The greenhouse effekt
The "greenhouse effect" is the
warming of climate that results when the atmosphere traps heat radiating from Earth toward
space. Certain gases in the atmosphere resemble glass in a greenhouse, allowing sunlight
to pass into the "greenhouse," but blocking Earth's heat from escaping into
space. The gases that contribute to the greenhouse effect include water vapor, carbon
dioxide (CO2), methane, nitrous oxides, and chlorofluorocarbons (CFCs).
Life on Earth depends on energy coming from
the Sun. About half the light reaching Earth's atmosphere passes through the air and
clouds to the surface, where it is absorbed and then radiated upward in the form of
infrared heat. About 90 percent of this heat is then absorbed by the greenhouse gases and
radiated back toward the surface, which is warmed to a life-supporting average of 59
dedrees Fahrenheit (F) (15 degrees Celsius (C).
On Earth, human activities are changing the
natural greenhouse. Over the last century the burning of fossil fuels like coal and oil
has increased the concentration of atmospheric CO2. This happens because the coal or oil
burning process combines carbon (C) with oxygen (O2) in the air to make CO2. To a lesser
extent, the clearing of land for agriculture, industry, and other human activities have
increased the concentrations of other greenhouse gases like methane (CH4), and further
increased (CO2).
The consequences of changing the natural
atmospheric greenhouse are difficult to predict, but certain effects seem likely:
* On average, Earth will become warmer.
Some regions may welcome warmer temperatures, but others may not.
* Warmer conditions will probably lead to
more evaporation and precipitation overall, but individual regions will vary, some
becoming wetter and others dryer.
* A stronger greenhouse effect will
probably warm the oceans and partially melt glaciers and other ice, increasing sea level.
Ocean water also will expand if it warms, contributing to further sea level rise.
* Meanwhile, some crops and other plants
may respond favorably to increased atmospheric CO2, growing more vigorously and using
water more efficiently. At the same time, higher temperatures and shifting climate
patterns may change the areas where crops grow best and affect the makeup of natural plant
communities.
INCREASES IN ATMOSPHERIC (CO2)
Human Activities Add to Atmospheric (CO2)
Atmospheric CO2 has increased about 25
percent since the early 1 800s.
Climatologists at NASA's Goddard Space Flight Center,
Greenbelt, Md., estimate the increase since 1958 has been about 10 percent. Currently, the
level of atmospheric C 2 is increasing at a rate of about 0.4 percent a year.
Human beings add CO2 to the atmosphere
mainly by burning fossil fuels like coal and oil. Deforestation is the second major way we
increase atmospheric CO2.
Felled timber releases CO2 as it burns or decays, and disturbed
soils produce CO2 from burned organic matter. Forests give way largely to annual crops
that store CO2 for only a season, or to cities with little vegetation at all.
Natural Processes Affect the Amount of
Atmospheric (CO2)
Human activities add appreciably to
atmospheric CO2, but natural processes are fundamental in regulating the amount of CO2 in
the atmosphere and elsewhere.
Besides the atmosphere, Earth has several
other reservoirs of CO2 including vegetation and the oceans. Through photosynthesis,
plants take up (CO2) and store it in their leaves, fruit, stems, and roots. Oceans
constitute by far the largest reservoir, however.
Atmospheric CO2 dissolves into surface
waters, and over time, ocean circulation carries it to great depths and distant locations.
Eventually, biochemical processes may turn it into shells and sediments that drift toward
the bottom of the oceans.
THE RISE OF ATMOSPHERIC GREEN HOUSE GAS
LEVELS
Because high levels of greenhouse gases may
have a significant effect on climate, vegetation, and other aspects of our lives, it is
important to determine when major increases will occur. The concentration of atmospheric
CO2 will certainly continue to rise so long as fossil fuels remain our principal source of
energy; but pinpointing the rate of future increases is difficult, due mostly to
uncertainties about future human behavior and technological developments.
Predictions become more complicated when
they take other greenhouse gases into account. While increasing CO2 is expected to be the
main factor in enhancing Earth's atmospheric greenhouse, gases like methane, nitrous oxide
and the CFCs almost certainly will contribute to changing climate as well.
How quickly
these gases will increase is also uncertain.
Predictions From the GISS Model
The NASA Goddard Institute for Space
Studies' (GISS) computer model has been used to calculate the temperature increase during
the next 50 years in response to gradual increases in greenhouse gas concentrations. The
simulation shows a change of 35.6 degrees F (2degrees C), which would make Earth warmer
than it is thought to have been at any point in history.
HOW INCREASING ATMOSPHERIC (CO2) MAY AFFECT
CLIMATE
Globally, as the concentration of
atmospheric (CO2) rises, temperatures will increase and other climate characteristics will
probably change as well:
* On average, surface air temperature
around the world will increase. When the climate has adjusted to a doubling of the CO2
concentration, researchers estimate a global average temperature increase of 3 to 8
degrees F (-13.
3 to 16.1 degrees C). The GISS computer model is close to the higher end of
this range.
* Overall, rain and perhaps snow at high
altitudes may increase, but this trend may not appear everywhere.
* Snow cover may recede.
Changes in worldwide averages interest most
of us less, however, than shifts in regional and local climate.
Conditions locally may
differ from global averages in temperature, amounts of rain and snow, frequency or
severity of major storms. Local conditions like these profoundly influence the quality of
our lives, and they certainly affect important activities like agriculture. Farmers need
to know, for example, whether the amounts of soil moisture available for their crops will
increase or decrease and whether they should worry more or less about frosts. In some
regions, concerns for safety and property could require measures to deal with more
frequent flooding, more severe storms, or the like. Scientists know how important these
concerns are. Nevertheless, they cannot yet produce good estimates of future changes in
regional climate because their tools are not yet sophisticated enough to forecast changes
in regional conditions.
IS THE GREENHOUSE EFFECT AT WORK?
To determine the effects produced from
greenhouse gases, scientists look for changes such as warmer weather, warmer ocean
temperatures, and a cooler stratosphere. The ability to predict these effects presents
many difficulties because the Earth's temperature fluctuates for a variety of reasons.
Instruments and techniques used to measure changes have not always been consistent.
Although it is difficult to determine how much of the warming effect is man-made, it is
fairly certain that humans have contributed to the growth of atmospheric CO2. The increase
in CFCs is entirely man-made. The cause of the increase in methane is less well
understood.
There is evidence that average surface air
temperature has increased worldwide by nearly 1 F (-1 7.2 C) since 1850. Given the
increase of about 25 percent in atmospheric CO2 between the early 1800s and the present,
it might be concluded that the greenhouse effect is producing a global warming.
However, there has been little increase in
the last 50 years, which raises questions about whether we really have experienced the
effect of increasing CO2. The pattern of changing global temperatures suggests that there
may be other factors influencing climate. There is also the possibility that the
sensitivity to greenhouse gases is less than what most climate models indicate.
Scientists
feel an increase of 1degree F (-17.2 degrees C) in 140 years is not necessarily outside
the range of natural climate variability.
HOW GREENHOUSE EFFECTS ARE PREDICTED
Goddard scientists use computer models to
simulate the known workings of Earth's environment in response to increases in certain
gases. Computer models divide the planet into hundreds of parts that scientists commonly
refer to as "boxes." Each box is represented by mathematical equations for such
variables as wind, temperature, incoming and outgoing radiation. Each of the boxes are
linked to neighboring boxes so they can respond to changing conditions around them with
changes of their own.
Thus, scientists can enter variables into the model such as
increased levels of certain atmospheric gases to see what changes these cause.
Computer models of Earth's environment have
some problems. For example, scientists do not yet completely understand the relationship
that clouds play in the heating and cooling process of the Earth's atmosphere. Also, the
role of the ocean in this process is not completely understood. However, scientists
continue to use the models while these difficulties are being studied .
FUTURE GREENHOUSE STUDY
Two NASA projects, managed by the Goddard
Space Flight Center, which will contribute to further study of the greenhouse effect, are
the Upper Atmosphere Research Satellite (UARS), which will look at the role of the
stratosphere, and the Earth Observing System (EOS).
UARS, launched in September 1991, is
providing NASA and a broad, international scientific community with comprehensive data
sets on the upper atmosphere's chemistry, energy and dynamics, including data crucial to
expanding our understanding of the ozone layer. It is the spearhead of a long term
international program of space research into global atmospheric change. The UARS program
is designed to carry out the first systematic, detailed satellite study of Earth's upper
atmosphere, establish the comprehensive data base needed for an understanding of
stratospheric ozone depletion, and bring together scientists and governments from around
the world to assess the role of human activities in atmospheric change. The mission will
also lay the foundations for a broader study of upper atmosphere influence on climate and
climatic variations.
EOS is a planned NASA program for observing
Earth from space using a new generation of spacecraft called polar orbiting platforms. EOS
will collect data to help us understand the processes which control our global environment
and to determine which natural events can be predicted, what things humans influence, and
the consequences of these activities.
| Anmerkungen: |
| impressum | datenschutz
© Copyright Artikelpedia.com
