Causes of Bond Events: A song of ice and fire

While there is still an ongoing debate about the presence of the 1500-year Bond Cycle, the processes triggering this oscillation remain even more mysterious. A number of hypotheses have been put forward, but all of them lack palaeological evidence. In this post I will give a brief overview of the ideas that scientists have come up with so far...

1. Orbital insolation

It is pretty common knowledge that our planet spins around its axis and orbits around the Sun and that is the exact reason we have things like times of the day and different seasons. If your familiarity with astronomy ends here, the next thing might slightly blow your mind, but... the Earth's position in relation to the Sun has not actually been the same in the past! We can blame it all on a series of complex gravitational interactions between the Sun and the Earth.

Taking it a step further, an absolute genius of a man, Serbian geophysicist and astronomer Milutin Milankovic mathematically theorised that variations in Earth's position relative to the Sun (aka its orbital parameters) have determined climatic patterns on our planet. Firstly, the Earth's orbit is NOT actually circular in shape, but varies from near circular to an ellipse over a period of about 100,000 years with a long cycle of about 400,000 years. The length of the long axis of the ellipse is described by a parameter known as eccentricity. Secondly, the Earth's axis of rotation with respect to the plane of its orbit varies between 21.8° and 24.4° every 41,000 years. This process, known as the change in obliquity, is responsible for the extent of the differences between seasons. Finally, there is precession (i.e. moving around a full circle), which could relate to either the elliptical orbit of the planet or to its axis of rotation. The combination of the two different precessions then produces the classically quoted precessional periodicity of 21,700 years.

Overall, variations in eccentricity, obliquity and precession significantly affect the amount of solar radiation a particular place on the planet will receive and therefore are some of the key reasons for climatic fluctiations.

I know that is a lot of new terms for you already and you probably wish for a little bit more detail on this topic, but sadly we have a lot to cover today. If all of the above sounds pretty confusing to you I suggest you have a look at my colleague's blog dedicated solely to the Milankovic cycles. Alternatively here is the best video on orbital parameters I've ever seen:

So what's the weakness of this hypothesis? Some scientists (e.g. Butikofer, 2007) believe that the periodicities of the orbital forcing are far too long to be the cause for the formation of the Holocene Bond events. The only possible option, in their opinion, is to associate the Bond Cycles with subharmonics of the orbital forcing.

2. Solar irradiance

The amount of radiation emitted by the Sun is also not constant throughout the time but expresses a significant cyclic variability. There is a very obvious gradual increase and more rapid decrease of the number of sunspots over the period of roughly 11 years, however Hathaway (2010) mentions numerous authors who have noted multi-cycle periodicities in the sunspot cycle amplitudes, which I have summarised for you in the table below:

Solar Cycle	Periodicity	Notes
Schwabe cycle	11 years	The most obvious sunspot cycle caused by the differential rotation of the Sun's convection zone
Halle Cycle	22 years	The magnetic field of the Sun reverses during each Schwabe cycle, so the magnetic poles return to the same state after 2 reversals
Gleissberg Cycle	70-100 years	Proposed periodicity of 7 or 8 cycles
Suess/de Vries Cycle	210 years
Halstatt Cycle	~2300 years

Source: Brooke (2012), http://www.youtube.com/watch?v=hhy9KjI-fLk

Given that the amount of radiation emitted by the Sun significantly influences the climate of the planet today it is logical to suggest that variations in the sunspot activity (shown on the graph below) could have been a trigger for the Holocene climatic variability.

Source: Solanski et al. (2005)

Gerard Bond himself looks like a big fan of this hypothesis, as well as several other authors (e.g. Chapman and Shackleton, 2000; Niggeman et al., 2003; Lamy et al., 2006; Allen et al., 2007), who considered that solar forcing was the determining or at least one of the determining factors of their reconstructed multicentury to millennial scale climate fluctuations. Bond et al. (2001) linked their drift-ice records with the production rate of the two nuclides ¹⁴C and ¹⁰Be (hope you remember me talking about them being proxies for solar irradiance) in Greenland ice cores and showed the both isotopic time series match very well with their drift-ice reconstructions.

3. Volcanic activity

When volcanoes erupt they can emit things like ash and sulphur dioxide into the air, which reflect sunlight away from the earth and therefore cause a cooling effect. This effect is especially marked in the case of large eruptions able to blast sun-blocking particles all the way up to the stratosphere. Therefore a hypothesis has been put forward that abrupt Holocene cooling events could have been caused by major volcanic eruptions.

Nevertheless, Cronin (2010) suggests that while having large impacts on climate, usually individual stratospheric eruptions lower the mean global temperature only by 0.2-0.3° and last only one to three years. He, however, admits that extended periods of more frequent stratospheric eruptions might, in theory, alter the climate over longer timescales. Still, no definitive link between the observed Holocene climate variability and volcanic eruptions is yet available. Nonetheless, Wanner et al. (2008) provides a reconstruction of strong tropical volcanic eruptions over the last 6000 years and it is striking that 12 out of the 19 eruptions observed during that time span occured in the last millennium with a considerable clustering during the Little Ice Age, which corresponds with the most recent Bond event.

4. Ice-sheet dynamics

When an ice sheet is thin the temperature at its base is <0°, but as the ice sheet thickens its basal temperature  increases until it reaches 0° and the basal melting occurs. Basal melting can lubricate the bed, causing the ice to surge, eventually thinning and refreezing (Cronin, 2010). Ice-surging in turn can lead to the discharge of iceberg armadas into the North Atlantic surface layer, lowering the salinity and reducing deep-water formation form the conveyor belt. As THC diminishes, less heat is drawn to high northern latitudes, and this acts as a negative feedback leading to increased ice growth in the Laurentide area and a self-sustaining cycle is created (Cronin, 2010).

This theory extends the explanation put forward by MacAyeal (1993) for the cause of the glacial Dansgaard-Oeschger oscillations into the Holocene and has now also acquired a decent number of supporters (e.g. Seidov and Maslin, 1999; Jennings et al., 2002).

5. Other processes

Other catalysts for Bond Cycles have been proposed such as tidal forcing (Keeling and Whorf, 1997), the influence of cosmic rays and the global electric circuit (Kirkby, 2007) and changes in the Earth's magnetic field (Courtillot et al., 2007). If you want to undertake some further research on this topic, I suggest you have a look at pages 176-183 in Chronin (2010), where he provides a good review for a number of other proposed trigger mechanisms.

Overall, although lack of supporting evidence means there is still no consensus on the cause for Holocene climatic fluctuations, it is most likely that more than one factor was responsible for the rapid cooling events. Wanner and Butikoffer (2008) tried to represent the complex mechanism of a formation of a cold Holocene event through the following diagram:

Nonetheless, they admit that their representation of Bond event trigger mechanisms is still speculative and must be investigated with further analyses of marine and other palaeoclimatic records as well as suitable model runs.

List of references:

Allen, J., A. Long, C. Ottley, D. Pearson, and B. Huntley (2007) 'Holocene Climate Variability in Northernmost Europe', Quaternary Science Reviews, 26, 1432-1453.
Bond, G., B. Kromer, J. Beer, R. Muscheler, M. Evans, W. Showers, S. Hoffmann, R. Lotti-Bond, I. Hajdas and G. Bonani (2001) 'Persistent Solar Influence on North Atlantic Climate during the Holocene', Science, 278, 1257-1266.
Butikofer, J. (2007) 'Millennial Scale Climate Variability during the Last 6000 Years - Tracking Down the Bond Cycles', Geographichesches Institut, Universitat Bern.
Chapman, M. and N. Shackleton (2000) 'Evidence of 550-Year and 1000-Year Cyclicities in North Atlantic Circulation Patterns during the Holocene', The Holocene, 10, 287-291.
Courtillot, V., Y. Gallet, J.-F. le Mouel, F. Fluteau and A. Genevey (2007) ' Are There Connections between the Earth's Magnetic Field and Climate?', Earth and Planetary Science Letters, 253, 328-339.
Cronin, T. (2010) Paleoclimates: Understanding Climate Change Past and Present, Columbia University Press: New York.
Hathaway, D. (2010) 'The Solar Cycle', Living Reviews in Solar Physiscs, 7, 1, 5-56.
Jennings, A., K. Knudsen, M. Hald, C. Hansen and J. Andrews (2002) 'A Mid-Holocene Shift in Arctic Sea Ice Variability on the East Greenland Shelf', The Holocene, 12, 49-58.
Kirkby, J. (2008) 'Cosmic Rays and Climate', Surveys in Geophysics, 28, 333-375.
Lamy, F., H. Arz, G. Bond, A. Bahr and J. Patzold (2006) 'Multicentennial-Scale Hydrological Changes in the Black Sea and Northern Red Sea during the Holocene and the Arctic/North Atlantic Oscillation', Paleoceonography, 21, PA1008.
MacAyeal, D. (1993) 'Binge/Purge Oscillations of the Laurentide Ice Sheet as a Cause of the North Athlantic's Heinrich Events', Paleoceonography, 8, 775-784.
Niggemann, S., A. Mangini, M. Mudelsee, D. Richter and G. Wurth (2003) 'Sub-Milankovitch Climatic Cycles in Holocene Stalagmites from Sauerland, Germany', Earth and Planetary Science Letters, 216, 4, 539-547.
Keeling, C. and T. Whorf (1997) 'Possible Forcing of Global Temperature by the Oceanic Tides', Proceedings of the National Academy of Sciences, 94, 8321-8328.
Seidov, D. and M. Maslin (1999) 'North Atlantic Deep Water Circulation Collapse during the Heinrich Events', Geology, 27, 23-26.
Solanski, S., I. Usoskin, B. Kromer, M. Schussler and J. Beer (2004) 'An Unusually Active Sun during Recent Decades Compared to the Previous 11,000 Years', Nature, 431, 7012, 1084-1087.
Wanner, H., J. Beer, T. J. Crowley, U. Cubasch, J. Flückiger, H. Goose, M. Grosjean, J. Kaplan, M. Küttel, O. Solomina, T. Stocker, P. Tarasov, M. Wagner, and M. Widmann (2008) 'Mid- to Late Holocene Climate Change: An Overview', Quaternary Science Reviews, 27, 19-20, 1791-1828.