Some external forcing factors with the ability to distrupt the general ocean circulation

As I have briefly discussed before, with reference to Heinrich events, the current ocean circulation pattern that exisits has not always do so and alterations to it in the past have resulted in huge shifts in the worlds climate. As well as those previously mentioned, there are some other external forcing factors that have the ability to affect the oceanic and atmospheric circulation. I think some of this stuff, like volcanic influence and Milankovich, comes up in the A2 syllabus although I don't know how much detail we go into - probably a bit more than I have done here, but hopefully the basics will still be useful!

At varying points in time, external forcing factors have and will continue to provoke variability in the ocean circulation. Understanding them is, again, crucial as some explain the past changes that occurred, whilst others endorse future predictions.

There are two types of Precession: this diagram illustrates Axial Precession
 and this alters the hemispheric distribution of insolation. For more on
Milankovich and the individual cycles see seperate blog post on
The race for a million year old ice sample 

Many proxy-climate records suggest that the Milankovich ‘pacemaker’ has driven alterations in the oceans, atmosphere and the cryosphere; due to the moderation in the hemispheric distribution of insolation provoked by the three cycles, precession (23ka and 19ka), obliquity (41ka) and eccentricity (100ka). The spectral analysis of d¹⁸O variations present in marine carbonates over the last 10Ma, contain signals of all three cycles moderating high northern latitude insolation variations.  Milankovich used this to explain the initiation and termination of ice ages and his theory states that the alterations in high-latitude insolation in the Northern Hemisphere are crucial in regulating the survival of snow through successive summers to warrant ice accumulation; a theory authenticated by the coincidence of glacial terminations and rapid increases in the solar insolation values of high latitudes in the Northern Hemisphere. The control Milankovich cycles exercise over hemispheric insolation distribution therefore dictates, although on a delayed timescale, the extension/shrinkage of ice masses (whether that be sea ice or ice-sheet/shelves) and it is because of this that they alter ocean circulation; although it is believed that precession has the largest influence. Generally speaking, the cycles range between two extremes, one favouring glaciation and the other deglaciation. When they favour glaciation, the THC is forced to downwell further south and thereby reduces northwards heat transfer and the associated ablation processes, to only further accelerate cooling. During glacial termination, the opposite is true and baseline increases in NADW intensity, beginning approximately 18ka, that parallel increasing Northern Hemisphere insolation, have been inferred from the RC11-83 South Atlantic Ocean floor sediment core. Many believe that it is the alterations provoked by this extrinsic forcing factor to the thermohaline circulation that drives, all atleast contributes, to the temperature changes that occur during transitions between glacial and interglacials.

Spectral analysis of variations of the chemical composition of ice cores have revealed that cycles shorter than those of Milankovich have occurred (periodicities of 11100, 6100 and 1470 years); with these sub-Milankovich cycles believed to be overtones generated within the climate system due to the influence longer cycles. The sub-Milankovich cycles can also be classed as an external forcing factor and can be attributed to shorter, abrupt alterations to the oceanic circulation. Precessional-driven climate alterations are believed to be linked to the 11100 year cyclicity which is being held responsible for temperature maximums being experienced every 11100ka in continents straddling the tropics. The 6100 year cyclicity seems to have a relationship with Heinrich events and other coolings such as the Little Ice Age and it is thought to only be amplified by ice-sheet presence. The shortest periodicity, because of the lethargic nature of ice-sheets, presumed to communicate readjustment of atmospheric circulation, is possibly due to solar output variations; something suggested by ¹⁴C records in tree-rings. When the sun is at its most ‘’energetic’, the Earth’s magnetic field is strengthened, blocking more cosmic rays. ¹⁴C is formed when cosmic rays hit plants, therefore measured in tree rings; with high levels of ¹⁴C suggesting an ‘inactive’ sun. Bond documented increases in icebergs and ice drift coinciding with the increase in ¹⁴C, indicating the sun was weaker at such times. Alterations in the volume of ice-rafted debris, in North Atlantic, also coincide with the 1470 year cyclicity, although are only 1/10 the size of the inconsistencies witnessed during the last glacial. Overall, there is agreement amongst scientists that these millennial-scale cycles, which have also been detected within ENSO, have a solar inception affiliated to the THC.

Volcanic eruptions, principally those that disperse ash and SO₂ into the stratosphere which are most commonly high latitude (due to lower tropopause) explosive eruptions, can provoke a period of cooling that can last for a few years; like Pinatubo did after it erupted in 1991, releasing 20 million tonnes of SO₂, cooling the Northern Hemisphere 0.5°C over  5 years.  Despite their apparent ability to affect the climate, the effect they have on ocean circulation is debatable due to the variability produced by GCM. Some models show a connection between volcanic eruptions and a reduction in MOC intensity. However, these reductions appear to be small and short term and, this fact combined with other models not picking up the link above, suggest that the impacts of volcanic eruptions on ocean circulation, if there is one at all, is of little consequence and do not seem to pose a threat to the stability of the ocean circulation.

The race for a million year old ice sample

Around every 100,000 years the Earth enters an Ice Age but it hasn't always been this way...... Up until about a million years ago the Earth swung between glacial and interglacial periods a lot faster, with this switch occuring every 40,000 years. However, no one knows why the time taken for Ice Age's to occur slowed and this is why scientists are so eager to find a million year old ice sample.



Axial Precession



At present, the switch between glacial and interglacial periods is believed to be influenced by three cyclical changes to the Earth's motion known as the Milankovitch Cycles, which were named after the Serbian astronomer who is credited with discovering their magnitude. The first of these cycles is called Precession and relates to the wobble of the Earth as it spins on its axis provoked by the gravitional interaction between the Sun, Moon and Earth. Precession occurs on a 26,000 year cylce and there are two forms - axial and apsidal precession. Axial precession is linked with the tilt of the axis in relation to the fixed place of the stars Vega and the North Star as the Earth wobbles from pointing to the North Star to pointing at Vega. If the axis tilts towards Vega then then the winter solstice in the Northern Hemisphere will coincide with the aphelion (point at which the Earth is furthest away from the Sun) and the summer solstice with perihelion (the point at which the Earth is closest to the Sun) thereby creating the greatest seasonal differences. When this occurs, the Southern Hemisphere experiences warmer winters and cooler summers and so a smaller seasonal difference. However, when the tilt of the Earth allows for the aphelion and perihelion to, respectively, occur near the autumn and spring equinoxes, the seasonal contrasts experienced in the Northern and Southern Hemisphere become similar.  At present, the perihelion is closer to the Northern Hemisphere's winter solstice therefore meaning a small seasonal difference in the Northern Hemisphere. Apsidal precession occurs when the Earth's orbit, as a result of the influences of the Moon, Jupiter and Saturn, starts to precess in space. This movement is known as the precession of the equinoxes and it effects the intensity of the seasons. So, in summary, Precession does not effect the amount of solar energy recieved by the Earth but the way it is distributed between the two hemispheres, therefore altering the seasonal differences experienced.



Apsidal Precession

 

Obliquity

The second cycle is the tilt of the Earth's axis which, I think, is normally known as Obliquity or the Angle of Inclination. Currently, the axis lies at a 23.4 degree angle and over a 41,000 year period this varies between 22 degrees and 24.5 degrees, thereby altering the latitundinal distribution of solar energy. This fluctuation in the angle of incidence causes changes in the intensity of the seasons experienced. As the angle of incidence increases, during summer, areas at high latitudes experience more solar energy whilst in the winter they experience a decrease in insolation (insolation is a measure of solar radiation energy recieved on a given surface area in a given time). This allows for permanent snow fields to form in the Northern Hemisphere.  In relation to low latitudes, changes in Obliquity have little effect as the strength decreases the closer you get to the equator. Therefore, changes in Obliquity alter the strength of the latitudinal temperature gradient. When the axial tilt is lower the Sun's solar radiation is more evenly distributed between the seasons but the difference in radiation recieved between the equator and polar regions is greater. A smaller degree of axial tilt would provoke the formation of ice sheets because warmer winters would result in more warm air, which has the potential to hold more moisture and so produce more snowfall. Ontop of this, milder summers would mean that less of the ice formed over winter would melt.



Circular orbit

 


Ellipictal orbit 

 

The third, and final, cycle is known as Eccentricity which is the shape of the Earth's orbit around the Sun. The changes in Eccentricity occur due to the gravitional influences of Jupiter and Saturn and the shape of the Earth's orbit around the sun changes from being ellipictal (eccentricity of 0.0607) to less ellipictal/more circular (eccentricity of 0.0005) on a cycle of around 100,000 years. This is of great importance to climate and glaciation as it alters the distance between the Earth and the Sun, thereby changing the distance that the Sun's radiation has to travel before reaching the Earth. This subsequently reduces or increases the amount of radiation recieved on the Earth in different seasons as this variation has a direct impact on the amount of solar energy recieved at perihelion in constrast to aphelion. Currently the Earth's Eccentricity is 0.016 which results in a 6.4% increase in the level of insolation recieved in January in comparison to July. This increase has been provoked by the 3% difference in the distance between perihelion and aphelion. When the Eccentricity is higher (so a more ellipictal orbit) the difference between the solar energy recieved at perihelion can be anything between 20% to 30% greater than that recieved at aphelion. The variations in Eccentricity also impacts on the length of the seasons and, at present, in the Northern Hemisphere, summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn. Overall, Eccentricity influences the amount of solar radiation that reaches the Earth and so the fluctuations in Eccentricity play a key role in determining climate and the occurance of glaciation. 

So, a short summary about Milankovitch cycles......  basically the changes in Procession, Obliquity and Eccentricity alter the intensity and distribution of solar radiation hitting the Earth which then affects the climate, with particular reference to the extent of glaciation. Milankovitch used these variations to develop a mathematical model which linked insolation to the corresponding surface temperatures and from this model he came to the conclusion that variations in insolation at high latitudes were responsible for the increase and decrease in the size of the ice caps at the poles. One crucial thing that I have yet to mention is the importance of landmass when talking about the Milankovitch cycles as it helps to explain why fluctuations in Precession, Obliquity and Eccentricity are harder to locate in older records. The Northern Hemisphere is known as a Milankovitch sensitive region and, as mentioned above, the effects of alterations in the three cycles decreases as you get closer to the equator and lower latitudes - which don't lie in Milankovitch sensitive latitudes. Therefore, when Pangea existed, which was centred around the equator, the cycles did not have such a prominent effect............. and so the question is, what did? 

This is perhaps the most puzzling question surrounding the shift to a slower pace, by the Earth, around a million years ago as records suggest that their was no obvious change to any of the three cycles and this is yet another reason as to why finding a million year old ice sample is so important. Understanding this shift would enable us to understand why we have the climate we do today and, perhaps, even help us make better predictions for the future climate. One of the most common possible explanations for this shift, at present, is the idea of the slow decline in concentration of the carbon dioxide in the atmosphere that is believed to have started to occur around 3 million years ago. This would have reduced the greenhouse effect and, possibly, cooled the Earth to the extent that the tilt of the Earth towards the Sun, every 41,000 years, was no longer able to provide sufficient heat to melt the glaciers that formed in between.  Confirmation of this is required though and is dependent on the finding of a direct record of the ancient atmosphere. This can only be uncovered from the analysis of the air that became trapped in tiny bubbles within ice as the snow it formed fell to Earth. In 2005, the European Consortium for Ice Coring in Antarctica discovered, to date, the oldest ice core which has stretched our records of the ancient atmosphere back 800,000 years - however, this is short of the crucial time period in which the key transition from a 40,000 year ice age pulse to an 100,000 year one occured.  And so, the race is on to find this crucial million year old ice core........

The EPICA have been joined in the race by an Australian Antarctic Division, an American contigent and a research team from the Chinese Arctic and Antarctic Administration. The Chinese have already secured a location in east Antarctica but have been set back by the discovery that the ice sheets in this chosen location are growing from the bottom up which means that the ancient ice has most likely melted or been replaced already. The Australians are close to securing a site in the Aurora basin, also in east Antarctica, which is believed to be home to the thickest ice in Antarctica, however research needs to be done to ensure that they too don't experience the same set back as the Chinese. Despite this, climatologists remain optmistic that a million year old ice core will be found eventually as it is one of those things that is going to take time. Current drilling methods, which are very similar to those used in the oil industry, mean that to reach this million year old ice core, which is hoped to lie at around 3000 metres deep, will take three summer seasons due to the remote locations of potential sites, but advances in technology mean that this process could be sped up.

This race for the million year old ice core is clearly no where near finishing and, despite the competition that exists between the four teams, the international collaboration that exists will hopefully allow for this increasingly important clue, that will be provided by this crucial ice core, to be uncovered and consequently provide information as to why the climate we presently experience exists and perhaps even how and why, due to physical influences, it could change in the future.