Climate, dispersal, civilisation and collapse

Dr Jonathan Dean has been a Lecturer in Physical Geography at the University of Hull since February 2017. Here he draws on work carried out with the British Geological Survey to investigate the links between climate and humans.

Why did some Homo sapiens, after evolving in eastern Africa and living there for tens of thousands of years, decide it was time to up sticks and move to Asia? Why did cities and vast empires in the Middle East collapse suddenly around 4,000 years ago, and again 3,000 years ago? People have often proposed a link between climate change and the course of human history, but to test these theories we need to know exactly how the climate changed back through time. That’s where people like me come in. My job is to work out how climate changed in the past. Because there are no meteorological records going back more than a few hundred years, we have to come up with clever ways to reconstruct past climate. I use lakes as a historical rain gauge. In some lakes, carbonate – which is like the limescale in your kettle at home – forms every year in the surface waters. In this carbonate, there are different types of oxygen, and the ratio of one type of oxygen to another varies depending on factors such as how deep or shallow the lake was at the time it formed. This carbonate then falls through the water to the lake bed and is locked away as an archive of lake level change…until scientists come along. We drill into the sediments to take cores. We then analyse the ratio of one type of oxygen to another at different points back through time from the carbonates in these sediments, and from that can reconstruct changes in lake level, and hence climate changes between wet and dry, back through time.

Lake sediments about to be analysed in the lab.

Let’s consider two examples of how climate change might have changed the course of human history. Firstly, why did our species, Homo sapiens, leave Africa after evolving there? Scientists have found evidence of Homo sapiens in the Middle East as far back as 130,000 years ago. However, other researchers have analysed the DNA of modern humans and concluded that modern non-Africans are likely to be descended from people who left Africa via Egypt around 60,000 years ago, suggesting the people who left 130,000 years ago died out before they could successfully populate the rest of the world. But why did Homo sapiens leave Africa? Maybe climate change played a role. We have used sediments taken from an old lake on the border between Ethiopia and Kenya. We showed in a paper published last year that there was a climate shift in eastern Africa at the time the successful dispersals out of Africa occurred around 60,000 years ago – the climate was changing from being very variable with multiple fluctuations between wetter and drier conditions, to a more stable climate where there was less change. During the more variable times, it was difficult for Homo sapiens, and only those who adapted to each climate change survived. This led to natural selection for the most flexible, highly skilled individuals and populations. When the climate then became more stable, it was easier for Homo sapiens to survive so populations increased. This led to pressure, as more and more humans tried to survive on the food and water resources of eastern Africa. This may have therefore pushed some people out of the region in order to try to find new lands to live on, and because of the natural selection during the times of variable climate they had the skills required to migrate out of Africa.

A dry lake bed in Ethiopia that we drilled to retrieve sediment cores (Photo Julian Ruddock).

Our second example brings us much closer to the present day. Between the time of the migration out of Africa and 5,000 years ago, humans had started playing musical instruments, developed farming and invented the wheel. But at approximately 4,000 years ago and again at 3,000 years ago it seems big civilisations ‘collapsed’ – the archaeological evidence suggests they either went into decline or ceased to exist all together. Again, climate change has been used to help account for these sudden events. A drought lasting several hundred years has been identified ~4,000 years ago in climate records from lakes in the Middle East – for example in our record from a Turkish lake that was published in a paper in 2015. In Egypt, the Nile floods failed, leading to famine and political upheaval, and they even stopped building pyramids for a few hundred years. Around 3,000 years ago we identify another drought, at the time the Hittites, who lived in central Turkey, went into decline. Nowadays in central Turkey, there is only roughly 300 mm of precipitation a year and even with modern technology agriculture is difficult. But at the times of these ‘collapses’ we have shown it would have been even drier. These droughts may have weakened civilisations and combined with civil conflict, invasions and population pressures to cause the ‘collapses’. We will never know for sure what killed off these civilisations, but what we can say is that it would have become much more difficult to grow crops, and hence for people to feed themselves, during these droughts.

Therefore, climate seems to have been a major force in shaping the course of human history – from explaining the migration of early Homo sapiens out of Africa, to contributing to the collapse of civilisations. There are important lessons for the future here. The Middle East is likely to bear the brunt of climate change this century, with drier conditions due to falling precipitation and increased summer evaporation. Eastern African is predicted to see some large climate changes too. Already politically volatile regions, fighting over water resources is likely to intensify conflicts this century. In the Middle East, it may become as dry as it was at the times of the droughts 4,000 and 3,000 years ago, and the question is whether modern technology and politics will prevent the ‘collapses’ of civilisations that we saw in the past.

(A version of this blog was first published on the BGS Geoblogy.)

Past climate in the north Atlantic: a new paper on old mud

by Jane Bunting (@DrMJBunting)

Almost twenty-five years ago, in the late Autumn of 1990, I travelled to Orkney with my supervisor Keith Bennett to collect the first sediment cores for my PhD research project. Results from analysis of one of the core sequences have just been published in the journal Quaternary Science Reviews (Whittington et al. 2015 - can be accessed here if you have a subscription, or email/tweet me).  This isn’t quite as bad as it sounds – I published my PhD findings from the site in 1994, focusing on what the pollen preserved in the sediments could tell us about the vegetation history of the islands since the last Ice Age (see here for Michelle's post on pollen analysis) – but rather reflects how long it can take to acquire data and write up collaborative research which is not supported by a single, comprehensive research grant, just by interested academics. 

Location of Orkney in the North Atlantic (bad screengrab from google maps)

Orkney is an archipelago situated to the north of mainland Scotland, and has a hyperoceanic climate (see map left: the google marker shows the location of Crudale Meadow, my study site, on the west of Mainland, the largest island). This means that the environment is very strongly affected by north Atlantic weather systems, and therefore that sediments accumulating in basins on the islands potentially contain a sensitive record of past climate oscillations. This latest paper focuses on the record of past climate, and particularly on abrupt changes in climate, recorded by sediments spanning the period from about 14000 years ago to about 8000 years ago. This covers the end of the last ice age and the beginning of the current warm period. 

Crudale Meadow is today a valley mire, a low point in the landscape supporting a waterlogged plant community of reeds, sedges and other fen species. However, coring down into the sediments showed that this wasn’t always the case. Under about two metres of tough, fibrous peat full of the preserved stems of the reeds and sedges, we found a pale cream-coloured mud, containing obvious fragments of shells. This was lime mud, which forms in shallow lakes and ponds when the water flowing in is alkaline – it’s closely related to the flaky stuff that clogs up kettles in a hard-water area. In Orkney, most former and present lakes deposit acidic muds with minimal carbonate content, but in some places the till left behind by the retreating glaciers contains lumps of chalk, which dissolve in rainwater filtering through the soil, raise its pH, and lead to the formation of lime mud through chemical reactions within the lakes and ponds. 

As a beginner pollen analyst, I was not very fond of those lime muds –they don’t preserve pollen as well as acidic lake muds – but for researchers looking to reconstruct the palaeoclimate of the North Atlantic they were actually very useful. This is because of a process called isotopic fractionation (click on this link for more information about isotopes and climate reconstruction). Oxygen naturally occurs in two forms, one lighter (oxygen-16) and one heavier (oxygen-18), and the ratio between these forms in the water that arrives through rainfall varies according to factors such as the source of the rain and the regional temperature. The carbonate part of the chemical structure of lime muds includes oxygen molecules from the lake water, and therefore locks in a record of the ratio of isotopes in the water. ‘Reading’ this record allows us to reconstruct past temperature, so when towards the end of my PhD I was asked to share my core with a larger team who had the capacity to carry out the analyses, I was interested to see what they’d find. We didn’t realise it would be quite such a long wait…

 The figure below is redrawn from the article, and summarises the reconstructed temperature trends. Looking from left to right, we’re moving back in time. The dashed black line shows a smoothed long term trend and the jagged blue line shows the underlying data. The blue boxes identify periods which are statistically unusually cold, and seem to reflect short-lived climate changes. 

Redrawn from Whittington et al. 2015. Oxygen isotope ratios from Crudale Meadow (see post for details)

Most of these can be found elsewhere in the North Atlantic, in sediment records and even in the Greenland ice cap record, but one is apparently not yet described from other records, and the record is one of the best demonstrations of abrupt events early in the current warm period yet found in Britain. By looking at the remains of molluscs preserved in the alkaline sediments, at the pollen record, and at other sedimentological evidence, we are also able to explore the extent to which these shorter climate episodes affected life in and around the basin.  Climate is the longer-term average of the weather, and in Orkney, you can often literally see changes in the weather blowing in across the sea. Locals say ‘if you don’t like the weather, wait half an hour’ or ‘there is one season a year, but four seasons a day’. Climate records like the one presented in this new paper show long term trends – the proportions of rain and sun, the mean of temperatures over multiple years, the natural variability of the complex and dynamic dance of atmosphere and ocean that creates climate and weather. Twenty five years is roughly equivalent to one sample in the results presented here, even if it seems a long time from a human perspective, or a publishing one!

 

View from Yesnaby, looking west (photo M. Farrell 2007)

After a long, wet, muddy day in 1990 collecting sediment cores from Crudale, Keith and I drove a mile or so to the nearby cliffs at Yesnaby and ate a very late packed lunch watching the dusk settle over the rolling Atlantic, nothing but ocean between us and north America. At the time I had no idea that there was a record of 6000 years of the climate over that ocean in the core-box, just a hope that there was enough data for a chapter in a PhD. As I write this, a PhD student at Royal Holloway (Rhys Timms) is analysing a new core from another of my PhD sites, Quoyloo Meadow, using the remains of non-biting midges along with other methods to further investigate the climate changes at the end of the last ice age. I wonder what new techniques will have come along by 2040, another 25 years in the future? 



Bunting, M.J 1994.  Vegetation history of Orkney, Scotland: pollen records from two small basins in west Mainland.  New Phytologist 128 771-792
 
Whittington, G., Edwards, K.J., Zanchetta, G., Keen, D.H., Bunting, M.J., Fallick, A.E. and Bryant, C.L. (2015). Lateglacial and early Holocene climates of the Atlantic margins of Continental Europe: stable isotope, mollusc and pollen studies from Orkney, Scotland. Quaternary Science Reviews 122, 112-130