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Ice Age

Ice age

An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers ("glaciation"). Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age (because the Greenland and Antarctic ice sheets still exist). More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term ice age in the former, glaciological, sense; and use the term 'glacial periods' for colder periods during ice ages and 'interglacial' for the warmer periods. During the last few million years, there have been many glacial periods, occurring initially at 40,000-year frequency but more recently at 100,000-year frequencies. These are the best studied. There have been four major ice ages in the further past.

Origin of ice age theory

The idea that, in the past, glaciers had been far more extensive was folk knowledge in some alpine regions of Europe (Imbrie and Imbrie, p25, quote a woodcutter telling de Charpentier of the former extent of the Swiss Grimsel glacier). No single person invented the idea [http://academic.emporia.edu/aberjame/histgeol/agassiz/glacial.htm]. Between 1825 and 1833, Jean de Charpentier assembled evidence in support of this idea. In 1836 Charpentier convinced Louis Agassiz of the theory, and Agassiz published it in his book Étude sur les glaciers of 1840. At this early stage of knowledge, what were being studied were the glacial periods within the past few hundred thousand years, during the current ice age. The far earlier ice ages' very existence was unsuspected.

Major ice ages

There have been at least four major ice ages in the Earth's past. The earliest hypothesized ice age is believed to have occurred around 2.7 to 2.3 billion (10⁹) years ago during the early Proterozoic Age. :Main article: Snowball Earth. The earliest well-documented ice age, and probably the most severe of the last 1 billion years, occurred from 800 to 600 million years ago (the Cryogenian period) and it has been suggested that it produced a Snowball Earth in which permanent sea ice extended to or very near the equator. It has been suggested that the end of this ice age was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial. A minor ice age occurred from 460 to 430 million years ago, during the Late Ordovician Period. There were extensive polar ice caps at intervals from 350 to 260 million years ago, during the Carboniferous and early Permian Periods. early Permian The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica, but intensified during the Pleistocene (starting around 3 million years ago) with the spread of ice sheets in the Northern Hemisphere. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000 and 100,000 year time scales. The last glacial period ended about 10,000 years ago. 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. Due to the positions of Greenland, Antarctica, and the northern portions of Europe, Asia, and North America in polar regions, the Earth today is considered prone to ice age glaciations. Evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. It took some time for the current theory to be worked out. Analyses of ice cores and ocean sediment cores unambiguously show the record of glacials and interglacials over the past few million years.

Interglacials

glacial erratic In between ice ages, there are multi-million year periods of more temperate climate, but also within the ice ages (or at least within the last one), temperate and severe periods occur. The colder periods are called 'glacial periods', the warmer periods 'interglacials', such as the Eemian interglacial era. We are in an interglacial period now, the last retreat ending about 10,000 years ago. There appears to be a folk wisdom that "the typical interglacial period lasts ~12,000 years" but this is hard to substantiate from the evidence of ice core records. For example, an article in Nature [http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v429/n6992/abs/nature02599_fs.html] argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Nonetheless, fear of a new glacial period starting soon does exist (See: global cooling). However, many now believe that anthropogenic (manmade) forcing from increased "greenhouse gases" would outweigh any Milankovitch (orbital) forcing; and some recent considerations of the orbital forcing have even argued that in the absence of human perturbations the present interglacial could potentially last 50,000 years.

Causes of ice ages

The cause of ice ages remains controversial for both the large-scale ice age periods and the smaller ebb and flow of glacial/interglacial periods within an ice age. The general consensus is that it is a combination of up to three different factors: atmospheric composition (particularly the fraction of CO₂ and methane), changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy), and the arrangement of the continents. The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO₂ levels in the atmosphere. However, the other two factors do matter. An abundance of land within the Arctic and Antarctic Circles appears to be a necessity for an ice age, probably because the landmasses provide space on which snow and ice can accumulate during cooler times and thus trigger positive feedback processes like albedo changes. The Earth's orbit does not have a great effect on the long-term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within the current ice age. The complex pattern of changes in Earth's orbit and the change of albedo may influence the occurrence of glacial and interglacial phases — this was first explained by the theory of Milutin Milankovic. The present ice ages are the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milankovic orbital forcing periods is so good that orbital forcing is the generally accepted explanation. The combined effects of the changing distance to the sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis can change and significantly redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which impact the intensity of seasons. For example, the amount of solar influx in July at 65 degrees north latitude is calculated to vary by as much as 25% (from 400 W/m² to 500 W/m², see graph at [http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html]). It is widely believed that ice sheets advance when summers become too mild to melt all of the accumulated snowfall from the previous winter. Some workers believe that the strength of the orbital forcing appears to be too small to trigger glaciations, but feedback mechanisms like CO₂ may explain this mismatch. While Milankovic forcing predicts that cyclic changes in the Earth's orbital parameters can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial/interglacial periods. In particular, during the last 800 thousand years, the dominant inter/glacial oscillation has been 100 thousand years, which corresponds to changes in Earth's eccentricity and orbital inclination, and yet is by far the weakest of the three frequencies predicted by Milankovic. During the period 3.0 — 0.8 million years ago, the dominant pattern of glaciation corresponded to the 41 thousand year period of changes in Earth's obliquity (tilt of the axis). The reasons for preferring one frequency to another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system. The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000-year cycle over the last 8 cycles. Richard A. Muller and Gordon J. MacDonald [http://www.pnas.org/cgi/content/full/94/16/8329] [http://muller.lbl.gov/pages/glacialmain.htm] [http://muller.lbl.gov/papers/sciencespectra.htm] and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100 thousand year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in insolation, as the earth moves in and out of known dust bands in the solar system. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Rial [http://pangea.stanford.edu/Oceans/GES290/Rial1999.pdf]. Another worker, Ruddiman has suggested a plausible model that explains the 100,000 cycle by the modulating effect of eccentricity (weak 100,000 year cycle) on precession (23,000 year cycle) combined with greenhouse gas feedbacks in the 41,000 and 23,000-year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41,000-year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the 2nd or 3rd cycle triggers an ice age. This would imply that the 100,000-year periodicity is really an illusion created by averaging together cycles lasting 80 and 120 thousand years. This theory is consistent with the existing uncertainties in dating, but not widely accepted at present (Nature 434, 2005, [http://web.mit.edu/~phuybers/www/Doc/Obliquity_HuybersWunsch.pdf]).

Recent glacial and interglacial phases

Richard A. Muller See Timeline of glaciation.

Glaciation in North America

The Wisconsinan glaciation has had a considerable effect on the landscape of the Northern Hemisphere. In North America, the Great Lakes and the Finger Lakes were carved by ice's deepening of old valleys. The old Teays River drainage system was radically altered and largely reshaped into the Ohio River drainage system. Other rivers were dammed and diverted to new channels, such as the Niagara, which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall near Syracuse, New York is now dry. Long Island was formed from glacial till, and the watersheds of Canada were so severely disrupted that they are still sorting themselves out — the plethora of lakes on the Canadian Shield in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of loess many dozens of feet thick in the Missouri Valley. Isostatic rebound continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets. The Driftless Zone, around the junction of Wisconsin, Minnesota, and Iowa, was not covered by glaciers.

Reference

- Imbrie, John and Katherine Palmer Imbrie. Ice ages: Solving the Mystery. Cambridge, Massachusetts: Harvard University Press, 1979, 1986 (reprint). ISBN 089490020X; ISBN 0894900153; ISBN 0674440757.

External links

- [http://www.pbs.org/wgbh/nova/ice/ Cracking the Ice Age] from PBS
- http://www.globalchange.umich.edu/globalchange1/current/lectures/samson/climate_patterns/ Category:Glaciology Category:History of climate ms:Zaman air batu ja:氷河期

Temperature

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. Physically, temperature is a measure of the random agitation of matter and ambiant photons, under the effect of thermal fluctuations. It is a fundamental parameter in thermodynamics and it is conjugate to entropy. More quantitatively, the order of magnitude of the fluctuations of the energy associated with an atom, molecule or another elementary constituant of a physical system is

k_B T

, where

k_B

is Boltzmann's constant, and T is temperature, expressed in Kelvins.

Overview

The formal properties of temperature are studied in thermodynamics and statistical mechanics. The temperature of a system at thermodynamic equilibrium is defined by a relation between the amount of heat

\delta Q

incident on the system during an infinitesimal quasistatic transformation, and the variation

\delta S

of its entropy during this transformation. :

\delta S = \frac

Contrarly to entropy and heat, whose microscopic definitions are valid even far away from thermodynamic equilibrium temperature can only be defined at thermodynamic equilibrium, or local thermodynamic equilibrium (see below). As a system receives heat its temperature rises, similarly a loss of heat from the system tends to decrease its temperature (at the - uncommon - exception of negative temperature, see below). When two systems are at the same temperature, no heat transfer occurs between them. When a temperature difference does exist, heat will tend to move from the higher-temperature system to the lower-temperature system, until they are at thermal equilibrium. This heat transfer may occur via conduction, convection or radiation (see heat for additional discussion of the various mechanisms of heat transfer). Temperature is also related to the amount of internal energy and enthalpy of a system. The higher the temperature of a system, the higher its internal energy and enthalpy are. Temperature is an intensive property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intensive properties include pressure and density. By contrast, mass and volume are extensive properties, and depend on the amount of material in the system.

Role of temperature in nature

Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology. Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted. Temperature-dependence of the speed of sound in air c, density of air ρ and acoustic impedance Z vs. temperature °C

Temperature measurement

Main article: Temperature measurement Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Rømer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 10³² K). In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electronvolts (eV) or kiloelectronvolts (keV), where 1 eV = 11,605 K. In the study of QCD matter one routinely meets temperatures of the order of a few hundred MeV, equivalent to about 10¹² K. For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from degrees Celsius to kelvins. :

\mathrm

In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F. The following formula can be used to convert from Fahrenheit to Celsius: :

\mathrm

See temperature conversion formulas for conversions between most temperature scales. ¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.
² Some numbers in this table have been rounded off.
³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F.

Negative temperatures

:See main article: Negative temperature. For some systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero, but rather it is, in a sense, hotter than infinite temperature (sic).

Articles about temperature ranges:

- 10⁻¹² K = 1 picokelvin (pK)
- 10⁻⁹ K = 1 nanokelvin (nK)
- 10⁻⁶ K = 1 microkelvin (µK)
- 10⁻³ K = 1 millikelvin (mK)
- 10⁰ K = 1 kelvin
- 10¹ K = 10 kelvins
- 10² K = 100 kelvins
- 10³ K = 1,000 kelvin = 1 kilokelvin (kK)
- 10⁴ K = 10,000 kelvins = 10 kK
- 10⁵ K = 100,000 kelvins = 100 kK
- 10⁶ K = 1 megakelvin (MK)
- 10⁹ K = 1 gigakelvin (GK)
- 10¹² K = 1 terakelvin (TK) See Orders of magnitude (temperature).

Theoretical foundation of temperature

Zeroth-law definition of temperature

While most people have a basic understanding of the concept of temperature, its formal definition is rather complicated. Before jumping to a formal definition, let us consider the concept of thermal equilibrium. If two closed systems with fixed volumes are brought together, so that they are in thermal contact, changes may take place in the properties of both systems. These changes are due to the transfer of heat between the systems. When a state is reached in which no further changes occur, the systems are in thermal equilibrium. Now a basis for the definition of temperature can be obtained from the so-called zeroth law of thermodynamics which states that if two systems, A and B, are in thermal equilibrium and a third system C is in thermal equilibrium with system A then systems B and C will also be in thermal equilibrium (being in thermal equilibrium is a transitive relation; moreover, it is an equivalence relation). This is an empirical fact, based on observation rather than theory. Since A, B, and C are all in thermal equilibrium, it is reasonable to say each of these systems shares a common value of some property. We call this property temperature. Generally, it is not convenient to place any two arbitrary systems in thermal contact to see if they are in thermal equilibrium and thus have the same temperature. Also, it would only provide an ordinal scale. Therefore, it is useful to establish a temperature scale based on the properties of some reference system. Then, a measuring device can be calibrated based on the properties of the reference system and used to measure the temperature of other systems. One such reference system is a fixed quantity of gas. The ideal gas law indicates that the product of the pressure and volume (P · V) of a gas is directly proportional to the temperature: :

P \cdot V = n \cdot R \cdot T

(1) where 'T is temperature, n is the number of moles of gas and R is the gas constant. Thus, one can define a scale for temperature based on the corresponding pressure and volume of the gas: the temperature in kelvins is the pressure in pascals of one mole of gas in a container of one cubic metre, divided by 8.31... In practice, such a gas thermometer is not very convenient, but other measuring instruments can be calibrated to this scale. Equation 1 indicates that for a fixed volume of gas, the pressure increases with increasing temperature. Pressure is just a measure of the force applied by the gas on the walls of the container and is related to the energy of the system. Thus, we can see that an increase in temperature corresponds to an increase in the thermal energy of the system. When two systems of differing temperature are placed in thermal contact, the temperature of the hotter system decreases, indicating that heat is leaving that system, while the cooler system is gaining heat and increasing in temperature. Thus heat always moves from a region of high temperature to a region of lower temperature and it is the temperature difference that drives the heat transfer between the two systems.

Temperature in gases

As mentioned previously for a monatomic ideal gas the temperature is related to the translational motion or average speed of the atoms. The kinetic theory of gases uses statistical mechanics to relate this motion to the average kinetic energy of atoms and molecules in the system. For this case 7736 K = 7463 degrees Celsius corresponds to an average kinetic energy of one electronvolt; to take room temperature (300 K) as an example, the average energy of air molecules is 300/7736 eV, or 0.0388 electronvolt. This average energy is independent of particle mass, which seems counterintuitive to many people. Although the temperature is related to the average kinetic energy of the particles in a gas, each particle has its own energy which may or may not correspond to the average. However, after an examination of some basic physics equations it makes perfect sense. The second law of thermodynamics states that any two given systems when interacting with each other will later reach the same average energy. Temperature is a measure of the average kinetic energy of a system. The formula for the kinetic energy of an atom is:
:

K_t = \begin \frac \end mv^2

(Note that a calculation of the kinetic energy of a more complicated object, such as a molecule, is slightly more involved. Additional degrees of freedom are available, so molecular rotation or vibration must be included.)

Thus, particles of greater mass (say a neon atom relative to a hydrogen molecule) will move slower than lighter counterparts, but will have the same average energy. This average energy is independent of the mass because of the nature of a gas, all particles are in random motion with collisions with other gas molecules, solid objects that may be in the area and the container itself (if there is one). A visual illustration of this [http://intro.chem.okstate.edu/1314F00/Laboratory/GLP.htm from Oklahoma State University] makes the point more clear. Not all the particles in the container have different velocities, regardless of whether there are particles of more than one mass in the container, but the average kinetic energy is the same because of the ideal gas law. In a gas the distribution of energy (and thus speeds) of the particles corresponds to the Boltzmann distribution. An electronvolt is a very small unit of energy, approximately 1.602×10^-19 joule.

Temperature of the vacuum

When a satellite in empty space is heated by sunshine and cooled by radiating energy away it is not in thermodynamic equilibrium and has no well-defined temperature. A system in a vacuum will radiate its thermal energy, i.e. convert heat into electromagnetic waves. If vacuum is filled with electromagnetic waves (say, radiation from walls of vacuum chamber, or relic microwave radiation in space) then the system will exchange by energy with these waves and thermally equilibrates at some finite (non zero) temperature. Cosmic microwave background radiation being remnant of radiation of hot early universe when radiation was in thermal equilibrium with matter has Planck spectrum (black body spectrum) with the temperature (at present) of about 2.7 K.

Second-law definition of temperature

In the previous section temperature was defined in terms of the Zeroth Law of thermodynamics. It is also possible to define temperature in terms of the second law of thermodynamics, which deals with entropy. Entropy is a measure of the disorder in a system. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability. Consider a series of coin tosses. A perfectly ordered system would be one in which every coin toss would come up either heads or tails. For any number of coin tosses, there is only one combination of outcomes corresponding to this situation. On the other hand, there are multiple combinations that can result in disordered or mixed systems, where some fraction are heads and the rest tails. As the number of coin tosses increases, the number of combinations corresponding to imperfectly ordered systems increases. For a very large number of coin tosses, the number of combinations corresponding to ~50% heads and ~50% tails dominates and obtaining an outcome significantly different from 50/50 becomes extremely unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy. Now, we have stated previously that temperature controls the flow of heat between two systems and we have just shown that the universe, and we would expect any natural system, tends to progress so as to maximize entropy. Thus, we would expect there to be some relationship between temperature and entropy. In order to find this relationship let's first consider the relationship between heat, work and temperature. A heat engine is a device for converting heat into mechanical work and analysis of the Carnot heat engine provides the necessary relationships we seek. The work from a heat engine corresponds to the difference between the heat put into the system at the high temperature, q_H and the heat ejected at the low temperature, q_C. The efficiency is the work divided by the heat put into the system or: :

\textrm = \frac  = \frac = 1 - \frac

(2) where w_cy is the work done per cycle. We see that the efficiency depends only on q_C/q_H. Because q_C and q_H correspond to heat transfer at the temperatures T_C and T_H, respectively, q_C/q_H should be some function of these temperatures: :

\frac = f(T_H,T_C)

(3) Carnot's theorem states that all reversible engines operating between the same heat reservoirs are equally efficient. Thus, a heat engine operating between T₁ and T₃ must have the same efficiency as one consisting of two cycles, one between T₁ and T₂, and the second between T₂ and T₃. This can only be the case if: :

q_ = \frac

which implies: :

q_13 = f(T_1,T_3) = f(T_1,T_2)f(T_2,T_3)

Since the first function is independent of T₂, this temperature must cancel on the right side, meaning f(T₁,T₃) is of the form g(T₁)/g(T₃) (i.e. f(T₁,T₃) = f(T₁,T₂)f(T₂,T₃) = g(T₁)/g(T₂)· g(T₂)/g(T₃) = g(T₁)/g(T₃)), where g is a function of a single temperature. We can now choose a temperature scale with the property that: :

\frac = \frac

(4) Substituting Equation 4 back into Equation 2 gives a relationship for the efficiency in terms of temperature: :

\textrm = 1 - \frac = 1 - \frac

(5) Notice that for T_C = 0 K the efficiency is 100% and that efficiency becomes greater than 100% below 0 K. Since an efficiency greater than 100% violates the first law of thermodynamics, this implies that 0 K is the minimum possible temperature. In fact the lowest temperature ever obtained in a macroscopic system was 20 nK, which was achieved in 1995 at NIST. Subtracting the right hand side of Equation 5 from the middle portion and rearranging gives: :

\frac  - \frac = 0

where the negative sign indicates heat ejected from the system. This relationship suggests the existence of a state function, S, defined by: :

dS = \frac

(6) where the subscript indicates a reversible process. The change of this state function around any cycle is zero, as is necessary for any state function. This function corresponds to the entropy of the system, which we described previously. We can rearranging Equation 6 to get a new definition for temperature in terms of entropy and heat: :

T = \frac

(7) For a system, where entropy S may be a function S(E) of its energy E, the temperature T is given by: :

\frac = \frac

(8) The reciprocal of the temperature is the rate of increase of entropy with energy.

References

External links

- [http://www.unitconversion.org/unit_converter/temperature.html Online Temperature Converter] - convert between various units of temperature, such as kelvin, Celsius, Fahrenheit, Rankine, Reaumur, and even Triple point of water
- [http://www.unitconversion.org/unit_converter/temperature-v.html Interactive Temperature Conversion Table] - convert selected unit to all other units of temperature
- [http://www.indiana.edu/~animal/fun/conversions/temperature.html Temperature Conversions: Celsius, Fahrenheit, Kelvin, Réaumur and Rankine]
- [http://www.unidata.ucar.edu/staff/blynds/tmp.html An elementary introduction to temperature aimed at a middle school audience]
- [http://www.straightdope.com/mailbag/mtempscales.html Why do we have so many temperature scales?]
- [http://thermodynamics-information.net A Brief History of Temperature Measurement] Category:Meteorology Category:Physical quantity Category:Thermodynamics Category:Heat ko:온도 ja:温度 th:อุณหภูมิ

Climate

The climate (ancient Greek: κλίμα) is the weather averaged over a long period of time. The Intergovernmental Panel on Climate Change (IPCC) glossary definition is: : Climate in a narrow sense is usually defined as the “average weather”, or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.[http://www.grida.no/climate/ipcc_tar/wg1/518.htm]

Climate vs weather

In the most succinct words, weather is the combination of events in the atmosphere and climate is the overall accumulated weather in a certian location. The exact boundaries of what is climate and what is weather are not well defined and depend on the application. For example, in some senses an individual El Niño event could be considered climate; in others, as weather. When the original conception of climate as a long-term average came to be considered, perhaps towards the end of the 19th century, the idea of climate change was not current, and a 30 year average seemed reasonable (but see note 1). Given the current availability of long-term trends in the temperature record, it is harder to give a precise contradiction-free definition of climate: over a 30 year period, averages may shift; over a shorter period, the statistics are less stable.

Climate determinants

In a given geographical region, the climate generally does not vary over time on the scale of a human life span. However, over geological time, climate can vary considerably for a given place on the Earth. For example, Scandinavia has been through a number of ice ages over hundreds of thousands of years (the last one ending about 10,000 years ago). Paleoclimatology is the study of these past climates, their origin, and by extension, the origin of today's climate. Over historic time spans there are a number of static variables that determine climate including: altitude, proportion of land to water, and proximity to oceans and mountains. Other climate determinants are more dynamic: The Thermohaline circulation of the ocean distributes heat energy between the equatorial and polar regions; other ocean currents do the same between land and water on a more regional scale. Degree of vegetation coverage affects solar heat absorption, water retention, and rainfall on a regional level. Alterations in the quantity of atmospheric greenhouse gases determines the amount of solar energy retained by the planet, leading to global warming (or cooling). The variables which determine climate are numerous and the interactions complex but there is general agreement that the broad outlines are understood, at least in so far as the determinates of historical climate change are concerned.

Climate indices

Scientists use climate indices in their attempt to characterize and understand the various climate mechanisms that culminate in our daily weather. Much in the way the Dow Jones Industrial Average, which is based on the stock prices of 30 companies, is used to represent the fluctuations in the stock market as a whole, climate indices are used to represent the essential elements of climate. Climate indices are generally identified or devised with the twin objectives of simplicity and completeness, and each typically represents the status and timing of the climate factor they represent. By their very nature, indices are simple, and combine many details into an generalized, overall description of the atmosphere or ocean which can be used to characterize the factors which impact the global climate system. Because the climate indices are generally determined from measurements made in a localized area, they can have impacts in other areas around the globe, through processes sometimes called teleconnections. References:
- [http://www.arctic.noaa.gov/essay_bond.html Why and how do scientists study climate change in the Arctic? What are the Arctic climate indices?]
- [http://www.arctic.noaa.gov/climate.html Climate index and mode information]

Classifications

In the original sense, climate is a concept used to divide the world into regions sharing similar climatic parameters. Climate regions can be classified on the basis of temperature and precipitation alone. Examples of such climate schemes are the Köppen climate classification or the Thornthwaite climate classification schemes. For more details about specific climates, please see:
- Tropical climate
- Subtropical climate
- Arid climate
- Semiarid climate
- Mediterranean climate
- Temperate climate
- Oceanic climate
- Continental climate
- Alpine climate
- Subarctic climate
- Polar climate
- Climate of Antarctica To understand a climate of a specific place or area, please see the article on that place or area.

Historical climates

- Climate changes of 535-536
- Medieval climate optimum

National climates

- Climate of the Alps
- Climate of India
- Climate of the United Kingdom

External links

- [http://climateapps2.oucs.ox.ac.uk/cpdnboinc/ Climate Prediction Project]
- [http://www.worldclimate.com WorldClimate]
- [http://www.atmosphere.mpg.de/enid/1442 ESPERE Climate Encyclopaedia]
- [http://www.weatherbase.com Weatherbase]
- [http://www.climate-zone.com Global Climate Data]
- [http://www.limaperunet.com/climate/climateall.html The Climate of Peru]
- [http://www.arctic.noaa.gov/climate.html Climate index and mode information]
- [http://www.arctic.noaa.gov/essay_bond.html Why and how do scientists study climate change in the Arctic? What are the Arctic climate indices?]
- [http://www.arctic.noaa.gov/detect/ A near-realtime Arctic Change Indicator Website]
- [http://www.beringclimate.noaa.gov/ A current view of the Bering Sea Ecosystem and Climate]

Notes

# In "Climatology" by W G Kendrew (OUP; 3rd edition 1949; chapter 38; page 359) we find: "A well-known cycle is one with a mean period of about 35 years... which was worked out by Bruckner... the reality of this cycle seems to be well established, though it is of little use for actual forecasting; it is a basis of the choice of 35 years as the period estimated to give true mean values of climate elements." Category:Ecology ko:기후 ja:気候 simple:Climate

Ice sheet

An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² (19,305 mile²). The only current ice sheets are Antarctic and Greenland; during the last ice age at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada and North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern Chile. Ice sheets are bigger than ice shelves or glaciers. Masses of ice covering less than 50,000 km² are termed an ice cap. An ice cap will typically feed a series of glaciers around their periphery. Although the surface is cold, the base of an ice sheet is generally warmer, in places it melts and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet - these are ice streams. The present-day polar ice sheets are relatively young in geological terms. The Antarctic Ice Sheet first formed as a small ice cap (maybe several) in the early Oligocene, but retreating and advancing many times until the Pliocene, when it came to occupy almost all of Antarctica. The Greenland ice sheet did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation. This had the unusual effect of allowing fossils of plants that once grew on present-day Greenland to be much better preserved than with the slowly forming Antarctic ice sheet.

Antarctic ice sheet

The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km² and contains 30 million km³ of ice. Around 90% of the fresh water on the Earth's surface is held in the ice sheet, and, if melted, would cause sea levels to rise by 61.1 metres. [http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113] In East Antarctica the ice sheet rests on a major land mass, but in West Antarctica the bed is in places more than 2500 m below sea level. It would be seabed if the ice sheet were not there.

Greenland ice sheet

The Greenland ice sheet occupies about 82% of the surface of Greenland, and if melted would cause sea levels to rise by 7.2 metres. [http://www.grida.no/climate/ipcc_tar/wg1/412.htm#tab113]

Possible effects of global warming

If global warming occurs, over the next century the Antarctic ice sheet is predicted to gain mass primarily because it is so cold that the extra warmth will not melt it significantly but will supply extra moisture; conversely the Greenland ice sheet is expected to lose mass through melting. These effects are expected to approximately cancel [http://www.grida.no/climate/ipcc_tar/wg1/418.htm]. See: sea level rise. Category:Bodies of ice Category:Geology ja:氷床

Glacier

:This article is about the geographical formation. For the professional wrestler, see Ray Lloyd A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity. A glacier is formed by multi-year ice accretion in sloping terrain. Glacier ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers can be found on every continent except Australia. Geologic features associated with glaciers include end, lateral, ground and medial moraines that form from glacially transported rocks and debris; U-shaped valleys and corries (cirques) at their heads, and the glacier fringe, which is the area where the glacier has recently melted. cirques

Types of glaciers

cirques There are two main types of glaciers: alpine glaciers, which are found in mountain terrains, and continental glaciers, which are associated with ice ages and can cover large areas of continents. Most of the concepts in this article apply equally to alpine glaciers and continental glaciers. A temperate glacier is one where liquid water is present at least part of the year. Polar glaciers are always below the freezing point. The smallest alpine glaciers form in mountain valleys and are referred to as valley glaciers. Larger ice layers can cover an entire mountain, mountain chain or even a volcano; this type is known as an ice cap. Ice caps feed outlet glaciers, tongues of ice that extend into valleys below, far from the margins of those larger ice masses. Outlet glaciers are formed by the movement of ice from a polar ice cap, or an ice cap from mountainous regions, to the sea. The largest glaciers are continental ice sheets, enormous masses of ice that are not affected by the landscape and extend over the entire surface, except on the margins, where they are thinnest. Antarctica and Greenland are the only places where continental ice sheets currently exist. These regions contain vast quantities of fresh water. The volume of ice is so large that if the Greenland ice sheet melted, it would cause sea levels to rise some six meters all around the world. If the Antarctic ice sheet melted, sea levels would rise up to 65 meters. Plateau glaciers resemble ice sheets, but on a smaller scale. They cover some plateaus and high-altitude areas. This type of glacier appears in many places, especially in Iceland and some of the large islands in the Arctic Ocean, and throughout the northern Pacific Cordillera from southern British Columbia to western Alaska. Tidewater glaciers are glaciers that flow into the sea. As the ice reaches the sea pieces break off, or calve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous splash as the iceberg strikes the water. If the water is deep, glaciers can calve underwater, causing the iceberg to suddenly explode up out of the water. The Hubbard Glacier is the longest tidewater glacier in Alaska and has a calving face over ten kilometers long. Yakutat Bay and Glacier Bay are both popular with cruise ship passengers because of the huge glaciers descending to them. Piedmont glaciers occupy broad lowlands at the base of steep mountains, and form when one or more alpine glaciers surge from the confining walls of mountain valleys. The size of piedmont glaciers varies greatly: among the largest is the Malaspina Glacier, which extends along the length of the southern coast of Alaska. It covers more than 5,000 km² of the coastal plain at the foot of the Saint Elias range. And it is only a part of the much bigger Kluane Icecap, which spans the Mount St. Elias and Chugach groups of mountain ranges all the way from the Malaspina Glacier to the Copper River and well into the southwestern Yukon, as well as southeast from the Malaspina towards the Iskut River in British Columbia. The highest alpine glacier in the world is the Siachen Glacier, which is also a zone of political conflict between India and Pakistan.

Formation of glaciers

Siachen Glacier The snow which forms glaciers is subject to repeated freezing and thawing, which changes it into a form of granular ice called névé. Under the pressure of the layers of ice and snow above it, this granular ice fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacial ice contains minute air bubbles as a result, giving it a distinctive blue tint due to Rayleigh scattering. The lower layers of glacial ice flow and deform plastically under the pressure, allowing the glacier as a whole to move slowly like a viscous fluid. Glaciers do not need a slope to flow, being driven by the continuing accumulation of new snow at their source. The upper layers of glaciers are more brittle, and often form deep cracks known as crevasses as they flex. These crevasses make travel over glaciers dangerous. Glacial meltwaters flow throughout and underneath glaciers, carving channels in the ice similar to caves in rock and also helping to lubricate the glacier's movement. In the summer, the melted ice from the glacier alone may be enough to create a stream, and while the glacier may be a barren waste of dense ice, fertile land is often nearby.

Anatomy of a glacier

cave The upper part of a glacier that receives most of the snowfall is called the accumulation zone. As a rule of thumb, the accumulation zone accounts for 60-70% of the glacier's surface area. The depth of ice in the accumulation zone exerts a downward force sufficient to cause deep erosion of the rock in this area. After the glacier is gone, this often leaves a bowl or amphitheater-shaped depression called a cirque. On the opposite end of the glacier, at its foot or terminal, is the deposition or ablation zone, where more ice is lost through melting than gained from snowfall and sediment is deposited. The place where the glacier thins to nothing is called the ice front. The altitude where the two zones meet is called the equilibrium line. At this altitude, the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. The downward erosive forces of the accumulation zone and the tendency of the ablation zone to deposit sediment also cancel each other out. Erosive lateral forces are not canceled; therefore, glaciers turn v-shaped river-carved valleys into u-shaped glacial valleys. The "health" of a glacier is defined by the area of the accumulation zone compared to the ablation zone. Healthy glaciers have large accumulation zones. Several non-linear relationships define the relation between accumulation and ablation. The worldwide shrinking of 70% of glaciers [http://www.grida.no/climate/ipcc_tar/wg1/064.htm] is among the evidence for global warming. Approximately 30% of glaciers are advancing. Even in very cold climates, there may be unglaciated areas, which receive too little precipitation to form permanent ice. This was the case in most of Siberia, central and northern Alaska and all of Manchuria during glacial periods of the Quaternary, and occurs today in that part of the Andes between 19°S and 27°S above the hyperarid Atacama Desert where, although the mountains reach 6700 metres above sea level, the cold Humboldt Current competely suppresses precipitation. During ice ages, continental glaciers may be as much as 1500 meters thick. A more extreme instance of glacial growth may have occurred during the Snowball Earth period. In the past several centuries the Earth's glaciers have generally been retreating, often dramatically.

Glacial motion

Earth Ice behaves like an easily breaking solid until its thickness exceeds about 50 meters (160 ft). Below that depth the increased pressure causes ice to become plastic and flow. The glacial ice is made up of layers of molecules stacked on top of each other, with relatively weak bonds between the layers. When the stress exceeds the inter-layer binding strength, the layers start to slide past each other. Another type of movement is basal gliding. In this process, the whole glacier moves over the terrain on which it sits, lubricated by thawed ice. As the pressure increases toward the base of the glacier, the melting point of water decreases, and the ice melts. Friction between ice and rock and geothermal heat from the Earth's interior also contribute to thawing. The top 50 meters of the glacier are more rigid. In this section, known as the fracture zone, there are no layers which slide past each other; instead the ice mostly moves as a single unit. Ice in the fracture zone moves over the top of the lower section. When the glacier moves through irregular terrain, cracks form in the fracture zone. These cracks can be up to 50 meters deep, at which point they meet the plastic flow underneath that seals them.

Speed of glacial movement

The speed of glacial displacement is partly determined by friction. Friction makes the ice at the bottom of the glacier move slower than the upper portion. In alpine glaciers, friction is also generated at the valley's side walls, which slows the edges relative to the center. This has been confirmed by experiments in the 19th century, in which stakes were planted in a line across an alpine glacier, and as time passed, those in the center moved further. Mean speeds vary; some have speeds so slow that trees can establish themselves among the deposited scourings. In other cases they can move as fast as many meters per day, as is the case of Byrd Glacier, an overflowing glacier in Antarctica which moves 750-800 meters per year (some 2 meters (6 ft) per day), according to studies using satellites. Many glaciers have periods of very rapid advancement called surges.[http://www.geog.leeds.ac.uk/research/glaciology/maths.htm] These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous state. During these surges, the glacier may reach velocities up to 1000 times greater than normal.

Moraines

Glacial moraines are formed by the deposition of material from a glacier and are exposed after the glacier has retreated. These features usually appear as linear mounds of till, a poorly-sorted mixture of rock, gravel and boulders within a matrix of a fine powdery material. Terminal or end moraines are formed at the foot or terminal end of a glacier, lateral moraines are formed on the sides of the glacier, and medial moraines are formed down the center. Less obvious is the ground moraine, also called glacial drift, which often blankets the surface underneath much of the glacier downslope from the equilibrium line. Glacial meltwaters contain rock flour, an extremely fine powder ground from the underlying rock by the glacier's movement. Other features formed by glacial deposition include long snake-like ridges formed by streambeds under glaciers, known as eskers, and distinctive streamlined hills, known as drumlins. Stoss-and-lee erosional features are formed by glaciers and show the direction of their movement. Long linear rock scratches (that follow the glacier's direction of movement) are called glacial striations, and divots in the rock are called chatter marks. Both of these features are left on the surfaces of stationary rock that were once under a glacier and were formed when loose rocks and boulders in the ice were transported over the rock surface. Transport of fine-grained material within a glacier can smooth or polish the surface of rocks, leading to glacial polish. Glacial erratics are rounded boulders that were left by a melting glacier and are often seen perched precariously on exposed rock faces after glacial retreat. The most common name for glacial sediment is moraine. The term is of French origin, and it was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French Alps. Currently, the term is used more broadly, and is applied to a series of formations, all of which are composed of till.

Drumlins

till Drumlins are asymmetrical hills with aerodynamic profiles made mainly of till. Their heights vary from 15 to 50 meters and they can reach a kilometer in length. The tilted side of the hill looks toward the direction from which the ice advanced (stoss), while the longer slope follows the ice's direction of movement (lee). Drumlins are found in groups called drumlin fields or drumlin camps. An example of these fields is found east of Rochester, New York, and it is estimated that it contains about 10,000 drumlins. Although the process that forms drumlins is not fully understood, it can be inferred from their shape that they are products of the plastic deformation zone of ancient glaciers. It is believed that many drumlins were formed when glaciers advanced over and altered the deposits of earlier glaciers.

Glacial erosion

Rocks and sediments are added to glaciers through various processes. Glaciers erode the terrain principally through two methods: abrasion and plucking. plucking As the glacier flows over the bedrock's fractured surface, it softens and lifts blocks of rock that are brought into the ice. This process is known as plucking, and it is produced when subglacial water penetrates the fractures and the subsequent freezing expansion separates them from the bedrock. When the water expands, it acts as a lever that loosens the rock by lifting it. This way, sediments of all sizes become part of the glacier's load. Abrasion occurs when the ice and the load of rock fragments slide over the bedrock and function as sandpaper that smoothens and polishes the surface situated below. This pulverized rock is called rock flour. This flour is formed by rock grains of a size between 0.002 and 0.00625 mm. Sometimes the amount of rock flour produced is so high that currents of meltwaters acquire a grayish color. Another of the visible characteristics of glacial erosion are glacial striations. These are produced when the bottom's ice contains large chunks of rock that mark trenches in the bedrock. By mapping the direction of the flutes the direction of the glacier's movement can be determined. The velocity of a glacier's erosion is variable. The differential erosion undertaken by the ice is controlled by four important factors:
- Velocity of glacial movement
- Thickness of the ice
- Shape, abundance and hardness of rock fragments contained in the ice at the bottom of the glacier
- Relative ease of erosion of the surface under the glacier. Material that becomes incorporated in a glacier are typically carried as far as the zone of ablation before being deposited. Glacial deposits are of two distinct types:
- Glacial till: material directly deposited from glacial ice. Till includes a mixture of undifferentiated material ranging from clay size to boulders, the usual composition of a moraine.
- Fluvial and outwash: sediments deposited by water. These deposits are stratified through various processes, such as boulders being separated from finer particles. The larger pieces of rock which are encrusted in till or deposited on the surface are called glacial erratics. They may range in size from pebbles to boulders, but as they may be moved great distances they may be of drastically different type than the material upon which they are found. Patterns of glacial erratics provide clues of past glacial motions.

Glacial valleys

glacial erratics glacial erratics. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades.]] Before glaciation, mountain valleys have a characteristic "V" shape, produced by downward erosion by water. However, during glaciation, these valleys widen and deepen, which creates a "U"-shaped glacial valley. Besides the deepening and widening of the valley, the glacier also smoothes the valley due to erosion. This way, it eliminates the spurs of earth that extend across the valley. Because of this interaction, triangular cliffs called truncated spurs are formed. Many glaciers deepen their valleys more than their smaller tributaries. Therefore, when the glaciers stop receding, the valleys of the tributary glaciers remain above the main glacier's depression, and these are called hanging valleys. In parts of the soil that were affected by abrasion and plucking, the depressions left can be filled by paternoster lakes, from the Latin for "Our Father", referring to a station of the rosary. At the head of a glacier is the corrie, which has a bowl shape with escarped walls on three sides, but open on the side that descends into the valley. In the corrie, an accumulation of ice is formed. These begin as irregularities on the side of the mountain, which are later augmented in size by the coining of the ice. After the glacier melts, these corries are usually occupied by small mountain lakes called tarns. There may be two glaciers separated by a diving ridge. This, located between the corries, is eroded to create an arête. This structure may result in a mountain pass. Glaciers are also responsible for the creation of fjords (deep coves or inlets) and escarpments that are found at high latitudes. With depths that can exceed 1,000 metres caused by the postglacial elevation of sea level and therefore, as it changed the glaciers changed their level of erosion. sea level

Arêtes and horns

An arête is a narrow crest with a sharp edge. Pointed pyramidal peaks are called horns. Both features may have the same process behind their formation: the enlargement of cirques from glacial plucking and the action of the ice. Horns are formed by cirques that encircle a single mountain. Arêtes emerge in a similar manner; the only difference is that the cirques are not located in a circle, but rather on opposite sides along a divide. Arêtes can also be produced by the collision of two parallel glaciers. In this case, the glacial tongues cut the divides down to size through erosion, and polish the adjacent valleys.

Sheepback rock

Some rock formations in the path of a glacier are sculpted into small hills with a shape known as roche moutonnée or sheepback. An elongated, rounded, asymmetrical, bedrock knob produced can be produced by glacier erosion. It has a gentle slope on its up-glacier side and a steep to vertical face on the down-glacier side. The glacier abrades the smooth slope that it flows along, while rock is torn loose from the downstream side and carried away in ice. Rock on this side is fractured by combinations of forces due to water, ice in rock cracks, and structural stresses.

Alluvial stratification

The water that rises from the zone of ablation moves away from the glacier and carries with it fine eroded sediments. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water then gradually deposits the sediment as it runs, creating an alluvial plain. When this phenomenon occurs in a valley, it is called a valley train. alluvial plain Alluvial plains and valley trains are usually accompanied by basins known as kettles. Glacial depressions are also produced in till deposits. These depressions are formed when large ice blocks are stuck in the glacial alluvium and after melting, they leave holes in the sediment. Generally, the diameter of these depressions does not exceed 2 km, except in Minnesota, where some depressions reach up to 50 km in diameter, with depths varying between 10 and 50 meters.

Deposits in contact with ice

When a glacier reduces in size to a critical point, its flow stops, and the ice becomes stationary. Meanwhile, meltwater flows over, within, and beneath the ice leave stratified alluvial deposits. Because of this, as the ice melts, it leaves stratified deposits in the form of columns, terraces and clusters. These types of deposits are known as deposits in contact with ice. When those deposits take the form of columns of tipped sides or mounds, which are called kames. Some kames form when meltwater deposits sediments through openings in the interior of the ice. In other cases, they are just the result of fans or deltas towards the exterior of the ice produced by meltwater. When the glacial ice occupies a valley it can form terraces or kame along the sides of the valley. A third type of deposit formed in contact with the ice is characterized by long, narrow sinuous crests composed fundamentally of sand and gravel deposite by streams of meltwater flowing within, beneath or on the glacier ice. After the ice has melted these linear ridges or eskers remain as landscape features. Some of these crests have heights exceeding 100 meters and their lengths surpass 100 km.

Loess deposits

Very fine glacial sediments or rock flour is often picked up by wind blowing over the bare surface and may be deposited great distances from the original fluvial deposition site. These eolian loess deposits may be very deep, even hundreds of meters, as in areas of China and the midwestern United States.

Isostatic rebound

loess This rise of a part of the crust is due to an isostatic adjustment. A large mass, such as a glacier, depresses the Earth's crust. After the glacier melts, the crust begins to rise to its original position. This is post-glacial rebound and is currently occurring in measurable amounts in Scandinavia and the Great Lakes region of the United States.

Ice ages

:Main article: Ice age.

Ice age divisions

A quadruple division of the Quaternary glacial period has been established for North America and Europe. These divisions are based principally on the study of glacial deposits. In North America, each of these four stages was named for the state in which the deposits of these stages were well exposed. In order of appearance, they are the following: Nebraskan, Kansan, Illinoisan, and Wisconsinan. This classification was refined thanks to the detailed study of the sediments of the ocean floor. Because the sediments of the ocean floor, in contrast to that of the Earth's surface, are less affected by stratigraphic discontinuities, they are useful to determine the climatic cycles of the planet. In this matter, geologists have come to identify over twenty divisions, each of them lasting approximately 100,000 years. All these cycles fall within the Quaternary glacial period. During its peak, the ice left its mark over almost 30% of Earth's surface, covering approximately 10 million km² in North America, 5 million km² in Europe and 4 million km² in Siberia. The glacial ice in the Northern hemisphere was double that found in the Southern hemisphere. This is because in the South Pole the ice cannot advance beyond the Antarctic landmass. It is now believed that the most recent glacial period began between two and three million years ago, in the Pleistocene era.

Causes of ice ages

Little is known about the causes of glaciations. Generalized glaciations have been rare in the history of Earth. However, the Ice Age of the Pleistocene was not the only glaciative event, since tillite deposits have been identified. Tillite is a sedimentary rock formed when glacial till is lithified. These deposits found in strata of differing age present similar characteristics as fragments of fluted rock, and some are superposed over bedrock surfaces of channeled and polished rock or associated with sandstone and conglomerates that have features of alluvial plain deposits. Two Precambrian glacial episodes have been identified, the first approximately 2 billion years ago, and the second (Snowball Earth) about 600 million years. Also, a well documented record of glaciation exists in rocks of the late Paleozoic (of 250 million years of age). Although there are several scientific hypotheses about the determining factors of glaciations, the two most important ideas are plate tectonics and variations in Earth's orbit (Milankovitch cycles).

Plate tectonics

Because glaciers can form only on dry land, plate tectonics suggest that the evidence of previous glaciations is currently present in tropical latitudes due to the drift of tectonic plates from tropical latitudes to circumpolar regions. Evidence of glacial structures in South America, Africa, Australia, and India support this idea, because it is known that they experienced a glacial period near the end of the Paleozoic Era, some 250 million years ago. The idea that the evidence of middle-latitude glaciations is closely related to the displacement of tectonic plates was confirmed by the absence of glacial traces in the same period for the higher latitudes of North America and Eurasia, which indicates that their locations were very different than today. Climatic changes are also related to the positions of the continents, which has made them vary in conjunction with the displacement of plates. That also affected ocean current patterns, which caused changes in heat transmission and humidity. Since continents drift very slowly (about 2 cm per year), similar changes occur in periods of millions of years. A study of marine sediment that contained climatically sensitive microorganisms until about half a million years ago were compared with studies of the geometry of Earth's orbit, and the result was clear: climatic changes are closely related to periods of obliquity, precession, and eccentricity of the Earth's orbit. In general it can be affirmed that plate tectonics is only applicable to very long periods of time, while Milankovitch's proposal, backed up by the work of others, adjusts to the periodic alterations of glacial periods of the Pleistocene. These proposals are subject to uncertainty and there may be other factors involved.

References

- This article draws heavily on the corresponding article in the Spanish-language Wikipedia, which was accessed in the version of July 24, 2005.
- Michael Hambrey and Jürg Alean, Glaciers, 2nd ed. (Cambridge University Press, 2004, ISBN 0-521-82808-2) An excellent less-technical treatment of all aspects, with superb photographs and firsthand accounts of glaciologists' experiences. All images of this book can be found online (see Weblinks: Glaciers-online)
- Douglas I. Benn and David J. A. Evans, Glaciers and Glaciation (Arnold, 1999)
- M. R. Bennett and N. F. Glasser, Glacial Geology: Ice Sheets and Landforms (John Wiley & Sons, 1996)
- Michael Hambrey, Glacial Environments (University of British Columbia Press, UCL Press, 1994) An undergraduate-level textbook.
- Robert Walley, Introduction to Physical Geography (Wm. C. Brown Publishers, 1992) A textbook devoted to explaining the geography of our planet.
- W. S. B. Paterson, Physics of Glaciers, 3rd ed. (Pergamon Press, 1994) A comprehensive reference on the physical principles underlying formation and behavior.

External links

- [http://www.glaciers-online.net/ Swisseduc - Glaciers online]
- [http://www.nsidc.org/glaciers/ National Snow and Ice Data Center - Glaciers]
- [http://www.glaciers.er.usgs.gov/ USGS Glacier Studies Project]
- [http://vulcan.wr.usgs.gov/Glossary/Glaciers/description_glaciers_hazards.html Glaciers and Glacial Hazards - USGS]
- [http://www.sciencedaily.com/releases/2003/08/030814071654.htm 2003-08-15 Scientists Rewrite Laws Of Glacial Erosion]
- [http://www.nps.gov/kefj#Kenai Fjords National Park, Alaska]
- [http://www.pbs.org/wgbh/nova/sciencenow/3210/03.html NOVA scienceNOW] - A 7 minute video of the NOVA broadcast that aired on PBS, July 26, 2005. Hosted by Robert Krulwich, the video is about the world's fastest glacier and why it is moving too fast. Category:Glaciology Category:Bodies of ice Category:Glaciers ja:氷河

Glaciology

Glaciology is the study of glaciers, or more generally the study of ice and natural phenomena that involve ice. The word glacier is derived from the Latin glacies, meaning ice or frost. Glaciology is an interdisciplinary earth science that integrates geology, climatology, meteorology, hydrology, biology, and ecology. The discipline also forms a part of physical geography. The presence of ice on Mars and Europa brings in an extraterrestrial component to the field.

Overview

Areas of study within glaciology include glacial history and the reconstruction of past glaciation patterns, effects of glaciers on climate and vice versa, the dynamics of ice movement, the contributions of glaciers to erosion and geomorphology, lifeforms that live in the ice, and so forth. Unsurprisingly, glaciology is one of the key areas supported by polar research.

Types

There are two general categories of glaciation which glaciologists distinguish: alpine glaciation, accumulations or "rivers of ice" confined to valleys; and continental glaciation, unrestricted accumulations which once covered much of the northern continents.
- Alpine - ice flows down the valley of mountainous areas and forms an apron of moving ice onto the plains below. Alpine glaciers tend to make the topography more rugged.
- Continental - an ice sheet found only in in high latitudes (Greenland/Antarctica), thousands of square kilometers wide and thousands of meters thick. These tend to smooth out the landscape.

Zones of glaciers

- Accumulation
- Wastage

Movement

; Ablation : wastage through evaporation and melting ; Bergshrund : crevasse formed at the head of a glacier, where it breaks away from the mountain face. ; Fracture (30-60m thick) : brittle fracturing creates crevasses due to central part of glacier moving faster than sides/floor. ; Flow : plastic due to extreme pressure. ; Plucking/Quarrying : moving glacier picks up large block of stone and transports them, sometimes up to a mile. ; Cirques : bowl shaped depression where the glacer started and has now melted away. ; Tarn : lake that forms in the cirque floor ; Horn : spire of rock formed by the headward erosion of a ring of cirques around a single mountain ; Arěte : a number of cirques gnaw into a ridge from opposite sides (become jagged)

Glacial Deposits

Stratified

; Outwash sand/gravel : from front of glaciers, found on a plain ; Kettles : block of stagnant ice leaves a depression or pit ; Eskers : steep sided ridges of gravel/sand, possibly caused by streams running under stagnant ice ; Kames : stratified drift builds up low steep hills ; Varves : thin sedimentary beds (coarse to fine), summer deposits more material and in the winter, less.

Unstratified

; Till-unsorted : (glacial flour to boulders) deposited by receding/advancing glaciers, forming moraines, and drumlins ; Moraines : (Terminal) material deposited at the end; (Ground) material deposited as glacier melts; (lateral) material deposited along the sides. ; Drumlins : smooth elongated hills composed of till.

References

- Hambrey, Michael and Jürg Alean. Glaciers 2nd ed. Cambridge University Press, 2004. ISBN 0-521-82808-2
- Benn, Douglas I. and David J. A. Evans. Glaciers and Glaciation. London; Arnold, 1998. ISBN 0-340-58431-9

External links

- [http://www.igsoc.org/ International Glaciological Society]
- [http://www.glaciers-online.net/ Glaciers online]
- [http://wdcgc.spri.cam.ac.uk/ World Data Centre for Glaciology, Cambridge, UK]
- [http://www.nsidc.org/ National Snow and Ice Data Center, Boulder, Colorado]
- [http://www.glims.org/ Global Land Ice Measurements from Space, USGS]
- [http://www.aber.ac.uk/~glawww/ Centre for Glaciology, University of Wales]
- [http://skua.gps.caltech.edu/ Caltech Glaciology Group]
- [http://www.glaciology.gfy.ku.dk/ Glaciology Group, University of Copenhagen]
- [http://www.lowtem.hokudai.ac.jp/english/ Institute of Low Temperature Science, Sapporo]
- [http://www.nipr.ac.jp/ National Institute of Polar Research, Tokyo] Category:Earth sciences Category:Glaciology

Antarctic

:For the Kim Stanley Robinson novel, see Antarctica (novel) Antarctica (from Greek ἀνταρκτικός, "opposite the Arctic") is a continent surrounding the Earth's South Pole. It is the coldest place on Earth and is almost entirely covered by ice; however, it is also the world's largest desert. Although myths and speculation about a Terra Australis ("Southern Land") go back to antiquity, the first commonly accepted sighting of the continent occurred in 1820 and the first verified landing in 1821 by the Russian expedition of Mikhail Lazarev and Fabian Gottlieb von Bellingshausen. (See also History of Antarctica.) With an area of 13,200,000 km², Antarctica is the fifth largest continent, after Asia, Africa, North America, and South America. However, it is by far the smallest in population: indeed, it has no permanent population at all. It is also the continent with the highest average altitude, and the lowest average humidity of any continent on Earth, as well as the lowest average temperature. It has been assigned the Internet ccTLD .aq.

Antarctic climate

.aq Antarctica is the coldest place on earth. Temperatures reach a minimum of between -85 and -90 degrees Celsius in the winter and about 30 degrees higher in the summer months. Weather fronts rarely penetrate far into the continent, leaving the center cold and dry. There is little precipitation over the central portion of the continent, but ice there can last for extended time periods. However, heavy snowfalls are not uncommon on the costal portion of the continent, where snowfalls of up to 48 inches in 48 hours have been recorded. Nearly all of Antarctica is covered by an ice sheet that is, on average, 2.5 kilometers thick. At the edge of the continent, strong katabatic winds off the polar plateau often blow at storm force. In the interior, however, windspeeds are often moderate. Depending on the latitude, long periods of constant darkness, or constant sunlight, mean that climates familiar to humans are not generally available on the continent.

Geography

katabatic wind The continent of Antarctica is located mostly south of the Antarctic Circle, surrounded by the Southern Ocean. Physically Antarctica is divided in two by mountains close to the neck between the Ross Sea and the Weddell Sea. The portion of the continent west of the Weddell Sea and east of the Ross Sea is called Western Antarctica and the remainder Eastern Antarctica, since they correspond roughly to the eastern and western hemispheres relative to the Greenwich meridian. Western Antarctica is covered by the West Antarctic Ice Sheet. See also: Extreme points of Antarctica, Antarctic territories.

Population

It is usually estimated that at a given time there are at least 1,000 people living in Antarctica. This varies considerably with season. Generally, stations use their home country's time zone, but not always; where known, a base's UTC offset is listed. Although Antarctica has no permanent residents, a number of governments maintain permanent research stations throughout the continent. Many of the stations are staffed around the year. These include: staffed
- Akademik Vernadsky Station, Galindez Island, (), ( UKR)
- Amundsen-Scott South Pole Station, South Pole United States Antarctic Program
- Belgrano II, () Laboratory and meteorological station Argentine southernmost base (since 1979).
- Bellingshausen Station, King George Island ()
- Bernardo O'Higgins Station, Antarctic Peninsula, Chilean Army.
- Casey, Vincennes Bay ( Australian Antarctic Division) (UTC+8)
- Comandante Ferraz Station, King George Island ()
- Concordia Research Station, (75° S 123° E),
- Dakshin Gangotri Station, Indian Antarctic Program
- Davis, Princess Elizabeth Land ( Australian Antarctic Division) (UTC+7)
- Dumont d'Urville Station () (UTC+10)
- Eduardo Frei Montalva Station and Villa Las Estrellas, King George Island, Chilean Air Force.
- Esperanza () Laboratory and meteorological station (since 1952). Radio LRA Arcángel, School #38 Julio A. Roca (since 1978), tourist facilities.
- General Artigas Station ()
- Georg von Neumayer Station, () (Atka-Bay) (Alfred Wegener Institute )
- Great Wall Station (), King George Island ()
- Halley Research Station () British Antarctic Survey
- Henryk Arctowski Polish Antarctic Station (), King George Island
- Jubany, (), since 1953 ()
- King Sejong Station (), King George Island, since 1988 ()
- Machu Picchu Research Station, Admiralty Bay, King George Island, summer base established in 1989.
- Macquarie Island ( Australian Antarctic Division)
- Maitri Station, () near Schirmacher Region ( Indian Antarctic Program)
- Marambio Base, () Seymour-Marambio Island. Laboratory, meteorological station, 1.2 km long, 30 m wide landing track (since 1969) () [http://www.marambio.aq website]
- Mawson Station, Mac Robertson Land ( Australian Antarctic Division) (UTC+6)
- McMurdo Station, Ross Island () (UTC+12, follows New Zealand DST)
- Mirny Station () ()
- Mizuho Station () (National Institute of Polar Research )
- Molodezhnaya Station () ()
- Novolazarevskaya Station, Dronning Maud Land () ()
- Orcadas () Orcadas Islands (since 1904)()
- Palmer Station, Anvers Island () (UTC-4, follows Chilean DST)
- Professor Julio Escudero base, King George Island.
- Progress Station () ()
- Rothera Research Station () British Antarctic Survey (UTC-3)
- San Martín Station () (since 1951) Laboratory and Meteorological measurements ()
- SANAE (South African National Antarctic Expeditions), on the Fimbul Coastal Ice Shelf in Queen Maud Land
- Saint Climent Ohridski () (since 1988) Biology Research, Laboratory and Meteorological measurements. First Orthodox Church - St. Ivan Rilski ()
- Scott Base, () Ross Island () (UTC+12, follows New Zealand DST)
- Showa Station () (National Institute of Polar Research ) (GMT+3)
- Troll Station (Norwegian Polar Institute), () Queen Maud Land ()
- Vostok, Antarctica () () (UTC+6)
- Zhongshan (Sun Yet-Sen) Station () () Emilio Marcos Palma was the first person born in Antarctica (Base Esperanza) in 1978, his parents being sent there along with seven other families. Emilio Marcos Palma

Communications

The international dialing code for Antarctica is +672. Antarctica has wireless telephone services. There is a single cell tower using AMPS technology at Argentina's Marambio Base and an Entel Chile GSM tower on King George Island. Communications are otherwise limited to satellite connections. Radio frequencies that can be used are FM2 and shortwave 1.

Military

The Antarctic Treaty prohibits any measures of a military nature in Antarctica, such as the establishment of military bases and fortifications, the carrying out of military maneuvers, or the testing of any type of weapon. It permits the use of military personnel or equipment for scientific research or for any other peaceful purposes. The United States military issues the Antarctica Service Medal to those members of the military or civilians who perform research duty on the Antarctica continent. The medal, including the winter-over bar issued to those who remain on the continent for two complete, six-month seasons, is properly awarded by the United States Congress. The only documented large-scale land military maneuver was "Operación 90," undertaken 10 years before the Antarctic Treaty by the Argentinian military.

External links

- [http://www.70south.com 70South]
- [http://www.ats.org.ar Antarctic Treaty Secretariat]
- [http://www.anetstation.com ANetStation]
- [http://www.add.scar.org The Antarctic Digital Database - a source of digital topographic map data for Antarctica]
- [http://www.ejercito.mil.ar/antartico/historia/antarti_hist.htm Argentine Antarctic history]
- [http://www.aad.gov.au/ Australian Antarctic Division]
- [http://www.antarctica.ac.uk British Antarctic Survey]
- [http://www.comnap.aq/ Council of Managers of National Antarctic Programs (COMNAP)], official homepage.
- [http://www.awi-bremerhaven.de/Polar/index.html German Antarctic Ships and Stations]
- [http://www.loc.gov/rr/international/frd/antarctica/antarctica.html Portals on the World - Antarctica] from the Library of Congress
- [http://www.polarmuseum.sp.ru/Eng/ The Russian State Museum of Arctic and Antarctic]
- [http://www.scar.org The Scientific Committee for Antarctic Research - coordinating body for Antarctic Science]
- [http://members.eunet.at/castaway/stations/aa-bases.html Antarctic Research Stations]
- [http://www.cia.gov/cia/publications/factbook/geos/ay.html The World Factbook – Antarctica] from the U.S. Central Intelligence Agency
- [http://www.70south.com Latest Antarctic news and information by 70South]
- [http://www.planetavivo.org/english/ResearchPrograms/Antarctica/SlideShows/ArdleyIsland/ArdleyIsland1.html Biodiversity at Ardley Island, South Shetland archipelago, Antarctica]
- [http://www.iaato.org International Association of Antarctic Tour Operators (IAATO)] Category:Continents Category:Antarctica Category:Special territories Category:Lists of coordinates ja:南極大陸 ko:남극 ms:Antartika simple:Antarctica th:ทวีปแอนตาร์กติกา zh-min-nan:Lâm-ke̍k-tāi-lio̍k

North America

North America is a continent in the northern hemisphe