Concluding thoughts

Sadly, all good things must come to an end. It is now time for me to finish my blog and carry on with my undergraduate thesis. But before I say goodbye to you all, I feel that it would be appropriate to summarise what we have learned through the course of this investigation and to draw some conclusions regarding the future of the Bond Cycles theory.

So far we have looked at the science behind the Bond events - what they are, how they are expressed in the palaeoclimatic records and what causes them. The key reading on this topic was the original paper by Bond et al. (1997), where the theory is proposed, as well as a number of subsequent research articles (e.g. Bond et al., 2001). Since there is no consensus on how to interpret Gerard Bond's findings, I have tried to familiarise you with the ongoing debate surrounding this new theory. The key papers that provided an excellent review of the vast majority of the scientific work in that area were by Butikofer (2007) and Wanner and Butikofer (2008), even despite the fact that the authors decided on the lack of supporting evidence to conclude on the presence of the 1500-years climate cycles. Finally, I have provided some examples of how Bond event have modified the contours of human history in the Holocene. From the adoption of agriculture to the collapse of Akkadian Empire, to ice skating on the Thames it has definitely been an exciting journey through the millennia.

Unfortunately, strict time frames meant being highly selective when choosing a topic. In trying to give you a flavour of as many things as possible, I feel like some discussions could definitely benefit from more depth. Thus, the debate, surrounding the Bond oscillations theory, is much more complex than presented, and probably needs a separate blog dedicated to it. Similarly, I have provided an overview of only some of the historical events, triggeres by the colder conditions in the North Atlantic, while in reality the influence of those conditions on the whole human activity has been truly collossal! However, for every post I have provided a list of references, so that if you found the topic pretty interesting you could undertake some additional research.

Although we have spent most of the time in the past travelling through the Holocene, in my previous post I have quickly looked at the future possibility of abrupt Younger Dryas-like climate change event and I have concluded that the thermohaline circulation is unlikely to shut down in the near future. Here are some more opinions on whether the Younger Dryas could happen again:

NASA: Unlikely. When it occured previously the world was a very different place with major ice sheets in areas that are now much warmer. The differences in freshwater in the North Atlantic projected for the next 50 to 100 years are much less than the melting that was going on then. However, while this scenario is unlikely, scientists continue to make observations that are relevant to this question and refine the models that are used for climate change scenarios.

NSIDC*: Past events, like the Younger Dryas, show that climate can respond in rapid and hard-to-predict ways to slow persistent forcings (a push on the climate system, such as excess heat in the atmosphere or ocean, wind shifts, or density changes in ocean water). One of the major differences between the present-day situation and the past is that the current forcing, greenhouse gases in the atmosphere, is unlike forcings that led to past climate changes (such as changes in Earth's orbit, or solar influences). Thus, the future course of Earth's climate is hard to forecast, since no simple analog exists in the recent record of natural changes.

So what was the point of all this? Why did I choose to prove that the Dansgaard-Oeschger events were not limited to the last glacial rather than talking about things that are currently in the news: pollution, ocean acidification or hurricane Sandy?

Just like in many disciplines, in climatology the past is very often the key to the present and/or the future. It is only after gaining a solid understanding of the way the climate system operates, environmental policies can be developed. The politicians may promote the view of a global warming armageddon or argue that it is a myth, but what the public really needs is the background knowledge that will help them to make an informed decision. This video has been brought up by my fellow bloggers on many occassions and is just too good not to share. It is actually talking about sea-level rise, rather that climate dynamics, but still illustrates my point perfectly:

The Bond Cycles theory is one of the main topics for the The UK Geological Society Conference on the topic of Holocene Climate Change this April. The conference aims to determine the extent to which Holocene rapid cooling events fit into a proposed 1500-year pattern. I feel like this could be a very important date for the future of Gerard Bond's idea and impatiently await the future debate.

Although this research is now over, my interest in palaeoclimatology is definitely not. I really hope you enjoyed my first attempt at blogging and I would greatly appreciate any feedback you may have for me! Finally, if you have any questions, please write them in the comments section and I will do my best in trying to answer them.

*National Snow & Ice Data Center

Future predictions

Although this short post is slightly off-topic, today I want to share my thoughts on something still pretty relevant. I want to talk about about abrupt climate change, and especially the future forecasts.

Last night I finally watched Al Gore's "An Inconvenient Truth". While it was definitely an hour and a half well spent and I recommend the film to everyone, I have been aware of the controversy surrounding some of Al Gore's statements. In particular, he claimed that global warming could shut down the Ocean Conveyor, throwing the planet into some sort of a Younger Dryas-like event. That is definitely not the point of view the British High Court shares and you can read the full story here http://www.telegraph.co.uk/earth/earthnews/3310137/Al-Gores-nine-Inconvenient-Untruths.html.

There are still scientists out there advocating the idea of the thermohaline circulation shutdown. In partucular, a paper by Schwartz and Randall (2003) gives a rather apocalyptic vision of the Earth's future climate and associated geopolitical issues. They predicted that the MOC collapse would begin in 2010, but I am pretty sure it has not happened yet.

The IPCC Fourth Assessment Report sounds slightly less dramatic. After reviewing current research in the field, IPCC states that the thermohaline circulation shutdown is very unlikely to occur for at least another century. The report also stresses the fact that there are "no indications of an imminent change in the North Atlantic THC".

List of references:

IPCC (2007) 'Fourth Assessment Report: Climate Change 2007' (WWW), IPCC (http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch12s12-6-2.html), 7/01/2013.

Only a Little Ice Age

And back to history! Unfortunately we will have to skip two subseqent Bond events as this blog is not really about the impact of climate change on human activity. However, as a closing note I could not resist the topic that well be pretty familiar for most of you - the Little Ice Age.

Those who have never heard about LIA might think that I'm implying that just a few centuries ago Europe once again had 3-4 km of ice on top of it. In reality this term, intoduced by F.Matthews in 1939, describes a period of cooling that took place roughly between the 16th and 19th centuries. Saying that, there is actually no consensus on the exact timing of the Little Ice Age onset, and as you will see later, the proposed dates start from the mid-13th century.

Since this is the most recent abrupt cooling event, there is actually plenty of evidence for its occurrence. Mann (2002) points out multiple documentary accounts of dramatic mountain glacier retreats and advances during past centuries, as well as widespread historical documentation of weather conditions and even a handful of several centuries-long thermometer measurements. For example, in London the freezing of the Thames River became a rather common occurrence and each time it was celebrated with a winter carnival, which was depicted in many paintings:

Source: Fine Art America, (http://fineartamerica.com/featured/frozen-thames-granger.html).

The Little Ice age brought colder winters to large regions of North America and Europe. Mann (2002) points out many farms and villages, that were lost to the advancing glaciers in the mid-17th century. He also gives a mention to increased reports of famine, disease and child mortality in Europe during that period. Further illustrating this point, here is the graph of rye prices in Germany, where we can observe some very pronounced peaks, possibly atributing them to years of low agricultural productivity:

Source: S. Mandia (2013), (http://www2.sunysuffolk.edu/mandias/lia/little_ice_age.html).

However, Mann (2002) believes that in Europe The Little Ice Age may have been more pronounced in terms of variability of the climate, rather than changes in the average climate itself. Thus, a severe winter preceded the hot summer that precipitated the Great Fire of London in 1666.

Finally, deteriorating climatic conditions during the Little Ice Age have frequently been blamed for the demise of Norse population in Greenland (e.g. Brown, 2000). However, although the LIA cooling has definitely created additional pressure for the Viking colonies in Greenland, it is very likely that the collapse of the Norse Greenland can also be attributed to the complex societal dynamics (Brown, 2000). If this topic seems interesting to you, my friend has a great blog about the fate of Nordic settlements in Greenland.

But was the LIA really a global phenomenon?

Mann (2002) highlights the fact that outside the North Atlantic region the large-scale signature of the Little Ice Age becomes rather unclear. In general, available evidence suggests generally colder conditions anywhere from the 13th through 19th century, quite variable in timing from region to region, and in most cases punctuated with intermittent periods of warmth. There are some long instrumental climate records in North America going back to the mid 18th century, nonetheless, Mann (2002) believes they provide a mixed signal. Thus the 17th century, the coldest century in Europe, does not appear to have been unusually cold in North America. By contrast, during the 19th century, as Europe was recovering from Little Ice Age conditions, North America was experiencing some of its coldest temperatures. The paper also points out the evidence of receding glaciers, left by the position of moraines, which suggests increased glaciation in certain regions of the world outside Europe prior to the 20th century, including Alaska in North America, and New Zealand and Patagonia in the Southern Hemisphere. However, the precise timing of glacial advances in these regions differs considerably from region to region, leading Mann (2002) to conclude that they possibly represent roughly coincident, but independent regional climate changes, rather than globally synchronous increased glaciation.

The existence of the Little Ice Age (whether defined by the particularly cold conditions in Europe during the 16th–18th centuries, or the more modest large-scale cooling of the 15th–19th centuries) invites questions as to what factors may have led to such a cooling. I have already given an overview of the suggested triggers for Bond Cycles, but let's see what event-specific mechanisms have been proposed...

In terms of volcanic forcing, many scientists give a particular mention to the 1815 eruption of Tambora in Indonesia, when the following year came to be known as a Year Without a Summer. However, a recent paper by Miller et al. (2012) presents the evidence of precisely dated records of ice-cap growth from Canada and Iceland, arguing that LIA ice growth began earlier than thought between 1275 and 1300 AD, followed by an intensification 100-150 years later. The authors use the results to conclude about the link between the onset of the LIA and an unusual 50-year-long period with four large sulfur-rich explosive eruptions. The same paper states that "large changes in solar irradiance are not required".

Nevertheless, a group of scientists does not share this opinion, as there has been a continuous effort to link the LIA cooling event to a period of low solar activity, known as the Maunder Minimum (e.g. Ineson et al., 2011). Other proposed causes include orbital forcing, a slowdown of the thermohaline circulation, possibly attributed to the preceeding Medieval Warm Period (e.g. Broecker, 2000) or even decreased human populations. Thus, Ruddiman (2003) identified the Black Death and the colonisation of the Americas as one of the main causes of reduced human populations, which he links to the decreased carbon dioxide levels and lower temperatures.

And finally, a pretty amusing fact about the Little Ice Age... I am sure you have all hear about Antonio Stradivari, the famous violin maker, who lived in the 17th century and whose instruments still sound superior to their modern analogues. The popular opinion was that the musician had some sort of a "secret ingridient", giving the violins their unique sound. However, recently two American scientists suggested the climate was to blame. They say that lower temperatures during the LIA made the trees grow slower and therefore their wood was denser than today, contributing to the tone of Stradivari's instruments (Whitehouse, 2003).

List of references:

Broecker, W. (2000) 'Was a Change in Thermohaline Circulation Responsinle for the Little Ice Age?', Proceedings of the National Academy of Sciences of the United States of America, 97, 4, 1339-1342.
Brown, D. M. (2000) 'The Fate of Greenland's Vikings' (WWW), Archaeological Institute of America (http://archive.archaeology.org/online/features/greenland/), 7/01/2013.
Ineson, S., A. Scaife, J. Knight, J. Manners, N. Dunstone, L. Gray and J. Haigh (2011) 'Solar Forcing of Winter Climate Variability in the Northern Hemisphere', Nature Geoscience, 4, 753-757.
Mann, M. (2002) 'Little Ice Age' in T. Munn (ed.) Encyclopedia of Global Environmental Change, Wiley & Sons Ltd: Chichester, 504-509.
Miller, G., A. Geirsdottir, Y. Zhong, D. Larsen, B. Otto-Bliesner, M. Holland, D. Bailey, K. Refsnider, S. Lehman, J. Southon, C. Anderson, H. Bjornsson and T. Thordarson (2012) 'Abrupt Onset of the Little Ice Age Triggered by Volcanism and Sustained by Sea/Ocean Feedbacks', Geophysical Research Letters, 39, 2.
Ruddiman, W. (2003) 'The Anthropogenic Greenhouse Era Began Thousands of Years Ago', Climatic Change 61, 261–293.
Whitehouse, D. (2003) 'Stradivarius' Sound due to Sun' (WWW), London: BBC News (http://news.bbc.co.uk/2/hi/science/nature/3323259.stm), 06/01/2013.

Global warming - it's only natural

I hope you all, my dear readers, have been enjoying our history lesson. However, I shall give it a bit of a break as I try myself as a book critic. Okay, not really a book critic... I will just explain how two guys tried to claim that humans have nothing to do with current global warming and in doing so got quite a lot of facts wrong.

So I recently started reading a book by Dennis Avery, the director of the Center for Global Food Issues at the Hudson Institute, and an atmospheric physicist Fred Singer. The book is called "Unstoppable Global Warming: Every 1,500 Years". And I think now you might start seeing why I am talking about global warming, which is none of my business really (at least in the context of this blog!). Yes, Avery and Singer are trying to convince us that the Bond Cycles are responsible for the dramatic increase in temperatures over the recent decades.

Have to be honest, I am only halfway through. Still, the idea is pretty much explained in the first chapter. The supporters of the book call the IPCC "environmental extremists", the opponents point out that Fred Singer used to receive funding from the fossil fuel industry. If you feel like joining the debate google the book's reviews - some of them are actually really amusing! I will just do the boring bit and actually explain the science behind the authors' claims.

The book states that the 1,500-year cycle in the Earth's can account for most of the global warming over the last hundred years and this cycle is linked to fluctuations in solar energy. We are currently in the warming phase, climbing out of the Little Ice Age, but in a few centuries the temperatures will be falling. Since there is nothing humans can do to modify the amount of solar radiation, we should simply accept the inevitable and live with it.

In reality Bond events did not cause the same global warming pattern observed today - they have acted to redistribute Earth's warmth through the bipolar seesaw mechanism. Ice cores from Antarctica show that equal-and-opposite cooling in the Southern Hemisphere balanced out the warming in the Northern Hemisphere during Bond events. At present, however, the evidence of dramatically rising temperatures can be seen all over the world. Here is a good video to help you understand what I am talking about:

The curse of Akkad

The large fields produced no grain

The flooded fields produced no fish

The watered garden produced no honey and wine...

    -From "The Curse of Akkad", written c. 4000 BP

Around 2,300 years BC, Sargon of Akkad united Mesopotamian city-states into what is considered to be the world's first empire. Stretching from Cyprus to Oman during the days of its might, the Akkadian empire suddenly collapsed after just a century of prosperity, as the whole northern region was abandoned for more than 300 years.

In Egypt, the Old Kingdom, which began with the 3rd Dynasty of kings in 2686 BC, was a period of internal security and prosperity. That is exactly when the pharaohs built their staggering pyramids, which are now one of the 7 wonders of the world. However, after the death of King Pepy II in 2184 BC, 19 pharaons took and lost the throne in just 25 years! By the end of this chaotic period, the Old Kingdom had completely collapsed.

Around the same time the Neolithic Cultures around Central China collapsed, the Longshan culture around the Yishu River and the Harappan civilisation of the Indus Valley have both majorly declined.

Of course, if we look deeper into every case study, we will find out that the extremely long reign of Pepy II meant he outlived many of his heirs, creating succession problems and throwing Egypt into a vortex of civil wars. We will also see that shortly before the demise Mesopotamia was invaded by hostile tribes. However, isn't it strange that all those cultures around the world collapsed roughly at the same time? Maybe there is something else that can explain this phenomenon? Well, if you have a look at my post a few weeks ago, you will see that all of those dramatic happenings correspond with yet another phase of cooling in the North Atlantic, namely the Bond Event 3.

So have we found the suspect?

Weiss et al. (1993) looked at the archaeological evidence from the town of Tell Leilan on the Habur Plains in northern Mesopotamia, the political capital of region in the 2nd millennium BC. Studying soil-stratigraphic units from the site, Weiss and his colleagues discovered soil conditions diagnostic of significant climatic change as well as alterations in hydrological regime. They identified increased aridity, intensified wind turbulence and increased dust veil as the conditions leading to low agricultural productivity in the region. The desertification caused the abandonment or reduction of Tell Leilan and other settlements on the Habur plains, and the collapse of the Akkadian economy dependant on imperialised agriculture of the region. The displaced populations continued to arrive to the southern villages, which were also suffering from the reduced flow from Euphrates. Overall, Weiss et al. (1993) attribute the collapse of Akkadian Empire solely to the ameliorating climatic conditions.

Another paper by Staubwasser et al. (2003) looks at the reasons behind the Harrapan civilisation decline. Around 4.2 ka BP this culture, located on the territory of modern Pakistan, transformed from a highly organised urban phase to a post-urban phase of smaller settlements accompanied by a southeastward migration of the population. Many large cities and cultural centres were almost completely abandoned while locations in northern India grew in population. Staubwasser and his colleagues have analysed a laminated sediment core from a site off the formerly active Indus Delta and have identified a corresponding sharp reduction in annual rainfall over the Indus watershed. The concordance of Harappan habitat tracking with this event has led them to suggest a causal relationship. A possible explanation was put forward that a reduction of the average annual rainfall over the Indus River watershed restricted Harappan farming in the Indus Valley and left large city populations unsustainable.

There is no direct evidence that Bond cycles were responsible for the deteriorating environmental conditions at the time, however, if you remember, the previous Bond event also corresponded with a very intense aridification period... Just a coincidence?

List of references:

Staubwasser, M., F. Sirocko, P. M. Grootes and M. Segl (2003) 'Climate Change at the 4.2 ka BP Termination of the Indus valley Civilization and Holocene South Asian Monsoon Variability', Geophysical Research Letters, 30, 8, 1425.
Weiss, H., M.-A. Courty, W. Wetterson, F. Guichard, L. Senior, R. Meadow and A. Curnow (1993) 'The Genesis and Collapse of Third Millennium North Mesopotamian Civilisation', Science, 261, 2154, 995-1004.

How did Sahara become a desert?

Okay, we have now looked at the Younger Dryas and the 8,200 year event - those two are pretty famous and are expected to be in the vocabulary of every geography/geology/earth science student. However, asking any of them whether they have heard about the 5.9 kiloyear event, I would expect hardly any nods. And yet it is one of the most intense aridification events during the Holocene!

You might find it hard to believe, but once upon a time the Sahara desert was actually a steppe, covered in grasses and shrubs! Pretty attractive for human hunters, it is exactly where the buffaloes and goats got domesticated. Then, around 3900 BC, an abrupt decrease in Saharan vegetation took place:

Source: Hoelzmann et al. (1998)

According to Hoelzmann et al. (1998) orbital variability was responsible for the monsoon strength in Africa and other parts of the tropics. Therefore, as rainfall gradually decreased eventually conditions became too dry for the plants and a transition to dusty, desert conditions took place.

So what about the humans?

Brooks (2006) explored the links between mid-Holocene socio-cultural and environmental change in 6 key regions: central Sahara, Egypt, Mesopotamia, South Asia, northern China and coastal Peru. He argued that the 5.9 kiloyear event (coinciding with Bond Event 4) has triggered worldwide migration to river valleys and has eventually led to the emergence of the first complex, highly organised, state-level societies in the Afro-Asiatic monsoon belt and northern South America in the 4th millennium BC.

List of references:

Brooks, N. (2006) 'Cultural Responses to Aridity in the Middle Holocene and Increased Social Complexity', Quaternary International, 151, 1, 29-49.
Hoelzmann, P., D. Jolly, S. P. Harrison, F. Laarif, R. Bonnefille and H.-J. Pachur (1998) 'Mid-Holocene Land-Surface Conditions in Northern Africa and the Arabian Peninsula: A Data Set for the Analysis of Biogeophysical Feedbacks in the Climate System', Global Biogeochemical Cycles, 12, 1, 35-51.

Just to sum up...

I came across this table today in Alley et al. (1997), which compares the 8.2 kiloyear event and Yonger Dryas with present climate, showing the percentage deviations of various environmental parameters. Thought it would help you lot to better imagine the scope of those two very dramatic events.

List of references:

Alley, R., P. Mayewski, T. Sowers, M. Stuvier, K. Taylor and U. Clark (1997) 'Holocene Climatic Instability: A Prominent Widespread Event 8200 Years Ago', Geology, 25, 6, 483-486.