[FoRK] Global Cooling, Are we doing enough to prevent it?
Albert S. <
albert.scherbinsky at rogers.com
> on >
Fri Jun 9 20:27:42 PDT 2006
Ice Ages, periods in Earths history when sea ice or
glaciers have covered a significant portion of the
planets surface and significant cooling of the
atmosphere has occurred. Earth has existed for about
4.5 billion years. During that time it has experienced
several ice ages, each lasting tens of millions of
The total of these episodes may account for as much as
15 to 20 percent of the planets history. The icy
cover has ranged from about 10 percent to about 30
percent of the entire surface of the planet.
The most recent ice age, the Pleistocene Epoch, lasted
from about 1.6 million years to 10,000 years before
present. During that time at least 20 glaciations, or
periods when the ice cover increased, occurred. Each
of these periods was followed by an interglaciation,
or a period when the ice cover shrank. The most recent
glaciation in North America, called the Wisconsin
glaciation, lasted from about 115,000 years ago to
years ago. The climate during that time was much
different from what it is today, with temperatures on
the continents as much as 15° C (27° F) colder. In
areas that are currently occupied by subtropical
deserts, cooler and wetter climates caused large lakes
to form from increased rainfall and glacial runoff.
The past 10,000 years have been part of a relatively
warm interglacial period. However, the presence of
massive continental ice sheets on Greenland and
Antarctica, along with numerous smaller glaciers in
mountainous regions throughout the world, indicates
that Earth is still in the grip of an ice age.
The most likely causes of ice ages are changes in
Earths orbit and orientation. The tilt of Earths
axis increases and decreases over a 41,000-year cycle.
A relatively large tilt generally leads to hotter
summers and colder winters. Meanwhile, the shape of
Earths orbit around the sun varies on a 96,000-year
cycle. When the orbit is at its most elliptical, the
amount of sunlight hitting Earth increases and
decreases more intensely over the year. Finally,
Earths axis wobbles on a 26,000-year cycle. Its
changing direction alters the season when Earth is
closest to the sun. Glaciations over the last 1.6
million years have occurred when the variables line up
to give the Northern Hemisphere the least amount of
summer warmth. At those times, snows from previous
winters do not melt completely, eventually
accumulating into miles-thick ice sheets. The ice
advances, then retreats when the Northern Hemisphere
begins to experience particularly warm summers again.
The last glaciation ended 11,000 years ago. Most
interglacial periods have persisted for 10,000 to
15,000 years, so it seems likely that a glaciation
will begin, but perhaps not for thousands of years.
Human-caused global warming may prevent or stall the
next ice age, although no one knows how much of a
factor this will be.
Are we doing enough to prevent the next Glaciation?
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