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
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 Niс o events (and their opposition
phases, La Niс 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
