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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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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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