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We
are pleased to present the following
excerpt from the book
Know It All: The
Little Book of Essential
Knowledge
by Elizabeth Humphrey, et.
al.
Reader's Digest Books -
October 2008
Earth's
Climate: What Gives?
The Earth's average temperature has
fluctuated greatly throughout its history.
Today we worry about polar ice caps and
glaciers melting more quickly than ever
before. Still, there have been times in
the past when ice and snow were virtually
absent from the planet. Could we be headed
for another iceless age?
The term ice age sometimes refers to
periods when ice sheets were more
extensive than usual. But these times are
more accurately called glacials, and they
occur within an ice age; the periods
between glacials are called interglacials.
We are now in an interglacial in what is
probably the Earth's fourth great ice age.
What has distinguished the last 200 years
is the melting of ice at apparently
unprecedented rates as the temperature of
the Earth gradually grows warmer.
Climate Change
In the early nineteenth century the
Swiss-German geologist Jean de Charpentier
suggested that the Alpine glaciers he had
been studying had at one time been far
larger. Later a Swiss-American geologist,
Louis Agassiz, built on Charpentier's
notion and proposed that Earth at one time
had been completely covered by ice.
Ice Ages Past . . .
Since then, scientific advances have
contributed to our understanding of the
Earth's ice ages, and it is now thought
that the first major ice age occurred some
2 billion years ago. Another ice age, 850
to 630 million years ago -- probably the
most severe -- may have covered the entire
globe in ice, a frosty scenario known as
"Snowball Earth."
The end of that ice age seems to have
coincided with the evolution of a great
many tiny organisms, although whether
there is a causal link between these
events and what they might be remains a
matter of debate.
Then, between 400 and 300 million years
ago, another ice age struck, and the
planet was again plunged into a cold
period, known as the Karoo Ice Age, named
for the glacial till (sediment) found in
the Karoo hills of South Africa.
. . . and Present
The current ice age began some
40,000,000 years ago, reaching its coldest
period about 3,000,000 years ago. The last
glacial period (often referred to
inaccurately as an ice age) ended about
10,000 years ago, and the first human
civilizations began to flourish shortly
after. How global warming will affect
Earth's cooling and warming cycles -- and,
more urgently, sea level as glaciers and
the polar ice caps melt -- is the pressing
issue of our age.
The Global Greenhouse
Without the greenhouse effect, a
natural process that heats the Earth's
surface and atmosphere, our average
temperature would be a frigid 0°F
(&endash;18°C) -- ensuring a
permanent ice age, to say the least. The
warmed globe radiates what is called
"infrared radiation," most of which should
travel through atmospheric layers to
space. With the advent of the Industrial
Revolution in the late 1700s, more and
more infrared radiation began to be
absorbed by naturally occurring greenhouse
gases, including carbon dioxide (CO2). The
increase of average concentrations of CO2,
from about 280 parts per million in 1700
to about 380 parts per million in 2005 is
the major cause of global warming.
In 2007 the Intergovernmental Panel on
Climate Change (IPCC) asserted that human
activities -- including the use of fossil
fuels -- was "very likely" the catalyst
for global warming.
Some scientists estimate that the
Earth's temperature will rise by as much
as 9°F (5°C) by 2050, while
others heatedly disagree. What isn't in
dispute is that the world's ice is in a
literal meltdown. For instance, the
largest single block, the Ward Hunt Ice
Shelf in the Arctic, lasted some 3,000
years before it started to crack in 2000;
a mere two years later it was split
through and is now breaking apart.
About
the Authors
Susan
Aldridge has been a freelance science and
medical writer for more than 15 years and
has contributed to a number of magazines
and websites. She lives in
London.
Elizabeth
King Humphrey has been a contributing
writer, editorial advisor, copy editor,
and co-designer for several magazines,
books, and PBS documentaries. She lives in
Wilmington, North Carolina.
Julie
Whitaker has a master's degree in
anthropology and American studies.
Whitaker has contributed to many books,
including several encyclopedias. She lives
on Vancouver Island, Canada.
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Dr. Dolhenty's Review of this Book
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