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Introduction
Half of the Earth's atmosphere lies within 5 km of the surface of the
earth, three-fourths is located within ten km and 90% is within 16 km (10 miles).
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 on the planet over the past million years.
Earth's Changing Atmosphere
Recent measurements have revealed that the characteristics of this
atmosphere are changing. Figure 1 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. It is an uncontested fact
that CO2 in the atmosphere has risen from a pre-industrial (1780) level of 280 parts per million (ppm) to
the current (2007) level of 382 ppm - a 34% increase.
There are other gases in the earth's atmosphere whose composition has
changed in recent years. Methane is an example. Figure 2 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.
Global-Average Surface Temperature
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.
Figure 3 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 1998 has been
the hottest year in the 138-year historical record and that six of the
eight warmest years have occurred since 1998. The extraordinary warmth
in 1998 was largely due to a very strong El Niño. The demise of this
El Niño allowed global temperatures to drop back somewhat, but then
resume an increase to the point where at the end of 2005, global mean
temperatures equaled those of the 1998 El Niño year even though
we had no El Niño in progress at this time.
There is now good evidence that at least part of this rise in temperature is caused
by the increases in carbon dioxide, methane and other trace gases that are produced as a result of anthropogenic
(human) activities.
There is good physical evidence to support this because we know carbon
dioxide and methane absorb infrared radiation and are contributing to this recent rise in temperature.
Temperature and CO2
Correlation
Reconstructed Earth Surface
Temperature, 1000-2000
If we plot current atmospheric CO2 levels on the historical record of Figure 4 we see that
current
levels far exceed levels of the last 400,000 years (Figure 6). If
human-induced emissions of the CO2
continue at current rates of increase, atmospheric CO2 will surpass 300% of pre-industrial levels by 2100
(Figure 7). The question then is what happens to global mean
temperatures on the graph in Figure 7? Is the recent
rise in temperature shown in Figure 3
the early sign of the
trend to be expected in Figure 7?
From a plot of atmospheric CO2 and Earth surface temperature over the
last 400,000 years (Figure
4) we see a strong correlation between temperature and CO2. It
is noteworthy that the concentration of CO2 during this period did
not exceed 300 ppmv, in contrast to recent measurements that show
concentrations exceeding 370 ppmv and rising steadily.
A 1999 report by Mann et al (1999) shows that the surface temperature
change for Planet Earth over the last 1,000 years as reconstructed from ice
cores, lake sediments and tree rings (Figure 5). The
yellow bars on this graph show the range of uncertainty for each proxy
measurement. Overall the graph shows that the temperature of the planet has
decreased gradually over the period from year 1000 to about 1900. Since then, the
temperature has risen abruptly at a rate uncharacteristic of the behavior of the
preceding 900 years. This graph provides strong evidence that conditions of the
planet have changed dramatically in the last 100 years.
Other Chemicals for Concern
We also have other chemicals in the earth's atmosphere that are
increasing. Chlorinated fluorocarbons (CFCs) were invented in the late
1930's, so they 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.
Population, Consumption, and Sustainability
The root cause of these faster-than-normal changes in our global environment
is a rapidly rising global human population with increasing amounts of per
capita
consumption and energy resource use. These
anthropogenic (i.e., human)
demands put strains on natural and managed (agricultural and forest) ecosystems
that raise questions about irreversible change to the global environment that
might jeopardize the ability of future generations to meet their basic
human
needs. Serious questions are raised about the "sustainability" (indefinitely
continuing current practices that deplete resources or degrade the environment)
of our use of energy, agricultural and forestry practices, use of fresh water,
and others.
Global Change Issues
Issues commonly listed under global environmental change include
The US Global Change Research
Program
was established by Presidential
order to coordinate research and related activities in the US to help
provide a well-founded scientific understanding of the Earth system to
ensure the availability of future resources essential for human well-being,
including water, food, fiber, ecosystem, and human health. It provides
the foundation for increasing the skill of predictions of climate change and
climate variations and sponsors research to understand vulnerabilities to
changes in climate, ultraviolet radiation at the Earth's surface, and land
cover. Such scientific knowledge is an essential basis for informed
decision making on environmental issues and to ensure the social and
economic health of future generations. In this course we will examine the
scientific basis for global change, explore the implications of these
changes and discuss some of the political implications of global
change.
Summary
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
planet. 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 is 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?
Course Objectives
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 authoritative 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 and finally, we will learn to use higher-level critical
thinking skills to gain an
understanding of these global problems and the difficultly of finding solutions. A central objective of this
course is to recognize and use such higher-level skills.
Updated atmospheric CO2 datasets and graphs can be obtained here