Ice Ages & Global Warming in Context
From: New Scientist, August 1997
J. Geffen
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1.
Ice ages and ice caps are an unusual phenomenon in the 4.5 billion year history
of the earth. Fossils in the geological record show that, in the past, warm-temperate
forests have grown beyond the polar circles and warm-temperate seas extended as far,
in all likelihood, as the poles themselves. Without those great expanses of polar ice to
reflect the warmth of the sun back into space, the world as a whole was a warmer
place. Thus, in a pre-historical context, it seems that we are now living in our own ice
age.
2.
During the Permo-Carboniferous times, an ice age occurred which appears to
have been at least as long as the one we are currently enjoying. At that time, it is
believed, the poles were in different places relative to the continents, and one of the
largest ice caps covered part of an Afro-Indian continent which included what is now
the Sahara Desert. This theory is based on evidence of past glaciations, including
ancient morraines and erratic boulders as well as fossils.
3.
Our own ice age began 20-30 million years ago, according to some estimates,
and peaked 3-4 million years ago. It began with a long period of slow cooling, during
which time tremendous changes were taking place in the distribution of the continents
about the earth’s surface, and in the circulation of the oceans. Antarctica, once part of
the super-continent of Gondwanaland, had recently detached itself from Australia. It
had forests and grasslands, but it was slowly drifting southwards towards the cold
polar area. As it moved, lateral pressures gave birth to high mountains – so high that
snow settled and became permanent, and glaciers formed.
4.
By 15-20 million years ago, Antarctica was isolated within the growing
Southern Ocean, and had developed substantial ice sheets. These spread across the
continent and formed the first polar ‘mirror’, reflecting energy from the sun back into
space off its vast white, shining surface. Circulation in the atmosphere and in the
ocean spread the resulting cold around, and the world as a whole began to chill.
Glaciation then began in the north, leading to further reflection and loss of incoming
energy. By 2-3 million years ago, the highlands of Alaska, Greenland and Eurasia
were in turn developing glaciers. During the last 2 million years, ice sheets spread
widely over the lowland areas of the north until the second polar mirror was in place.
5.
Having covered the whole continent, the ice sheets of Antarctica have had little
room to move much – except seasonally. The separate ice caps of southern Patagonia,
however, as well as those of New Zealand and some of the Southern Ocean islands
have fluctuated, as have the ice sheets of the northern hemisphere. The edges of the
northern sheets have advanced and retreated at least four times during the last million
years. At their maximum, they covered Britain as far south as the Thames estuary,
Ice Ages & Global Warming / 2
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North America down to the Great Lakes, and much of northern Europe. Some 30% of
the earth’s surface was blanketed in ice, whereas today only about 11% of the planet’s
surface is frozen.
6.
Over the last 7,000 years we have passed from the so-called ‘climactic
optimum’, when mean temperatures were 2°C (4°F) higher than they are today,
through several cycles to the ‘Little Ice Age’ of AD 1400-1700, when they were
almost 2°C (4°F) lower. Between 1880 and 1940 there was an overall rise in the
northern hemisphere of 1°C (2°F), and between 1940 and 1970 a drop of 1°C (2°F).
7.
We seem currently to be in an ‘interglacial’ period, with the ice gradually
retreating – a phenomenon very much in keeping with past cycles. But it is thought
that only a slight drop in mean summer temperature, of as little as 2-3°C (4-5°F),
would allow seasonal snow fields to persist year round and to develop into ice sheets.
Similarly, only a slight rise in mean temperature could cause whole areas of
continental ice to disperse and melt. The ice could return, as it left, or it could
disappear altogether. Our impact on the earth, especially our release of atmospherealtering gases, may be enough to amplify natural forces, or counteract them.
The Greenhouse Effect
8.
Among the man-made phenomena considered to be particularly dangerous for
the Antarctic environment is the so-called Greenhouse Effect. This is related to the
production of carbon dioxide.
9.
Under normal circumstances, plants absorb the carbon dioxide in the air through
photosynthesis, and plankton absorbs the carbon dioxide dissolved in sea water.
However, our cars and factories are producing more carbon dioxide than ever before,
and at the same time more and more green areas are disappearing. As a result, there is
a surplus of carbon dioxide in the atmosphere. When the heat of the sun reaches the
earth, it is normally reflected back into space by the earth’s surface. High levels of
carbon dioxide, however, trap the sun’s heat within the earth’s atmosphere, causing a
progressive increase in the earth’s average temperature. It is thought that an increase
of 5°C (9°F) would be sufficient to melt large areas of Antarctica’s ice sheet and raise
sea levels by 5-7 metres (16.5-23 feet) across the planet.
Ice Ages & Global Warming / 3
Answer in your own words.
1.
2.
Answer the question below in English.
In what sense could the information provided in paragraph 1 be understood as
suggesting that ours is, in a sense, an exceptional period in the long history of
the earth?
Answer: ____________________________________________________________
Answer the question below in English.
What surprising piece of information could the average student gather after
reading paragraph 2?
Answer: ____________________________________________________________
Answer the question below in Hebrew.
3.
How did Antarctica – paragraph 3 – come into being?
Answer: ____________________________________________________________
4.
5.
Answer the question below in English.
Where have the ice sheets that have covered large parts of Britain in the last
million years – paragraph 5 – originated from?
Answer: ____________________________________________________________
Answer the question below in English.
In what sense could all those predicting inevitable catastrophe in consequence
of global warming draw comfort from the information provided in paragraph 6?
Answer: ____________________________________________________________
Complete the sentence below.
6.
When the Ice Age had reached its high point, the amount of the earth’s surface
covered by ice was about _________________________ times as large as it is today
(paragraph 5).
Ice Ages & Global Warming / 4
7.
Answer the question below in English.
What would be the earth’s prospects – paragraph 7 – were it not for the impact
of human interference?
Answer: ____________________________________________________________
Answer the question below in Hebrew.
8.
Explain the Greenhouse Effect.
Answer: ____________________________________________________________