Attributing extreme or unusual weather events to a specific cause is not straightforward because extreme events can be caused by natural variability in climate systems, anthropogenic activity or, in most cases, a complex interplay between the two.
A first step towards understanding how the world’s future climate might change is to look to the past to evaluate the extent to which human actions have influenced climate systems. The science of attributing significant weather events primarily to climate change or to natural fluctuations is a growing field of academic study (Otto et al., 2018a), though it presents a mixed picture and remains subject to substantial uncertainties and data limitations. For example, although climate change is recognised as having significantly increased the risk of prolonged drought in the Western Cape, South Africa (Otto et al., 2018b), analysis of the part played by climate change in the two failed rainy seasons that contributed to the 2010–2011 drought in Somalia was inconclusive (Otto et al., 2018a), and scientific evidence has so far not attributed increased rainfall in the Congo Basin to anthropogenic activity (Otto et al., 2013). Although there is also no clear evidence that the low rainfall during the 2016 drought in Kenya was caused by human activity, the high temperatures experienced at the time, and which are thought to have been related to climate change, may well have contributed to the severity of the drought (Uhe et al., 2018).
4.1 Future temperature projections
Climate projections are just that, projections. Challenges in making generalisations in relation to Africa’s future climate include the continent’s varied weather patterns (which, as noted in section 1.1, spans humid, arid, desert and subtropical Mediterranean climates), the difficulty in accurately projecting anthropogenic greenhouse gas emissions, and the extent to which different climate systems may react to climate forcing (Hulme et al., 2001). Climate modellers use observational data and the latest computers but there will always be some degree of uncertainty in making projections (Aloysius et al., 2016). For an explanation of climate modelling, see Box 3.
The African continent was warmer at the beginning of the twenty-first century than it was at the beginning of the twentieth century. The temperature trend is projected to increase across the continent in the twenty-first century (Fig. 3). Temperature is projected to rise faster than the global average across most of the continent to 2100 in comparison to a late twentieth-century baseline (data were averaged from 1985–1999) (James & Washington, 2013). Early estimates suggested that the African continent warmed on average by 0.5 ℃ in the twentieth century (Hulme et al., 2001), though more recent estimates suggest a much more substantive and now accelerating increase (for example, NOAA, 2020). The accuracy of such estimates for the African continent as a whole are inevitably limited by poor availability of observational data for many regions. From a global perspective, from 1880 until 1970, the global average rate of increase was 0.07 ℃ per decade. Then, during the final decades of the twentieth century, warming increased to a global average rate of 0.18-0.19 ℃ per decade from 1971 to the present day (Blunden & Arndt, 2020). In comparison, the most recent estimates from NOAA (2020) suggest an average for the African continent of 0.12 ℃ per decade til 1981, rising to 0.31 ℃ per decade in more recent years. Projections from Blunden & Arndt (2020) suggest that the range of warming in Africa through the twenty-first century will likely fall within the boundaries of less than 0.2 ℃ per decade to more than 0.5 ℃ per decade, which is consistent with the estimates above.
In terms of temperature records, the African continent has followed the global trend, with the continent’s ten hottest years having all been recorded since 2005 (Blunden & Arndt, 2020) (see section 2.0). The extent of average annual warming will depend upon future greenhouse gas emissions; the average annual temperature for Africa could be 2 ℃ to 6 ℃ warmer in 2100 in relation to the 1961–1990 temperature average (Hulme et al., 2001). Although these estimates were made some years ago, more recent projections are broadly in agreement (Niang et al., 2014). The most recent IPCC report, AR5, discusses CMIP5 modelling projections which suggest that changes in mean annual temperature may exceed 2 ℃ above the late twentieth-century baseline over most land areas of Africa in the mid-twenty-first century under the RCP8.5 scenario, and exceed 4 ℃ over most land areas under the same scenario by the late twenty-first century. In addition, despite a relatively slower average increase compared to global average in past decades, temperatures across most of the African continent are projected to rise faster than the global average over the twenty-first century (James & Washington, 2013; Niang et al., 2014).
A number of papers published within the past five years discuss the importance of relative humidity in combination with temperature. Although increasing temperatures and periods of extreme heat are in themselves detrimental to health (see section 5.1), the combination of high temperatures with high relative humidity presents additional health challenges (see section 5.1.1). Metrics that incorporate both humidity and air temperature can be translated into a ‘feels like’ temperature that can be used to communicate periods of extreme heat. Researchers suggest that communicating warming extremes such as 1.5 ℃ or 2 ℃, as discussed in the Paris agreement targets, does not relay a sufficient sense of urgency among non-scientists/experts for action to be taken to stop greenhouse gas emissions (Matthews et al., 2017).
Heat stress occurs when a person’s core temperature increases as a result of external factors and homeostasis cannot be maintained. A study of global megacities found that, of those in Africa, Khartoum in Sudan is already experiencing periodic heat stress according to data analysis from the reference period 1979–2005. Under a global warming scenario of 1.5 ℃, the city of Lagos, Nigeria, would experience heat stress for the first time, as would Abidjan on the Ivory Coast. Under a warming scenario of 2.7 ℃ Luanda in Angola would be added to the list of African cities experiencing heat stress; and under 4 ℃ global warming Kinshasa in the Democratic Republic of Congo would also become heat stressed (Matthews et al., 2017).
4.2 Future rainfall projections
Projections for future rainfall patterns over the African continent are more uncertain than the projections for future temperature changes (see Box 2). That said, the general consensus is that under RCP8.5, Southern and Northern African regions are predicted to experience decreases in mean annual rainfall by the mid and late twenty-first century. Others project that South Africa is likely to become drier in the west and southwest, and wetter in the east (Scholes et al., 2015). In contrast to the Northern and parts of Southern Africa, the Central and East African regions are likely to experience increases in mean annual rainfall under RCP8.5 from around 2050 (Fig. 3). Predictions of future rainfall patterns for West Africa are more uncertain because models have different outcomes. However, some regional-scale models predict an increase in the number of extreme rainfall days over West Africa and areas of the Sahel during the twenty-first century (Niang et al., 2014).
One study that analysed the likelihood of extreme rainfall events in Africa under global heating scenarios and using regional climate model data projected an increase in the frequency and intensity of rainfall over the Sahel during the summer rainy season from the mid-twenty-first century; an increase in the frequency, duration and intensity of rainfall in East Africa from the mid-century; and reduced rainfall and increased duration of dry periods in southeastern Africa. The study has limitations because it used only one regional climate model (Han et al., 2019). Other studies project an increase in rainfall over the central Sahel and a decrease over the western Sahel. However, precipitation changes over the Sahel are uncertain for reasons that include long-term lack of observational data that can be used for modelling to project future scenarios, and uncertainties in projecting future changes in atmospheric circulation patterns (Monerie et al., 2020).
Accurate rainfall predictions are necessary to help manage extreme weather events. Variations in the predictions made by computer modelling seem to arise if the various weather systems are not accurately represented in the models; therefore further work to understand how weather systems have behaved over past decades and centuries is needed. Part of the work will involve more extensive data gathering; the region as a whole is data-sparse. Research is ongoing to understand extreme weather patterns, particularly of rainfall, in East Africa (and elsewhere) (Ummenhofer et al., 2018).
Models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are not always in agreement regarding projections of rainfall over East Africa. Modellers test the accuracy of a model by comparing the model outcome with historical observations. The models tend to underestimate rainfall in the long rains (March–April–May) and overestimate rainfall in the short rains (October–November–December) (Yang et al., 2014). Accurate modelling is essential to help countries to prepare for future periods of extreme weather (King et al., 2019).
To mitigate the impact of unpredictable and/or extreme rainfall events in future decades, the scientific consensus is that restriction of the average global temperature rise to no more than 1.5 ℃ above pre-industrial temperatures is necessary. Doing so would likely reduce the number of projected extreme precipitation days in many regions around the world, including Africa.
4.3 Climate projections from the Intergovernmental Panel on Climate Change
At the time of publication of the report Facing the Weather Gods, the most up-to-date projections from the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4), were that the African continent would be likely to “experience higher temperature rises than the global average, becoming warmer and drier,” and that there were “some indications of increasing variance in rainfall across the global tropics as a whole, suggesting that extremes of wet and dry may be becoming more commonplace”.
One of the main findings in the IPCC’s Fifth Assessment Report (AR5) that built on the projections from AR4 was the increased evidence that anthropogenic activities have impacted the global climate system, which is seen in increased land surface and sea surface temperatures (SPM 1.2 in IPCC, 2014a). Specific to Africa, the IPCC AR5 found that most African governments have implemented some form of climate change adaptation strategies such as disaster risk management, making adjustments in technologies and infrastructure, and mitigating vulnerability through basic public health measures and livelihood diversification (section 4.4.2.1 in IPCC, 2014a). Key risks of a changing climate for Africa identified in AR5 (Fig. 2.4 in section 2.3, IPCC, 2014a) are those of water stress on river and lake systems for floods and drought; an increased risk of water-borne disease; and impacts on food security from reduced crop productivity. The IPCC also concluded that risks in all global regions (apart from the Polar regions and the ocean) can be reduced through a combination of mitigation and adaptation strategies, and more fundamentally by limiting the increase in global temperature.
West Africa, East Africa, Southern Africa and the Sahara are all projected by the IPCC (2014b) as ‘likely’ to experience an increase in the number of hot days and a decrease in the number of cold days per year in the latter three decades of this century (2071–2100). However, data for all four regions of the continent are inconsistent for some other factors, including drought and precipitation. The IPCC suggests that there is likely to be an increase in heavy precipitation and a decrease in dryness in East Africa. In Southern Africa the evidence points towards an increase in dryness (IPCC, 2014b, Table TS.6).
In summary: Global climate models have improved since AR4, but for Africa there is no clear evidence that improvements in resolution have led to significantly improved climate predictions so far. The projection from the CMIP5 models (the most up-to-date at the time of the AR5) for the twenty-first century was that further warming in all seasons across the continent is very likely and there will probably be seasonal changes in rainfall patterns Southern Africa is very dry and is very likely to remain very dry. There is no clear consensus among model projections regarding rainfall in West Africa. Rainfall over East Africa is likely to increase in the short rains season (Christensen et al., 2013, section 14.8.7).
These projections are also underpinned by the World Meteorological Organisation in its ‘State of the Climate in Africa 2019’ report (WMO, 2020).
4.4 Records of observational data
There is evidence that extreme heat events in sub-Saharan Africa are not routinely reported, which can mean that many people living in the region are unaware of the dangers posed by extreme heat until such episodes occur. In turn, this may lead to excess deaths. For instance, the emergency events database has recorded only two extreme heat events in sub-Saharan Africa between 1900 and 2019, yet has recorded 83 heatwaves in Europe between 1980 and 2019. The failure to implement heat-detection systems in less developed sub-Saharan African countries has been attributed to poor governance frameworks and lack of expertise (Harrington & Otto, 2020).
Box 6: Advances in climate science since Facing the Weather Gods
Facing the Weather Gods, published by Greenpeace in 2013, highlighted the lack of climate and weather data available for much of Africa, especially in central Africa, in comparison to other continents. Although there have been improvements in scientific modelling, some regions, the African continent included, are still data poor, which means that verifying the modelling projections using historical data is difficult. A lack of data, or the existence of unreliable data, for most areas of the African continent over the past century mean that it is difficult to reach conclusions about trends, most notably of rainfall (Niang et al., 2014). The situation will persist until a time when sufficient records are available to make judgments.
One response to the lack of health data for low- and middle-income countries (including Africa) was the establishment of The International Network for the Demographic Evaluation of Populations and their Health (INDEPTH), a health surveillance system (Sankoh & Byass, 2012). The network is now beginning to produce research and results (Coates et al., 2019). Any associations between health and climate change, and in particular with extremes such as heatwave days, will be difficult to determine, at least for some years, because of the many variables involved.
Since 2013, many climate modellers have endeavoured to understand the various discrepancies between model simulations and observational data. By understanding the processes involved, new understanding of the climate system is uncovered and models are improved (see Box 2).
The very first climate predictions that used computer modelling were in the 1970s. Scientific understanding and computing have both advanced significantly since then. The latest state-of-the-art supercomputers can now more accurately represent the Earth’s climate at higher resolution by incorporating complex global weather systems, land-use changes, snow and ice cover, changing sea levels and temperature, and the interactions between these. Analysis of 17 climate models published between 1970 and 2007 to assess how well they projected global heating found that over the past five decades the models were “generally quite accurate” in predicting changes to global mean surface temperature (Hausfather et al., 2020).
The Coupled Model Intercomparison Project Phase 5 (CMIP5) that was used in the IPCC Fifth Assessment Report (AR5) – and since then the latest CMIP6 – can incorporate more information still and has greater capability than previous models, such as those used in AR4. CMIP5 includes Earth System Models with a more complete representation of forcings, new Representative Concentration Pathways (RCP) scenarios and more output for analysis (Collins et al., 2013).
Improvements in understanding the response of the climate system to increasing anthropogenic greenhouse gas emissions have led to new estimates of likely future scenarios. For example, in AR4, the climate sensitivity (see section 5.0) was given in the range 2 ℃-4.5 ℃, whereas in AR5, improvements in observational and model studies of temperature change meant that the estimated range broadened to between 1.5 ℃-4.5 ℃ (IPCC, 2013; p16).