The Chinese philosopher
Confucius some 26 centuries ago proclaimed "study the past
to divine the future." From
the paleo perspective that looks at climate and environmental
systems over thousands and millions of years, climate change is
normal and part of the Earth's natural variability related to
interactions among the atmosphere, ocean, and land, as well as
changes in the amount of solar radiation reaching the earth.
There is evidence to
suggest the Earth's temperature has been significantly hotter
than it is today. During the mid Cretaceous Period about 100 million
years ago, the mean temperature may have been as high as 21 degrees
Celsius as compared to about 15 C today.
The planet has also experienced colder temperatures, not only
during the Ice Ages of the past several million years, but also
during the end of the Carboniferous and beginning of the Permian
periods about 300-260 million years ago.
The
largest abrupt
climate
event of the last 100 million years ago likely occurred when a
meteor hit the region of Yucatan, causing mass extinctions about
65 million years ago including the extinctions of the dinosaurs,
allowing mammals to become dominant. About 55 million years ago,
a warm spell led to significant global warming, with palm trees
in Alaska and crocodiles in the Arctic.
Around 25 million years ago, the Antarctic ice shelf began to
form, and some 20 million years back, major modern mountain ranges
such as the the Cordilleras, the Andes, and the Himalayan range
were formed, with mammals becoming dominant.
Massive terrestrial ice sheets throughout the Northern Hemisphere
indicate cold conditions during the last glacial maximum (21,000
years ago). Warm climate, vegetation, dinosaurs, and corals living
at high latitudes during the mid-Cretaceous (120-90 million years
ago) indicate globally warm conditions. (See Beyond
for more.)
More
recently during the Little Ice Age
(AD 1450 -1890) historic and instrumental record, predominately
around the North Atlantic, indicate colder than modern temperatures.
Based
on past climate patterns and current understanding of how orbital
dynamics of the Earth such as precession,
eccentricity and variations in Earth axial tilt influence glacial
and interglacial cycles, it
would be expected that Earth's climate would be heading back into
an Ice Age within the next few thousand years. But evidence now
indicates that much of the climate system is in fact heating up.
The
paleoclimate record also shows that climate doesn't necessarily
change slowly over hundreds or thousands of years. In some cases,
major increases or decreases in precipitation and/or temperature
can occur in periods less than a decade. Since the end of the
last Age there have been at least four abrupt changes, including
the "Younger Dryas" event dated by ice cores to between
12,800 and 11,500 years B.P., which chilled and dried out much
of the northern hemisphere. The study of abrupt change in climate
is currently one of the hottest areas of climate research. There
are few theories and no models that address how such large changes
can occur in such short periods of time.
Is
it possible that such abrupt climate changes will occur in the
future? The short answer is, yes... and that the change may not
necessarily be warming. Some scientists warn that it is possible
that global warming caused by increased greenhouse gases in the
atmosphere could actually produce an abrupt cooling in some parts
of the world-- such as Europe or North America-- by shutting down
the Gulf Stream. As
oceanographer Jochem Marotzke has stated "We are in a state
now where the more we know, the more it becomes clear how little
we really understand about the (climate) system." (See Marotzke,
2000)
Predicting
the Future
Additional
Resources and References
