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So we've established that we can use ice cores as a record of regional temperature through the Quaternary. But regional temperature is subject to many different variables. If only we had information about global temperature change? Well, the last post mentioned that we'd concentrate on just the polar ice cores. Isotope ratios from ice cores in both Greenland and Antarctica have been compared and it seems that they correlate relatively well, implying that whatever is forcing the climate in the Northern Hemisphere is doing exactly the same thing in the Southern Hemisphere, a global climate forcing. Now for the promised tangent.
We mentioned solar forcing. Cyclic changes in received solar radiation (known as insolation) are caused by the rhythmic variation in the Earth's orbit and axial tilt. These are known as the Milankovitch cycles, named after Milutin Milankovitch. The eccentricity of the Earth's orbit (how elliptical it is) fluctuates on a 100,000 year cycle and affects both the total solar radiation received in a year (due to the change in distance from the sun) and seasonality (difference in temperature between Summer and Winter). Obliquity is the angle of the tilt of the Earth's axis. This fluctuates between 22.1 and 24.5 degrees on a cycle of 42,000 years and is another control on seasonality. If the North Pole is further away from the sun during Southern Hemisphere Summer, Northern Hemisphere Winter is longer and colder than if it were closer to the sun. Do you follow? The third cycle is the Precession of the Equinoxes. This occurs on a cycle of 26,000 year cycle and is the cycle at which the Earth's axis rotates about an imaginary axis perpendicular to Earth's orbit. The precession is caused by the combined gravitational forces of the Sun and the Moon and is another control on seasonality. For a really great tutorial on Milankovitch Cycles check out this link, it's a great introduction to the concept (you do need flash player I think).
How do these Milankovitch cycles relate to the fluctuations in the climate proxy records we see in ice cores? It is the current paradigm that these cycles in the relative position of the Earth to the Sun control the cycles in the Earth's climate from glacial conditions back to interglacial conditions.
The prevailing cycle in the climate proxy record is the 100,000 year glacial cycle. However, it has been noted by many researchers that the fluctuations in received solar radiation caused by the 100,000 year eccentricity cycles would not be enough to control the massive fluctuations in global air temperature seen between the interglacial and glacial climate extremes. Many summarise that it is probably the combined effect of the precession and obliquity cycles that control or trigger the 100,000 year climate cycle observed over the past 400-800,000 years.
There is however, a hitch. Work by Carl Wunsch analysing the temperature proxy record leads to the conclusion that most of the temperature variation over the past 400,000 years is indistinguishable from a stochastic (internally complex with unidentified feedback mechanisms) system and not a result of solar forcing. In my opinion, the jury is still out on this. Also, as highlighted in the work by Wunsch, an 800,000 year sample size isn't that big when you're only considering eight 100,000 year cycles.
So, make of that what you will! The best place to start reading about this is probably from the beginning, with the Hays et al. (1979) paper in Science, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". I know it talks about marine sediment cores, but the concept is exactly the same. It's definitely a great introduction to the concept of solar forcing of past climate.
There we have it, a brief introduction to ice core temperature proxies. Tomorrow I will discuss some of the other climate proxies we extract from ice cores: CO2, CH4 and dust flux, and what these tell us about the environments at the time.
We mentioned solar forcing. Cyclic changes in received solar radiation (known as insolation) are caused by the rhythmic variation in the Earth's orbit and axial tilt. These are known as the Milankovitch cycles, named after Milutin Milankovitch. The eccentricity of the Earth's orbit (how elliptical it is) fluctuates on a 100,000 year cycle and affects both the total solar radiation received in a year (due to the change in distance from the sun) and seasonality (difference in temperature between Summer and Winter). Obliquity is the angle of the tilt of the Earth's axis. This fluctuates between 22.1 and 24.5 degrees on a cycle of 42,000 years and is another control on seasonality. If the North Pole is further away from the sun during Southern Hemisphere Summer, Northern Hemisphere Winter is longer and colder than if it were closer to the sun. Do you follow? The third cycle is the Precession of the Equinoxes. This occurs on a cycle of 26,000 year cycle and is the cycle at which the Earth's axis rotates about an imaginary axis perpendicular to Earth's orbit. The precession is caused by the combined gravitational forces of the Sun and the Moon and is another control on seasonality. For a really great tutorial on Milankovitch Cycles check out this link, it's a great introduction to the concept (you do need flash player I think).
How do these Milankovitch cycles relate to the fluctuations in the climate proxy records we see in ice cores? It is the current paradigm that these cycles in the relative position of the Earth to the Sun control the cycles in the Earth's climate from glacial conditions back to interglacial conditions.
The prevailing cycle in the climate proxy record is the 100,000 year glacial cycle. However, it has been noted by many researchers that the fluctuations in received solar radiation caused by the 100,000 year eccentricity cycles would not be enough to control the massive fluctuations in global air temperature seen between the interglacial and glacial climate extremes. Many summarise that it is probably the combined effect of the precession and obliquity cycles that control or trigger the 100,000 year climate cycle observed over the past 400-800,000 years.
There is however, a hitch. Work by Carl Wunsch analysing the temperature proxy record leads to the conclusion that most of the temperature variation over the past 400,000 years is indistinguishable from a stochastic (internally complex with unidentified feedback mechanisms) system and not a result of solar forcing. In my opinion, the jury is still out on this. Also, as highlighted in the work by Wunsch, an 800,000 year sample size isn't that big when you're only considering eight 100,000 year cycles.
So, make of that what you will! The best place to start reading about this is probably from the beginning, with the Hays et al. (1979) paper in Science, "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". I know it talks about marine sediment cores, but the concept is exactly the same. It's definitely a great introduction to the concept of solar forcing of past climate.
There we have it, a brief introduction to ice core temperature proxies. Tomorrow I will discuss some of the other climate proxies we extract from ice cores: CO2, CH4 and dust flux, and what these tell us about the environments at the time.
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