Let's get started... Pleistocene vs Holocene

I hope you all enjoyed the video from earlier on and, as it was promised last week, we are now going to get down to business.

But firstly, since I have a feeling that not every person reading my blog has a degree in palaeoclimatology, I feel obliged to bring you all to the same starting point (might also be useful for those geographers, who slept through their 1st year lectures). We are going to take a small step away from our main topic and transfer ourselves to the more ancient times of the last glacial period (aka the Pleistocene) as I familiarise all the non-climatologists out there with the concept of Dansgaard-Oeschger cycles.

Greenland ice cores reveal that much of the last glacial period was punctuated by millennial-scale climatic fluctuations, known as Dansgaard-Oeschger stadials-interstadials. These events occured 25 times througout Pleistocene and were characterised by a very abrupt high amplitude warming to near-interglacial conditions, occuring in a matter of decades, which were followed by a gradual cooling. Next, a very rapid cooling occured, followed by a gradual warming phase leading up to the next sudden warming. Shifts of 9 to 16 degrees Celcius were observed over Greenland during a cycle (Wang and Mysak, 2005). D-O oscillations are quiasi-periodic in nature, with the period being modulated by the glacial background climate: relatively long when the background climate was either relatively warm or cold; or relatively short when the background climate state was at an intermediate phase (NCDC, 29/10/2012).

D-O stadials are characterised by the process known as ice-rafting, which is essentially a substantial ice output of the continental ice sheets. Such iceberg discharges seriously affect the North Atlantic thermohaline circulation, switching it to the weaker mode trough its partial shut-down. The NA thermohaline circulation plays an important role in redistributing heat around the globe and its reduced intensity leads to a cooling in the Northern Hemisphere and a warming of Antarctica. This can be explained by the bipolar seesaw mechanism where changes in the strength of the THC result in changes in the interhemispheric heat transport (Stenni et al., 2010).

To confuse you even more, the 6 most extreme Dansgaard-Oeschger stadials are known as Heinrich events, named after the marine geologist Hartmut Heinrich who discovered them. During these periods ice-rafting took place on a much larger scale, which eventially led to the full shut-down of the North Atlantic thermohaline circulation and therefore no heat redistribution took place.

Contrary to the Pleistocene, until very recently the current interglacial was considered to be a period of comparatively stable climate. A number of studies  (O'Brien et al., 1995; Alley et al., 1997; Overpeck, 1997) used the records from Greenland ice cores to suggest that climate variability was considerably more subdued during the Holocene than it was during the last glaciation.

It wasn't until 1997, when Bond and his colleagues looked at the Holocene period and discovered a similar bipolar seesaw effect, although of a lesser magnitude. The discovery has led them to suggest that the pervasive millennial-scale climate cycle operates independently of the glacial-interglacial climate state. So far eight of such events have been identified, dating back to 1400, 2800, 4200, 5900, 8100, 9400, 10300 and 11000 years ago (Bond et al., 1997). Bond cycles are sometimes referred to as "mini Dansgaard-Oeschger cycles" and are thought to have a magnitude of approximately 15-20% of the D-O events.

To illustrate what I have just been talking about, here is a graphical representation of climatic variations throughout Pleistocene and Holocene. We can notice a series of very pronounced peaks for both periods, indicating the abrupt climate change events.

Source: Rahmstorf (2003)

 

Since we are talking about the temperature variations, some of you would expect to see the y-axis of the graphs named something like "degrees Celcius" and would get rather frustrated by the complicated sounding labels we have here. The reason for expressing climate data in such a way is that the direct meteorological observations began approximately 300 years ago. The scientists simply do not know the Earth's climatic conditions a thousand years ago, let alone going back a few millions. For this reason in trying to reconstruct past environmental conditions palaeoclimatologists have to use sources of documentary data known as proxies.

Oxygen isotopes in ice records are one of the most powerful and widely used proxies, when determining the periods of abrupt climate change. However, I am sure you already need a break to digest all the information you have received today. Therefore we shall leave the all the isotopes and proxies to the next post, where I will be talking about how the scientists know for sure that those abrupt climatic fluctuations actually took place.

List of references:

Alley, R., C. Shuman, D. Meese, A. Gow, K. Taylor, K. Cuffey, J. Fitzpatrick, P. Grootes, G. Zielinski, M. Ram, G. Spinelli and B. Elder (1997) 'Visual-Stratigraphic Dating of the GISP2 Ice Core: Basis, Reproducibility and Application', Journal of Geophysical Research, 102, CP12, 26, 367.
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.
National Climatic Data Center (2008) 'A Palaeo Perspective on Abrupt Climate Change: Heinrich and Dansgaard-Oeschger Events' (WWW), National Oceanic and Atmospheric Adimistration (http://www.ncdc.noaa.gov/paleo/abrupt/data3.html), 29/10/2012.
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.
Overpeck, J. (1997) 'Arctic Environmental Change of the Last Four Centuries', Science, 278, 5341, 1251-1256.
Rahmstorf, S. (2003) 'Timing of Abrupt Climate Change: A Precise Clock', Geophysical Research Letters, 30, 10, 17-1-4.
Stenni, B., D. Buiron, M. Frezotti, S. Albani, C. Barbante, E. Bard, J. Barnola, M. Baroni, M. Bonazza and E. Capron (2011) 'Expression of the Bipolar See-Saw in Antarctic Climate Records during the Last Deglaciation', Nature Geoscience, 4, 46-49.
Wang, Z. and L. Mysak (2006) 'Glacial Abrupt Climate Changes and Dansgaard-Oeschger Osciallations in a Coupled Climate Model', Paleoceonography, 21, PA2001, 9PP.