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GLOBAL WARMING
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Figure 1: Average sea surface temperatures
around the world. (NCEP and Univ. Wisc.) Global warming has caused
the sea surface temperatures to increase and the ice cap at the
north pole to become thinner.
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Nearly every day, new evidence is presented showing that
the globally averaged temperature of Earth has increased over the last few
centuries. According to the Intergovernmental Panel on Climate Change (IPCC),
the globally averaged surface temperature has increased by 0.6°C over the
last 100 years. There is evidence that not only is the atmosphere warming
but the ocean temperatures are increasing as well. The ice cap on the
North Pole has become significantly thinner.
The global warming has increased dramatically in the last
20 years. The IPCC report estimates that the 1990s were the warmest years
since the beginning of instrumental records in 1861 and that 1998 may have
been the warmest year on record. This increase in temperature over the
last century is likely to have been the largest 100-year increase in the
last 1000 years. Because of these dramatic climate changes of the last 100
years many scientists believe that human activities, such as burning
fossil fuels, have contributed to global warming.
Two of the questions that now face scientists studying
climate change are…
- How has human activity influenced the climate?
- How would the global climate change without human influence?
In order to answer the first question, scientists must
answer the second question.
SOLAR VARIABILITY
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Figure 2: This composite figure (prepared by
Lockheed) shows a sequence of solar x-ray images taken with the
Yohkoh satellite about six months apart from solar maximum (lower
left) to solar minimum (upper right). This is a dramatic example
of how the sun changes over the 11-year solar cycle. NOAA will
start making similar observations in July 2001 with the Solar
X-ray Imager on the GOES spacecraft.
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The total energy output of the sun is nearly constant. At
the top of Earth’s atmosphere the total irradiance from the sun is about
1366 W/m˛. Imagine thirteen 100 Watt light bulbs shined all of their
energy onto a square meter. During the course of an 11-year solar cycle,
the average output of the sun changes by about 1-2 W/m˛ or about 0.1%.
Thus, the solar constant varies between 1365 and 1367 W/m˛ and is
therefore, not really a constant.
In other wavelengths such as the ultraviolet and extreme
ultraviolet parts of the solar spectrum, the solar variability can be
quite large. In the x-ray wavelengths, the sun can change brightness by a
factor of 100 or even 1000 in just a few minutes but these wavelengths
only affect the upper reaches of our atmosphere. Figure 2 shows a 5-year
sequence of x-ray images of the sun from solar maximum to solar minimum.
It is thought that the total solar output of the sun has
changed by larger amounts over longer time scales. There is evidence that
the total solar output may have been as low as 1360 W/m˛ during the 19th
century and even lower than that during the 17th century. Thus over
centennial time scales, the solar output may have changed by 0.5%.
SOLAR VARIABILITY AND CLIMATE CHANGE
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Figure 3: (a) The northern hemisphere land
temperatures are plotted with the solar cycle length (Friss-Christensen
and Lassen; 1991).
(b) The globally averaged sea surface
temperatures are plotted with the sunspot numbers (Reid; 1999).
Both sunspot number and solar cycle length are proxies for the
amount of solar energy that Earth receives. The similarity of
these curves is evidence that the sun has influenced the climate
of the last 150 years.
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Variability in the amount of energy from the sun has
caused climate changes in the past. It is now accepted that the global
cooling during Ice Ages is the result of changes in the distribution and
amount of sunlight that reaches Earth. During the last Ice Age, the
globally averaged temperature of Earth was about 6°C colder than it is
today. While this may not sound like much, the effect was to cover large
parts of Canada, Alaska, and Siberia with huge sheets of ice up to a mile
thick.
Even the climate changes of the 20th century may have a
significant solar component. Figure 3 shows comparisons of globally
averaged temperature and solar activity. Many scientists find that these
correlations are convincing evidence that the sun has contributed to the
global warming of the 20th century. Some say that as much as 1/3 of the
global warming may be the result of an increase in solar energy. So, while
it is becoming clear that human activity is changing the climate today,
solar activity may also be contributing to climate change and probably
changed the climate in the past.
In order to accurately predict how future human activities
will change the climate, it is critical to understand the variability of
the natural system. Therefore, even though solar activity may not be the
dominant factor in global warming, it is important enough that
understanding how the climate responds to small changes in solar
irradiance will help scientists predict the climate changes caused by
human activity.
The NOAA Space Environment Center (SEC) combines
scientific research and an operational Space Weather Center to maintain a
vigilant watch on solar activity. SEC’s primary mission is studying the
affects of a variable sun on the upper atmosphere and the near-Earth space
environment. Monitoring and understanding the solar effects on the middle
and lower atmosphere is a new component of SEC’s mission. Present
NOAA/SEC activities include monitoring the sun in x-ray and ultraviolet
wavelengths as well as sunspots. NOAA recognizes the need for new efforts
in this area and will include solar extreme ultraviolet measurements on
the next generation of GOES spacecraft and total solar irradiance and
solar spectral irradiance measurements as part of its upcoming NPOESS
spacecraft mission.
DID SUNSPOTS SINK THE TITANIC?
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Figure 4: The weather and sea conditions that
lead to icebergs in the path of the Titanic were typical of the
early 1900s. Since then, the climatic parameters have changed and
icebergs are rarely observed so far south.
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It is well documented that the early part of the 20th
century was much colder than it is today. This can be seen in the plots in
Figure 3. A consequence of these colder temperatures is that there are
changes in sea currents and temperatures and in the strength and direction
of the winds at sea. As a result, large icebergs from the Greenland ice
sheet would often drift southward into the Atlantic Ocean and into the
shipping lanes between Europe and America. It was much more likely that a
vessel would encounter icebergs back in the early part of the century than
it is now. This is in part a consequence of a cooler climate 80 years ago.
In a scientific paper, written on the subject of the
weather on that night in 1912 when the Titanic struck an iceberg and sunk,
E. N. Lawrence concludes that there is a link between sunspots and the
icebergs found in shipping lanes in the early 1900s. Figure 5 is a plot
from Lawrence’s paper showing the correlation between sunspots and
icebergs.
While most scientists would agree that sunspots did not
really sink the Titanic, there is significant evidence to show that the
cold climate of 1912 may have been in part due to the lower level of solar
energy reaching Earth relative to today. The cold climate may have
provided the conditions needed for large icebergs to drift far south of
Greenland and into the shipping lanes of the North Atlantic. These
icebergs were a severe hazard to early ships without radar especially at
night when they could not see the icebergs. To state the connection more
clearly, increases in globally averaged temperature, produced in part by
increased solar and human activity, may have reduced the number of
icebergs in the North Atlantic thereby preventing other disasters such as
the sinking of the Titanic.
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Figure 5: An article by Lawrence in the
scientific journal Weather, published by the Royal Meteorological
Society of London concludes that “The date of the Titanic
disaster fatally coincided with a climax in the
iceberg-weather-sunspot link system” (Lawrence, 2000).
Click on graph for larger image.
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