Past climate and people

In these climatically uncertain times, its worth looking at how our ancestors responded to change.  If we want to investigate this theme, arguably the best interval of time worth probing is the last 11,500 years: a period rich in archaeological remains and synonymous with the development of agriculture and the emergence of civilisation.  Although it makes a lot of sense that the conditions had to be stable enough for these advances to be made, we now know that significant climatic changes took place throughout this period.  If we can get a handle on what effect these had, we should get an idea of how to deal with the changes predicted for the future.

One particularly impressive study has just been reported on east Saharan archaeological sites (DOI: 10.1126/science.1130989).  Looking at nearly 500 radiocarbon ages from 150 sites and comparing the results to climate reconstructions for the region, researchers have been able to show that there was very little human occupation of the Sahara at the end of the last ice age because of the extremely arid conditions.  Things changed abruptly 8500 years ago when increased monsoonal rainfall caused a greening of much of the desert; prehistoric peoples rushed in to exploit the resources that suddenly became available.  This rich landscape seems to have persevered for a few thousand years, but it all turned sour when the rains started to fail. 5300 years ago, the monsoon became less effective at penetrating the Sahara and the desert began to expand.  The result was that many people slowly migrated south into northern Sudan, following the retreating monsoon.  Migration was the key; the mantra was keep with the rains and you should be right.  The Nile valley was one of the few refugees left beyond the retreating monsoon; the sudden settlement and development of the pharaohs civilization along the Nile looks like it happened when people could no longer survive the full desert conditions around 3500 years ago.

Quarrying in the Egyptian desert
If we go further north, we can look at an impressive reconstruction of past climate based on fossil Irish bog oaks that overlap in time and stretch back year by year for 7468 years. Over some 30 years, researchers at Queens University Belfast have done backbreaking work, extracting oaks buried in bogs across Northern Ireland. Over this time, the research group found there were curious periods when hardly any of the trees seemed to grow at all. Other times seemed to show a tree-like Utopia where even boggy environments could be colonized: the population soared. When I was in Belfast, we looked at this more closely and realized that there was a climate signal in this variability (Journal of Quaternary Science; DOI: 10.1002/jqs.927). During times of lots of trees, the climate was dry enough that they could move onto the bogs and flourish. When it got too wet and the water tables on the bogs rose, the trees died and no saplings could get established: the numbers crashed. Because of Irelands position on the western seaboard of Europe, its climate is highly sensitive to what’s happening in the North Atlantic. If the ocean sneezes, Ireland catches a cold. When the Atlantic was spluttering in the past, the trees seemed to show the land was verging on pneumonia. But if there were times in the past when the trees weren’t happy, how did the Irish people feel?

The Irish archaeological record has over 450 radiocarbon ages for forts, crannogs (homes built on artificial islands on lakes and in marshes) and settlements. We converted these ages to calendar dates so we could directly compare when the structures were being built to the climate signal preserved by the trees (Journal of Archaeological Science, DOI: 10.1016/j.jas.2005.05.014). What we found stunned us.  Refuges were constructed when the climate took a dive. Almost without fail, when times got bad, people would congregate together in permanent locations, defending what little food and other resources they had.

Scenarios for climate change suggest a potentially worse future than that with which our ancestors had to contend. Will we be able to manage mass migrations of people? Can we respond more sensibly than waging war on our neighbours to steal what little they have? I hope so.

The past is the key to the future and we need all the time we can get to see it.

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