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Paleoclimatology

From Academic Kids

Paleoclimatology is the study of climate change taken on the scale of the entire history of the earth.

Contents

The beginning of Earth's climate

Life flourishes in the Cambrian

The earth's history dates back more than 4.5 billion years (Ga) which is divided roughly into 2 eons called the "cryptozoic" [Gr. kryptos, hidden + zoo, life] and the "Phanerozoic" [Gr. phanerous, visible + zoo, life]. [Paleoclimatologists usually use geological time divisions, which would be the Priscoan, Archean, Proterozoic, and Phanerozoic Eons.] The division of these two eons is the boundary of the Precambrian and the Cambrian which occurred about 570Ma (570Ma means 570 million years before present, ie 570,000,000 BP). Originally it was thought that life began with the Cambrian. Evidence suggests that during this time the earth experienced perhaps the biggest explosion of diverse life forms that ever have been on the planet, and that since that time the total number of phyla has been gradually reduced with extinction after extinction (ref. Stephen Jay Gould: Wonderful life - the Story of the Cambrian and the Burgess Shale). Some influential researchers, including Briton Simon Conway Morris, an expert in the Burgess Shales, dispute the assertion that there were significantly more phyla at this time, suggesting that a number of creatures assigned to new phyla (particularly Hallucigenia) were actually arthropods or onychophores. Later work established that life evolved rather early in the history of the planet, perhaps as long as 4 billion years ago. Recent research suggests that life appeared on Earth just as soon as the planet cooled sufficiently for it to do so, and indeed may have arisen and been destroyed multiple times by planetismal impacts before the planet stabilized. This possibility has prompted some scientists to suggest that life is common in the universe, since it appeared so quickly here.

Techniques of paleoclimatology

Paleoclimatologists employ a wide variety of skills to arrive at their theories and conclusions.

Glaciers are a widely employed instrument in Paleoclimatology. The ice in glaciers has hardened into an identifiable pattern, with each year leaving a distinct layer in an ice core. It is estimated that the polar ice caps have 100,000 of these layers or more.

  • Inside of these layers scientists have found pollen, allowing them to estimate the total amount of plant growth of that year by the pollen count. The thickness of the layer can help to determine the amount of rainfall that year.
  • Because evaporation rates of water molecules with slightly heavier isotopes of hydrogen and oxygen are slightly different during warmer and colder periods, changes in the average temperature of the ocean surface are reflected in slightly different ratios between those isotopes. Various cycles in those isotope ratios have been detected.

Petrified tree rings give Paleoclimatology data over a much larger stretch of time. The fossil itself is dated with radioactive dating within a wide margin of error. The rings themselves can give some information about rainfall and temperature during that epoch.

Sediment layers have been studied, particularly those in the bottom of lakes and oceans. Characteristics of preserved vegetation, animals, pollen, and isotope ratios provide information.

Sedimentary rock layers provide a more compressed view of climate, as each layer of sediment is made over a period of hundreds of thousands to millions of years. Scientists can get a grasp of long term climate by studying sedimentary rock. Some scientists believe each layer designates a major change in climate.

Planet's timeline

Main article: geologic time scale

Some of the mile stones that mark the history of the planet are as follows:

4,000Ma earliest biogenic carbon
3,700Ma oldest rocks
3,500Ma oldest stromatolites
3,500Ma first evidence of sex [ref. Origins of Sex : Three Billion Years of Genetic Recombination]
3,450Ma earliest bacteria
3,800Ma banded iron formations (with reduced iron)
3,000Ma earliest precambrian ice ages [need ref]
[?] Chuos Tillites of South-West Africa
[?] Sturtian Tillites of the Finders Range, South-central Australia
3,000Ma earliest photosynthetic bacteria
2,700Ma oldest chemical evidence of complex cells
2,300Ma first green algae (eukaryotes)
2,000Ma free oxygen in the atmosphere
2,000Ma to 1600Ma Gowganda tillites in the Canadian shield
1,700Ma end of the banded iron formations and red beds become abundant (non-reducing atmosphere)
700Ma first metazoans late Proterozoic (Ediacaran Epoch) - first skeletons
570Ma to present Phanerozic Eon [history of the phanerozoic goes in here]
100Ma development of the angiosperms (flowering plants)
2Ma to present modern world and man's appearance on earth [whole history of man goes in here about 1/2000th of the time scale]
0.01Ma end of the last ice age
0.001Ma warming trend of the middle ages
0.0001Ma end of the mini ice age
0.00022Ma to present industrialized world and the introduction of man made greenhouse gases.

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Millions of Years

History of the atmosphere

Earliest atmosphere

The earliest atmosphere of the Earth was probably blown off early in the history of the planet. These gases were later replaced by an atmosphere derived from outgassing from the Earth. It is in this way that the oceans and the present atmosphere came to be.

Carbon dioxide and free oxygen

Free oxygen did not exist until about 1,700Ma and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. The early atmosphere and hydrosphere (up until about 2,000Ma) were devoid of free Oxygen. Gradually early photosynthesis managed to convert the abundant CO2 releasing O2.

The very early atmosphere of the earth contained mostly carbon dioxide (CO2) : about 80%. This gradually dropped to about 20% by 3,500Ma. This coincides with the development of the first bacteria about 3,500Ma. By the time of the development of photosynthesis (2,700Ma), CO2 levels in the atmosphere were in the range of 15%. During the period from about 2,700Ma to about 2,000Ma, photosynthesis dropped the CO2 concentrations from about 15% to about 8%. By about 2,000Ma free O2 was beginning to accumulate. This gradual reduction in CO2 levels continued to about 600Ma at which point CO2 levels were below 1% and O2 levels had risen to more than 15%. 600Ma corresponds to the end of the Precambrian and the beginning of the Cambrian, the end of the cryptozoic and the beginning of the Phanerozic, and the beginning of oxygen-breathing life.

Precambrian climate

The climate of the late Precambrian was typically cold with glaciation spreading over much of the earth. At this time the continents were bunched up in a supercontinent called Rodinia. Massive deposits of tillites are found and anomalous isotopic signatures are found which are consistent with the idea that the earth at this time was a massive snowball. Click here for a map of Rodinia at the end of the Precambrian [Click Link (http://www.scotese.com/precambr.htm)] after Australia and Antarctica rotated away from the southern hemisphere. [- can we get a picture of the geological section from the Flinders Range?]

As the Proterozoic Eon drew to a close the earth started to warm up. By the dawn of the Cambrian and the Phanerozoic Eon, Earth was experiencing average global temperatures of about +22C. Hundreds of millions of years of ice were replaced with the balmy tropical seas of the Cambrian Period within which life exploded at a rate never seen before or after. [ref. Stephen Jay Gould - Wonderful life, the story of the Burgess Shale].

Phanerozoic Climate

500 Million Years of Climate Change
Enlarge
500 Million Years of Climate Change

The establishment of CO2-consuming (and Oxygen-producing) photosythesizing organisms in the Precambrian led to the production of an atmosphere much like today's, though generally much higher in CO2 than today. The atmospheric concentration of CO2 has been gradually decreasing from a concentration of 7000 ppm 530 million years ago. In fact, only the Carboniferous Period and our present age, the Quaternary Period, have witnessed CO2 levels less than 400 ppm (see article "Climate and the Carboniferous Period" (http://www.geocraft.com/WVFossils/Carboniferous_climate.html|)). Earth's atmosphere today contains about 370 ppm CO2 (0.037%). Click this Link (http://www.geocraft.com/WVFossils/PageMill_Images/image277.gif) to see a graph of the world's average temperature and and the CO2 levels throughout the Phanerozoic.

The Earth's temperature during the Phanerozoic has been either ~12C or ~22C, but it has seldom stayed at an intermediate value for any significant length of time. For most of the past 600 million years the world's average temperature was about 22C. The exceptions to that are as follows. For a short period at the end of the Ordovician (~440 mybp) there was a drop to about 12C that lasted a few million years. In the Upper Devonian through most of the Mississippian (~370 mybp to ~310 mybp) the temperature dropped to about 20C. Then, during the Pennsylvanian and Permian (~310 mybp to ~250 mybp) there was a protracted period when the world's average temperature again dropped to about 12C. For a few million years around the end of the Jurassic and the beginning of the Cretaceous there was another drop, but this time only to about 17C. For the last 40 million years or so the world's temperature has been dropping and is now again hovering around 12C.


Quaternary subera

The Quaternary subera includes the current climate. There has been a cycle of ice ages for the past 2.2-2.1 million years (starting before the Quaternary in the late Neogene Period).

Ice core data for the past 400,000 years. Note length of glacial cycles averages ~100,000 years.  Blue curve is temperature, green curve is CO2, and red curve is windblown glacial dust (loess). Today's date is on the left side of the graph.
Enlarge
Ice core data for the past 400,000 years. Note length of glacial cycles averages ~100,000 years. Blue curve is temperature, green curve is CO2, and red curve is windblown glacial dust (loess). Today's date is on the left side of the graph.
Note in the graphic on the right the strong 120,000 year periodicity of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed that ice ages deepen by progressive steps, but the recovery to interglacial conditions occurs in one big step.

Controlling Factors

Geologically short-term (<120,000 year) temperatures are believed to be driven by orbital factors (see Milankovitch cycles). The arrangements of land masses on the Earth's surface are believed to reinforce these orbital forcing effects.

The timing of ice ages throughout geologic history is in part controlled by the position of the continental plates on the surface of the Earth. When landmasses are concentrated near the polar regions, there is an increased chance for snow and ice to accumulate. Small changes in solar energy can tip the balance between summers in which the winter snow mass completely melts and summers in which the winter snow persists until the following winter. See the web site Paleomap Project (http://www.scotese.com/earth.htm) for images of the polar landmass distributions through time.

Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as is the case today. Today, Greenland, Antarctica, and the northern portions of Europe, Asia, and North America are situated such that a minor change in solar energy will tip the balance between year-round snow/ice preservation and complete summer melting. The presence of snow and ice is a well-understood positive feedback mechanism for climate. The Earth today is considered to be prone to ice age glaciations.

References

  • Bradley, R.S. (1985). Quaternary paleoclimatology: Methods of paleoclimatic reconstruction. Allen & Unwin.
  • Crowley, T.J., and North, G.R. (1991). Paleoclimatology. Oxford. ISBN 0195105338
  • Imbrie, J., and Imbrie, K.P. (1979). Ice ages: Solving the mystery. Enslow.
  • Template:Book reference

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