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Paleoclimatology
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Paleoclimatology (also Palaeoclimatology) is the study of climate change taken on the scale of the entire history of Earth. It uses records from ice sheets, tree rings, sediment, and rocks to determine the past state of the climate system on Earth.
Reconstructing ancient climates
Paleoclimatologists employ a wide variety of techniques to deduce ancient climates.
Ice:Mountain Glaciers and the polar ice caps/ice sheets are a widely employed source of data in paleoclimatology.
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Encyclopedia
Paleoclimatology (also Palaeoclimatology) is the study of climate change taken on the scale of the entire history of Earth. It uses records from ice sheets, tree rings, sediment, and rocks to determine the past state of the climate system on Earth.
Reconstructing ancient climates
Paleoclimatologists employ a wide variety of techniques to deduce ancient climates.
Ice:Mountain Glaciers and the polar ice caps/ice sheets are a widely employed source of data in paleoclimatology. Recent ice coring projects in the ice caps of Greenland and Antarctica have yielded data going back several hundred thousand years -- over 800,000 years in the case of the EPICA project.
- 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 precipitation that year. Certain layers contain ash from volcanic eruptions.
- Air trapped within fallen snow becomes encased in tiny bubbles as the snow is compressed into ice in the glacier under the weight of later years' snow. This trapped air has proven a tremendously valuable source for direct measurement of the composition of air from the time the ice was formed.
- 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.
Dendroclimatology: This science retrieves climate information from tree rings. Rings from living trees of great age give data about recent centuries back to a few millennia. Older intact wood that has escaped decay can extend the time covered by identifying patterns that match rings of known age from live trees. 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.
On a longer time scale, geologists must refer to the sedimentary record for data.
Sedimentary content:
- Sediments, sometimes lithified to form rock, may contain remnants of preserved vegetation, animals, plankton or pollen, which may be characteristic of certain climatic zones.
- Biomarker molecules such as the alkenones may yield information about their temperature of formation.
- Chemical signatures, particularly Mg/Ca ratio of calcite in foramanifera tests, can be used to reconstruct past temperature.
- Isotopic ratios can provide further information. Specifically, the record responds to changes in temperature and ice volume, and the record reflects a range of factors, which are often difficult to disentangle.
Sedimentary facies:On a longer time scale, the rock record may show signs of sea level rise and fall; further, features such as "fossilised" sand dunes can be identified. Scientists can get a grasp of long term climate by studying sedimentary rock going back billions of years. The division of earth history into separate periods is largely based on visible changes in sedimentary rock layers that demarcate major changes in conditions. Often these include major shifts in climate.
Corals:Coral "rings" are similar to tree rings, except they respond to different things, such as the water temperature and wave action. From this source, certain equipment can be used to derive the sea surface temperature and water salinity from the past few centuries. The of coraline red algae provides a useful proxy of sea surface temperature at high latitudes, where many traditional techniques are limited.
Limitations
All records decrease in utility back in time. The oldest ice core taken was from the Antarctic and dates to 800,000 years old. An international effort is currently being made in the same location to core to 1.2 million years ago. The deep marine record, our source of most isotopic data, only exists on oceanic plates, which are eventually subducted - the oldest remaining material is old. Older sediments are also more prone to corruption by diagenesis. Consequently, our resolution and confidence in data decrease over time.
Planet's timeline
Our knowledge of precise climatic events decreases as we go further back in time. Some notable events are noted below, with a timescale for context.
History of the atmosphere
Earliest atmosphere
The outgassings of the Earth was stripped away by solar winds early in the history of the planet till a steady state was established, the first atmosphere. Based on todays volcanic evidence, this atmosphere would have contained 80% water vapor, 10% carbon dioxide, 5 to 7% hydrogen-sulfur, and smaller amounts of nitrogen, carbon monoxide, hydrogen, methane and inert gases.
A major rainfall lead to the buildup of a vast ocean, enriching the other agents, first carbon dioxide and later nitrogen and inert gases. A major part of carbon dioxide exhalations were soon dissolved in water and built up carbonatic sediments.
Second atmosphere
As early as 3,8 Billion years ago, water related sediments have been found . About 3,4 Billion years ago, nitrogen was the major part of the then stable second atmosphere. An influence of life has to be taken into account rather soon since hints on early life forms are to be found as early as 3,5 Billion years ago. The fact that this is not in line with the - compared to today 30% lower - solar radiance of the early sun has been described as the Faint young Sun paradox.
The geological record however shows a continually relatively warm surface during the complete early Temperature record of the earth with the exception of one cold glacial phase about 2,4 billion years ago. Sometime during the late Archean Era an oxygen containing atmosphere began to develop from photosynthesizing algae. The early basic carbon isotopy is very much in line with what is found today and the fundamental features of the carbon cycle were established as early as 4 billion years ago. As Jan Veizer assumed 2005, that not only did we have life as far back as we had rocks, but there was as much life then as today and the fundamental features of the carbon cycle were established as early as 4 billion years ago.
Third atmosphere
The accretion of continents about 3,5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1,7 Ga ago 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. O2 showed major ups and downs until reaching a steady state of more than 15%. The following time span was the Phanerozoic, during which oxygen-breathing metazoan life forms began to appear.
Climate during geological ages
Precambrian climate
In the first three quarters of the earth history, only one major glaciation is to be found in the geological record. Since about 950 million years, the Earth's climate has varied regularly between large-scale or just polar cap wide glaciation and extensively tropical climates.
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 and compared to the previous time, sigificantly reduced Solar Wind.
The climate of the late Precambrian showed some major glaciation events 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 gave rise to the (controversial) Snowball Earth hypothesis. As the Proterozoic Eon drew to a close, the Earth started to warm up. By the dawn of the Cambrian and the Phanerozoic, life forms were abundant in the Cambrian explosion with average global temperatures of about +22 °C.
Phanerozoic climate
Major drivers for the preindustrial ages have been variations of the sun, volcanic ashes and exhalations, relative movements of the earth towards the sun and tectonically induced effects as for major sea currents, water sheds and ocean oscillations. Especially the early Phanerozoic shows a decoupling of a rather high carbon dioxide content and significant global glaciation . Royer et al. 2004 found a climate sensitivity for the rest of the Phanerozoic which was calculated to be similar to today's modern range of values.
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 requirement for the development of large scale ice sheets seems to be 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 or absence of land masses at the poles is not sufficient to guarantee glaciations or exclude polar ice caps. 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.
The relatively warm local minimum between Jurassic and Cretaceous goes along with widespread tectonic activity, e.g. the breakup of Supercontinents.
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 climate changes due to sudden collapses of natural methane reservoirs in the oceans (see methane clathrates).
A similar, single event induced severe climate change after a metorite impact is discussed as reason for the Cretaceous-Tertiary. Other major threholds are the Permian-Triassic, and Ordovician-Silurian extinction events with various reasons suggested.
Quaternary sub-era
The Quaternary sub-era 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).
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
Short term (104 to 106 years)
Geologically short-term (<120,000 year) temperatures are believed to be driven by orbital factors (see Milankovitch cycles) amplified by changes in greenhouse gases. The arrangements of land masses on the Earth's surface are believed to influence the effectiveness of these orbital forcing effects.
Medium term (106 to 108 years)
Continental drift 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 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 Uplift-Weathering Hypothesis, first put forward by T. C. Chamberlin in 1899 and later independently proposed in 1988 by Maureen Raymo and colleagues, where upthrusting mountain ranges expose minerals to weathering resulting in their chemical conversion to carbonates thereby removing CO2 from the atmosphere and cooling the earth. Others have proposed similar effects due to changes in average water table levels and consequent changes in sub-surface biological activity and PH levels.
Long term (108 to 109 years)
It has been proposed that long term galactic motions of the sun have a major influence on earth's climate. There are two principal motions, the first and most significant is the orbit of the sun around the galactic centre with a period of the order of 240 million years. Since this period is different from the rotation period of the galactic spiral arms, the sun, and the earth with it, will periodically pass through the arms (estimates of the period are uncetain and vary from 143 million years to 176 million years). The second is an oscillatory bobbing motion, similar to a floating buoy, which will periodically take the sun through the galactic disc. The period of this bobbing motion is 67 million years, so a pass through the galactic plane will occur every 33 million years. The causal link between these galactic motions and climate is postulated to be the effect that entering a denser region of the galaxy will have on reducing cosmic ray flux (CRF). Veizer claims that an increase in CRF correlates with temperature decrease and suggests a possible causal link to be increased ionisation aerosols in the atmosphere providing more condensation nucleii for cloud formation. Hence the earth's albedo is increased, reflecting the sun's radiation and lowering earth's temperature. The claims by Henrik Svensmark that CRF also strongly affects short term climate changes is, however, highly controversial and has been challenged by many.
It has also been suggested that there is some correlation between these galactic cycles and geological periods. The reason for this is postulated to be that the earth experiences many more impact events while passing through high density regions of the galaxy. Both the climate changes and sudden impacts may cause, or contribute to, extinction events.
Very long term (109 years or more)
Jan Veizer and Nir Shaviv have proposed the interaction of cosmic rays, solar wind and the various magnetic fields to explain the long term evolution of earths climate. According to Shaviv, the early sun had emitted a stronger solar wind with a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to todays would have been sufficient to explain an ice free earth and the faint young sun paradox. The solar minimum around 2.4 billion years ago is consistent with an established cosmic ray flux modulation by a variable star formation rate in the Milky Way and there is also a hint of an extinction event at this time. Within the last billion years the solar wind has significantly diminished. It is only within this more recent time that passages of the heliosphere through the spiral arms of the galaxy have been able to gain a strong and regularly modulating influence as described above.
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.
See also
Bibliography
- Bradley, Raymond S. (1985.) Quaternary paleoclimatology : methods of paleoclimatic reconstruction (Boston: Allen & Unwin) ISBN 0-04-551067-9, ISBN 0-04-551068-7.
- Imbrie, John. (1986) Ice ages : solving the mystery (Cambridge, Massachusetts: Harvard University Press, 1986, c1979).
- Margulis, Lynn, and Dorion Sagan. (c1986) Origins of sex: three billion years of genetic recombination (New Haven : Yale University Press) Series : The Bio-origins series; ISBN 0-300-03340-0.
- Gould, Stephen Jay. (c1989) Wonderful life, the story of the Burgess Shale (New York: W.W. Norton) ISBN 0-393-02705-8.
- Crowley, Thomas J., and North, Gerald R. (1996) Paleoclimatology (Oxford, UK: Clarendon Press) Series : Oxford monographs on geology and geophysics no. 18; ISBN 0-19-510533-8.
- Karl-Heinz Ludwig: Eine kurze Geschichte des Klimas. Von der Entstehung der Erde bis heute, (A short history of climate, From the evolution of earth till today) Herbst 2006, ISBN 3-406-54746
- William F. Ruddimann, Earth's Climate - Past and Future, Palgrave Macmillan, 2001, ISBN 0716737418
- , Christian-Dietrich: Klima im Wandel, Tatsachen, Irrtümer, Risiken; (Climate Change, Facts, Errors, Risks) Deutsche-Verlags-Anstalt GmbH, 1992
- B. Windley: The Evolving Continents. Wiley Press, New York 1984
- Drummond, Carl N. and Wilkinson, Bruce H., (2006) Interannual Variability in Climate Data, Journal of Geology, v. 114, p. 325-339.
External links
- , an excellent overview by Prof. Richard A Muller of UC Berkley.
- News Archive
- Ice-driven CO2 feedback on ice volume by W. F. Ruddiman
- Rapid Climate Change
- Short history of climate
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