global change

Global Change



Contents of Entire Course

Solar Radiation and the Atmosphere
Ozone (O3)
Greenhouse Gases
Volcanic Effects
Carbon Dioxide in the Atmosphere
Methane
The Carbon Cycle
Global Warming
Effects of Global Warming
Global Warming in the Past
Mid-Cretaceous
Eocene

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

The Earth is a complex system. Changes that take place in one part of the system have effects on other parts. The tectonic system 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 Earth.

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.


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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) Volcanoes produce several things that result in changing atmosphere and atmospheric temperatures. 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 of atmosphere.
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 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. 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 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 produce energy.
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 global warming. 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. 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 currently resides. 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: 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
 
  Mid-Cretaceous
 
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 that time.
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) 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: 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. 

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