Paleoclimatology

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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</center>


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. After photosynthesis developed, photoautotrophs began 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 [1] 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

Image:Phanerozoic Climate Change.png Image:Phanerozoic Carbon Dioxide.png

Qualitatively, the Earth's climate was varied between conditions that support large-scale continental glaciation and those which are extensively tropical and lack permanent ice caps even at the poles. The time scale for this variation is roughly 140 million years and may be related to Earth's motion into and out of galactic spiral arms (Veizer and Shaviv 2003). The difference in global mean temperatures between a fully glacial earth and ice free Earth is estimated at approximately 10 °C, though far larger changes would be observed at high latitudes and smaller ones at low latitudes. One key requirement for the development of large scale ice sheets is the arrangement of continental land masses at or near the poles. With plate tectonics constantly rearranging the continents, it can also shape long-term climate evolution. However, the presence of land masses at the poles is not sufficient to guarantee glaciations. Evidence exists of past warm periods in Earth's climate when polar land masses similar to Antarctica were home to deciduous forests rather than ice sheets.

Changes in the atmosphere may also exert an important influence over 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 for most of this period it was much higher in CO2 than today. Similarly, the Earth's average temperature was also frequently higher than at present, though it has been argued that over very long time scales climate is largely decoupled from carbon dioxide variations (Veizer et al. 2000). Or more specifically that changing continental configurations and mountain building probably have a larger impact on climate than carbon dioxide. Others dispute this, and suggest that the variations of temperature in response to carbon dioxide changes have been underestimated (Royer et al. 2004). However, it is clear that the preindustrial atmosphere with only 280 ppm CO2 is not far from the lowest ever occurring since the rise of macroscopic life.

Superimposed on the long-term evolution between hot and cold climates have been many short-term fluctuations in climate similar to, and sometimes more severe than, the varying glacial and interglacial states of the present ice age. Some of the most severe fluctuations, such as the Paleocene-Eocene Thermal Maximum, may be related to rapid increases in atmospheric carbon dioxide due to the collapse of natural methane reservoirs in the oceans (see methane clathrates). Severe climate changes also seem to have occurred during the course of the Cretaceous-Tertiary, Permian-Triassic, and Ordovician-Silurian extinction events; however, it is unclear to what degree these changes caused the extinctions rather than merely responding to other processes that may have been more directly responsible for the extinctions.


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).

Image:Vostok-ice-core-petit.png
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.

Contental drift obviously affects the thermohaline circulation, which transfers heat between the equatorial regions and the poles, as does the extent of polar ice coverage.

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 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.

Another proposed factor in long term temperature change is the alteration of the carbon cycle balance favoring the production of greenhouse gases and reduction of the geological carbon reservoir. The Uplift-Weathering Hypothesis, first put forward by T. C. Chamberlin in 1899 and later independently proposed in 1988 by Maureen Raymo and colleagues, attributes increased geochemical erosion of limestone and other calcium carbonate geological deposits to increased mountain formation and higher average surface elevation. Others have proposed similar effects due to changes in average water table levels and consequent changes in sub-surface biological activity and PH levels.

Over the very long term the energy output of the sun has gradually increased, on the order of 5% per billion (109) years, and will continue to do so until it reaches the end of its current phase of stellar evolution.

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.
  • {{{Author|}}}{{|{{{3}}}}}}|show1| (1990)}}{{{{{Year|}}}}}}|show1|.}} {{|{{{3}}}}}}|show1|[{{{URL}}}}} Origins of Sex : Three Billion Years of Genetic Recombination{{|{{{3}}}}}}|show1|]}}{{|{{{3}}}}}}|show1|, {{{Pages}}}}}{{|{{{3}}}}}}|Show1|, Yale University Press, Hartford, Connecticut}}. {{{ID|}}}
  • Royer, Dana L., Robert A. Berner, Isabel P. Montañez, Neil J. Tabor, and David J. Beerling (2004). CO2 as a primary driver of Phanerozoic climate. GSA Today 14 (3): 4-10. DOI:<4:CAAPDO>2.0.CO;2 10.1130/1052-5173(2004)014<4:CAAPDO>2.0.CO;2
  • Shaviv, N. and Veizer, J. (2003) Celestial driver of Phanerozoic climate? GSA Today July 2003, 4-10.
  • Veizer, J., Godderis, Y. and Francois, L.M. (2000) Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon. Nature 408, 698-701.

External links

Retrieved from "http://en.freepedia.org/Paleoclimatology.html"


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