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Mechanisms: Positive and Negative Feedbacks


THE MECHANISMS



           PROCESSES WHICH MAKE GLACIAL PERIODS MORE SEVERE

               (Positive and Negative feedbacks in glacial periods)

                                 

 

Each glacial period is subject to positive feedback which makes it more severe and negative feedback which mitigates and (in all cases so far) eventually ends it.

 

What is a climate feedback?

 

This happens when a change in our climate causes an impact which changes our climate further — a knock-on effect which feeds back into our climate. There can be both negative and positive feedbacks:

 

   

* Negative feedback:  This is an impact which offsets the prevailing change in climate. Under global warming, this would create a cooling effect, balancing out the changes. If the climate was getting colder, it would create a heating effect.

* Positive feedback:  This is an impact which increases the change in the climate. It would add to global warming by creating further heating or, if our climate was cooling, would cool the climate further.


              

     

For example, suppose that a disturbance caused global temperatures to increase. In a warm atmosphere, more water could evaporate from the oceans, leading to larger amounts of water vapour in the atmosphere absorbing more radiation from the Earth's surface and emitting more radiation back, thereby enhancing the greenhouse effect and further increasing the air temperature. This would also make it possible for the air to hold even more water vapour as evaporation increases. If this feedback is not stopped, it would be considered a runaway greenhouse effect; one in which the Earth's temperature could increase until the oceans eventually evaporated away.

 

In addition to positive feedbacks, there are negative feedbacks that act to restore the climate system to its initial state. In the example of a warmer atmosphere with more water vapour, more clouds could form which would increase the amount of solar radiation reflected back to space and decrease the solar radiation absorbed by the Earth's surface, thereby slowing the rate of warming slow. Although it may not be enough to push the Earth completely back toward its initial state, it might lead to a new energy balance, one that is in equilibrium with an overall increase in energy.

 

It is still unclear how the climate on a regional or even global scale can change as rapidly as present evidence suggests.  It appears that the climate system is more delicately balanced than had previously been thought, linked by a cascade of powerful mechanisms that can amplify a small initial change into a much larger shift in temperature and aridity.  At present, the thinking of climatologists tends to emphasise several key components:

 

 

North Atlantic circulation (Thermohaline circulation): Positive Feedback

 

The circulation of the North Atlantic Ocean is presently seen as playing a major role in either triggering or amplifying rapid climate changes in the historical and recent geological record.  The North Atlantic has a peculiar circulation pattern; the north-east trending Gulf Stream carries warm and relatively salty surface water from the Gulf of Mexico up to the seas between Greenland, Iceland and Norway.  Upon reaching there, the surface water cools off and (with the combination of being cooler and relatively salty) becomes dense enough to sink into the deep ocean.  The "pull” exerted by this dense sinking water is thought to help maintain the strength of the warm Gulf Stream, ensuring a current of warm tropical water into the north Atlantic that sends mild air masses across to the European continent.

 

              

 

·         If the sinking process in the North Atlantic were to diminish or cease (due to too much freshwater), the weakening of the warm Gulf Stream would mean that Europe had colder winters. 

·         A sudden "switching off” of deep water formation in the North Atlantic could take the form of an exceptionally wet year on the landmasses which have rivers draining into the Arctic sea (Siberia, Canada and Alaska)

·         Not only affecting Europe, Antarctica would be even colder than it is now, due to lack of heat from the Gulf Stream water that sinks in the north Atlantic.  The effect of switching off the deepwater heat source would be cooler air and a greater sea ice extent around the Antarctica, reflecting more sunlight and further cooling the region. 

·         A thermohaline circulation shutdown could have other major consequences apart from cooling of Europe, such as an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica (including those from intensified El Niño effect), more frequent and intense El Niño events, or an oceanic anoxic event (oxygen (O2) below surface levels of the stagnant oceans becomes completely depleted - a probable cause of past mass extinction events).

 

 

Carbon dioxide and methane concentration as a feedback in sudden changes:  Positive Feedback

 

Analysis of bubbles in ice cores shows that at the peak of glacial phases, CO2 was about 30% lower than during interglacial conditions.  This is thought to be due to some change in plankton activity or ocean circulation patterns that occur under colder climates, drawing more carbon down out of the atmosphere once climate began to cool.  The lower carbon dioxide concentrations resulting from this would cool the atmosphere, and allow more snow and ice to accumulate on land.  Relatively rapid changes in climate, occurring over a few thousand years, could have resulted from changes in the atmospheric CO2, concentration.  The actual importance of carbon dioxide in terms of the climate system is unknown, though computer climate simulations tend to suggest that it directly cooled the world by less than 1 deg. C on average, but due to amplification of this change by various factors within the climate system such as the water vapour content, the resulting change in global climate could have been more than 2 deg. C (ref.).  

 

A problem with invoking carbon dioxide as a causal factor in sudden climate changes is that it generally seems to have varied too slowly, following on the timescale of millennia what often occurred on the timescale of decades.  Methane, a less important greenhouse gas, was also 50% lower during glacial phases, probably due to reduced biological activity on the colder, drier land surfaces.  However, it does seem to have increased rapidly in concentration in association with changes in climate, reaching its normal Holocene levels in around 150 years or less during the global climate warming at the end of the Younger Dryas, around 11,500 years ago (Taylor et l. 1997).  Such sudden rises in methane concentration were probably not important in affecting climate; the warming effect of a 50% change in methane would have been much less than an equivalent change in CO2, because methane is at such a low overall concentration in the atmosphere.  It has been suggested that another mechanism, involving sudden and short-lived releases of massive amounts of methane from the ocean floors, could sometimes have resulted in rapid warming phases that do not leave any trace in terms of raised methane levels in the ice core data, where the trapped gas bubbles generally only indicate methane concentrations at a time resolution of centuries rather than the few years or decades that such a "methane pulse” might last for.  However, more recently obtained ice cores from areas where the ice sheet built up particularly rapidly (Chapellaz et al.  1993, Taylor et al. 1997) show a more detailed time resolution of the record of methane concentration in the atmosphere.  These records fail to show any evidence of sudden "bursts” of methane.

 

 

Surface albedo (reflectivity) of ice, snow and vegetation:  Positive Feedback

 

The intensely white surface of sea ice and snow will reflect back much of the sun’s heat, hence keeping the surface cool.  In general, it has the potential to set off rapid climate changes because they can appear or disappear rapidly given the right circumstances.  Ice sheets are more permanent objects which, whilst they reflect a large proportion of the sunlight that falls upon them, take hundreds of years to melt or build up because of their sheer size.  When present, sea ice or snow can have a major effect in cooling regional and global climates, but with a slight change in conditions (slightly warmer summer) they will each disappear rapidly, giving a much greater warming effect because sunlight is now absorbed by the much darker sea or land cover underneath.  In an unusually cold year, the opposite could happen, resulting in a cooler summer climate.  It is possible that by slow changes over millennia or centuries, the climate could be brought to a break point involving a runaway change in snow and ice reflectivity over a few decades.  These slow background changes might include variations in the earth’s orbit (affecting summer sunlight intensity), or gradual changes in carbon dioxide concentration, or in the northern forest cover which affects the amount of snow that is exposed to sunlight.

 

It is possible that the relatively long-lived ice sheets might occasionally help bring about very rapid changes in climate, by rapidly "surging” outwards into the sea and giving rise to large numbers of icebergs that would reflect back the sun’s heat and rapidly cool the climate.

 


 

Holes containing cryocronite deposits visible in the bed of a melt water lake on the Petermann glacier in north Greenland. The surface of the glacier is covered with pot holes like these, which are often perfectly circular to the eye. The black granular cryocronite accelerates melting because the darker part of the spectrum lowers the albedo or reflectance of the usually white ice. A combination of these darker deposits, warmer melt water, the sun and the wind conspire to make these intriguingly geometric pot-holes in the ice. The sizes typically vary between 3cm and 30 cm in diameter and are between 3 and 500 cm deep. A team of three scientists are on board the Arctic Sunrise during the 1st leg of Greenpeace's 3 month long Arctic Impacts expedition around Greenland, to document the effects of climate change on the Arctic environment ahead of the Copenhagen summit which will be held in December 2009

 

 

 

Another, possible neglected, factor in rapid regional or global climate changes may be the changes in the albedo of the land surface that result from changes in vegetation or algal cover on desert and polar desert surfaces.  An initial spreading of dark-coloured soil surface algae or lichens following a particularly warm or moist year might provide a "kick” to the climate system by absorbing more sunlight and thus warming the climate, and also reducing the dust flux from the soil surface to the atmosphere.  Larger vascular plants and mosses might have the same effect on the timescale of years or decades.  The recent detailed analysis of the ending of the Younger Dryas by Taylor et al. 1997, suggests that warming occurred around 20 years earlier in lower and mid latitudes, perhaps due to some initial change in vegetation or snow cover affecting land surface albedo.  Some of the earlier climate warming events during the last 130,000 years show similar signs of changes in dust flux followed by changes in high-latitude temperature. 

 

 

Processes which mitigate glacial periods:  Positive Feedback

 

Ice sheets that form during glaciations cause erosion of the land beneath them.  After some time, this will reduce land above sea level and thus diminish the amount of space on which ice sheets can form.  This mitigates the albedo feedback, as does the lowering in sea level that accompanies the formation of ice sheets.

 

Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation.  The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advances.

 

 

Water Vapour:  Positive Feedback

 

Water vapour is a more important greenhouse gas than carbon dioxide, and as its atmospheric concentration can vary rapidly, it could have been a major trigger or amplifier in many sudden climate changes.  For example, a change in sea ice extent or in carbon dioxide would be expected to affect the flux of water vapour into the atmosphere from the oceans, possible amplifying climate changes that would otherwise have occurred anyway.  Large rapid changes in vegetation cover might also have added to these changes in water vapour flux to the atmosphere.  Geologist W.S Broecker has suggested that water vapour may act as a global "messenger”, co-ordinating rapid climate changes, many of which seem to have occurred all around the world fairly simultaneously, or in close succession.  Broecker notes the evidence for large changes in the water vapour content of the atmosphere in terms of changes in the 180 content of tropical high Andean ice cores, suggesting that the air’s overall content of water vapour was about half of what it is at present. 

 

 

Dust and particulates:  Positive Feedback

 

Particles of mineral dust, plus the aerosols formed from fires and from chemicals evaporating out of vegetation and the oceans, may also be a major feedback in co-ordinating and amplifying sudden large climate fluctuations.  Ice cores from Greenland, Antarctica, tropical mountain glaciers, and deep ocean cores, show greater concentrations of mineral dust during colder phases.  This suggests that there was more dust around in the world’s atmosphere during cold periods that during warm phases.  It seems that the atmospheric content of dust and sulphate particles changed very rapidly, over just a few decades, during sudden climate transitions in the Greenland ice core record.   The drier and colder the world gets, the more desert there is and the higher the wind speeds, sending more desert dust into the atmosphere where it may reinforce the cold and dryness by forming stable "inversion” layers that block sunlight and prevent rain-giving convective processes.  A run of wet years in the monsoon belt could trigger rapid revegetation of desert surfaces by algae or vascular plants, and a sudden decrease in the amount of dust blown into the atmosphere.  Less dust could help make conditions still warmer and wetter, pushing the climate system rapidly in particular direction (though dust and other particles might actually tend to warm the surface if they blow over lighter-coloured areas covered by snow or ice.

 

 

Wavering sun:  Positive Feedback

 

It is known that the Sun’s output and the Earth’s relationship to the Sun are quite complicated to quantify.  To begin with, the sun regularly fluctuates in intensity by about a fourth of a percent during an eleven-year cycle.  But also, studies of other stars have shown that they can regularly go through extended quiet periods where a star’s intensity fell by up to five percent for a number of years.  These quiet periods are measured by observing the reduction in sunspot activity of a star.  Of thirteen stars routinely measured since 1966, four have exhibited this extended "flat” period throughout the  Study (Gribbin 19).  Not to mention the face that our Sun is constantly getting hotter, and thus affecting Earth’s climate, it  also has quite the potential (based on studies of other similar stars) to fluctuate its intensity over years, decades, and even centuries.  It is believed that these fluctuations are a function of the Sun’s rotation.  More specifically, "the rotation speed of the Sun is inversely correlated with sunspot numbers ”because an increased rotation” inhibits transport of the magnetic field from the deep interior to the surface and could cause a reduction of solar wind” (Grove 366-367). 

 

Support for Sun fluctuations as the source of the Little Ice Age comes from a recorded decrease in sunspots during part of the Little Ice Age.  As early as the 1880’s, a German astronomer, Gustav Sporer, began wondering why it was that very few sunspots were seen in parts of the 17th and 18th century, even though there were "so many observers of the Sun, as were then perpetually peeping in upon (it) with their telescopes in England, France, Germany and Italy” (Eddy 1976, in Grove 366).  The prolonged absence of sunspots between 1645 and 1715 came to be known as the Mauder Minimum.  During this period, based on drawing of early astronomers, the Sun’s rotations was faster during the Little Ice Age than present, which accounts for the low numbers of sunspots.  Still, the Mauder Minimum does not occur through the whole period of the Little Ice Age, thus making a definitive correlation problematic. 

 

 

Variations in earth’s orbit (Milankovitch cycles) :  Positive Feedback    

 

A major background factor in some, but not all, sudden climate switches seems to have been the set of "Milankovitch” rhythms in seasonal sunlight distribution.  Although this factor changes gradually over many thousands of years, it may take the earth’s climate to a "break point” at which other factors will begin to amplify change into a sudden transition.

 

The Milankovitch cycles are a set of cyclic variations in characteristics of the Earth’s orbit around the sun.  Each cycle has a different length, so at times their effects reinforce each other and at other times they (partially) cancel each other.

 

It is very unlikely that the Milankovitch cycles can start or end an ice age (series of glacial periods):

 

·         Even when their effects reinforce each other they are not strong enough.

·         The "peaks” (effects reinforce each other) and "troughs” (effects cancel each other) are much more regular and much more frequent than the observed ice ages.

 

In these cases their ultimate trigger must lie in other factors, probably a combination of many processes that sometimes lines up to set the climate system on a runaway course in either the direction of cooling or warming. 

 

In contrast, there is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial period within an ice age.  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 Milankovitch orbital forcing periods is so close that orbital forcing is generally accepted.

 

In one of the Milankovitch rhythms, the shape of the Earth’s orbit shifts from more elliptical to more nearly circular.  In another the degree of tilt of the Earth’s axis changes, and in the third the timing of the seasons changes relative to the Earth’s elliptical track nearer and further from the sun.  These rhythms, respectively work on 21,000, 42,000 and 100,000-year timescales, alter the relative amount of solar radiation reaching the Earth’s Northern and Southern Hemispheres during summer and winter.  Times when summer sunlight in the Northern Hemisphere is strong (but when the winter sunlight is correspondingly weak) tend to be the times when the rapid global transition from glacial to interglacial conditions occurs.  This is thought to be due to the effects of summer temperatures on various of the factors mentioned above; for example, it ensures melting back of snow and sea ice in summer, helping the earth to absorb more solar radiation and thus to heat up further.  It is probably the combination of amplifying factors that brings the earth out of glacial and into interglacial conditions.  These big glacial-interglacial transitions tend roughly to follow the 100,000-year timescale, when the three different rhythms (and possibly other poorly understood factors such as the internal structure of ice-sheets) line up to give a big increase in northern summer warmth, but the lesser individual rhythms can also be detected in the temperature record on the 19,000 and 42,000-year timescales, and in fact the timing of interglacial onset tends to more closely follow multiples of the 19,000 year cycle than an exact correspondence to the 100,000 year cycle.     

 

 

Volcanism:  Positive Feedback  

 

It is well known that volcanic injection of micro-particles and gases into the stratosphere reduce the amount of solar energy that reaches the surface of the Earth.  Successive volcanoes (or even one) can temporarily dust the sky of the entire Earth and thus affect the Climate of the Earth on a variety of time-scales.  For example, the eruption of Mt. Agung in 1963 had a marked effect on the climate of the American high Arctic as well as in the tropical troposphere, where temperature decreased almost one degree Celsius from august 1964 to august 1965.  But other equally large eruptions, like Bezymyannyy in 1956, have not been followed by a similar decline in temperature, most likely due to difference like the location and chemical make-up of the eruption, which make simply equating more eruptions with lower temperatures a bit troublesome.  Consequently, "there seems little doubt that volcanic activity influences climate but the extent of this influence is controversial” (grove 368).

 

Nevertheless, an ice-core from Crete, Greenland shows a rather provocative correlation between increased volcanic activity and lower than average temperatures over the past fourteen hundred years, there was lower-than average acidic levels of the Greenland core (e), which refers to decreased amounts of sulphate in the atmosphere due to volcanic activity, matches almost perfectly with the warm period of this era.  "The quietist period volcanically was from CE 1100 to 1250 that is in the medieval warm period.  The most active period volcanically came between CE 1250 and 1500 and between CE 1550 and 1700, suggesting that it had an important role in the causation of the Little Ice Age” (Grove 376).

 

 

Position of the continents:  Positive Feedback

 

The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form.  The ice sheets increase the Earth’s reflectivity and thus reduce the absorption of solar radiation.  With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a positive feedback loop.  The ice age continues until the reduction in weathering causes an increase in the greenhouse effect. 

 

There are three known configurations of the continents which block or reduce the flow of warm water from the equator to the poles:

 

·         A Continent sits on top of a pole, as Antarctica does today.

·         A polar sea is almost land-locked, as the Arctic Ocean is today.

·         A super continent covers most of the equator, as Rodinia did during the Cryogenian period.

 

Since today’s Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth will continue to endure glacial periods in the geologically near future. 

 

Some scientists believe that the Himalayas are a major factor in the current ice age, because these mountains have increased Earth’s total rainfall and therefore the rate at which CO2 is washed out of the atmosphere, decreasing the greenhouse effect.  The Himalayas formation started about 70 million years ago when the Indo-Australian plate collided with the Eurasian Plate, and the Himalayas are still rising by about 5mm per year because the Indo-Australian plate is still moving at 67mm/year.  The history of the Himalayas broadly fits the long-term decrease in earth’s average temperature since the mid-Eocene, 40 million years ago.    

 

 

Uplift of the Tibetan plateau and surrounding mountain areas above the snowline

 

Matthias Kuhle’s geological theory of Ice Age development was suggested by the existence of an ice sheet covering the Tibetan plateau during the Ice Ages (Last Glacial Maximum).  The plate-tectonic uplift of Tibet past the snow-line has led to a c.2.4 million km2 ice surface with a 70% greater albedo than the bare land surface.  The reflection of energy into space resulted in a global cooling, triggering the Pleistocene Ice Age.  Because this highland is at a subtropical latitude, with 4 to 5 times the insolation of high-latitude areas, what would be Earth’s strongest heating surface has turned into a cooling surface.

 

Kuhle explains the interglacial periods by the 100 000-year cycle of radiation changes due to variations of the Earth’s orbit.  This comparatively insignificant warming, when combined with the lowering of the Nordic inland ice areas and Tibet due to the weight of the superimposed ice-load, has led to the repeated complete thawing of the inland ice areas.

 

 

Changes in Earth’s atmosphere

 

There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect.  Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism. 

 

The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO2 levels in the atmosphere, and some supporters of Snowball Earth argue that it was caused by a reduction in atmospheric CO2.  The hypothesis also warns of future Snowball Earths.

 

The August 2009 edition of Science provides further evidence that changes in solar insolation provide the initial trigger for the Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change. 

 

William Ruddiman has proposed the early anthropocene hypothesis, according to which the anthropocene era, as some people call the most recent period in the Earth’s history when the activities of the human race first began to have a significant global impact on the Earth’s climate and ecosystems did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors.  It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the Milankovitch cycles.  In his overdue-glaciation hypothesis Ruddiman states that an incipient ice age would probably have begun several thousand years ago, but the arrival of that scheduled ice age was forestalled by the activities of early farmers.  

 

 

Other factors include:  

 

·         Cloud Cover

·         Eco Systems

·         Forest Fires

·         Thermal Radiation

·         Methane (peat bogs, organic matter and hydrates)

  

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