4.3 Holocene climate change

Tipping et al. (2013) have recently summarised the evidence for climate changes that are likely to have affected Scotland and the people living in it. The current paradigm is that climate in the past changed rapidly or even abruptly, as in the switch from the cold of the Loch Lomond Stadial to the warmth of the earliest Holocene, though infrequently (Mayewski et al. 2004). Current reconstructions stress short 'moments of crisis' separated by long periods of stability or smaller climatic excursions. The rapidity of the crises gave little time for people to adapt, and prolonged stability meant that coping strategies will have been forgotten (Tipping 2005).

Mechanisms of change at slow temporal scales are the precisely rhythmic very long–term variations of the Earth round the sun, called Milankovitch cycles. At shorter temporal scales variations in solar activity are probably a key driver but feedback processes with many properties of the Earth's surface are complex. Periods of rapid climate change are re–organisations of the atmospheric circulation system, not simply changes in temperature or precipitation. The North American or Laurentide ice sheet, for instance, collapsed abruptly at c. 6200 cal BC and altered global atmospheric circulation through its effect on North Atlantic Ocean circulation, probably shutting down the "gulf stream". Stager and Mayewski (1997) and Debret et al. (2009) suggested that the present atmospheric circulation system only emerged with the collapse of this huge ice sheet. Before c. 6200 cal BC weather charts, could people have plotted them, would have looked very different to today.

The west coast of Scotland experiences alternating cold–dry–still 'continental' weather when a blocking high pressure zone settles over the North Sea and the near–continent, and warm–wet–stormy weather from the Atlantic. Today, these alternate frequently, determined in large part by semi–periodic changes in the North Atlantic Oscillation (NAO), a term which describes the oscillating system of quasi–stable low pressure zones over Iceland and high pressure zones over the Azores. The NAO varies in strength, and our weather also varies because of this, but Trouet et al. (2009) have suggested that the climate can get locked into one or other of these phases for very long periods. So the warm Middle Ages in north west Europe, for example, have been seen as one 200–year or so period when westerly winds prevailed on a strong jetstream, with the subsequent 'little ice age' characterised by cold easterlies.

Tipping et al. (2013) tried to summarise for archaeologists and historians the types of data being assembled to construct a new chronology of climate change relevant to Scotland. Charman (2010) summarised data for the British Isles and Swindles et al. (2013) have assembled data for Ireland. Many data–sets are being generated from ocean and sea–loch sediment to the west of the Argyll coast because the North Atlantic Ocean is fundamental to understanding these patterns (below). No modern–standard terrestrial climate proxy records exist for the region. Undisturbed parts of the raised moss of the Moine Mhor are suitable for generating a proxy record of hydrological change in late prehistory and history (cf. Charman et al. 2006), which could become important in linking the quite different records between Ireland and Scotland in the later Neolithic and Bronze Age (Swindles et al. 2013). The invasion and demise of Scot's pine populations on blanket peat in prehistory on Rannoch Moor (Bridge et al. 1990) are best interpreted as responses of these trees to hydrological change but detail is lacking. Scots pine trees preserved in peat bogs are, however, a critical data source for annually resolved palaeo–climatic records (Bridge et al. 1990; Moir et al. 2010).

Schematically, we might recognise the following periods of climatic deterioration as critical:

  • Early Holocene climatic events include short–lived deteriorations in climate before c. 9300 cal BC, at c. 8350 cal BC and c. 7550 cal BC (Bos et al. 2007; Whittington et al. 2015)
  • The major early Holocene climatic reversal, the 8.2 ka event (Alley et al. 1997; Alley and Ágústdóttir 2005) at c. 6200 cal BC, had widespread, hemispheric impacts, felt intensely at the latitude of Argyll (Seppa et al. 2007) and involved a temperature depression in parts of the North Atlantic of 2–3°C. North–west Europe was markedly more arid.
  • From c. 4400 cal BC until c. 3800 cal BC a series of complex changes occurred at the Mesolithic–Neolithic transition (Bonsall et al. 2002; Tipping 2010 but see Schulting 2012).
  • Recent work has emphasised an event centred at c. 3200 cal BC (Roland et al. 2015).
  • Many diverse analyses identify the period 2200–1800 cal BC as one of abrupt global climate change (Mayewski et al. 2004) though Swindles et al. (2013) argue for its weak expression in the eastern North Atlantic region.
  • The early Iron Age c. 800–500 cal BC, long seen as a period of marked climatic deterioration (Burgess 1985; Barber 1998; van Geel et al. 1998; Charman 2010; Swindles et al. 2013). Armit et al. (2015) have recently questioned the impact of this event on societal change in Ireland.
  • A 'Dark Age' climatic deterioration is recognised in some reconstructions centred on cal AD 500 (Swindles et al. 2013)
  • The 'little ice age' commenced at c. cal AD 1400 and ceased by c. cal AD 1850 and can be resolved as a series of short–lived climatic extremes: see Section 10.2 for a detailed analysis of the climate change from the Medieval Warm Anomaly to the 'Little Ice Age'.