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Earth’s Climate: What
Gives?
from Know It All: The Little Book of Essential Knowledge 
by Susan Aldridge, Elizabeth King Humphrey, and Julie Whitake |
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About
Mommy Nature
Order
Know It All: The Little Book of Essential Knowledge
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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
(–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.
The above is an excerpt
from the book
Know It All: The Little Book of Essential Knowledge,
A Reader's Digest book
published in association with Quid Publishing. Copyright ©
Quid Publishing 2008.
Author Bios
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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