Developer blog for the Holocene section, which aims to explore likely changes in the local sound environment since the end of the last glacial period 12,000 years ago.

2 JANUARY 2013

Reconstructing the Holocene climate

Climate research is resolving the prehistoric Holocene trends of rainfall and temperature to millennia and individual centuries.

WITHOUT THE PRESENCE of human beings, the sound environment of the Thames floodplain and its modest, surrounding hills would have changed slowly and with little drama throughout the Holocene epoch.

In this alternative timeline, birch and pine trees begin to colonise the steppe-tundra after the cold period of the Younger Dryas ends 11,500 years ago. Britain is still a peninsula of Europe, and the Thames only a tributary of a greater European river system feeding into the Rhine. Vast herds of bison and wild horses thunder across the grasslands, free to roam the Eurasian steppe.

As the climate warms, sea levels begin to rise. Around 8,000 years ago the Norwegian coastal shelf collapses and a tsunami rolls across the marshlands and low-lying plains of what’s now the North Sea, and Britain becomes an island.

The pine forests give way to mixed broadleaf trees. The Thames turns into a tidal river, with mudflats in its lower reaches luring flocks of wading birds during winter. The estuary inundates and retreats from a plain of peat bogs, which in turn are colonised in places by yew trees, making an environment with no exact parallel in the present. Beneath the dense yew canopy all is strangely quiet.

Britain’s terrestrial animals and non-migrating birds are now marooned. In time, the throttling of gene flow between them and their European cousins will cause them to become separate species, and the calls of some begin to sound different. Or else sea levels fall once again and glaciers return to scour the land like a palimpsest before a new cycle of warmth and colonisation. All this passes unremarked.

The cumulative effects of human activity which make Stratford or Richmond sound the way they do depend on the climate having been fairly stable for the past few thousand years. The ultimate causes of this likely lie with changes in the Earth’s orbit and its angle of rotation. Around 11,000 years ago, the amount of sunlight falling on what would eventually become a London rooftop or stretch of pavement was around 9% higher in summer and 14% lower in winter than today.

There’s no simple correspondence between orbital changes and surface temperatures [1], but the general trend during the early Holocene epoch was for winter and summer temperatures to diverge less, with much of the gap’s closing due to the winter months growing warmer [2].

The most widely publicised climate graphs tend to show global or hemispheric temperature trends. Fortunately, there’s been a great deal of lesser-known research focusing on the British and European climates of the past. One particularly accessible book is Hubert Lamb’s Climate, History and the Modern World. Although published in 1982 it can still be read profitably today.

Lamb was the climatologist responsible for founding the Climatic Research Unit at the University of East Anglia and recent weather station data released by them can give some clues as to whether there might be current European and Fennoscandian analogues for earlier British conditions. In Climate, History and the Modern World, Lamb presented estimates of Central England summer and winter temperatures derived from fossil pollen remains:

Central England temperatures derived from pollen remains


The smooth curves depicted by Lamb result from them being drawn along data points between 500 and 1,000 years apart. Short-lived perturbations to the climate may not show up. For comparison, an annual mean temperature reconstruction from later work has been shown also [1], based on the analysis of beetle remains. It suggests that the depths of the Younger Dryas reglaciation, some 12,000 years ago, were rather colder than Lamb’s estimates.

Another accessible climate history book, published in 2003, is Philip Eden’s Weather and climate in history. Some of the dating seems as if calendar years and radiocarbon years are being conflated, but nonetheless Eden provides a very useful summary of the Holocene climate. He also provides temperature estimates for southern England.

According to Eden, the middle of the Younger Dryas saw January temperatures of between -16 and -20°C and Julys of 8 to 12°C. They then climbed rapidly, in a period of perhaps little more than 50 years, to Januarys of 0 to 4°C and Julys of 14 to 18°C. The warming trend slowed but didn’t reach a peak in central and northern Europe until between 6,000 and 3,500 years ago, a period termed the Climatic Optimum. Winters were between 2 and 2.5°C warmer, and summers 1.5 to 2°C warmer.

The tree-line in the Scottish highlands was as much as 180 metres above its present level of 600 metres. Around 4,000 years ago on Dartmoor, Bronze Age farmers were able to cultivate their crops around 150–200 metres higher than would be possible now. Eden states that it was generally drier and less stormy during the Climatic Optimum. The semi-permanent area of high pressure which today loiters over the Azores then often extended across western Europe and Britain. In winter, the Atlantic storm track went much further north too. Mean annual rainfall was initially around 15–20% lower, although this began to increase later on during the Optimum.

Conditions began to deteriorate around 3,500 years ago with the climate becoming wetter and average temperatures falling by around 1.5°C. Bronze Age settlements on high ground were abandoned. Eden thinks it likely that gales become more common as the Atlantic storm track shifted south towards its present-day position.

By 300 BC, well into the Iron Age, the stormy weather was its worst, with rainfall some 40–50% higher than today. The Somerset Levels became marshland and it seems reasonable to guess that the environment of the Thames floodplain was affected in the same way. The inhabitants of the time responded by building wooden causeways through the marshes and by using roads on high ground. Eden states that summer temperatures were ‘at least as low’ as they were during the Little Ice Age of 16–17th centuries, when the Thames in London coould freeze to a depth of nearly a foot.

Conditions began to improve slightly after 250 BC, growing warmer and producing a period of relatively stable climate lasting until 400 AD. Sea levels rose by between one and two metres, inundating the Fens and Norfolk Broads. Eventually written records begin to tell the story of Britain’s climate, with some accounts of the earliest times being entertainingly dubious. Dr Thomas Short’s A General Chronological History of the Air, Weather, Seasons, Meteors, etc., published in 1749, tells us that in AD 4 there was a ‘rain of blood’ in London lasting five hours, heavy casualties from a Thames flood in AD 7 or 9, and that Westminster was destroyed by a hurricane in AD 18 – all some years before the arrival of literate chroniclers during the Roman conquest of AD 43.

Access to greater computer power and the development of new sources of data for past rainfall and temperature mean that researchers can now speculate about the prehistoric Holocene climate to a resolution of individual centuries. Methods include gathering information from river deposits [3], including those from several sites near the Thames, and integrating wide arrays of different proxy-climate records such as mineral deposits in caves, insect remains, and levels of preserved oxygen isotopes [4].

REFERENCES

1. Briffa, K.R. and Atkinson, T.C., 1997. Reconstructing Late-Glacial and Holocene Climates. Climates of the British Isles: Past, Present and Future, 84–111.

2. Davis, B.A.S., Brewer, S., Stevenson, A.C. and Guiot, J., 2003. The temperature of Europe during the Holocene reconstructed from pollen data. Quaternary Science Reviews, 22, 1701–1716.

3. Macklin, M.G. and Lewin, J., 2003. River sediments, great floods and centennial-scale Holocene climate change. Journal of Quaternary Science, 18, 101–105.

4. Charman, D.J., 2010. Centennial climate variability in the British Isles during the mid-late Holocene. Quaternary Science Reviews, 29, 1539-1554.

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