Geoscience Reference
In-Depth Information
over the past two million years. Carbon dioxide dropped to concentrations
as low as 180 parts per million (ppm) during the peak of the glacials and rose
to about 280 ppm during the interglacials.
Carbon dioxide causes a climate feedback by amplifying the small changes
in solar radiation received by changes in the earth's orbit. During glacial
periods, carbon dioxide is absorbed by the oceans, decreasing its concentra-
tion in the atmosphere, leading to further cooling, in a positive feedback
loop. During the transitions from glacials to interglacials, carbon dioxide is
released back to the atmosphere, further warming the earth, in yet another
positive feedback loop. Carbon dioxide levels reached their “interglacial”
value of 280 ppm about 6,000 years ago but have climbed to the unprec-
edented level (over the past two million years) of 390 ppm over the past
century. Many climate scientists warn that a continued rise in atmospheric
carbon dioxide will warm the earth's climate for the foreseeable future.
h e ice cores also show that, during glacial periods, the climate changed
quite abruptly—warming rapidly and then cooling—every few thousand
years. Although we have not experienced such drastic climate swings during
the Holocene, climate scientists are actively studying these past events in
order to understand what caused them and to determine if they might recur
in a future warmer world.
the last big thaw
At the peak of the last glacial period, some 20,000 years ago, ice sheets
reached a maximum size (and sea level its lowest level) during a period called
the Last Glacial Maximum. Solar radiation in the Northern Hemisphere
summer dropped to a minimum at that time, and the global climate was as
cold as it had been in 100,000 years. Gigantic ice sheets blanketed North
America, Europe, Asia, and Scandinavia. h ese ice sheets were between one
and two miles thick; their weight was so great that they depressed portions
of the continents by as much as 1,200 feet. h e water locked up in these enor-
mous ice sheets originated from the oceans, causing global sea levels to drop
by about 360 feet.
Gradual shit s in aspects of the earth's orbit explain the thawing and
melting of the ice ages beginning 20,000 years ago as well as the broad cli-
mate patterns that have occurred since then. Summer solar radiation in the
Northern Hemisphere began increasing from a minimum 20,000 years ago
