What is the greenhouse effect, and is it affecting our
The greenhouse effect is unquestionably real, and is
essential for life on Earth. It is the result of heat absorption by certain
gases in the atmosphere (called greenhouse gases because they trap heat) and
re-radiation downward of a part of that heat. Water vapor is the most important
greenhouse gas, followed by carbon dioxide and other trace gases. Without a
natural greenhouse effect, the temperature of the Earth would be about zero
degrees F (-18°C) instead of its present 57°F (14°C). However, the concern is
not with the fact that we have a greenhouse effect, but it is with the question
regarding whether human activities are leading to an enhancement of the
Are greenhouse gases increasing?
Human activity has been increasing the concentration of
greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of
coal, oil, and gas; plus a few other trace gases). There is no scientific
debate on this point. Pre-industrial levels of carbon dioxide (prior to the
start of the Industrial Revolution) were about 280 parts per million by volume
(ppmv), and current levels are about 370 ppmv. According to the IPCC
"business as usual" scenario of carbon dioxide increase (IS92a) in
the 21st century, we would expect to see a doubling of carbon dioxide over
pre-industrial levels around the year 2065.
Is the climate warming?
Global surface temperatures have increased about 0.6°C (plus
or minus 0.2°C) since the late-19th century, and about one half degree F (0.2
to 0.3°C) over the past 25 years (the period with the most credible data). The
warming has not been globally uniform. Some areas (including parts of the
southeastern U.S.) have cooled. The recent warmth has been greatest over N.
America and Eurasia between 40 and 70°N. Warming, assisted by the record El
Niсo of 1997-1998, has continued right up to the present.
Linear trends can vary greatly depending on the period over
which they are computed. Temperature trends in the lower troposphere (between
about 2,500 and 18,000 ft.) from 1979 to the present, the period for which
Satellite Microwave Sounding Unit data exist, are small and may be
unrepresentative of longer term trends and trends closer to the surface.
Furthermore, there are small unresolved differences between radiosonde and
satellite observations of tropospheric temperatures, though both data sources
show slight warming trends. If one calculates trends beginning with the
commencement of radiosonde data in the 1950s, there is a slight greater warming
in the record due to increases in the 1970s. There are statistical and physical
reasons (e.g., short record lengths, the transient differential effects of
volcanic activity and El Niсo, and boundary layer effects) for expecting
differences between recent trends in surface and lower tropospheric
temperatures, but the exact causes for the differences are still under
investigation (see National Research Council report "Reconciling
Observations of Global Temperature Change").
An enhanced greenhouse effect is expected to cause cooling
in higher parts of the atmosphere because the increased "blanketing"
effect in the lower atmosphere holds in more heat. Cooling of the lower
stratosphere (about 30-35,000ft.) since 1979 is shown by both satellite
Microwave Sounding Unit and radiosonde data, but is larger in the radiosonde
There has been a general, but not global, tendency toward
reduced diurnal temperature range (the difference between high and low daily
temperatures) over about 50% of the global land mass since the middle of the
20th century. Cloud cover has increased in many of the areas with reduced
diurnal temperature range.
Relatively cool surface and tropospheric temperatures, and a
relatively warmer lower stratosphere, were observed in 1992 and 1993, following
the 1991 eruption of Mt. Pinatubo. The warming reappeared in 1994. A dramatic
global warming, at least partly associated with the record El Niсo, took place
in 1998. This warming episode is reflected from the surface to the top of the
Indirect indicators of warming such as borehole
temperatures, snow cover, and glacier recession data, are in substantial
agreement with the more direct indicators of recent warmth.
Arctic sea ice has decreased since 1973, when satellite
measurements began but Antarctic sea ice may have increased slightly.
Are El Ninos related to Global Warming?
El Ninos are not caused by global warming. Clear evidence
exists from a variety of sources (including archaeological studies) that El
Ninos have been present for hundreds, and some indicators suggest maybe
millions, of years. However, it has been hypothesized that warmer global sea
surface temperatures can enhance the El Niсo phenomenon, and it is also true
that El Ninos have been more frequent and intense in recent decades.Recent
climate model results that simulate the 21st century with increased greenhouse
gases (using the IPCC IS92a greenhouse gas increase scenario) suggest that El
Niсos are likely to become more common in the future.
Is the hydrological cycle (evaporation and precipitation)
There has probably been only a small (1%) increase in global
precipitation over land during the 20th century. Precipitation has increased
over land in high latitudes of the northern hemisphere, especially during the
cold season, concomitant with temperature increases. A step-like decrease of
precipitation occurred after the 1960s between the equator and about 35 degrees
latitude, from Africa to Indonesia, as temperatures increased. These changes
are consistent with observed changes in streamflow, lake levels, and soil
moisture (where data are available and have been analyzed).
Northern Hemisphere snow cover extent has consistently
remained below average since 1987.
Pan evaporation, a measure of potential evaporation, has
decreased since 1951 over much of the former Soviet Union and the U.S. However,
actual evaporation, which is dependant on available water, may have increased.
Evaporation appears to have increased over the tropical oceans (although not
everywhere). The evidence suggests an increase of atmospheric water vapor in
the tropics, at least since 1973.
In general, cloud amount has increased both over land and
ocean in recent decades. Over the ocean, increases in convective and middle-
and high-level clouds have been reported.
Is the atmospheric/oceanic circulation changing?
A rather abrupt change in the El Niсo - Southern Oscillation
behavior occurred around 1976/77 and the new regime has persisted. There have
been relatively more frequent El Niсo episodes. This behavior is highly unusual
in the last 120 years (the period of instrumental record). Changes in
precipitation over the tropical Pacific are related to this change in the El
Niсo - Southern Oscillation, which has also affected the pattern and magnitude
of surface temperatures.
Is the climate becoming more variable or extreme?
On a global scale there is little evidence of sustained
trends in climate variability or extremes. This perhaps reflects inadequate
data and a dearth of analyses. However, on regional scales, there is clear
evidence of changes in variability or extremes.
In areas where a drought usually accompanies an El Niсo,
droughts have been more frequent in recent years. Other than these areas and
the few areas with longer term trends to lower rainfall (e.g., the Sahel),
little evidence is available of changes in drought frequency or intensity.
In some areas there is evidence of increases in the
intensity of extreme rainfall events, but no clear global pattern has emerged.
Despite the occurrence in recent years of several regional-scale extreme floods
there is no evidence of wide-spread changes in flood frequency. This may
reflect the dearth of studies, definition problems, and/or difficulties in
distinguishing the results of land use changes from meteorological effects.
There is some evidence of recent (since 1988) increases in
extreme extratropical cyclones over the North Atlantic. Intense tropical
cyclone activity in the Atlantic appears to have decreased over the past few
decades. Elsewhere, changes in observing systems confound the detection of
trends in the intensity or frequency of extreme synoptic systems.
There has been a clear trend to fewer extremely low minimum
temperatures in several widely-separated areas in recent decades. Widespread
significant changes in extreme high temperature events have not been observed.
There is some indication of a decrease in day-to-day
temperature variability in recent decades.
How important are these changes in a longer-term context?
For the Northern Hemisphere summer temperature, recent
decades appear to be the warmest since at least about 1000AD, and the warming
since the late 19th century is unprecedented over the last 1000 years. Older
data are insufficient to provide reliable hemispheric temperature estimates.
Ice core data suggest that the 20th century has been warm in many parts of the
globe, but also that the significance of the warming varies geographically,
when viewed in the context of climate variations of the last millennium.
Large and rapid climatic changes affecting the atmospheric
and oceanic circulation and temperature, and the hydrological cycle, occurred
during the last ice age and during the transition towards the present Holocene
period (which began about 10,000 years ago). Based on the incomplete evidence
available, the projected change of 3 to 7°F (1.5 - 4°C) over the next century
would be unprecedented in comparison with the best available records from the
last several thousand years.
Is sea level rising?
Global mean sea level has been rising at an average rate of
1 to 2 mm/year over the past 100 years, which is significantly larger than the
rate averaged over the last thousand years. Projected increase for the 21st
century is about 0.5 meter, but estimates range widely.
Can the observed changes be explained by natural
variability, including changes in solar output?
Some changes, particularly part of the pre-1960 temperature
record, show some relationship with solar output, but the more recent warm era
is not well correlated. The exact magnitude of purely natural global mean
temperature variance is not known precisely, but model experiments excluding
solar variation indicate that it is likely less than the variability observed
during this century.
Global Warming or Global Cooling the Threat for the Future?
Has the climate of the United States changed significantly
during the century that is about to end? In what ways and by how much? Have
national trends emerged that agree--or perhaps disagree--with what is expected
from projections of global greenhouse warming? These are questions addressed in
a report entitled "Trends in U.S. Climate during the Twentieth
Century," by Thomas R. Karl, Richard W. Knight, David R. Easterling,
Robert G. Quayle who serve on the scientific staff of the National Oceanic and
Atmospheric Administration's National Climatic Data Center (NCDC), in
Asheville, North Carolina. Thomas "The challenge to the climatologist is
to separate any meaningful signals from ever-present noise, and to discern, if
possible, whether there is indeed at work the sometimes slow and subtle hand of
significant change. The second task, which is even harder, is to identify,
unequivocally, the cause," according to the scientists was the focus of
"Before such questions can be answered, we need to
remind ourselves that 'climate', as it is defined for a specific region and
time, includes more than the simple average of weather conditions. Either
random events or long-term persistent change, or more often combinations of
them, can bring about significant swings in a variety of climate indicators
from one time period to the next. Examples include a year dominated by severe
drought and the next excessively wet; a series of bitterly cold winters
followed by winters more mild; one scorching summer preceded by a summer
pleasantly warm; years with numerous severe storms followed by years with few
severe storms. The temptation at each time and place is often to attribute any
of these temporal and sometimes local variations to a wider and more pervasive
change in climate..."
In their assessment they noted that the so-called
"greenhouse" gases "have all been markedly increasing in amount
since about the time of the industrial revolution, that began in earnest some
150 years ago. The largest and best-known contributor is carbon dioxide,
originating principally from the burning of wood and coal and petroleum
derivatives. However, other climatic trends include "changes in the
composition of the atmosphere in ways that act to cool the surface temperature.
This includes the anthropogenic decrease of stratospheric ozone, and an
increase in anthropogenic microscopic sulfate particles, often readily apparent
during the warm season as smog. The effect of these additional atmospheric
constituents on global climate is less certain than that of the better known
greenhouse gases, but models suggest that in some areas they may have already
acted to significantly retard greenhouse warming. It is important to note,
however, that the global-scale warming predicted in climate modeling
experiments from future greenhouse gas increases is substantially larger on a
global average than the regional cooling expected from these other sources.
Measurements of past and current levels of carbon dioxide
and other greenhouse gases indicate that we should have already increased the
global greenhouse effect by man-made, or anthropogenic additions, by nearly 40%
in the last 150 years. If these changes were the only process of importance,
then the same mathematical climate models suggest that the average global
surface temperature should have risen by about 1° C during this time. Available
climate data suggest that the mean global temperature has indeed risen, but
unsteadily and by only about half that amount.
"Confounding any search for anthropogenic effects are
the natural changes and variations of climate that will constantly add to or
subtract from the expected signal. Examples include changes in upper
atmospheric steering winds (commonly known as the jet stream) due to
ocean-atmosphere interactions; changes in the circulation of the ocean that can
influence air temperatures; effects of major volcanic eruptions; feedbacks from
changes in the land surface, as in soil moisture, snow cover, and plant cover;
and changes in the energy received from the Sun.
Precipitation and drought
Another factor in the climatic equation is precipitation and
drought. Studies indicate that, "since about 1970 precipitation has tended
to remain above the twentieth century mean, averaging about 5% higher than in
the previous 70 years. Such an increase hints at a change in climate.
Statistical analysis suggests that the change is unusual, but there is still
about a 10% chance that such a change could arise from a stable or
quasi-stationary climate without any real long-term changes."
While during the 1930's there was a sharp rise in
temperature, there was a modest cooling trend from the 1950's to the 1970 when
the temperature began to rise again. There has been a rise in temperature since
the 1970's . The report states, " A straightforward statistical average of
mean temperatures across the U.S. gives evidence of a rise through the century
of about 0.3 to 0.4° C (0.6 to 0.8° F), although so crude a characterization of
mean temperature change in the U.S. would be indeed a gross oversimplification."
"The increase in annual temperatures after the 1970s is
mainly the result of significant increases of temperature during the first six
months of the year (winter and spring). Temperatures during summer and autumn
have changed little after dropping from conditions of the warm 1930s. Unusually
high precipitation and cloud amount tend to cool the air, especially during the
second half of the year. It is rare to find much above normal precipitation and
cloud amount during these two seasons when temperatures are higher than normal.
"On a regional basis the West contributes most to the
increase of annual average nation-wide temperatures. As with drought and
excessive moisture, portions of the country can be extremely cold at the same
time that others are unusually warm, leading to an average national temperature
that is near-normal. Similarly, abnormally high daytime maximum temperatures
can occur while nighttime temperatures remain below normal, or vice-versa,
although these are not usually the case."
"Changes and variations of destructive storms are of
particular interest because of their socio-economic and biophysical impact.
Reliable records of the number and intensity of tropical hurricanes that reach
the U.S. go back to at least 1900.Based on a commonly used classification of
hurricane intensity, the studies indicates that the frequency of these violent
storms that make landfall in the U.S. has been relatively low over the past few
decades, as compared to the middle of the century. The decline is reflected in
both the total number of hurricanes making landfall in the U.S. and in the
occurrence of more destructive storms. It is difficult to discern any long-term
trend however, since the frequency of hurricanes was also low in the early part
of the century. Furthermore, recent studies indicate that even if significant
greenhouse induced warming were to occur, it is doubtful whether increases in
tropical storms would be detectable due to the large natural variability in
Changes in circulation
Another factor the climatologists have studied are changes
in circulation over the past few decades. Since the winter of 1976-77, the
sea-surface temperatures in the central and eastern equatorial Pacific have
remained anomalously warm. The report states:"Such events have been
directly linked to increased precipitation in the southeastern U.S. and warmer
than normal temperatures in the Pacific Northwest. During these same years a
large-scale redistribution of atmospheric mass has taken place in the North
Pacific, associated with a change of the upper-level steering winds over the
North Pacific and North America. El Niñ o events (and their opposition
phases, La Niñ a events) have been quantitatively linked to the 1988
drought, to increased precipitation in the South, and to other abnormal
temperature conditions in the U.S. Variations in the circulation of the North
Atlantic Ocean have also directly influenced the eastern U.S. climate in the
form of stronger than normal winds over these regions that seem to oscillate on
decadal time-scales. Such oscillations have been linked to colder than normal
temperatures in the region."
Climate change indices
"Most readers will by now agree that it is difficult
to draw a simple picture that summarizes the many parameters and
multidimensional aspects of observed climate change and variability, no matter
how complete the record. One approach toward simplification might be to
consider only long-term measurements of a few near-surface conditions: temperature
and precipitation, for example, are two primary elements of climate that affect
many aspects of our lives. But neither tells the whole story.
"Several indicators stand out most conspicuously in the
picture of surface climate variations and changes in the U.S. over the past
century. These include the rather steady increase in precipitation derived from
extreme 1-day precipitation events; the systematic decrease in the day-to-day
variations of temperature; and the increased frequency of days with
precipitation. Trends in other indicators of climate change are now neither
sufficiently large nor persistent enough to be considered as strongly
suggestive of systematic change, even though it remains a likely explanation.
These include the increase of total precipitation and the related increase in
cloud amount, as well as an overall increase in mean temperature. The area of
the country that has experienced an increase in mean temperature has risen
while the proportion of the country with much below normal mean minimum
temperatures has decreased. Many of these indicators appear to have undergone
significant change during the late 1970s and have more or less remained at
these levels to the present. In contrast, other surface climate change indicators
(such as the frequency of tropical cyclones) reflect the kind of climatic
variability that is completely consistent with the premise of a stable or
The increase in temperature across the U.S. in this century
is slightly smaller, but of comparable magnitude to the increase of temperature
that has characterized the world as a whole. The increase in minimum
temperature and the related increase in area affected by much above normal
minimum temperatures are also found in many other countries of the northern
hemisphere. Worldwide precipitation over land has changed little through the
twentieth century; increases noted in high latitudes have been balanced by
low-latitude decreases. By comparison, the change in precipitation in the U.S. is
still relatively moderate compared to some of the increases and decreases at
other latitudes. Decreases in the day-to-day differences of temperature
observed in the U.S. are also apparent in China and Russia, the only other
large countries analyzed as of this date. The persistent increase in the
proportion of precipitation derived from extremely heavy precipitation has not
been detected in these other countries.
Introduction in Global warming
“Global warming” has been introduced by the scientific
community and the media as the term that encompasses all potential changes in
climate that result from higher average global temperatures. Hundreds of
scientists from many different countries are working to understand global
warming and have come to a consensus on several important aspects. In general,
Global warming will produce far more profound climatic changes than simply a
rise in global temperature.
A recent study by an international panel of scientists suggested that if trends
in current emissions of greenhouse gases and aerosols continue, the globe may
warm by an average of 2°C by the year 2100. The average rate of warming would
probably be greater than any seen in the last 10,000 years
An analysis of temperature records shows that the Earth has warmed an average
of 0.5°C over the past 100 years. This is consistent with predictions of global
warming due to an enhanced greenhouse effect and increased aerosols. Yet, it
could also be within acceptable limits for natural temperature variation. The
twelve warmest years of the twentieth century have occurred since 1980. The
Earth’s warmest years since 1861 have been: 1981, 1983, 1987, 1988, 1989, 1990,
1991, 1994, 1995, 1996, 1997 and 1998. 1997 and 1998 were the two warmest years
recorded during that period. This lends support to the assumption that the
Earth’s climate is warming. However, it may take another decade of continued
increases in global temperatures to provide conclusive evidence that the
world’s climate is warming as a result of the enhanced greenhouse effect.
Global surface air temperature in 1997 was warmer than any previous year this
century, marginally exceeding the temperature of 1995. Part of the current
global warmth is associated with the tropical El NiЯo, without which a record
global temperature would probably not have occurred.
Global surface temperatures in 1998 set a new record for the period of
instrumental measurements, report NASA/GISS researchers who analyzed data
collected from several thousand meteorological stations around the world. The
global temperature exceeded that of the previous record year, by such a wide
margin that the 1998 calendar year is certain to also set a new record. The
United States experienced in 1998 its warmest year in the past several decades.
As for the Russia, global surface air temperatures in 1997-98 were not warmer
than previous years.
Until recently, researchers were uncertain whether Climate
developments reflected natural variations in the Earth, or whether in fact
human activities contributed to the warming. The latest observed data reveals
some striking trends:
- All 10 of the warmest years on record have occurred in the last 15 years.
- The 1990s have already been warmer than the 1980s - the warmest decade on
record - by almost 0.2°F (0.1°C), according to the Goddard Institute of Space Studies.
- The global average surface temperature has risen 0.5°-1.1°F (0.3°-0.6°C)
since reliable records began in the second half of the 19th century.
In 1995, scientists with the Intergovernmental Panel on Climate Change - the
authoritative international body charged with studying this issue-reached a
conclusion in the Second Assessment Report, which
summarizes the current state of scientific knowledge on global warming, also
called climate change.
For the first time ever, the Panel concluded that the observed increase in
global average temperature over the last century "is unlikely to be
entirely natural in origin" and that "the balance of evidence
suggests that there is a discernible human influence on global climate."
The Earth's climate is the result of extremely complex
interactions among the atmosphere, the oceans, the land masses, and living
organisms, which are all warmed daily by the sun's energy. This heat would
radiate back into space if not for the atmosphere, which relies on a delicate
balance of heat-trapping gases - including water vapor, carbon dioxide, nitrous
oxide, and methane - to act as a natural "greenhouse," keeping in
just the right amount of the sun's energy to support life.
For the past 150 years, though, the atmospheric concentrations of these gases,
particularly carbon dioxide, have been rising. As a result, more heat is being
trapped than previously, which in turn is causing the global temperature to
rise. Climate scientists have linked the increased levels of heat-trapping
gases in the atmosphere to human activities, in particular the burning of
fossil fuels (coal, oil, and natural gas for heating and electricity; gasoline
for transportation), deforestation, cattle ranching, and rice farming.
But Global Warming has received much press in the past decade. There are many
questions like these ones. Could the earth’s climate really heat up? What are
the causes if such a warming occurs? Is global warming a theory and thrue or
false theory at that?
These questions and more are what climate scientists are asking themselves
daily. So, there are two sides to every story and both are discussed in the media.
As the Earth's climate is the result of extremely complex
interactions, scientists still cannot predict the exact impact on the earth's
climate of these rising levels of heat-trapping gases over the next century.
But there is striking agreement among most climate scientists about what is
likely to occur. Poureful climate models suggest that the planet will warm over
the next century at a more rapid rate than ever before recorded. The current
best estimate is that if carbon dioxide concentrations double over
preindustrial levels, global average surface temperatures will rise between
1.8° and 6.3°F (between 1° and 3.5°C). According to the scientific possible
scenarios, an atmospheric doubling of carbon dioxide could occur as early as 2050.
Future impacts from this kind of warming will most likely include: - damage to
- severe stress on forests, wetlands, and other natural habitats
- dislocation of agriculture and commerce
- expansion of the earth's deserts
- melting of polar ice caps and consequent rise in the sea level
- more extreme weather events
The Future and Global Warming Policy
During the 1980-90s, evidence mounted that increased
atmospheric concentrations of heat-trapping gases could cause significant
disruptions of the earth's climate systems. These discoveries moved the global
warming issue into the arena of public policy
© Реферат плюс