This view of the Moon in orbit about the Earth was taken
from a distance of about 6.2 million kilometers (3.9 million miles), on
December 16, 1990 from the Galileo spaceship. The Moon is moving from left
to right. (Courtesy NASA/JPL)
To understand the environmental systems of the world around us, it's necessary to consider and understand the relationships that exist between the moving earth and the sun. For it is these relationships that largely determine why different places on earth receive different amounts of solar energy and why any given place on earth receives different amounts of solar energy at different times of the year. These spatial and temporal variations in solar energy receipts are the underlying cause of the different seasons of the year and the various types of weather that characterize different places on earth.
is
clockwise. At all times, the half of the earth facing away from the sun
is in darkness and the half facing toward the sun is illuminated by rays
of incoming solar radiation (or "insolation" for short). The boundary that
separates the dark half from the light half of the globe is called the
circle
of illumination. Along the circle of illumination, solar rays barely
skim the edge of the globe; these rays are called tangential rays
(like the tangent of a circle, which is a straight line that just touches
a circle's perimeter). In contrast to tangential rays, there are solar
rays that strike the earth at an angle of 90 degrees; these are called
direct
rays. Or, to put it more precisely, direct rays are solar rays that
strike a horizon plane at a 90 degree angle (a horizon plane is
a plane that is tangent to the planet at a particular location). The place
where direct rays strike the earth is called the subsolar point,
and the latitude of the subsolar point is called declination. On
the figure shown here, the subsolar point is on the Tropic of Cancer (23.5
degrees north latitude); therefore, declination is at 23.5 degrees north
latitude. The rays that strike the earth between the direct and tangential
rays are called oblique rays. The angle between a horizon plane
and an oblique ray is always greater than zero and less than 90 degrees.
Because of axial tilt and axial parallelism, earth's axis is constantly changing its orientation relative to the sun: sometimes the axis tilts towards the sun, sometimes it tilts away, and sometimes it does neither. These changes in orientation between the axis and the sun are the basic cause of the four seasons. The Northern Hemisphere's summer occurs when the North Pole is tilted toward the sun, causing the sun to be high in the sky and daylengths to be long. Winter occurs when the North Pole is tilted away from the sun, causing the sun to be low in the sky and daylengths to be short. Between winter and summer is the transitional season of spring, and between summer and winter is the transitional season of fall. During these seasons, earth's axis is neither tilted toward nor away from the sun, so that the sun rises to intermediate heights in the sky and days are intermediate in length. It is important to realize that when the North Pole is tilted toward (or away from) the sun, the South Pole is tilted away from (or toward) the sun. Thus, when it's summer in the Northern Hemisphere, it's winter in the Southern Hemisphere, and when it's winter in the Northern Hemisphere, it's summer in the Southern Hemisphere.
An important day during the solar year is the day when a pole (either
the North or South Pole) is tilted directly toward the sun. This day is
called the summer solstice, and during the summer solstice, the
sun is higher in the sky and daylength is longer than on any other day
of the year. The summer solstice marks the beginning of summer. Another
important day is the day when a pole is tilted directly away from the sun.
This day is called the winter solstice, and during the winter solstice,
the sun is lower in the sky and daylength is shorter than on any other
day of the year. The winter solstice marks the beginning of winter. Keep
in mind that the summer solstice in the Northern Hemisphere and the winter
solstice in the Southern Hemisphere occur on the same day (June 20 or 21);
furthermore, the winter solstice in the Northern Hemisphere and the summer
solstice in the Southern Hemisphere occur on the same day (Dec. 21 or 22).
Exactly in between the extremes of the solstices are two other important
days on which earth's poles are inclined neither toward nor away from the
sun.. On these days, the direct rays of the sun fall on the equator and,
with the exception of the poles, every place on earth experiences twelve
hours of light and twelve hours of darkness. For this reason these occurrences
are called equinoxes, which mark the beginning of spring and fall. Spring
begins with the spring or vernal equinox (around March 21 in the
Northern Hemisphere, Sept. 23 in the Southern Hemisphere) and fall begins
with the fall or autumnal equinox (Sept. 23 in the Northern Hemisphere,
March 21 in the Southern Hemisphere).
Our ancestors marked the seasons by watching the sun move across the horizon as the seasons changed. In the autumn you can watch sunrise (or sunset) and mark its location on the horizon. Every day the spot will be a little farther south as we head toward winter. The motion is easily seen in a week's time. As we near the winter solstice, the apparent motion gets slower and slower and then, for a few days before and after the solstice, it seems to stop. Following these days of no apparent movement, sunrise then heads back north. This pause in the movement of sunrise is the source of the name solstice, which means "sun stands still."
(image source:
USA
Today)
On
average, about 30% of the incoming shortwave radiation from the sun (insolation)
is reflected by the atmosphere back to space, and another 20% is absorbed
in the atmosphere by various gases and clouds. This leaves about half of
the solar radiation to pass through the atmosphere to the earth's surface,
where it is mostly absorbed. The earth's surface then radiates longwave
radiation (primarily infrared radiation), most of which is absorbed in
the atmosphere. Thus, the atmosphere absorbs much more longwave radiation
than shortwave radiation. So, for the most part, the atmosphere is not
directly heated by radiation from the sun. Instead, it is heated primarily
from below, by longwave radiation emitted by the earth's surface. Most
of the radiation emitted by the earth's surface is absorbed by a few important
trace gases and by clouds, which warms the atmosphere. The atmosphere then
emits longwave radiation toward outer space and back toward the surface.
The term the greenhouse effect refers to the process whereby longwave
radiation from the surface is absorbed in the atmosphere and longwave radiation
from the atmosphere is emitted back toward the surface. The greenhouse
effect is largely due to the absorption and emission of longwave radiation
by clouds and by two gases, carbon dioxide and water vapor, which are present
in only small amounts. Other gases present in even smaller amounts also
play important roles in absorbing and radiating longwave radiation; these
include methane, nitrous oxide, ozone, and chlorofluorocarbons (CFCs).
Together, clouds and the so-called greenhouse gases retard the loss
of heat from earth to space when no insolation is being received (i.e.,
during nighttime). In addition, clouds and the greenhouse gases radiate
a considerable amount of energy to the earth's surface -- in fact, the
atmosphere radiates more energy (as longwave radiation) to the earth's
surface than the sun does (as shortwave radiation). Thus, the earth's surface
is a lot warmer than it would be were there no clouds or greenhouse gases.
What this means is that, because of the greenhouse effect, temperatures
on earth are maintained at levels that make life possible. If our atmosphere
contained no greenhouse gases or clouds, earth's average surface temperature
would be about 33 degrees C colder than it presently is. So without the
greenhouse effect, earth would be way too cold for life (as we know it)
to survive.
It
should be pointed out that some longwave radiant energy always escapes
to space. In fact, on average from year to year, the total amount of longwave
radiation emitted to space by the earth-atmosphere system equals the amount
of insolation that gets absorbed in the atmosphere and at the surface.
Because the amount of insolation absorbed by the earth and its atmosphere
equals the amount of longwave radiation emitted by the earth and its atmosphere
to space, the average temperature of earth is not changing from year to
year. In other words, the earth is in a state of thermal equilibrium.
B. The human-enhanced greenhouse effect
Actually, there is considerable doubt that thermal equilibrium presently
prevails in the earth-atmosphere system, or, if it does, that it will continue
into the future. The reason for this doubt is based on the fact that human
activities are altering the composition of the atmosphere in ways that
will likely enhance the greenhouse effect, and thereby cause the earth
to become a significantly warmer place. Several studies have shown that
humans have already altered the composition of the atmosphere with respect
to its greenhouse gases; and some studies have indicated that the effects
of this alteration on temperatures is already notable. The subject of a
human-enhanced greenhouse effect and global warming is, however, a controversial
one and research on this issue continues. For example, most scientists
believe human activities are causing the atmosphere to become warmer, and
many believe that human enhancement of the greenhouse effect is the greatest
present-day threat to the world's environmental systems. However, some
scientists believe human activities are actually causing the atmosphere
to become cooler (by making it dirtier with particulate matter that reflects
insolation), while others believe that humans are having no impact at all
(because negative feedback mechanisms in the atmosphere tend to keep it
in a state of equilibrium).
Nevertheless, for the most part, few scientists disagree that humans
are adding greenhouse gases to the atmosphere. The main greenhouse gases
that humans are producing (the anthropogenic greenhouse gases) include
carbon dioxide, methane, and chlorofluorocarbons (CFCs) (ozone and nitrous
oxide are other human-produced greenhouse gases). The pie chart shown here
illustrates the predicted contribution that each of these gases will have
on future human-induced global warming. As you can see, increasing amounts
of carbon dioxide are expected to have far and away the greatest impact.
However, the impacts of other gases, most notably CFCs and methane, are
also expected to be quite substantial.
Concentrations of the major greenhouse gases have actually been increasing
in the atmosphere for some time, primarily as a result of industrialization
and increasing global populations. Increases in greenhouse gases have been
especially prominent during this century, as is apparent in the following
three illustrations prepared by the U.S. Environmental Protection Agency
based on data from the Intergovernmental Panel on Climate Change (IPCC),
the scientific advisory body created by the United Nations to analyze the
science of global climate change. For the three gases illustrated here,
carbon dioxide has undergone the most noteworthy increase in concentration.
The reason for this is that the largest source of anthropogenic greenhouse
gases is the burning of fossil fuels (e.g., gasoline and diesel fuel in
cars, trucks, buses, and train locomotives; coal in coal-fired electricity
generating plants; oil in home furnaces), which produces immense quantities
of carbon dioxide.
Most scientists agree that, as concentrations of greenhouse gases increase, the proportion of longwave radiation emitted by the surface and absorbed in the atmosphere will also increase, causing the atmosphere to become warmer. Some feel that such human-induced warming is already apparent in the records of temperature that span the last 100 to 150 years. The following graph shows a reconstructed record of annual global average surface temperatures prepared by atmospheric scientists at the U.S. National Aeronautics and Space Administration. This graph indicates that since about 1880, global average temperatures rose slightly for about sixty years, then cooled for the next twenty-five, and then began a warming trend that has continued to the present.
CONTACT: Patricia Viets, NOAA FOR IMMEDIATE RELEASE Dane Konop, NOAA 1/8/98
1997 WARMEST YEAR OF CENTURY, NOAA REPORTS
1997 was the warmest year of this century, based on land and ocean surface temperature data, reports a team of scientists from the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N. C.
Led by the center's Senior Scientist Tom Karl, the team analyzed temperatures from around the globe during the years 1900 to 1997 and back to 1880 for land areas. For 1997, land and ocean temperatures averaged three-quarters of a degree Fahrenheit above normal. (Normal is defined by the mean temperature, 61.7 degrees F, for the 30-years 1961-90.) The 1997 figure exceeds the previous record warm year, 1990, by 0.15 degrees Fahrenheit.
The record-breaking warm conditions of 1997 continues the pattern of very warm global temperatures. Nine of the past eleven years have been the warmest on record.
"Land temperatures did not break the previous record set in 1990, but 1997 was one of the five warmest years since 1880," said Karl. Including 1997, the top ten warmest years over the land have all occurred since 1981, and the warmest five years all since 1990. Land temperatures for 1997 averaged three-quarters of a degree above normal, falling short of the 1990 record by one-quarter of a degree. Ocean temperatures during 1997 also averaged three-quarters of a degree above normal, which makes it the warmest year on record, exceeding the previous record warm years of 1987 and 1995 by 0.3 of a degree Fahrenheit.
With the new data factored in, global temperature warming trends now exceed 1.0 degree Fahrenheit per 100 years, with land temperatures warming at a somewhat faster rate. "It is likely that the sustained trend toward increasingly warmer global temperatures is related to anthropogenic increases in greenhouse gases," Karl said.
(Link to image is http://www.noaa.gov/public-affairs/globalsurftemp.JPG.)
A variety of human activities are affecting the levels of greenhouse gases in the atmosphere. The following graph summarizes the contributions that different general types of activities are making to an enhanced greenhouse effect. Note in particular that the effect of energy use (primarily the burning of fossil fuels) is greater than all the other activities combined.
The primary greenhouse gas produced by energy use, as mentioned above, is carbon dioxide. In the following diagram, which illustrates the global annual carbon cycle, you can see that there are many sources of atmospheric carbon dioxide. You should observe that natural sources of carbon dioxide are actually much greater than anthropogenic sources. In addition, there are many natural "sinks" on earth that serve to remove carbon from the atmosphere (e.g., vegetation is a natural sink that utilizes atmospheric carbon in the process of photosynthesis). However, except for a small amount of carbon dioxide that dissolves in the world's oceans, carbon resides in earth-bound sinks only temporarily before it is returned to the atmosphere. Anthropogenic emissions of carbon dioxide have destroyed the natural balance that existed between carbon sources and carbon sinks by adding more carbon to the atmosphere than the earth's natural sinks can remove. For this reason, humans are causing atmospheric concentrations of carbon dioxide to increase each year by about 0.4%
In ways that are similar to the carbon cycle, other greenhouse gases also cycle between the atmosphere and sinks on the earth's surface. The following table summarizes, in the form of a budget, the annual cycling of methane. Note that, unlike carbon dioxide, anthropogenic sources produce most of the methane that moves from the surface to the atmosphere each year. Natural sinks remove all but about 6% of the methane added to the atmosphere each year. As a result, concentrations of methane are increasing by about 0.6% each year.
All countries on earth are, to some extent, responsible for emitting anthropogenic greenhouse gases into the atmosphere. But there are a select few countries that produce substantially more than all the rest. Of those countries that emit large quantities of anthropogenic greenhouse gases, the United States is by far the biggest producer, as is indicated in the following graph.
The United States especially exceeds all other countries in the emission of the most significant greenhouse gas -- carbon dioxide.
There is evidence, however, that the gap between the United States and other countries with respect to the production of greenhouse gases is closing. This is particularly true for lesser developed nations that are expanding their utilization of fossil fuels both to promote industrialization and to enhance living standards. The increasing consumption of fossil fuels in some of the world's countries is illustrated in the following graph. Between 1970 and 1989, use of fossil fuels in the U.S. increased by only 3%. In contrast, fossil fuel use in India and China increased over the same period by 150% and more.
While most scientists agree that human-induced increases in greenhouse gas concentrations will lead to global warming, there is a great deal of uncertainty as to how much global warming will likely occur. Predictions of warmer conditions in the future are based on computer models of the atmosphere, which are far from perfect in their ability to accurately simulate atmospheric processes. There are several reasons for this. Firstly, scientists have been systematically observing and studying the atmosphere for only a relatively short time, so computer modelers have only limited knowledge of the many factors that may affect the earth's climate in the long run. Secondly, the earth-atmosphere system is extremely large and complex, so it is virtually impossible to include in a computer model all the factors that could affect the system's response to increasing concentrations of greenhouse gases. The lack of accuracy of computer model predictions is indicated by the graph to the left, which shows actual global average temperatures for the period 1900 to 1989 and "predictions" (or "retrodictions") of global average temperatures made backward in time by various computer models of the earth-atmosphere system. During this period, there were times when the computer model retrodiction results were well above the actual observed temperatures, and times when the computer model results were well below the actual observed temperatures, indicating the computer models inability to simulate the year-to-year variability that characterizes temperatures in the real earth-atmosphere system.
Although imperfect, the retrodictions produced by computer models for average global temperatures for the 1900-1989 period do capture the general warming trend suggested by actual values of recorded temperatures. So while imperfect, computer models appear to do a good job at predicting general trends. And the general trend predicted for the future (specifically, for the period 1990 to 2050) is one of an increasingly warm earth. At the present time, predictions of average global temperature in the year 2050 range from 0.5 degrees warmer to 4.5 degrees warmer than the present-day average. While the lower end of this predicted range does not seem a very large increase, the upper end of the range is undeniably significant. Indeed, if global warming occurs as predicted, many of earth's natural environmental systems will be severely affected.
Do Exercise B
Links to global warming on the WWW:
Summary of Forum on Global Change Modeling
U.S. Environmental Protection Agency Global Warming Site
U.S. Global Change Research Information Office
World Resources Institute Climate Change Site
Union of Concerned Scientists Global Warming Site
