RealAudio version of
the learning unit
RealAudio version of
the learning unit
Portuguese translation of the learning unit.
| Half of the Earth's atmosphere lies within 5 km of the surface of the earth, and three-fourths is located within ten km. Among the many functions of this atmosphere are two that are critical to life as we know it: the atmosphere shields the surface of the earth from lethal ultraviolet radiation and allows life to exist on the surface of the planet. It also serves to process energy that comes from the sun and energy that is reradiated from the surface of the earth. Absorption of this energy establishes an average surface temperature that allows water to exist in all three phases -- liquid, solid, and vapor. The favorable environment established through these two functions allows for the flourishing of life as we know it on the planet. |
American Scientist, 78, 325.
Permission granted by Sigma, Xi, The Scientific Research Society.
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Recent measurements have revealed that the characteristics of this
atmosphere are changing. Above is a plot of atmospheric carbon dioxide,
in parts per million, as a function of time since 1972. You can see
that at all locations, the carbon dioxide concentration of the
atmosphere is increasing. We know why it's increasing: the burning of
fuels (coal, oil, natural gas), the burning of vegetation, and the production of cement.
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Adapted from Khalil and Rasmussen, C and E News, 64 (47), 23.
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There are other gases in the earth's atmosphere whose composition has
changed in recent years. Methane is an example. The accompanying plot
shows the concentration of methane in parts per million over the last
thousand years. You can see that for the first 700 years of this record, until the 1700's,
the concentration of methane in the earth's atmosphere was fairly
constant. But since the Industrial Revolution, methane
concentration has increased quite dramatically to the levels more than
double the pre Industrial Revolution values. The sources of methane are
reasonably well known: cattle, termites, rice paddies, boreal forests,
and high latitude tundras are sources of methane gas.
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 Changes in mean global surface
temperature from the 1951-1980 average from 1880-1994. (U.S. Global Change
Research Program.)
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Measurements of the global-averaged surface temperature of the Planet Earth show that this temperature has increased by about 0.5oC over the last 100 years. This increase with time has not been uniform. The accompanying plot shows there was a reasonably steady rise from 1860 to a peak in the 1940's and then a drop off to the 1970's followed by a very dramatic rise since about 1970. Recent data show that 1995 has been the hottest year in the 135 year historical record.
There are speculations that this rise in temperature might be caused by the increases in carbon dioxide, methane and other trace gases. There is good physical evidence to support this because we know carbon dioxide absorbs infrared radiation and could very well be contributing to this recent rise in temperature.
| We can estimate the average global surface temperature of the planet by analyzing bubbles of air trapped in ice centuries ago in the Antarctic and Greenland ice sheets. A team of French and Russian scientists analyzed the ratio of the isotopes of oxygen from an ice core over 2 km deep in the Antarctic ice mass and used this as a proxy measure of temperature at the time the ice was laid down. They also measured trace gases in these same bubbles and reported the changes in concentration of carbon dioxide over the last 160,000 years. The record shows that carbon dioxide in the earth's atmosphere rose from about 190 ppm 160,000 years ago to about 300 ppm 130,000 years ago and then gradually decreased to a minimum when the last ice age reached it peak about 15,000 years ago. Since then carbon dioxide has steadily risen to the pre- Industrial Revolution value of about 275 ppm. The temperature record has followed a pattern that looks suspiciously like the carbon dioxide record over this time period. |
 Changes in global
atmospheric CO2 and global surface temperature over the last
160,000 years. (U.S. Global Change Research Program.)
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The data are not resolved close enough in time to determine whether the temperature changes caused the carbon dioxide levels to change or vice versa. Noting the abrupt anthropogenically caused increase in carbon dioxide over the last 140 years, we must then ask what will happen to the temperature record in the future. Will it tend to follow the carbon dioxide curve or not?
The public press has called our attention to these issues through special issues of, for instance, Newsweek whose cover from an issue in 1988 showed a family in a glass house indicating that the green house effect might lead to more hotter summers. The cover of another magazine shouts "Save the earth and ruin the world!", the implication being that attempting to suppress these rises in carbon dioxide could wreck the economy. The cover of Forbes magazine on December 25, 1988 states that the global warming panic in a classic case of overreaction. So we see a considerable difference of opinion in the public press about the importance of global warming.
We also have other chemicals in the earth's atmosphere that are increasing. Chlorinated fluorocarbons (CFCs) were invented in the late 1930's, so did not exist in the earth's atmosphere before that time. They are very stable compounds and are removed from the atmosphere very slowly. This allows time for these compounds to diffuse into the stratosphere where they can be broken down by ultraviolet light creating free chlorine atoms that can combine with ozone to create diatomic oxygen and therefore serve as a means of depleting stratospheric ozone.
Nitrous oxide is another gas in the earth's atmosphere whose concentration is increasing. Natural sources of nitrous oxide in the soil are augmented by the use of nitrogen fertilizers leading to increased atmospheric concentrations. Nitrous oxide, like the CFCs, has a very long lifetime in the atmosphere and also can lead to destruction of stratospheric ozone. A loss of stratospheric ozone allows increased amounts of ultraviolet light to reach the surface of the earth and damage living tissue. Cases of skin cancer in New Zealand and Australia are experiencing a dramatic rise, in part due to this increase in ultraviolet light. Tiny ocean organisms, called phytoplankton, in regions where stratospheric ozone has been depleted are vulnerable to damaging levels of ultraviolet radiation.
But the public press is not of one mind on this issue either. One industry publication asserts that this is problem of vanishing facts and not vanishing ozone. It contends that the media and NASA are hyping this for their own purposes. Time magazine, on the other hand, takes a different view, by declaring Earth to be Planet of the Year in 1989, and calls attention to the "endangered earth". Another issue of TIME describes practices of deforestation which are destroying regions of the planet that house rich, diverse ecosystems. Tropical rain forests are homes to a rich diversity of species, many of which have not even been discovered. Transforming natural ecosystems such as the rainforests leads not only to destruction of biomass but also to changes in the soil chemistry, local climate, and the quality and movement of water in these areas. Land-use practices in other vulnerable areas have allowed overgrazing of land to the point that surface vegetation is irreversibly reduced, allowing weathering processes to permanently reduce the productivity of soil and promote soil erosion.
These problems spill into the human dimension and pit developed countries, which seem to have an insatiable demand for energy and resources, against developing countries whose citizens struggle to provide their families with even the bare essentials of human existence, leaving little time to be concerned about longer term implications of global environmental change.
In summary, we live on a planet that, from space, looks blue. It has water existing in all three phases. On closer inspection, we can see that we have a green planet, with a rich diversity of biological species. However, the habitable zone of this planet that supports biological activity is a very thin shell around the surface of the plant. The region of the earth in which humans can live without life support systems is a thin spherical shell about 3 kilometers thick on a planet of radius 6,370 km. If the earth was the size of a basketball, this zone of human habitation would be about the thickness of a sheet of paper. This the only known zone in the universe where humans can exist without life support systems. And now humans are performing a global experiment -- a chemistry experiment -- on this habitable zone without knowing what the consequences might be. As concerned citizens and future leaders of the planet, we have two options: we can cover our eyes and pretend there is no problem, or we can use the tools at our disposal to study this problem -- to evaluate the evidence, to look at the scientific results, and answer for ourselves what level of certainty we have about these issues. And what are the consequences -- environmentally, socially, and economically -- of acting or of failing to act in determining the nature of our global chemistry experiment. What future do we leave to our grandchildren?
This is a course in Global Change that addresses these and many other issues. A key objective of the course is to demonstrate the interconnectedness of the earth system. A second objective is to instill in students the value of peer-reviewed literature on global-change issues. A third objective is to engage students in dialog among themselves and with outside experts on the economic, social, political, and ethical implications of these changes.
Transcription by Theresa M. Nichols
