Prof. David J. Nash, University of Brighton, UK, and University of the Witwatersrand, South Africa
To grasp the significance of global warming, and to confirm its connection to human activity, you have to know how climate has changed in the past. Scholars of past climate change know that understanding how climate has varied over historical timescales requires access to robust long-term datasets. This is not a problem for regions such as Europe and North America, which have a centuries-long tradition of recording meteorological data using weather instruments (thermometers, for example). However, for large areas of the world the ‘instrumental period’ begins, at best, in the late 19th or early 20th century. This includes Africa, where, with the exception of Algeria and South Africa, instrumental data for periods earlier than 1850 are sparse. To overcome such data scarcity, other approaches are used to reconstruct past climates, most notably through analyses of accounts of weather events and their impacts in historical documents.
Compared to the wealth of documentary evidence available for areas such as Europe and China, there are relatively few collections of written materials that allow us to explore the historical climatology of Africa. Documents in Dutch exist from the area around Cape Town that date back to the earliest European settlers in 1652, and Arabic- and Portuguese-language documents from northern and southern Africa, respectively, are likely to include climate perspectives from even further back in time. However, the bulk of written evidence for Africa stems from the late 18th century onwards, with a proliferation of materials for the 19th century following the expansion of European colonial activity.
These documents are increasingly used by historical climatologists to reconstruct sequences of rainfall variability for the African continent. This focus on rainfall isn’t surprising, given that rainfall was – and is – critical for human survival. As a result, people tended to write about its presence or absence in diaries, letters, and reports. In turn, these rainfall reconstructions are now used by historians as a backdrop when exploring climate-society relationships for specific time periods. It is therefore critical that we understand any issues with rainfall reconstructions in case they mislead or misinform.
This article will take you under the hood of the practice of reconstructing past climate change. Its aim is to: (a) provide an overview of historical climatology research in Africa at continental to regional scales; and (b) point out how distinct approaches to rainfall reconstruction in different studies can potentially produce very different rainfall chronologies, even for the same geographical area (which of course alters the kinds of environmental histories that can be written about Africa). The article concludes with some personal reflections on how we might move towards a common approach to rainfall reconstruction for the African continent.
Different approaches to rainfall reconstruction in Africa
Most historical rainfall reconstructions for Africa use evidence from one or more source type (Figure 1). A small number of studies are based exclusively upon early instrumental meteorological data. Of these, some (the continent-wide analysis by Nicholson et al. in 2018, for example) combine rain gauge data published in 19th-century newspapers and reports with more systematically collected precipitation data from the 19th to 21st centuries, to produce quantitative or semi-quantitative time series. Others, such as Hannaford et al. (2015), for southeast Africa, use data digitized from ship logbooks to generate quantitative regional rainfall chronologies.
Most reconstructions, however, draw on European traditions by using narrative accounts of weather and related phenomena contained within documentary sources (such as personal letters, diaries/journals, reports, newspapers, monographs and travelogues) to develop semi-quantitative relative rainfall chronologies. Some of the most widely available materials are those written by early explorers, missionaries, and figures of colonial authority. The use of such evidence permits the reconstruction of rainfall for periods well before the advent of meteorological data collection.
The greatest numbers of regional documentary-based reconstructions are available for southern Africa, which forms the focus of this article. These draw on documentary evidence from a combination of published and unpublished sources, often using available instrumental data for verification and calibration, and span much of the 19th century. Where information density permits, it has been possible to reconstruct rainfall variability down to seasonal scales (see, for example, a study by Nash et al. in 2016). There are, in addition, continent-wide series that integrate narrative information from mainly published sources with available rainfall data (Nicholson et al., 2012, for 90 homogenous rainfall regions across mainland Africa).
An important point to note is that the various reconstructions adopt slightly different methodologies for analyzing documentary evidence. For example, all of the regional studies in southern Africa noted above use a five-point scale to classify annual rainfall (from –2 to +2; extremely dry to extremely wet). Scholars decide how to classify a specific rainy season in a region through qualitative analysis of the collective documentary evidence for that season. In other words, they take into account all quotations describing weather and related conditions. This contrasts with the approach used by Nicholson and colleagues in a 2012 continent-wide rainfall series. In that reconstruction, scholars attributed a numerical score on a seven-point scale (–3 to 3) to each individual quotation according to how wet or dry conditions appear to have been. They then summed and averaged the scores for each item of evidence for a specific region and year. As we will see, these distinct analytical approaches, which may draw on different documentary evidence, may introduce significant discrepancies between rainfall series.
Comparisons between rainfall series
A compilation of all the available annually-resolved rainfall series for mainland southern Africa is shown in Figure 2. This includes seven series (g-m) based exclusively on documentary evidence, four regional series (c-f) from Nicholson et al. (2012) based on combined documentary evidence and rain gauge data, the 19th-century portion of the ships’ logbook reconstruction series (b) by Hannaford et al. (2015), and, for comparison, the 19th-century section of a width-based tree ring rainfall reconstruction (a) for western Zimbabwe by Therrell et al. (2006). With the exception of the Cape Winter Rains series, all are for areas of southern Africa that receive rainfall predominantly during the summer months.
Fig. 2. Annually-resolved rainfall reconstructions for southern Africa, spanning the 19th century. (a) Tree-ring width series by Therrell et al. (2006); (b) Ships’ logbook-based reconstructions by Hannaford et al. (2015); (c-f). Combined documentary and rain-gauge reconstructions by Nicholson et al. (2012); (g-m) Documentary-based reconstructions by (g) Nash et al. (2018), (h) Grab and Zumthurm (2018), (i) Kelso and Vogel (2007), (j) Nash and Endfield (2002, 2008), (k) Nash and Grab (2010), (l) Nash et al. (2016), (m) Vogel (1989).
This compilation shows that, in the 19th century, rainfall varied from place to place across southern Africa. However, we can identify a number of droughts that affected large areas of the subcontinent. Droughts, for example, stretched across southern Africa in the mid-1820s, mid-1830s, around 1850, early-mid-1860s, late-1870s, early-mid-1880s and mid-late-1890s. We can also pinpoint a smaller number of coherent wetter years: in, for example, the rainy seasons of 1863-1864 and 1890-1891. Analyses that use many different climate "proxies" - that is, sources that register but do not directly measure past climate change - indicate that the early-mid 1860s drought was the most severe of the 19th century, and that of the mid-late-1890s the most protracted (see, for example, studies by Neukom et al., 2014, and Nash, 2017).
The inset map in Figure 2 reveals that a number of rainfall series overlap in their geographical coverage, which allows a direct comparison of results. In some cases, the overlap is between series created using very different methodologies. For the most part, there is good agreement between these overlapping series, but there are some significant differences. The rest of this article will focus on two of these periods of difference: the 1810s in southeast Africa, and the 1890s in Malawi.
How dry was the first decade of the 19th century in southeast Africa?
Four rainfall series are available for southeast Africa for the first decade of the 19th century (Figure 3) – documentary series for South Central Africa and the Kalahari (by Nicholson et al., 2012), a tree-ring series for Zimbabwe (Therrell et al., 2005), and a ships’ log series for KwaZulu-Natal (Hannaford et al., 2015). Collectively, these series suggest that there was at least one major drought that potentially affected much of the region.
This was a very important time in the history of southeast Africa. The multi-year drought is remembered vividly in Zulu oral traditions as the ‘mahlatule’ famine (translated as the time we were obliged to eat grass). Scholars have seen it as a trigger for political revolution and reorganization, one that ultimately led to the dominance of the Zulu polity.
Fig. 3. Comparison of three annually-resolved rainfall reconstructions for southeast Africa for the first half of the 19th century, including the tree ring series for Zimbabwe by Therrell et al. (2006), the combined documentary and rain-gauge reconstructions for South Central Africa and the Kalahari by Nicholson et al. (2012), and the ships’ logbook reconstructions for southeast South Africa by Hannaford et al. (2015). The inset map shows the location of each series.
Yet there are some discrepancies between the overlapping records, which have important implications for our understanding of relationships between climate change and society. For example, while the documentary-based South Central Africa series in Figure 3 suggests protracted drought from 1800 to 1811, the overlapping tree ring series for Zimbabwe infers periods of average or above-average rainfall, alternating with drought. A similar contrast is shown between the documentary-based Kalahari series (which encompasses the southern Kalahari but extends to the east coast of South Africa) and the overlapping ships’ logbook-based reconstruction for Royal National Park, KwaZulu-Natal.
Since these series are based on different evidence, it is impossible to tell which is more likely to be ‘right’. However, the rainfall series based on documentary evidence are clearly less sensitive to interannual rainfall variability than those based on ships’ log data or tree rings, at least for the early 19th century. This is surprising, as a major strength of documentary evidence is normally the way that it captures extreme events.
The reasons for these discrepancies are unclear, but are likely to be methodological. The Africa-wide rainfall series by Nicholson and colleagues, from which the South Central Africa and Kalahari series in Figure 3 are derived, is a model of research transparency – it identifies the evidence base for every year of the reconstruction, with all documentary and other data made available via the NOAA National Climatic Data Center. Inspection of this dataset indicates that the reconstructions for the early 1800s in southern Africa are based on a limited number of published monographs and travelogues, written mainly by explorers. While these are likely to include eyewitness testimonies, there is potential for bias towards drier conditions. The majority of authors were western European by birth and, in some cases, their writings reflected their first travels in the subcontinent. It wouldn’t be at all surprising if they found southern Africa significantly drier than home.
How dry was the last decade of the 19th century in Malawi?
The collective evidence for rainfall variability around present-day Malawi during the mid-late 19th century is shown in Figure 4. Here, two rainfall reconstructions overlap: the first, a reconstruction for three regions of the country based primarily on unpublished documentary evidence by Nash et al. (2018); and the South Central Africa series and adjacent rainfall zones of Nicholson et al. (2012).
Fig. 4. Comparison of two annually-resolved rainfall reconstructions for southeast Africa for the second half of the 19th century, including a documentary-based reconstruction for three regions of Malawi (Nash et al., 2018), and the combined documentary and rain-gauge reconstruction for South Central Africa by Nicholson et al. (2012). The inset map shows the location of each series.
Extreme events, such as the droughts of the early-1860s, mid-late-1870s, and mid-late-1880s, and a wetter period centred around 1890-91, are visible in both reconstructions. However, there are discrepancies in other decades, most notably during the 1890s where the Nicholson series indicates mainly normal to dry conditions, and the Nash series a run of very wet years.
Delving deeper into the documentary evidence underpinning the Nicholson series suggests that the discrepancies may again be methodological, and strongly influenced by source materials. As with the other regional reconstructions for southern Africa, the Nash study bases annual classifications on average conditions across a large body of mainly unpublished primary documentary materials. Nicholson, by contrast, uses smaller numbers of mainly published documentary materials, combined with rain gauge data. An over-emphasis of references to dry conditions in these documents, combined with an absence of gauge-data for specific regions and years, could therefore skew the results.
The way forward?
There are two main take-home messages from this article. First, on the basis of a comparison of annually-resolved southern African rainfall series, documentary data appear less sensitive to precipitation variability than other types of proxy evidence, even for some extreme events. Discrepancies are most apparent for periods of the early 19th century, where documentary evidence is relatively sparse.
Second, different approaches to reconstruction may produce different results, especially where documentary evidence is combined with gauge data. The summative approach used by Nicholson and colleagues, for example, where individual quotations are classified, summed and averaged, may be much more sensitive to bias from individual sources when data are sparse.
Having identified these potential issues, one way forward might be to run some experimental studies using different approaches on the same collections of documentary evidence to assess the impact of methodological variability on rainfall reconstructions. This would be no small task, as it would mean re-analyzing some large datasets. However, it would confirm or dismiss the suggestions made here about the relative effectiveness of different methodologies.
These experimental studies would help us to identify the "best practice" for reconstructing African rainfall. They would allow us to improve the robustness of the baseline data available for understanding historical rainfall variability in the continent likely to be most severely impacted by future climate change. They would also permit us to refine our understanding of past relationships between climatic fluctuations and the history of African communities. These relationships may offer some of our best perspectives on the future of African societies in a warming planet.
Brázdil, R. et al. 2005. "Historical climatology in Europe – the state of the art." Climatic Change 70: 363-430.
Grab, S.W. and Zumthurm, T. 2018. "The land and its climate knows no transition, no middle ground, everywhere too much or too little: a documentary-based climate chronology for central Namibia, 1845–1900." International Journal of Climatology 38 (Suppl. 1): e643-e659.
Hannaford, M.J. and Nash, D.J. 2016. "Climate, history, society over the last millennium in southeast Africa." Wiley Interdisciplinary Reviews-Climate Change 7: 370-392.
Hannaford, M.J. et al. 2015. "Early-nineteenth-century southern African precipitation reconstructions from ships' logbooks." The Holocene 25: 379-390.
Kelso, C. and Vogel, C.H. 2007. "The climate of Namaqualand in the nineteenth century." Climatic Change 83: 257-380.
Nash, D.J., 2017. Changes in precipitation over southern Africa during recent centuries. Oxford Research Encyclopedia of Climate Science, doi: 10.1093/acrefore/9780190228620.013.539.
Nash, D.J. and Endfield, G.H. 2002. "A 19th century climate chronology for the Kalahari region of central southern Africa derived from missionary correspondence." International Journal of Climatology 22: 821-841.
Nash, D.J. and Endfield, G.H. 2008. "'Splendid rains have fallen': links between El Nino and rainfall variability in the Kalahari, 1840-1900." Climatic Change 86: 257-290.
Nash, D.J. and Grab, S.W. 2010. "'A sky of brass and burning winds': documentary evidence of rainfall variability in the Kingdom of Lesotho, Southern Africa, 1824-1900." Climatic Change 101: 617-653.
Nash, D.J. et al. 2018. "Rainfall variability over Malawi during the late 19th century." International Journal of Climatology 38 (Suppl. 1): e649-e642.
Nash, D.J. et al. 2016. "Seasonal rainfall variability in southeast Africa during the nineteenth century reconstructed from documentary sources." Climatic Change 134: 605-619.
Neukom, R. et al. 2014. "Multi-proxy summer and winter precipitation reconstruction for southern Africa over the last 200 years." Climate Dynamics 42: 2713-2716.
Nicholson, S.E. et al. 2012. "Spatial reconstruction of semi-quantitative precipitation fields over Africa during the nineteenth century from documentary evidence and gauge data." Quaternary Research 78: 13-23.
Nicholson, S.E. et al. 2018. "Rainfall over the African continent from the 19th through the 21st century." Global and Planetary Change 165: 114-127.
Pfister, C. 2018. "Evidence from the archives of societies: Documentary evidence - overview". In: White, S., Pfister, C., Mauelshagen, F. (eds.) The Palgrave Handbook of Climate History. Palgrave Macmillan, London, pp. 37-47.
Therrell, M.D. et al. 2006. "Tree-ring reconstructed rainfall variability in Zimbabwe." Climate Dynamics 26: 677-685.
Vogel, C.H. 1989. "A documentary-derived climatic chronology for South Africa, 1820–1900." Climatic Change 14: 291-307.