History of Climate Change
History of Climate Change
Table of Contents
Climate change, meaning a change in average air temperature and precipitation,
is a significant aspect of global change. As a guide to the future consider
the past climate; we know most about temperatures.
These two figures show former temperatures with major periods of
glaciation labeled. The dashed lines are the present
global average temperature of about 15° C (59° F).
Thus the solid curves show small changes from this average;
note that the temperature drops only about 5° C
during a glaciation. This has occurred about every 100,000 years, with
smaller wiggles in between. That is, there has been a 100,000 year
for the past million years or so, and there may be shorter cycles
The most recent glaciation, 20,000 years
ago, is called the Laurentide, and Earth is still recovering from it.
This map from the
The Illinois State Museum exhibit on ice ages shows the extent of that ice.
Retreat of the Laurentide Ice is an animation made by the Museum.
The most recent small drop in average temperature caused the Little Ice
Age of 1500-1700 AD, which history describes. Mountain glaciers
advanced in Europe and rivers like the Thames in England froze solid,
which doesn't happen now.
Europeans first settled in America during these centuries.
They had a hard time as a result of the cold conditions in the
northern hemisphere, colder than they expected.
No one is certain what caused the lower temperatures of the Ice Ages,
and several different causes
probably worked together. The 100,000 year long cycle is likely caused by
variations in the Earth's orbit and tilt that change the amount of heat
reaching Earth from the Sun. A small change can make the few degrees
difference between a temperate climate and ice. Also, the Sun's total output of heat
may vary a little.
- The geological record of carved mountain valleys, scratched bedrock,
and glacial debris and moraines gives evidence of the past several
- Recently, cores have been removed from the ice at Vostok
Station in Antarctica. The longest cores are about 2000 meters,
sampling layers of ice deposited as early as 160,000 years ago.
The ice trapped bubbles of air when it froze. The ratio of oxygen isotopes
in this air indicates the average air temperature at the time the
bubble was trapped in ice.
The bubbles also trap atmospheric greenhouse gases that can be measured.
- Fossil plants and the distribution of pollen show that vegetation
has changed, consistent with changing climate.
Pollen from plants, buried in shallow deposits of earth, indicate
the distribution of vegetation since the last glaciation,
about 20,000 years ago. ||
Ancient Plant Distribution describes how vegetation
has changed as the ice retreated, and offers a student activity.
- Tree rings provide a record of the weather back 3,000 years
in some cases, and hundreds of years in many areas. See
Tree Rings, a Study of Climate Change.
- The historical
record speaks to us for some 2,000 years and there have been real
quantitative measurements since about 1850.
The amounts of the so-called atmospheric greenhouse gases,
primarily carbon dioxide and methane, affect temperature.
If nothing else changes, more greenhouse gases would make the temperature rise.
(Atmospheric Greenhouse Gases explains why.)
Data from the air bubbles in ice cores show that in the past when the temperature
was higher there was more carbon dioxide. Thus the temperature and the amount
of the greenhouse gas carbon dioxide
were correlated. (That is they changed together, and in an understandable way.)
But consider these important ideas!
Humans are putting the greenhouse gas carbon dioxide into the air by burning fossil fuel or wood.
We also add CFC (that is chlorofluorocarbons) compounds like Freon,
used in refrigeration systems. CFC's are greenhouse gases as
well as destroyers of ozone. We measure that carbon dioxide is currently
increasing, and it seems to be getting warmer the last few decades.
Although this suggests that
increased greenhouse gases are causing the warming, it is not certain.
Atmospheric Greenhouse Gases
discusses these topics and offers data and student activities.
If climate changes, the amount of frozen water is expected to change greatly.
Because of all its sea-ice, glaciers, and permafrost
(permanently frozen ground),
Alaska is a great place to study changes in climate.
Peat, tundra and boreal forests also respond sensitively to climate change.
- The observation that past temperature and the amount of carbon dioxide in the
atmosphere changed together doesn't tell us that one effect caused the other.
- Variations in solar energy could have changed the temperature.
Could temperature differences
alter the amount of carbon dioxide?
- Or did some unknown cause change the carbon dioxide, and this altered the
- Or did something unknown change both temperature and carbon dioxide?
- The cause and effect relationship might be different in different
geological eras. Specifically man-made carbon dioxide is changing temperature, but in the past
it was the other way around.
Recordings show that Alaska has been warming at 0.7 - 0.8° C every 10 years,
according to Alaska's Center for Global Change. If the trend s,
the temperature could be 7 - 8° C warmer in 100 years. This is a big difference.
In the last ice age it was only about 5 - 8° C cooler.
The Tables from Preparing for an
Uncertain Future show details of observed and predicted climate
The Alaska Center for Global Change provides further information.
Return to Global Change.
The Illinois State Museum exhibit on ice ages.
Graphs from Foland et al.; see reference below.
- NOAA's Paleo Climatology Program
archives data on these topics.
- Global Warming Update
- Preparing for an Uncertain Future: Impacts of short- and Long-term Climate
Change on Alaska, Workshop Proceedings, Arctic Science Conference-University of
Alaska Fairbanks, September 1995.
- Folland, C.K., et al., "Observed Climate Variations and Change", Chap 7 in
Climate Change: the IPCC Scientific Assessment, WMO, Cambridge University Press,
Cambridge, UK 1990.
- Climate Change 1995: The IPCC Second Assessment Report.
Summary for Policy Makers:
The Science of Climate change, IPCC Working Group I (1995).
Written by: Hugh Anderson and Bernard Walter.
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Friday 28 March 1997