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Ice Ages

Ice Ages

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

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

Glaciation

A glaciation (a created composite term meaning Glacial Period, referring to the Period or Era of, as well as the process of High Glacial Activity), often called an ice age, is a geological phenomenon in which massive ice sheets form in the Arctic and Antarctic and advance toward the equator. Conversely, the term interglacial or Interglacial Period, such as the current era, is used to denote the absence of large-scale glaciation on a global scale — i.e., a non-Ice Age. Interglacials are, in general, shorter than glacial epochs. Glaciations are characterized by cool, wet climates and thick ice sheets extending from each pole. Mountain or alpine glaciers in otherwise unglaciated areas expand and extend to lower elevations even in the lowest of latitudes. Sea levels drop due to the presence of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the earth has significant continental glaciation in the Arctic and Antarctic, we currently are in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an "interglacial". The current one is the Flandrian. In general, the Earth seems to have been ice-free even in high latitudes except during relatively rare glacial maximums such as the one from which we emerged 10 to 15 thousand years ago. The causes of glaciations have been much debated ever since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit, hypothesized periodic changes in solar output, and/or the effects of continental masses drifting into polar regions where Antarctica currently resides. Known periods of glaciation include the Huronian (2400 Ma - 2100 Ma), Sturtian-Varangian (950 Ma - 570 Ma), Andean-Saharan (450 Ma - 420 Ma), Karoo (360 Ma - 260 Ma), Cenozoic (30 Ma - Present). These can be further divided by location and time (e.g.: the names Riss (180,000 - 130,000 years bp) and Würm (70,000 - 10,000 years bp) refer specifically to glaciation at the headwaters of the Donau/Rhine rivers.) Not every year in each interval was a time of complete or even partial glaciation. The best-studied glaciation, that of the recent past, appears to have taken place in at least four separate ice incursions and retreats. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has led to a recent "Snowball Earth" hypothesis - that the Earth froze over completely in the late Proterozoic, then thawed very rapidly as trapped water and carbon dioxide were returned to Earth's atmosphere. An alternative hypothesis, sometimes called Slushball Earth, maintains that the Equator at least was ice-free. ----- The term ice age is used to refer either to a single glaciation or to an entire period of repeated glaciations such as the recent 30 million years of the Cenozoic period, especially the Pleistocene glaciations.

Greenland

:For the town in New Hampshire, see Greenland, New Hampshire. Greenland (Greenlandic: Kalaallit Nunaat, meaning "Land of the Greenlanders"; Danish: Grønland, meaning "Greenland") is a self-governed Danish territory. An Arctic island nation located in the continent of North America, both geographically and ethnically; politically and historically, however, Greenland is closely associated with Europe. The Atlantic Ocean and Iceland lie to Greenland's Southeast; the Greenland Sea to the East; the Arctic Ocean to the North; Baffin Bay and Canada to the West. Greenland is the world's largest island, and is the largest dependent territory by area in the world. It also contains the world's largest national park. About 81 percent of its surface is covered by ice, known as the Greenlandic ice cap. Nearly all Greenlanders live along the fjords in the south-west of the island, which has a milder climate. Most Greenlanders have both Kalaallit (Inuit) and Scandinavian ancestry, and speak Greenlandic (Kalaallisut) as their first language. Greenlandic is spoken by about 50,000 people, which is more than all the other Eskimo-Aleut languages combined. A minority of Danish migrants with no Inuit ancestry speak Danish as their first language. Both languages are official, with the West Greenlandic dialect forming the basis of the official form of Greenlandic. There is an on-going diplomatic sovereignty dispute between Canada and Greenland (represented internationally by Denmark) over the tiny Hans Island. Greenland was one of the Norwegian Crown colonies until 1815, when it formally became a Danish colony, although Norway and Denmark had been in a personal union for centuries (see Denmark-Norway). Greenland became an integral part of the Kingdom of Denmark in 1953. It was granted home rule (hjemmestyre) by the Folketing (Danish parliament) on May 1 1979. The law went into effect the following year. The Queen of Denmark, Margrethe II, remains Greenland's Head of state.

History

Greenland was home to a number of Paleo-Eskimo cultures in prehistory, the latest of which - the Early Dorset culture - disappeared around the year 200. Hereafter, the island seems to have been without humans for some eight centuries. Icelandic settlers found the land uninhabited when they arrived ca. 982. They established three settlements near the very Southwestern tip of the island, where they thrived for the next few centuries, disappearing after over 450 years of habitation. The name Greenland comes from those Scandinavian settlers. In the Norse sagas, it is said that Eiríkur Rauði (Erik the Red) was exiled from Iceland for murder. He, along with his extended family and slaves, set out in ships to find the land that was rumored to be to the northwest. After settling there, he named the land Greenland in order to attract more people to settle there. The fjords of the Southern part of the island were lush and had a warmer climate at that time, possibly due to what was called the Medieval Warm Period. These remote communities thrived and lived off farming, hunting and trading with the motherland, and when the Scandinavian monarchs converted their domains to Christianity, a bishop was installed in Greenland as well. The settlements seem to have coexisted relatively peacefully with the Inuit, who had migrated southwards from the Arctic islands of North America around 1200. In 1261, Greenland became part of the Kingdom of Norway, which was part of the Kalmar Union and later of the dual monarchy of Denmark-Norway. After almost five hundred years, the settlements simply vanished, possibly due to famine during the 15th century in the Little Ice Age, when climatic conditions deteriorated, and contact with Europe was lost. Bones from this late period were found to be in a condition consistent with malnutrition. Some believe the settlers were wiped out by plague or exterminated by Inuits. Other historians have speculated that Basque or English pirates or slave traders from the Barbary Coast contributed to the extinction of the Greenlandic communities. Denmark-Norway reasserted its latent claim to the colony in 1721. The island's ties with Norway were severed by the Treaty of Kiel of 1815, through which Sweden gained control over mainland Norway while Denmark retained all of their common overseas possessions (which at that time included small territories in India, West Africa and the West Indies, as well as lands in northwestern Europe). Norway occupied and claimed parts of (then uninhabited) Eastern Greenland in the 1920s, claiming that it constituted Terra nullius. Norway and Denmark agreed to settle the matter at the Permanent Court of International Justice in 1933, where Norway lost. Greenland was also called Gruntland ("Ground-land") on early maps. Whether Green is an erroneous transcription of Grunt ("Ground"), which refers to shallow bays, or vice versa, is not known. During World War II, Greenland was on its own, the connection to Denmark having been cut on April 9, 1940 when Denmark was occupied by Germany. Through the cryolite from the mine in Ivigtut, Greenland was able to pay for goods bought in the United States and Canada. The manner in which Greenland had been run prior to the war was altered. The Sirius Patrol, guarding the Northeastern shores of Greenland using dog sleds, was founded in 1941 and participated in defeating the Germans, which gave Denmark a better position in the postwar turmoil. In 1953 Greenland was made an equal part of the Danish Kingdom. In 1979 Greenland took one step further when home rule was granted. During the War Eske Brun was governor and ruled the Island via a 1925-law concerning the governing of the Island where, under extreme circumstances, the governors could take control. The other governor Aksel Svane was transferred to the USA as leader of the supply to Greenland commission.

Politics

Greenland's Head of State is the Danish Monarch, currently Margrethe II. The Queen's government in Denmark appoints a Rigsombudsmand (High commissioner) representing the Danish government and monarchy. Greenland has a 31 member elected parliament. The head of government is the Prime Minister, who is usually the leader of the majority party in Parliament. It is notable that Greenland is not part of the European Union, despite Denmark itself being a member state.

Geography

European Union European Union The total area of Greenland measures 2 099 988 km², of which the ice sheet covers 1 799 992 km² (85,7%). The coastline of Greenland is 24,430 miles long (39,330 km), about the same length as the Earth's circumference at the Equator. All towns and settlements of Greenland are situated along the ice-free coast, with the population being concentrated along the Western coast. Of the 18 municipalities, 15 are in West Greenland (Aasiaat, Ilulissat, Kangaatsiaq, Qasigiannguit, Qeqertarsuaq, Upernavik, Uummannaq in the northern part, Maniitsoq, Nuuk, Paamiut, Sisimiut in the central part, and Ivittuut, Nanortalik, Narsaq, Qaqortoq in the southern part), 2 in East Greenland (Ammassalik, Illoqqortoormiut) and 1 in North Greenland (Qaanaaq). Northeastern greenland, part of North Greenland, is not part of any municipalitiy, but is the site of the world's largest national park, Northeast Greenland National Park. At least four scientific expedition stations and camps had been established in the ice-covered central part of Greenland (indicated as pale blue in the map to the right), on the ice cap: Eismitte, North Ice, North GRIP Camp and The Raven Skiway. Currently, there is a year-round station, Summit Camp, on the ice cap, established in 1989. The radio station Brondlund Fjord was, until 1950, the northernmost permanent outpost of the world. The extreme north of Greenland, Peary Land, is not covered by an ice cap, because the air there is too dry to produce snow, which is essential in the production and maintenance of an ice cap. If the Greenland ice cap were to completely melt away, Greenland would most likely become an archipelago. Between 1989 and 1993, U.S. and European climate researchers drilled into the summit of Greenland's ice sheet, obtaining a pair of two-mile (3.2 km) long ice cores. Analysis of the layering and chemical composition of the cores has provided a revolutionary new record of climate change in the Northern Hemisphere going back about 100,000 years and illustrated that the world's weather and temperature have often shifted rapidly from one seemingly stable state to another, with worldwide consequences.

Economy

Greenland suffered economic contraction in the early 1990s, but since 1993 the economy has improved. The Greenland Home Rule Government (GHRG) has pursued a tight fiscal policy since the late 1980s which has helped create surpluses in the public budget and low inflation. Since 1990, Greenland has registered a foreign trade deficit following the closure of the last remaining lead and zinc mine in 1990. Greenland today is critically dependent on fishing and fish exports; the shrimp fishing industry is by far the largest income earner. Despite resumption of several interesting hydrocarbon and mineral exploration activities, it will take several years before production can materialize. Tourism is the only sector offering any near-term potential and even this is limited due to a short season and high costs. The public sector, including publicly owned enterprises and the municipalities, plays the dominant role in Greenland's economy. About half the government revenues come from grants from the Danish Government, an important supplement to the gross domestic product.

Demographics

Culture

The Greenland National Museum and Archives[http://www.natmus.gl] is located in Nuuk.

Miscellaneous topics

- Communications in Greenland
- Transportation in Greenland
- Military of Greenland
- Foreign relations of Greenland
- University of Greenland

References

- CIA World Factbook 2000

External links

- [http://www.nanoq.gl/english.aspx Greenland Homerule] - Official site
- [http://www.cia.gov/cia/publications/factbook/geos/gl.html Greenland] - CIA World Factbook
- [http://www.statgreen.gl/ Statistics Greenland]
- [http://www.norden.org/web/1-1-fakta/gr_kort.htm Greenland Map] - Hi-Res Map at the Nordic Ministerial Council
- [http://www.mapsofworld.com/lat_long/greenland-lat-long.html Latitude and Longitude of Important locations in Greenland] Category:Greenland Category:North Atlantic Islands Category:Islands of Denmark Category:Special territories of the EU Category:Danish dependencies Category:Former Norwegian colonies zh-min-nan:Chheⁿ-tē ko:그린란드 ja:グリーンランド simple:Greenland th:เกาะกรีนแลนด์

North America

North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the North Pacific Ocean. It covers an area of 24,497,994 km² (9,458,728 sq mi), or about 4.8% of the Earth's surface. As of July 2002, its population was estimated at more than 514,600,000. It is the third largest continent in area, after Asia and Africa, and is fourth in population after Asia, Africa, and Europe. Both North and South America are named after Amerigo Vespucci, who was the first European to suggest that the Americas were not the East Indies, but a previously undiscovered (by Europeans) New World. North America occupies the northern portion of the landmass generally referred to as the New World, the Western Hemisphere, the Americas, or simply America. North America's only land connection is to South America at the narrow Isthmus of Panama. (For geopolitical reasons, all of Panama – including the segment east of the Panama Canal in the isthmus – is often considered a part of North America alone.) According to some authorities, North America begins not at the Isthmus of Panama but at the Isthmus of Tehuantepec, with the intervening region called Central America and resting on the Caribbean Plate. Most, however, tend to see Central America as a region of North America, considering it too small to be a continent on its own. Greenland, although a part of North America geographically, is not considered to be part of the continent politically.

Physical features

Greenland, plutonic, metamorphic rock types of North America. ]] Plate tectonics recognizes the vast majority of North America as being the surface of the North American Plate. Parts of California and western Mexico are known for being the edge of the Pacific Plate, with the two plates meeting along the San Andreas fault. The continent can be divided into four great regions (each of which contains many sub-regions): the Great Plains stretching from the Gulf of Mexico to the Canadian Arctic; the geologically young, mountainous west, including the Rocky Mountains, the Great Basin, California and Alaska; the raised but relatively flat plateau of the Canadian Shield in the northeast; and the varied eastern region, which includes the Appalachian Mountains, the coastal plain along the Atlantic seaboard, and the Florida peninsula. Mexico, with its long plateaus and cordilleras, falls largely in the western region, although the eastern coastal plain does extend south along the Gulf. The western mountains are split in the middle, into the main range of the Rockies and the coast ranges in California, Oregon, Washington, and British Columbia with the Great Basin – a lower area containing smaller ranges and low-lying deserts – in between. The highest peak is

Ice Ages

Origin of ice age theory

Major ice ages

Interglacials

Causes of ice ages

Recent glacial and interglacial phases

Glaciation in North America

Reference

See also

External links

Temperature

Overview

Role of temperature in nature

Temperature measurement

Units of temperature

Negative temperatures

Articles about temperature ranges:

Theoretical foundation of temperature

Zeroth-law definition of temperature

Temperature in gases

Temperature of the vacuum

Second-law definition of temperature

See also

References

External links

Climate

Climate vs weather

Climate determinants

Climate indices

Classifications

See also

Historical climates

National climates

External links

Notes

Glacier

Types of glaciers

Formation of glaciers

Anatomy of a glacier

Glacial motion

Speed of glacial movement

Moraines

Drumlins

Glacial erosion

Glacial valleys

Arêtes and horns

Sheepback rock

Alluvial stratification

Deposits in contact with ice

Loess deposits

Isostatic rebound

Ice ages

Ice age divisions

Causes of ice ages

Plate tectonics

See also

References

External links

Glaciation

See also

Greenland

History

Politics

Geography

Economy

Demographics

Culture

Miscellaneous topics

See also

References

External links

North America

Physical features