Chapter 3. Precipitation variability over the last millennium
3.3 Summarising proxy-documentary precipitation variability
From the above overview, it is possible to characterise the predominant multi-decadal signals of the widely separated records in most periods, allowing some conclusions to be drawn. A summary of general precipitation fluctuations from the records considered is shown in Table 3.6, along with a classification of the agreement across the SRZ. Strong agreement indicates that nearly all records agree, good infers that a majority show correspondence, variable suggests that some records agree but a
majority do not, while weak refers to a lack of agreement between proxies. On present available evidence, this summary is considered likely to apply to most of the SRZ.
However, these multi-decadal scale fluctuations mask considerable inter-annual variability, while intra-regional differences are also present. The overall agreement of this table with the most recent similar undertaking by Stager et al. (2013) (Table 2.1;
Figure 2.15) is very good, despite the incorporation of several other widely dispersed datasets in this thesis.
TABLE 3.6 Multi-decadal SRZ proxy precipitation fluctuations over the last 1200 years.
Year AD Precipitation Agreement
800-830 Very wet Strong
830-850 Relatively dry Strong
850-970 Relatively wet, trending towards drier Variable
970-1020 Relatively dry Good
1020-1030 Variable, trending towards drier Weak
1030-1070 Very dry Good
1070-1110 Relatively dry, trending towards wetter Good
1100-1140 Very wet Variable
1140-1180 Relatively wet Good
1180-1220 Relatively dry Good
1220-1270 Relatively wet Strong
1270-1330 Relatively dry Variable
1330-1420 Variable, little agreement Weak
1420-1450 Relatively dry Variable
1450-1480 Relatively wet Strong
1480-1500 Very wet Good
1500-1520 Relatively wet, trending towards drier Strong
1520-1570 Variable, mostly drier Good
1570-1600 Variable, trending towards drier Variable
1600-1640 Relatively dry Strong
1640-1670 Variable, trending towards wetter Strong
1670-1720 Very dry Strong
1720-1770 Relatively dry Strong
1770-1800 Variable, trending towards wetter Good
1800-1820 Relatively wet Good
1820-1880 Relatively dry Good
1880-1900 Variable, trending towards wetter Good 1900-2000 Variable, trending towards drier Good
Analysis of this wide range of proxy sources has enhanced insight and understanding of precipitation variability over the last 1200-years, allowing greater confidence and reducing ambiguity for historical investigation. The general pattern of this variability is fairly clear, though it is also problematic to draw conclusions beyond a general level, particularly regarding spatial differences in SRZ palaeoprecipitation, as well as the precise (inter-annual) timing of variability. This is mainly a result of
limitations in age models, complex interactions between local- and regional-scale climate patterns and processes, and uncertainties relating to the multiple processes that affect proxy data (Holmgren et al. 2012). With these constraints in mind, though, it is now possible to use this updated review of regional palaeoprecipitation in conjunction with analysis of societal development over the same timeframe in Chapters 5 and 6.
The major advancement and contribution of this chapter to the existing literature is the incorporation of all available high-resolution palaeoprecipitation datasets into a comparative analysis of precipitation variability over the last 1200 years, thus reducing reliance on individual proxies. A range of qualitative and quantitative analysis techniques, several of which have not been previously performed in this region over this timespan, have enabled this variability to be assessed in a comprehensive manner. Importantly, the general correspondence of this variability across the SRZ found in earlier work remains prominent, even with the inclusion of more widely separated datasets. Relatively clear evidence for a MCA between c. AD
800-1200 was found by assessing multiple proxies using certain thresholds as indicators of wet and dry periods. The same analysis also provided clear evidence for the driest part of the LIA occurring between c. AD 1600-1800, yet ambiguities remain over its onset. Questions over the nature of early-nineteenth century variability were raised, particularly regarding the accuracy of the Nicholson et al. (2012) dataset, and will be examined further in Chapter 4. Independent comparison from written sources showed some coherence with proxy data, but should be treated with caution due to caveats concerning the sparse and temporally variable nature of the written sources.
Therefore, while problems remain in establishing precise detail over the timing and extent of climate variability, the use of a range of data and inclusion of independent written sources has reduced uncertainties in prescribing the timing and magnitude of cool-dry and warm-wet periods.
3.3.1 Palaeoclimate and society: towards scales of relevance
Societal investigations regarding the significance of climate variability can now be informed with greater confidence by the conclusions reached in this chapter. Key points will hereafter be summarised for the two main areas and timescales subject to investigation, with an added summary of how this research impacts upon previous climate-related hypotheses for state formation in the Shashe-Limpopo basin. These particularly concern the timing of precipitation variability and its coincidence with societal development, and how this impacts upon the investigations on the interaction between climate variability and societal change in the later chapters.
3.3.1.1 Shashe-Limpopo basin c. AD 900-1350
The Limpopo Baobab record suggests that the overall trend in the Shashe-Limpopo basin was towards drier conditions, and is reinforced in the analysis of faunal remains
by Smith (2005). Caution must be applied when relating other palaeoclimate data to the Shashe-Limpopo basin, as local environmental factors, such as the rain shadow effect from the Soutspansberg range, play a strong role (Smith 2005). Nonetheless, that conditions were warmer and wetter when trajectories towards state formation began in the Shashe-Limpopo basin is a point of little contention. From the analysis in this chapter, it is clear that the climate was generally wetter through to c. AD 1200, with generally wetter conditions observed in some records up to the late thirteenth century.
Despite the dominance of wet conditions, it is also apparent that a number of dry spells occurred in this area in the MCA. The most severe of these, picked up by the Limpopo Baobab tree-rings, lasted between c. AD 1030-1070, though the other local record of precipitation variability is not of a sufficient resolution to record this dry spell (Smith 2005). Smith also notes a dry spell sometime between c. AD 1200 and 1260, but this is not reflected in the Baobab, where conditions remained relatively wet.
A similar divergence occurs regarding the suggested drier conditions in the basin at c. AD 1300 (Huffman 2008). The Baobab record does show a decline at this time, but the isotope record of Smith (2005) suggests wetter conditions remained until at least c. AD 1415. Other records from further afield, including the Lake Sibaya conductivity record, the T7 and T8 δ13C series, and the T7 grey scale records also indicate a dry spell at this point, and therefore Smith’s relatively low resolution evidence alone is insufficient to rule out the link between the decline of Mapungubwe and a trend towards drier conditions in the basin. Whether this trend represents the onset of the regional LIA, though, is not of prime importance for societal investigation.
The uncertainties associated with linking climate variability and societal change in this period are particularly high for reasons of data availability, and thus will not form a significant component of the later analysis.
3.3.1.2 Zambezi-Limpopo region c. AD 1400-1830
Multi-centennial records from present-day Zimbabwe are unavailable prior to the late-eighteenth century, and thus inference from a wider range of proxies is required. The moisture inferences from the range of proxies suggest that it is too simplistic to assert that Great Zimbabwe was abandoned in a warm-wet period, and therefore that climate could not have played any role in state dynamics in the period running up to its decline. For instance, the early-fifteenth century was relatively dry in numerous records (Figure 3.5), and although conditions trended towards wetter from around AD
1450, this argument only allows for an abrupt collapse explanation, and rules out the possibility of persistent drier conditions eroding the resilience of the state.
Although the early-sixteenth century was generally wetter, there are a number of possible written references to climatic stress from the Mozambique coast, and these will be further investigated in Chapter 5. There are also a high number of written references to climatic stress in the Zambezi valley and Mutapa area in the mid- to late-sixteenth century, where agreement between proxies mostly suggests a trend towards
drier conditions. Importantly, this period was characterised by high variability in precipitation (Figure 3.9), which may have been equally as significant for society as consistently drier conditions. The seventeenth century marked the beginning of the LIA and was generally dry, yet there are fewer references to climatic stress in the Zambezi-Limpopo area. In the eighteenth century, however, written references increase markedly in the Zambezi valley, Mutapa state and Mozambique coast.
Although spatial differences in palaeoprecipitation were present in the LIA, it appears that the LIA imposed a general signal on patterns of local variability across the SRZ, thus providing a relatively consistent picture of climate variability. This either suggests that the severity of eighteenth century dry conditions translated into greater societal impacts, or that the differential vulnerability of societies over time was more important. These questions constitute key lines of investigation for Chapters 5 and 7.
3.3.1.3 KwaZulu-Natal area c. AD 1750-1830
The late-eighteenth century has been linked with the end of the regional LIA. Figure 3.8 does not suggest that this is the case, with only marginally improved conditions evident in the latter half of this century. The KwaZulu-Natal proxy records from Lake Sibaya and Karkloof, however, indicate that an earlier recovery of precipitation began from at least c. AD 1770, which does not rule out hypotheses suggesting that this facilitated higher crop yields. Both of these records also infer an extended dry spell in the 1790s, which may relate to the mahlatule famine. That the early-nineteenth century was a period of highly variability precipitation is relatively clear, and as previously suggested, this may well be as important as the severity of dry or wet conditions. Yet further investigation is needed into the early part of the nineteenth century, where there exists considerable disagreement between proxy sources and the precipitation dataset of Nicholson et al. (2012). Indeed, it is critical to establish the climatic conditions in the early-nineteenth century if the significance of climate variability is to be understood alongside conflict and political centralisation. This will be investigated using ships’ logbooks in the next chapter.