Friday, 27 May 2011

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
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.

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