precipitation, for example, are two primary elements of climate that affect
many aspects of our lives. But neither tells the whole story.
CONCLUSIONS
"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
