Download Paleoclimatology - Printer-friendly

Paleoclimatology - Printer-friendly - MSN Encarta Premium
1 of 4
http://encarta.msn.com/text_761568730___0/Paleoclimatology.html
Print
Paleoclimatology
Article View
On the File menu, click Print to print the information.
Paleoclimatology
I. INTRODUCTION
Paleoclimatology, study of past climates on Earth by examining Earth's crust, fossil records, and other physical
evidence. The climate of a region is defined by the range of temperature and precipitation (rain or snow)
occurring there. These factors result in characteristic patterns of vegetation, animal life, sedimentation (layers of
sediment that eventually form rock), and land forms and features. Paleoclimatologists search for the reasons
behind climatic change throughout Earth's history so that scientists may better predict future climate change and
evaluate the influence of humankind's activities on the atmosphere and climate.
Paleoclimatologists must find and date past events to piece together climatic history. The further they look into
the past, the more climatic evidence has faded as a result of erosion and chemical processes. Paleoclimatologists
have been able to assemble a detailed picture of climatic variation over the past 20,000 years and a more general
knowledge about climatic variation over the past 150,000 years. Further back in time the evidence is less
complete, but major, often catastrophic, climatic events have been identified at various times throughout Earth's
4.6-billion-year history.
II. PALEOCLIMATIC EVIDENCE
Climatologists can measure temperature and precipitation directly to study climatic changes on Earth today, but
less direct methods must be used to discern past climates and climatic changes. Information about past climates
comes from a variety of sources. For example, paleoclimatologists analyze the isotopes of different atoms to date
events (see Dating Methods: Radiometric Dating) and to determine past solar activity and temperatures.
Trees also record valuable information about climatic conditions. Annual growth rings allow precise dating of
climatic events going back 5000 years. The amount of carbon-14 isotope in wood indicates the intensity of solar
activity during the tree's growth and the amount of carbon dioxide in the atmosphere. Narrow growth rings in
trees indicate periods of climatic stress, while the absence of growth rings indicates a warm tropical climate with
no seasons.
Lake sediments and bogs, which may date back more than 20,000 years, are rich sources of information about
past climates. Spores, pollen, and insects separated from the sediment or peat reveal changing patterns of
vegetation and associated animal life.
Drilled cores retrieved from Earth's polar ice caps provide samples of ancient air trapped as bubbles in ice as old
as 160,000 years. Careful analyses of the air in the bubbles and trace elements in the ice give evidence of
atmospheric and climatic changes such as those caused by volcanic eruptions.
Cores retrieved from the sediment of the deep ocean floor provide evidence of changes in ocean temperature as
well as oceanic and atmospheric circulation patterns dating back 170 million years. The temperature of surface
and bottom waters can be measured by studying the fossilized shells and skeletons of ancient plankton
(microscopic marine life living near the ocean's surface) and benthonic organisms (marine life of the deep ocean).
Scientists use a method called oxygen isotope stratigraphy, in which they compare the temperature-controlled
ratio of two oxygen isotopes that were incorporated into the shells as the shells grew. This ratio is preserved in the
fossilized organisms.
Shells and skeletons extracted from deep-sea sediment also provide evidence of biodiversity (the variety of species
living in a region)—a useful indicator of climate. A greater number of species are found in tropical waters and
fewer species live in polar seas. Volcanic ash and mineral grains blown from the continents can also be identified
in deep-sea sediments, indicating variations in atmospheric circulation patterns.
The fossil record provides climatic clues dating back 550 million years. Scientists recognize that some animals and
3/8/2007 7:03 PM
Paleoclimatology - Printer-friendly - MSN Encarta Premium
2 of 4
http://encarta.msn.com/text_761568730___0/Paleoclimatology.html
plants are very susceptible to the effects of climate change. This places climatic limits on habitats available to
different animals and plants. This would also have been true in the past. Fossil evidence is particularly useful
when examining species from the past that are closely related to an existing species. One example is coral, which
typically requires warm, shallow seas. Corals, now found fossilized in limestone, probably lived in similar
conditions and hence are good indicators of past sea levels and temperatures.
Deposits of till (ground rock that ice sheets leave behind) provide evidence of global glaciations during Earth's
history. Paleoclimatologists look for tillite (ancient till that has turned into solid rock). Glacial periods can also be
recognized by studying geomorphologic features. Moraines and drumlins are ridges and hills of till that form at
the edge of glaciers, marking the former extent of ice sheets. Varved sediments are layers of sediment, ranging
from coarse on the bottom to fine on top, that are seasonally deposited in glacial lakes. These features—as well as
pingos (small hills of sediment), patterned ground, and permafrost (ground that remains frozen year-round)—all
indicate former glacial episodes.
III. CLIMATIC CHANGE
Some climatic changes have occurred on a regular basis throughout Earth's history. These regular, progressive
changes, known as Milankovitch cycles, are due to variations in the amount of solar radiation reaching Earth's
surface. The radiation varies as the distance between Earth and the Sun changes and as the angle at which Earth
faces the Sun changes.
These variations of solar radiation are periodic, and corresponding climatic effects occur in cycles of about
100,000 years, 40,000 years, and 25,000 years. The resulting changes in global temperature modify the size of
polar ice sheets, cause changes to the sea and to land atmospheric circulation patterns, and change rainfall
intensity and seasonality. These changes, in turn, modify vegetation and animal life.
Superimposed upon these effects is the variable output of the Sun itself. The amount of radiation produced by the
Sun varies due to the changing magnetic structure of the Sun's gaseous mass. Over the past 100 years, scientists
have noted an 11-year cycle of sunspot activity, but the link to climate change is inconclusive. Scientists have
found that there was very little sunspot activity reported during the 17th century, when the Little Ice Age caused
the River Thames in London and the Tajo (Tagus) River in Spain to freeze over regularly.
Global tectonic events on Earth (see Plate Tectonics) result in major changes in climate over long periods of
geological time. Continents and oceans move and change shape as giant sections of Earth's crust move and shift
on Earth's mantle. As plates crush into each other and move apart, they build mountains and open and close deep
and shallow water passageways. These actions affect air- and water-circulation patterns, which in turn affect
climate.
Changes in atmospheric chemistry affect climate in a number of ways. Oxygen, ozone, and carbon dioxide levels
have varied throughout time, with corresponding effects on solar heat absorption and, therefore, on global
temperature. High carbon dioxide levels trap heat from the Sun in the atmosphere, leading to a greenhouse effect.
Volcanic eruptions may cause abrupt changes in Earth's climate. Volcanoes emit large quantities of ash and sulfur
dioxide that block sunlight and can lead to widespread acid rain. In April 1815 the Indonesian volcano Tambora
spewed great quantities of ash and gas into the atmosphere, causing freezing conditions in the northeastern
United States and western Europe throughout the summer of 1815. Evidence for eruptions can be found through
analysis of tree growth rings, which are narrow during periods of stunted growth, and through the measurement
of acidity in dated ice cores from Greenland.
Scientists theorize that the impact of a meteorite or comet with Earth could cause catastrophic climate change. If a
large meteorite hit Earth, it would produce a huge crater, and rock from the crater could end up in the
atmosphere, locally heated to several thousand degrees. Sunlight would be blocked for many years, killing plant
and animal life. Resultant changes in atmospheric chemistry could deplete ozone and cause widespread acid rain.
Gases from fires and rotting vegetation could substantially enhance the greenhouse effect.
Scientists have identified a meteorite impact site in the Yucatán Peninsula, in Mexico, that they hypothesize is
linked to the final extinction of the dinosaurs as well as much other life on Earth about 65 million years ago—near
the end of the Cretaceous Period. Some scientists believe that widespread volcanic activity could also have
contributed to the catastrophic climate change at the same time. Other global mass extinctions found in the
geological record may also eventually be linked to catastrophic climate change brought on by meteorite impacts or
extreme volcanic activity.
3/8/2007 7:03 PM
Paleoclimatology - Printer-friendly - MSN Encarta Premium
3 of 4
http://encarta.msn.com/text_761568730___0/Paleoclimatology.html
IV. CLIMATIC HISTORY
Little is known about Earth’s climate during Precambrian time, which stretches from the planet’s formation 4.6
billion years ago to about 542 million years ago. Some of the oldest rocks, dated at 3.8 billion years old, were laid
down under water, suggesting that the average surface temperatures were between 0°C (32°F), the freezing point
of water, and about 100°C (212°F), the boiling point of water. There is some evidence that deep oceans of liquid
water have existed on Earth for much of its history. Fossils indicate that a flourishing biota of microorganisms has
existed for the past 3.5 billion years. Tillites, which indicate glacial episodes, occur in rocks formed from 2.7
billion to 2.3 billion years ago, and other Precambrian glaciations occurred 950 million years ago; between 820
million and 730 million years ago; and between 640 million and 500 million years ago.
Rocks from the Paleozoic Era, which followed Precambrian time, indicate that two further glacial episodes
occurred, at 430 million years ago and at 300 million years ago. Fossils become abundant from about 450 million
years ago, providing information about biodiversity, which can be a consequence of climate. The most significant
event in the Paleozoic Era was the colonization of land by plant life. A fully developed vegetation cover changed
Earth's color significantly. The new color altered the level of absorption and reflection of solar radiation by Earth's
surface, which in turn affected the climate.
The Mesozoic Era, the time of the dinosaurs, seems to have been a period of widespread, mild warmth, possibly
due to the main continental masses being joined together. This formed the supercontinent Pangaea, which was
surrounded by a single large ocean. Such an ocean, stretching from pole to pole, would have facilitated the
transfer of heat, resulting in a more even global temperature. A short-lived but catastrophic change in climate
occurred at the end of the Mesozoic Era, possibly due to the impact of a large meteorite.
Paleoclimatologists believe that, during the current Cenozoic Era, Earth's climate has been characterized by a
series of glacial and interglacial cycles superimposed upon overall global cooling. Global temperatures
progressively declined throughout the early Cenozoic and dropped quite suddenly about 2 million years ago,
heralding a series of glaciations. However, fossil plants in coals from places such as northern Alaska, Greenland,
and Svalbard (a group of islands belonging to Norway) suggest that there were warm episodes in the early and
mid-Cenozoic Era as well.
The distribution of continents and oceans has changed throughout the Cenozoic Era until reaching that of today,
with Antarctica becoming centered on the South Pole. About 30 million years ago, Antarctica had separated from
South America and Australia; and a cold circum-Antarctic current was established. This isolated the continent
from warm ocean currents and triggered the thickening and advance of the Antarctic ice cap. An extensive
Antarctic ice sheet existed seven million years ago. Northern Europe and America moved northward into colder
latitudes, and the ice sheets advanced from the South Pole.
The most recent glaciations have been more severe than those of the preceding portion of the Cenozoic Era.
Because it is possible to date more of the events of the past 2 million years, much more is known about climate
change during that period. The glacial-interglacial cycles have lasted about 100,000 years each, an interval that
closely corresponds to the duration of changes in Earth's orbital position relative to the Sun. Evidence from
deep-sea sediments reveals a pattern of slow advance of the polar ice caps, followed by sudden retreat due to rapid
warming at the end of each glacial episode. The last major glaciation—10,700 years ago—ended with a 7°C (13°F)
global temperature rise over a 50-year period.
Since the Industrial Revolution in the 19th century, humankind has been contributing to atmospheric and climate
change by burning fossil fuels such as coal, natural gas, and petroleum; clearing forested land; and adding new
gases to the atmosphere. The levels of carbon dioxide and other greenhouse gases have been rising rapidly (see
Global Warming). The identification of the hole in the stratospheric ozone layer in 1985 caused great concern
because ozone blocks harmful ultraviolet light. The discovery that this hole has been growing progressively larger
demonstrated that humankind's activities are capable of causing global changes that threaten the very existence of
life on Earth. An international panel of scientists was assigned to assess climate change, and the first report,
Climate Change: The IPCC Scientific Assessment, was published in 1990, with supplement in 1992. The scientists
made a number of predictions of how climate would change, based on their study of present and past climates and
on their calculations of the effect of emissions of greenhouse gases at various levels. Enhanced global warming
and a rise in sea level seem inevitable. Paradoxically, since humankind is almost certainly living in an interglacial
period, enhanced global warming may actually help prevent the onset of another glaciation, although at the cost of
a significant rise in sea level and consequent effects on low-lying areas.
Contributed By:
Graham Durant, B.Sc., Ph.D.
3/8/2007 7:03 PM
Paleoclimatology - Printer-friendly - MSN Encarta Premium
4 of 4
http://encarta.msn.com/text_761568730___0/Paleoclimatology.html
Deputy Director, Hunterian Museum, University of Glasgow. Director, Science Projects Scotland Limited.
"Paleoclimatology," Microsoft® Encarta® Online Encyclopedia 2007
http://encarta.msn.com © 1997-2007 Microsoft Corporation. All Rights Reserved.
© 1993-2007 Microsoft Corporation. All Rights Reserved.
3/8/2007 7:03 PM