Fact or fiction?

Almost every new proposed theory has to go through a lot of heavy criticism before it becomes accepted (think of plate tectonics, poor Wegener!). Due to the fact that the concept of cyclic Holocene climate oscillations is rather new and goes contrary to the view scientific community adopted until very recently there is a great deal of heated debate relating to this issue.

To understand the nature of this debate in more depth we are going to work with the three questions put forward by Butikofer (2007):

1) Where are Bond Cycles postulated in paleoclimate record?
2) Is there really a global imprint of millennial (∼1500 years) scale cycles with shifts from cold to warm (dry to wet, respectively) climate?
3) What are the possible processes causing Bond Cycles during the Holocene?

Feeling that the despite your passion for climatology there is a limited amount of information you can intake on a casual Tuesday evening, I decided to focus only on the first two questions in this post. Next time our discussion will be centred around all the proposed causes for the Bond events and I am sure you are already very excited for that. But for now let's at first look at how it all started...

The first ever research to suggest that Holocene atmospheric circulation above the ice cap was punctuated by a series of millennial-scale shifts was by O'Brien et al. (1995), which focused on the analysis of soluble impurities from Greenland ice cores. They argued that the increases in soluble impurities in the ice cores probably occured at times of lowered atmospheric temperatures. However, the main breakthrough came when, encouraged by the findings of O'Brien and her colleagues, Bond et al. (1997) launched an investigation of deep sea Holcene sediments in the North Athlantic. These sediments were analysed in terms of presence of lithic/petrologic tracers in them, which led Bond et al. (1997) to suggest that ice-rafting also occured during the Holocene and was caused by a series of ocean surface coolings with a cyclic signal centered on 1470 +/- 532 years. Overall, the investigation presents the evidence for "pervasive, at least quasiperiodic, climate cycle occuring independently of the glacial-interglacial climate stage".

Gerard Bond and his colleagues are not the only ones who believe in the existence of Holocene quasi-periodic climate fluctuations. Niggemann et al. (2003) also came to the conclusion about the operation of a 1450-years cycle after looking at stalagmites (another type of proxy!) in the Atta cave in Sauerland, Germany, which were analysed in terms of their δ¹⁸O composition. The peaks of δ¹⁸O in the stalagmites indicate sections where calcite formed during periods of lower humidity in the cave, which could be attributed to potentially drier winters; winter precipitation is in turn related to the strenght of the Northern Hemisphere annular mode (NAM) through the intensity of westerlies.

The final argument here are the findings of Willard et al. (2005), who, when interpreting pollen data from Chesapeake Bay in the mid-Atlantic region of the United States, discovered significant decreases in pine pollen every 1400 years. These decreases have been said to represent the decreases in January temperatures in between 0.2 and 2 degrees Celcius. The timing of pine minima is correlated with a series of quasi-periodic cold intervals documented by various proxies in Alaskan, Greenland and North Atlantic cores.

On the other hand, Butikofer (2007) reviewed a number of papers on Holocene climate variability (including the ones mentioned above) and concluded that there is not enough evidence to confirm the existence of globally extensive climate cycles operating with a distinct pacing near 1500 years during the past 6000 years and "the previously claimed similarity between Holocene millennial-scale variability and the dominant 1470-year glacial periodicity appears questionable". This idea was later further developed in Wanner and Butikofer (2008).

It is rather hard to summarise all the arguments for both supporters and opponents of this proposed theory in a single article. Therefore, if you feel like investigating this topic in more depth, I strongly suggest you look at the Table 1 in Wanner and Butikofer (2008), where a good overview of 28 research papers discussing the phenomenon of Bond Cycles is provided.

List of refecences:

Bond, G., W. Showers, M. Cheseby, R. Lotti, P. Almasi, P. deMenocal, P. Priore, H. Cullen, I. Hajdas and G. Bonani (1997) 'A Pervasive Millennial-Scale Cycle in the North Atlantic Holocene and Glacial Climates', 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.
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.
O'Brien, S., L. Meeker, D. Meese, M. Twickler and S. Whitlow (1995) 'Complexity of Holocene Climate as Reconstructed from a Greenland Ice Core', Science, 270, 5244, 1962-1964.
Wanner, H. and J. Butikofer (2008) 'Holocene Bond Cycles: Real or Imaginary?', Geografie - Sbornik Ceske Geograficke Spolecnosti, 113, 4, 338-350.
Willard, D., C. Berhardt, D. Korejwo and S. Meyers (2005) 'Impact of the Millennial-Scale Holocene Climate Variability on Eastern North American Terrestrial Ecosystems: Pollen-Based Climatic Reconstructions', Global and Planetary Change, 47, 1, 17-35.