Global warming
Global
warming
What is the greenhouse effect, and is it affecting our climate?
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 greenhouse effect.
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 data.
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 troposphere.
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) changing?
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).
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.
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
their study.
"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..."
GREENHOUSE WARMING
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."
TEMPERATURE
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."
TROPICAL STORMS
"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
these storms."
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
unchanging climate.
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.
Global
warming
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 Cause
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.
The
Impacts
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 human health
- 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