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How did past drought phases impact upon human perception of their environment?

In today’s blog on Climate and Society in Ireland, Gill Plunkett, David M. Brown and Graeme T. Swindles consider the impacts that droughts might have had on human populations in prehistoric Ireland.

Climate and Society in Ireland is a collection of essays, commissioned by the Royal Irish Academy, that provides a multi-period, interdisciplinary perspective on one of the most important challenges currently facing humanity. In Chapter 4, Gill Plunkett (Queen’s University Belfast), David M. Brown (Queen’s University Belfast) and Graeme T. Swindles (Carleton University, Ottawa) examine ways in which we can identify the past occurrence of droughts and evaluate whether the droughts may have triggered economic responses or population collapses.

Ireland is not a location known for its droughts. Rather, its frequent rainfall is internationally infamous, and a common source of conversation and consternation across the island. Yet the atypical hot, dry summer of 2018 (lasting from June to August), while lauded by many inhabitants, brought unexpected hardship to farmers, as grass growth declined and led to a shortage of fodder, and crops risked failing due to a lack of irrigation. There were health consequences too, as larger than usual numbers of individuals presented to hospitals with severe cases of sunburn, and in at least one instance, a child was admitted with a rare skin disease that may have been aggravated by the unusual heat. All this transpired just months after the ‘Beast from the East’ saw an anticyclonic arctic airmass bring exceptional cold and heavy snow that caused considerable societal and economic disruption.

While the extreme summer of 2018 may be symptomatic of the climate crisis that currently confronts us, droughts have featured in the Irish climate in the past. Instrumental climate records are of course a relatively recent innovation; for Ireland, the longest-running continuous sequence of weather data—from Armagh Observatory—was established a little over two centuries ago, within which time the climate has emerged from the Little Ice Age and post-Industrial warming began. For an understanding of longer-term natural climate behaviour, one must turn to historical and palaeoenvironmental proxy records. An entry in the Annals of Ulster for the year AD 749 tells of ‘Snow of unusual depth so that nearly all the cattle of the whole of Ireland perished, and the world afterwards was parched by unusual drought’, extremes that are rather reminiscent of the spring and summer of 2018. The consequences of droughts for farmers can also be gleaned from earlier chronicles: for example, the Annals of Ulster records for the year AD 773 an ‘Unaccustomed drought and heat of the sun so that nearly all bread [grain] failed. Abundance of oak-mast afterwards’. These references serve as salutary reminders of the detrimental societal impacts even one dry season can trigger. What then if such conditions persisted over many years as ‘drought phases’? Palaeoenvironmental data suggest that they did.

One of the upsides of Ireland’s pervasive wet climate is that it has resulted in ample bogs whose distinctive qualities have allowed them to capture records of past environmental change as they formed and accumulated peat through the millennia. Specifically, the inhibition of biological decay has ensured the partial preservation of plant and animal remains that once lived on the bogs, communities of which were strongly influenced by the degree of bog surface wetness. In peatlands that are independent of the water table (raised and blanket bogs), bog surface wetness is at least in part governed by climate: the drier and/or warmer the conditions, the drier the bog surface, while the colder and/or wetter the condition, the higher the water level at the bog surface. While the degree of wetness is therefore a product of both temperature and precipitation, it is thought that summer effective precipitation is the leading variable reflected in palaeohydrology records. Examining changes in biotic communities preserved within the peat, linked as they are to bog surface wetness, can therefore yield indirect (proxy) records of past climate variability. During drier phases, the peat itself will undergo a greater degree of decomposition (humification), so humification levels too are an index of past conditions. Subfossilised remains of bog oaks and pines also give insights into changing hydrological conditions through the dates of their establishment and die-off. Successful germination and establishment of such trees on bog surfaces will only occur during periods of lower water tables.

Using a multi-proxy approach on peat sequences extending back to the Early Bronze Age (2500 BC), Swindles, Blundell and Roe identified three periods of potentially extended drought dating respectively to 1150–800 BC, 320 BC–AD 150 and AD 250–470, in addition to an extended period of drought in the post-Industrial era. The climate signal in the upper levels of the bogs is, however, confounded by direct human impacts on bog hydrology, such as peatland drainage. A subsequent study entailing a wider selection of sites upheld the identification of these intervals as drier phases, although Plunkett saw the first of these events as two distinct dry events, separated by a wet shift. These findings imply that extended drought phases did indeed transpire in the past.

Here we examine three time-intervals for which there has been posited evidence for droughts phases during Irish prehistory, and for which we can avail of precisely-dated data from bog oaks and pines to evaluate the evidence for and timing of these events. We consider the impacts these changes might have had on human populations, specifically from an economic perspective, and examine whether environmentally-driven interpretations stand up to critical analysis.

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