Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 48

was some 25m higher 5 million years ago. Therefore, another global average tem-
perature rise of 1 C from today's climate might produce a huge rise in sea level.
However, there is considerable evidence that indicates that the global average
temperature 5 million years ago was far warmer than 1 C above current tempera-
tures. Robinson et al. (2008) said that about 3 million years ago, the global
average temperature was 2-3 C warmer than today. However, ''global warmth
was distributed differently.'' Three million years ago, ''temperatures at high north-
ern latitudes, above 70 N, were as much as 10 -20 C higher than today, but
tropical temperatures were near the same.'' This points out the limitation of using
a single global average temperature to characterize climate.
2.3.4 The Early Pliocene: 3 to 5 million years ago
According to Haywood and Williams (2005):
''Although the geography of our planet looked very similar to that of today
three million years ago, the world was undergoing momentous changes every-
where, from the Americas to Tibet. At about this time, animals from South
America first started to colonize North America, indicating that the Isthmus
of Panama had finally risen above sea level. Along this trans-continental highway
of migration, the armadillo was amongst the animals that migrated north, whilst
dogs, cats, bears and many other animals headed south. In Africa, the spread of
Savannah vegetation and retreat of forest habitats may have encouraged our
primate ancestors to come down from the trees, colonizing the open plains of the
rift valleys of east Africa and undergoing an evolutionary radiation into a
number of 'graceful' and 'robust' australopithecines. Their fossil remains are
found in modern Ethiopia and Tanzania. In Asia, the continued collision of
India with the Eurasian land mass pushed the Himalayas still higher, intensifying
the Asian monsoon. From the Americas, ancestral horses about the size of ponies
migrated west along the Aleutian archipelago into Asia and Europe. In the
Antarctic, the ice sheets and glaciers were not static, but fluctuated in size,
influencing the global climate and sea level. In the oceans too, there were changes.
The emerging Isthmus of Panama finally cut off the exit route for Atlantic water
into the Pacific, and this contributed to a saltier Atlantic Ocean which may have
encouraged the warm water current known as the 'Gulf Stream' in the North
Atlantic to flow vigorously.
For much of the past three million years our planet's global climate has been
cooler than today, particularly during the ice ages of the Pleistocene. However,
during the mid-Pliocene there is strong evidence for a period lasting 300,000
years, when the global climate was warmer than it is today. On Antarctica, along
the Trans-Antarctic Mountains, rocks of probable mid-Pliocene age yield fossils
of southern beech plants suggesting that parts of Antarctica were ice-free. The
question is, what caused this globally warmer climate and what relevance does it
have to our understanding of current global warming? There are two key ques-
tions. Did higher levels of greenhouse gases in the atmosphere cause Pliocene