makes it to the Earth's surface where it is converted to
heat in the form of infrared radiation. The greenhouse gases then scatter
this radiation with about half of it being scattered back into the
atmosphere. This keeps the atmospheric temperature relatively stable, so
long as the concentration of greenhouse gases remains relatively stable.
The most important green house gases are H2O (water vapor), CO2 (Carbon
Dioxide), and CH4 (methane). H2O is the most abundant greenhouse gas, but
its concentration in the atmosphere varies with temperature. Venus, which
has mostly CO2 in its atmosphere, has temperature of about 500oC (also
partly due to nearness to Sun)
produce several things that result in changing atmosphere and atmospheric
CO2 produced by volcanoes adds to the greenhouse gases and may
result in warming of the atmosphere.
Sulfur gases produced by volcanoes reflect low wavelength radiation
back into space, and thus result in cooling of the atmosphere.
Dust particles injected into the atmosphere by volcanoes reflect low
wavelength radiation back into space, and thus can result in cooling
of the atmosphere.
Chlorine gases produced by volcanoes can contribute to ozone
depletion in the upper atmosphere.
The Mt. Pinatubo eruption in 1991 and El Chichon eruption in 1981 released
large quantities of dust and sulfur gases - resulted in short term cooling
Volcanism in the middle Cretaceous produced large quantities of basalt on
the seafloor and released large amounts of CO2. The middle Cretaceous was
much warmer than present, resulting in much higher sea level
Carbon Dioxide in the Atmosphere
CO2 of atmosphere has been increasing since the mid 1800s. Correlates well
with burning of fossil fuels. Thus humans appear to have an effect.
Methane concentration in the atmosphere has also been increasing.
Naturally this occurs due to decay of organic matter, the digestive
processes of organisms, and leaks from petroleum reservoirs. Man has
contributed through domestication of animals, increased production of
rice, and leaks from gas pipelines and petrol.
The Carbon Cycle
In order to understand whether or not humans are having an effect on
atmospheric carbon concentrations, we must look at how carbon moves
through the environment. Carbon is stored in four main reservoirs.
In the atmosphere as CO2 gas. From here it exchanges with seawater
or water in the atmosphere to return to the oceans, or exchanges
with the biosphere by photosynthesis, where it is extracted from the
atmosphere by plants. CO2 returns to the atmosphere by respiration
from living organisms, from decay of dead organisms, from weathering
of rocks, from leakage of petroleum reservoirs, and from burning of
fossil fuels by humans.
In the hydrosphere (oceans and surface waters) as dissolved CO2.
From here it precipitates to form chemical sedimentary rocks, or is
taken up by organisms to enter the biosphere. CO2 returns to the
hydrosphere by dissolution of carbonate minerals in rocks and
shells, by respiration of living organisms, by reaction with the
atmosphere, and by input from streams and groundwater.
In the biosphere where it occurs as organic compounds in
organisms. CO2 enters the biosphere mainly through photosynthesis.
From organisms it can return to the atmosphere by respiration and by
decay when organisms die, or it can become buried in the Earth.
In the Earth's lithosphere as carbonate minerals, graphite, coal,
petroleum. From here it can return to the atmosphere by weathering,
volcanic eruptions, hot springs, or by human extraction and burning to
Cycling between the atmosphere and the biosphere occurs about every 4.5
years. Cycling between the other reservoirs probably occurs on an average
of millions of years.
For example, carbon stored in the Earth in sedimentary rocks or as fossil
fuels only re-enters the atmosphere naturally when weathering and erosion
expose these materials to the Earth's surface. When humans extract and
burn fossil fuels the process occurs much more rapidly than it would occur
by natural processes. With an increased rate of cycling between the Earth
and the atmosphere, extraction from the atmosphere by increased
interaction with the oceans, or by increased extraction by organisms must
occur to balance the input. If this does not occur, it may result in
Average global temperatures vary with time as a result of many processes
interacting with each other. These interactions and the resulting
variation in temperature can occur on a variety of time scales ranging
from yearly cycles to cycles with times measured in millions of years.
Such variation in global temperatures is difficult to understand because
of the complexity of the interactions and because accurate records of
global temperature do not go back more than 100 years. But, even if we
look at the record for the past 100 years, we see that overall, there is
an increase in average global temperatures, with minor setbacks that may
have been controlled by random events such as volcanic eruptions or El
Niño events. Records for the past 100 years indicate that average global
temperatures have increased by about 0.5oC. While this may not seem like
much, the difference in global temperature between the coldest period of
the last glaciation and the present was only about 5oC
In order to predict future temperature changes we first need to understand
what has caused past temperature changes. Computer models, called Global
Circulation Models have been constructed to attempt this. Although there
is still much uncertainty, most of these models agree that if the
greenhouse gases continue to accumulate in the atmosphere until they have
doubled over their pre-1860 values, the average global temperature
increase will be between 1 and 5oC. This is not a uniform temperature
increase. Most models show that the effect will be greatest at high
latitudes (near the poles) where yearly temperatures could be as much as
16oC warmer than present.
Again, because of the large number of uncertainties involved in the
computer models scientists are reluctant to rely on the models. Still,
there is a consensus that average temperatures have increased over the
last 100 years, and that if these increases are due to the added input of
greenhouse gases into the atmosphere, then temperatures will continue to
increase at a rate of about 0.3oC per decade. This will lead to average
temperatures about 1 degree warmer by the year 2025 and about 3 degrees
warmer by the year 2100.
Effects of Global warming
Global Precipitation changes - A warmer atmosphere will lead to
increased evaporation from surface waters and result in higher amounts
of precipitation. The equatorial regions will be wetter than present,
while the interior portions of continents will become warmer and drier
Changes in vegetation patterns - because rainfall will distributed
differently, vegetation will have to adjust to the new conditions. Mid
latitude regions are likely to be more drought prone, while higher
latitude regions will be somewhat wetter and warmer than normal,
resulting in a shift in agricultural patterns.
Increased storminess - A warmer, wetter atmosphere will favor
tropical storm development. Hurricanes will be stronger and more
Changes in Ice patterns. - Due to higher temperatures, ice in
mountain glaciers will melt. But, because more water will be
evaporated from the oceans, more precipitation will reach the polar
ice sheets causing them to grow.
Reduction of sea ice - Sea ice will be greatly reduced to the
increased temperatures at the high latitudes, particularly in the
northern hemisphere where there is more abundant sea ice. Ice has a
high albedo (reflectivity), and thus reduction of ice will reduce the
albedo of the Earth and less solar radiation will be reflected back
into space, thus enhancing the warming effect.
Thawing of frozen ground - Currently much of the ground at high
latitudes remains frozen all year. Increased temperatures will cause
much of this ground to thaw. Organic compounds and gas hydrates in the
frozen ground will be subject to decay, releasing more methane into
the atmosphere and enhancing the greenhouse effect. Ecosystems and
human structures currently built on frozen ground will have to adjust.
Rise of sea level - Warming the oceans results in expansion of water and
thus increases the volume of water in the oceans. Along with melting of
mountain glaciers and reduction in sea ice, this will cause sea level to
rise and flood coastal zones, where much of the world's population
Changes in the hydrologic cycle - With new patterns of precipitation
changes in stream flow and groundwater level will be expected.
Decomposition of organic matter in soil - With increasing
temperatures of the atmosphere the rate of decay of organic material
in soils will be greatly accelerated. This will result in release of
CO2 and methane into the atmosphere and enhance the greenhouse effect
Global Warming in the Past
From out study of glaciations in the past we know that climate can change
as result of natural processes, both becoming warmer and colder than
present. Although these climatic fluctuations appear to be caused by
eccentricities in the Earth's orbit, it is interesting to note that during
glaciations in the past the concentrations of greenhouse gas
concentrations in the atmosphere were lower, atmospheric dust was higher,
and the Earth's albedo was higher, all of these factors could have
contributed to cooler climates. Similarly, during past interglacial
episodes, the atmosphere contained less dust, higher concentrations of
greenhouse gases, and the Earth had a lower albedo, all of which
contribute to warmer climates. The questions that remain to be answered
Are there higher concentrations of greenhouse gases and lower dust
concentrations in the atmosphere due to the warmer temperatures or did
they cause the warmer temperatures?
Are these differences simply due to orbital variations, or is there
some other natural self regulating process that allows for cycles?
How do human affect these cycles?
Over the past 100 million years, geologists have been able to reconstruct
CO2 concentrations in the atmosphere and average atmospheric temperature
based on a wide variety of geologic and geochemical evidence. From this
reconstruction, it appears that temperature was much higher than present
during the Mid-Cretaceous, during the Eocene, and during the Pliocene. We
will next look at what might have caused these periods of global warming
During this period we note the following observations:
The rate of production of new oceanic crust between 120 and 90 million
years ago (mid Cretaceous) were nearly twice the rate prior to and after
Large volcanic plateaus were emplaced in the ocean basins. The total
volume of these eruptions of basalt are unknown, as some may have been
subducted, but many are greater than 10 million km3. (The Ontong Java
plateau of the southwestern Pacific alone has a volume of ~ 55 million
The time interval during which these volcanic plateaus were emplaced
Deposition of oxygen depleted sediments like black shales.
A peak in sea level stands, which became 100 to 200 m higher
This information can be interpreted in the following manner: Magnetic
polarity remained constant because a superplume
originated at outer core/mantle boundary taking with it a large amount of
heat. This resulted in increasing the Temperature gradient in the core and
thus resulted in vigorous convection in the core, which then became
resistant to magnetic polarity changes. (Convection currents in the core
are what are thought to cause the Earth's magnetic field. If the rate of
convection is high, then it is more difficult to change the polarity of
the magnetic field)
CO2 released from the magmas erupted on the ocean floor by these
plumes resulted in a super green house effect, causing mid Cretaceous
climates to increase to 10 to 12o C above current average global
Increased ocean temperatures resulted in an increase in productivity
of marine life which resulted in the formation of increased formation
Increased global temperatures resulted in sluggish circulation of
ocean water which resulted in oxygen depleted waters and the
deposition of Carbon rich black shales. These shales were preserved
because shallow seas flooding the continents.
The large volume of basalts erupted on the ocean floor displaced sea
water resulting higher stands of the sea.
This example serves to show how events deep within the Earth, (events
taking place at the core - mantle boundary) could have a drastic effect on
conditions at the Earth's surface.
Eocene Global Warming
During this period we note the following:
Fossils of alligators are found on Elsmere Island at 78o North
Tropical vegetation and tropical marine organism fossils occur up to
45 to 55o North and South Latitude, about 15o higher than today.
Estimates of atmospheric CO2 concentrations show values between 2
and 6 times current values.
The increased CO2 concentrations have been attributed to a large scale
metamorphic event that occurred as a result of the continent-continent
collision that began to uplift the Himalayas, and other metamorphic events
that occurred in the Mediterranean region and the circum-Pacific region
during the Eocene. Such metamorphic events, particularly in the upper
parts of the metamorphic areas where greenschist metamorphism would occur,
would release large amounts of CO2 into the atmosphere.
This example shows how the rock cycle itself, aided by tectonic processes
could affect atmospheric conditions
Hopefully this will give you an idea about how human beings can effect the
way the Earth works, and also give you an idea about the complexity of the
interactions between various parts of the Earth and processes that occur
throughout the Earth.
Unfortunately, the complexity of the processes are not completely
understood. This has major political implications. For example, scientists
are uncertain about the reliability of models that attempt to predict
future conditions. This uncertainty is taken by some political factions as
a denial that an event like global warming will take place. Most
scientists, however, agree that global warming is highly possible, but
they are unwilling to say that it will definitely occur. Politicians want,
or expect you to want, exact answers. The real question, however, is
whether or not we should be preparing for such events to avoid disaster if
it does occur, or, since we can't be certain, just wait until the disaster
has occurred and we can do nothing about it.