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The Big Chill
by Kirk A. Maasch
 During the past billion years, the Earth's climate
has fluctuated between warm periods - sometimes even completely ice-free - and
cold periods, when glaciers scoured the continents. The cold periods - or ice
ages - are times when the entire Earth experiences notably colder climatic
conditions. During an ice age, the polar regions are cold, there are large
differences in temperature from the equator to the pole, and large,
continental-size glaciers can cover enormous regions of the earth.
Ever since the Pre-Cambrian (600 million years ago), ice ages have occurred at
widely spaced intervals of geologic time - approximately 200 million years -
lasting for millions, or even tens of millions of years. For the Cenozoic
period, which began about 70 million years ago and continues today, evidence
derived from marine sediments provide a detailed, and fairly continuous, record
for climate change. This record indicates decreasing deep-water temperature,
along with the build-up of continental ice sheets. Much of this deep-water
cooling occurred in three major steps about 36, 15 and 3 million years ago -
the most recent of which continues today. During the present ice age, glaciers
have advanced and retreated over 20 times, often blanketing North America with
ice. Our climate today is actually a warm interval between these many periods
of glaciation. The most recent period of glaciation, which many people think of
as the "Ice Age", was at its height approximately 20,000 years ago.
Although the exact causes for ice ages, and the glacial cycles within them,
have not been proven, they are most likely the result of a complicated dynamic
interaction between such things as solar output, distance of the Earth from the
sun, position and height of the continents, ocean circulation, and the
composition of the atmosphere.
Climatic Cooling from 60 million years ago to present day
Between 52 and 57 million years ago, the Earth was relatively warm.
Tropical conditions actually extended all the way into the mid-latitudes
(around northern Spain or the central United States for example), polar regions
experienced temperate climates, and the difference in temperature between the
equator and pole was much smaller than it is today. Indeed it was so warm that
trees grew in both the Arctic and Antarctic, and alligators lived in Ellesmere
Island at 78 degrees North.
But this warm period, called the Eocene, was followed by a long cooling trend.
Between 52 and 36 million years ago, ice caps developed in East Antarctica,
reaching down to sea level in some places. Close to Antarctica, the
temperature of the water near the surface dropped to between 5 and 8 degrees
Celsius. Between 36 and 20 million years ago the earth experienced the first
of three major cooling steps. At this time a continental-scale temperate ice
sheet emerged in East Antarctica. Meanwhile, in North America, the mean
annual air temperature dropped by approximately 12 degrees Celsius.
Between 20 and 16 million years ago, there was a brief respite from the big
chill, but this was followed by a second major cooling period so intense that
by 7 million years ago southeastern Greenland was completely covered with
glaciers, and by 5-6 million years ago, the glaciers were creeping into
Scandinavia and the northern Pacific region. The Earth was once more released
from the grip of the big chill between 5 and 3 million years ago, when the sea
was much warmer around North America and the Antarctic than it is today.
Warm-weather plants grew in Northern Europe where today they cannot survive,
and trees grew in Iceland, Greenland, and Canada as far north as 82 degrees
North.
We are still in the midst of the third major cooling period that began around 3
million years ago, and its effect can be seen around the world, perhaps even in
the development of our own species. Around 2 and a half million years ago,
tundra-like conditions took over north-central Europe. Soon thereafter, the
once-humid environment of Central China was replaced by harsh continental
steppe. And in sub-Saharan Africa, arid and open grasslands expanded,
replacing more wooded, wetter environments. Many paleontologists believe that
this environmental change is linked to the evolution of humankind.
Possible Explanations for the Past 60 Million Years of Cooling
Climate change on ultra-long time scales (tens of millions of years) are more
than likely connected to plate tectonics. Plate motions lead to cycles of
ocean basin growth and destruction, known as Wilson cycles, involving
continental rifting, seafloor-spreading, subduction, and collision. Several
explanations of the latest cooling trend that involve a climate-tectonic
connection are summarized below.
Geographic Distribution and Size of Continents
Through the course of a Wilson cycle continents collide and split apart,
mountains are uplifted and eroded, and ocean basins open and close. The
re-distribution and changing size and elevation of continental land masses may
have caused climate change on long time scales. Computer climate models have
shown that the climate is very sensitive to changing geography. It is
unlikely, however, that these large variations in the Earth's geography were
the primary cause of the latest long-term cooling trend as they fail to
decrease temperatures on a global scale.
 Likewise, changing topography cannot, by itself,
explain this cooling trend. Computer model experiments performed to test the
climate's sensitivity to mountains and high plateaus show that plateau uplift
in Tibet and western North America has a small effect on global
temperature but cannot explain the magnitude of the cooling trend. Plateau
uplift does, however, have a significant impact on climate, including the
diversion of North Hemisphere westerly winds and intensification of monsoonal
circulation.
Geometry of Ocean Basins
Another theory explaining these changes in climate involves the opening and
closing of gateways for the flow of ocean currents. This theory suggests that
the redistribution of heat on the planet by changing ocean circulation can
isolate polar regions, cause the growth of ice sheets and sea ice, and increase
temperature differences between the equator and the poles.
Ocean modeling experiments suggest that the ocean could not have carried enough
heat to the poles to maintain the early warm climates. But atmospheric climate
modeling experiments show that even if the ocean did transport enough heat up
to the coast of Antarctica to maintain sea surface temperatures at 10 to 15
degrees Celsius, the interior conditions would still be much colder - and this
is contrary to the geologic record. It is possible, however, that changes in
heat transport caused by variations in ocean gateways may have played a
significant role in cooling trends over the last 60 million years, and, in
particular, may help explain some of the relatively sudden cooling events.
Atmospheric Carbon Dioxide
Changes in the concentration of carbon dioxide in the atmosphere are a strong
candidate to explain the overall pattern of climatic change. Carbon dioxide
influences the mean global temperature through the greenhouse effect. The globally averaged surface temperature
for the Earth is approximately 15 degrees Celsius, and this is due largely to
the greenhouse effect. Solar radiation entering earth's atmosphere is
predominantly short wave, while heat radiated from the Earth's surface is long
wave. Water vapor, carbon dioxide, methane, and other trace gases in the
Earth's atmosphere absorb this long wave radiation. Because the Earth does not
allow this long wave radiation to leave, the solar energy is trapped and the
net effect is to warm the Earth. If not for the presence of an atmosphere,
the surface temperature on earth would be well below the freezing point of
water.
Through a million year period, the average amount of carbon dioxide in the
atmosphere is affected by four fluxes: flux of carbon due to (1) metamorphic
degassing, (2) weathering of organic carbon, (3) weathering of silicates, (4)
burial of organic carbon. Degassing reactions associated with volcanic
activity and the combining of organic carbon with oxygen release carbon dioxide
into the atmosphere. Conversely, the burial of organic matter removes carbon
dioxide from the atmosphere.
 Plate collisions disrupt these carbon fluxes in a
variety of ways, some tending to elevate and some tending to lower the
atmospheric carbon dioxide level. It has been suggested that the Eocene, the
early warm trend 55 million years ago, was caused by elevated atmospheric
carbon dioxide and that a subsequent decrease in atmospheric carbon dioxide led
to the cooling trend over the past 52 million years. One mechanism proposed as
a cause of this decrease in carbon dioxide is that mountain uplift lead to
enhanced weathering of silicate rocks, and thus removal of carbon dioxide from
the atmosphere.
In addition, the collision of India and Asia led to the uplift of the Tibetan
Plateau and the Himalayas. While topography may not be enough to explain the
cooling trends, another mechanism may account for changing climate. The uplift
may have caused both an increase in the global rate of chemical erosion, as
well as erode fresh minerals that are rapidly transported to lower elevations,
which are warmer and moister and allow chemical weathering to happen more
efficiently. Through these mechanisms, then, it has been hypothesized that the
tectonically driven uplift of the Tibetan Plateau and the Himalayas is the
prime cause of the post-Eocene cooling trend.
Kirk A. Maasch is a professor at the University of Maine, in the Department of
Geological Sciences.
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