Paleoclimatology

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, corals, shells and rocks to determine the past state of the climate system on Earth.

Contents

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.
  • 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.
  • Layering can be observed due to seasonal pauses in ice accumulation and can be used to establish chronology; associating specific depths of the core with ranges of time.
  • Changes in the layering thickness can be used to determine changes in precipitation or temperature.
  • Oxygen-18 quantity changes (δ18O) in ice layers represent changes in average ocean surface temperature. Water molecules containing the heavier O-18 evaporate at a higher temperature than water molecules containing the normal Oxygen-16 isotope. The ratio of O-18 to O-16 will be relatively higher as temperature increases and relatively less as temperature decreases. Various cycles in those isotope ratios have been detected.
  • Pollen has been observed in the ice cores and can be used to understand which plants were present as the layer formed. Pollen is produced in abundance and its distribution is typically well understood. A pollen count for a specific layer can be produced by observing the total amount of pollen categorized by type (shape) in a controlled sample of that layer. Changes in plant frequency over time can be plotted through statistical analysis of pollen counts in the core. Knowing which plants were present leads to an understanding of precipitation and temperature, and types of fauna present. Palynology includes the study of pollen for these purposes.
  • Volcanic ash is contained in some layers, and can be used to establish the time of the layer's formation. Each volcanic event distributed ash with a unique set of properties (shape and color of particles, chemical signature). Establishing the ash's source will establish a range of time to associate with layer of ice.
Dendroclimatology
Climatic information can be obtained through an understanding of changes in tree growth. Generally, trees respond to changes in climatic variables by speeding up or slowing down growth, which in turn is generally reflected a greater or lesser thickness in growth rings. Different species, however, respond to changes in climatic variables in different ways. A tree-ring record is established by compiling information from many living trees in a specific area. Older intact wood that has escaped decay can extend the time covered by the record by matching the ring depth changes to contemporary specimens. Using this method some areas have tree-ring records dating back a few thousand years. Older wood not connected to a contemporary record can be dated generally with radiocarbon techniques. A tree-ring record can be used to produce information regarding precipitation, temperature, hydrology, and fire corresponding to a particular area.

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 Foraminifera tests, can be used to reconstruct past temperature.
  • Isotopic ratios can provide further information. Specifically, the δ18O record responds to changes in temperature and ice volume, and the δ13C 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 (see also sclerochronology)
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 δ18O of coraline red algae provides a useful proxy of sea surface temperature at high latitudes, where many traditional techniques are limited.[1]

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, the source of most isotopic data, only exists on oceanic plates, which are eventually subducted — the oldest remaining material is 200 million years old. Older sediments are also more prone to corruption by diagenesis. Resolution and confidence in the data decrease over time.

Planet's timeline

See also: Geologic time scale and History of Earth

Knowledge of precise climatic events decreases as the record goes further back in time. Some notable events are noted below, with a timescale for context.

Ediacaran Paleoproterozoic Mesoproterozoic Hadean Archean Proterozoic Phanerozoic Precambrian

Cambrian Ordovician Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleozoic Mesozoic Cenozoic Phanerozoic

Paleocene Eocene Oligocene Miocene Pleistocene Paleogene Neogene Quaternary Cenozoic

Millions of Years

History of the atmosphere

Earliest atmosphere

The outgassings of the Earth was stripped away by solar winds early in the history of the planet until a steady state was established, the first atmosphere. Based on today's volcanic evidence, this atmosphere would have contained 80% water vapor, 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, hydrogen, methane and inert gases.

A major rainfall led 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 carbonate sediments.

Second atmosphere

As early as 3.8 billion years ago, water related sediments have been found.[2] 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.[3] 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. In 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 [4] As Jan Veizer assumed 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.[4]

Third atmosphere

Oxygen content of the atmosphere over the last billion years

The accretion of continents about 3.5 billion years ago[5] 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 billion years 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%.[6] 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's history, only one major glaciation is to be found in the geological record. Since about 950 million years ago, 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, significantly reduced solar wind.[7]

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 the Rodinia supercontinent. Massive deposits of tillites are found and anomalous isotopic signatures are found, which gave rise to the 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

500 million years of climate change
500 million years of changes in carbon dioxide concentrations

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, watersheds and ocean oscillations. In the early Phanerozoic, increased atmospheric carbon dioxide concentrations have been linked to driving or amplifying increased global temperatures.[8] Royer et al. 2004[9] 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 an 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. The constant rearrangement of continents by plate tectonics 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.

Phanerozoic till today's temperature record
Ice age record

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 clathrate reservoirs in the oceans.

A similar, single event of induced severe climate change after a meteorite impact has been proposed as reason for the Cretaceous-Tertiary extinction event. Other major thresholds 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).

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

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 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 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)

Correlation between variations in cosmic ray flux (red) and change in sea temperature (black). Data as presented by Shaviv & Veizer[10].

It has been proposed that long term galactic motions of the sun have a major influence on the 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.[11] 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 uncertain and vary from 143 million years[10] to 176 million years[12]). 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.[13] The causal link between these galactic motions and climate is unclear but one (controversial) postulate is the effect that entering a denser region of the galaxy will have on increasing the cosmic ray flux (CRF).[10] This theory has been criticised, both for overstating the correlation with CRF and for failing to propose a believable mechanism that would allow CRF to drive temperature.[9] The claims by Henrik Svensmark that CRF also strongly affects short term climate changes is even more controversial and has been challenged by many.[14][15]

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.[12]

Very long term (109 years or more)

Jan Veizer[4] and Nir Shaviv[10] 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 today's would have been sufficient to explain an ice free earth and the faint young sun paradox[16]. 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

References

Notes

  1. ↑ Halfar, J.; Steneck, R.S.; Joachimski, M.; Kronz, A.; Wanamaker, A.D. (2008). "Coralline red algae as high-resolution climate recorders". Geology 36: 463. doi:10.1130/G24635A.1. 
  2. ↑ Windley, B. (1984). The Evolving Continents. New York: Wiley Press. ISBN 0471903760. 
  3. ↑ J. Schopf (1983). Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton NJ: Princeton University Press. ISBN 0691083231. 
  4. ↑ 4.0 4.1 4.2 Veizer, J. (2005). Celestial climate driver: a perspective from four billion years of the carbon cycle. Geoscience Canada
  5. ↑ Veizer (1976). Windley, B.F.. ed. The Early History of the Earth. London: John Wiley and Sons. p. 569. 
  6. ↑ Summary Chart for the Precambrian
  7. ↑ Shaviv N.J. (2002). "Cosmic Ray Diffusion from the Galactic Spiral Arms, Iron Meteorites and a possible Climatic Connection". Physical Review Letters 89 (5): 051102. doi:10.1103/PhysRevLett.89.051102. PMID 12144433. 
  8. ↑ Rosemarie E. Came, John M. Eiler, Jan Veizer, Karem Azmy, Uwe Brand & Christopher R. Weidman (September 2007). "Coupling of surface temperatures and atmospheric CO2 concentrations during the Palaeozoic era". Nature 449 (7159): 198–201. doi:10.1038/nature06085. PMID 17851520. 
  9. ↑ 9.0 9.1 Royer, Dana L. and Robert A. Berner, Isabel P. Montañez, Neil J. Tabor, David J. Beerling (July 2004). "CO2 as a primary driver of Phanerozoic climate". GSA Today 14 (3): 4–10. doi:10.1130/1052-5173(2004)014<4:CAAPDO>2.0.CO;2. http://www.gsajournals.org/gsaonline/?request=get-document&issn=1052-5173&volume=014&issue=03&page=0004. 
  10. ↑ 10.0 10.1 10.2 10.3 Shaviv, NJ, Veizer, J (July 2003). "Celestial driver of Phanerozoic climate?". GSA Today 7 (7): 4–10. , see also online version or online discussion
  11. ↑ Borrero, Hess et al. (2008). Earth Science: Geology, the Environment, and the Universe. Glencoe: McGraw-Hill. p. 348. ISBN 0-07-875045-8. 
  12. ↑ 12.0 12.1 Gillman, M, Erenler, H (2008). "The galactic cycle of extinction". International Journal of Astrobiology 7. doi:10.1017/S1473550408004047. 
  13. ↑ Huggett, RJ (2003). Environmental Change the Evolving Ecosphere. Routledge. p. 48. ISBN 0-415-14520-1. 
  14. ↑ Schmidt, Gavin (2007-06-01). "Clouding the issue of climate". Physics World. http://physicsworld.com/cws/article/print/30103. 
  15. ↑ K. S. Carslaw, R. G. Harrison, J. Kirkby (November 2002). "Atmospheric Science: Cosmic Rays, Clouds, and Climate". Science 298 (5599): 1732–7. doi:10.1126/science.1076964. PMID 12459578. 
  16. ↑ Shaviv, N. J. (2003). "Toward a solution to the early faint Sun paradox: A lower cosmic ray flux from a stronger solar wind". J. Geophys. Res. 108 (A12): 1437. doi:10.1029/2003JA009997. http://arxiv.org/abs/astro-ph/0306477v2. 

Bibliography

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External links

Subfields of physical geography

Biogeography · Climatology / Paleoclimatology · Coastal geography · Geomorphology · Glaciology · Hydrology / Hydrography · Landscape ecology · Limnology · Oceanography · Palaeogeography · Pedology · Quaternary science

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