Chapter 2
The climatic feature most relevant to Africa’s ecology is precipitation or, more specifically, rainfall. Conditions cold enough to produce snow occur only in Africa’s southern and northern extremes, and on a few of the highest peaks in between. Moreover, it is not merely the total annual amount of rainfall that is important, but particularly its seasonal distribution. Plant growth ceases for part of the year not because it is too cold, but because it is too dry. It does matter, though, whether the dry months occur during the cool (winter) period or warm (summer) period, recognising that concepts of summer and winter do not apply near the equator. Nevertheless, the annual seasonal cycle takes the form of an alternation between a wet season and a dry season even in the equatorial zone.
Seasonal variation in temperature and rainfall is governed by the tilt in the Earth’s axis of rotation, which determines which hemisphere receives most solar radiation during different stages of the Earth’s rotation around the sun. In mid-December, the sun is overhead at noon at latitude 23.5°S (the Tropic of Capricorn) in the southern hemisphere. In mid-June, it shines directly down at midday over 23.5°N (the Tropic of Cancer). Where the sun is overhead at noon draws together the easterly trade winds from the north and south, generating the Intertropical Convergence Zone, or ITCZ, in tropical latitudes. Where the ITCZ is located shifts north and south of the equator during the course of the year. Near the equator, this produces two rainfall peaks, timed shortly after the sun’s transit overhead – one in November and one in April. Towards the subtropics, there is a single rainy season timed during the warmer summer months. During the winter months, high pressures developed by descending air produce clear sunny days. Ocean temperatures also influence rainfall, because where the surface water is cooler less water evaporates. This results in deserts on the west coasts of southern Africa, South America and Australia.
Local topography has a further modifying influence on rainfall received. Higher-lying areas deflect air upwards, causing moisture to condense locally while producing rain shadows leeward. Drier conditions develop westward because the easterly winds have deposited much of their moisture by then. Winds also affect ocean currents and consequent sea surface temperatures, with ramifying effects on precipitation around the globe. The El Niño–Southern Oscillation (ENSO) is controlled by the Humboldt current, which conveys cold Antarctic water northwards off the western coast of South America. This affects atmospheric pressure cells located over Africa and hence the development of rain-forming clouds during the southern wet season. The rain that southern Africa does not receive falls further north, generating wetter conditions in eastern Africa.
Current Climates
Africa is prevalently drier than other continents, apart from Australia. The regions of Africa receiving a mean annual rainfall (MAR) above 2000 mm, supporting rainforest, are located mostly in low-lying regions of west-central Africa near the coast. They include coastal Cameroon and adjoining parts of Nigeria, plus the region extending from Ivory Coast to Guinea further west (Figure 2.1). Some regions of the central Congo Basin receive similarly high rainfall. Elsewhere, rainfall amounts of this magnitude occur only locally on mountain slopes.
Figure 2.1
Annual rainfall map of Africa, showing prevalence of mean annual rainfall totals under 800 mm through eastern and southern Africa, broken by a moist corridor extending from coastal Mozambique through Malawi.
(map produced by the Global Precipitation Climatology Center, from A. Siebert in Geography Compass, June 2014)
Over most of High Africa, MAR ranges between 500 and 1000 mm (see Table 1.1). Amounts below 600 mm occur close to the equator in the Eastern Rift valley and on the Serengeti Plains, situated in the shadow of the Ngorongoro highlands. Northern Kenya is dry due to the deflection of the north-easterly rain-bearing winds by the Ethiopian highlands. Rainfall diminishes towards desert conditions along the west coast of southern Africa, culminating in the Namib Desert. In the north-west, conditions grade from the Sahel into the Sahara Desert.
Within the African tropics, rainfall is fairly evenly distributed through the year only in parts of the central Congo basin, where moist air is drawn from both the Indian and Atlantic oceans (Figure 2.2). Nevertheless, quite substantial amounts of rain are received during the dry season months in much of western Africa (Table 1.1). In eastern Africa close to the equator, the dry season lasts from May through October and there is a variable lull in rainfall through January–February. Most rain falls in March–April during the northward movement of the ITCZ. The duration of the dry season gets shortened to 3 months in parts of Uganda near the Western Rift. Southward through Zambia, Zimbabwe and Malawi, as well as in parts of Angola in the west, the dry season is lengthened to 7 months and the pre-rain period from October into November is intensely hot as well as dry.
Figure 2.2
Seasonal rainfall patterns and associated temperature regimes for various African cities showing variation in the duration and intensity of the dry season (latitude and mean annual rainfall in brackets). Moisture deficits arise when the monthly rainfall in millimetres dips below the mean monthly temperature in degrees Celsius.
(panels drawn using data from climate-data.org)
In the summer rainfall region of South Africa, 85 percent or more of the annual rainfall typically falls during a wet season lasting from October or November through March or April. The cold winter months from May through July or August are dry and sunny because high-pressure conditions prevail and suppress cloud development. Hot ‘berg’ winds blow from the interior during August and September, promoting the spread of fires shortly before the rainy season commences. In western Africa, north-easterly trade winds blowing over the interior during the dry season generate dry and dusty conditions known as the Harmattan. Images captured from space by satellite depict the north to south alternation of green vegetation across the continent (Figure 2.3).
Figure 2.3
Contrasting shifts in cloud cover and greenness in southern versus northern sections of Africa between (A) March 2020 and (B) September 2020. Note the broad dry region in the south during September at the end of the dry season and the dry region extending through western Africa in March.
(© EUMETSAT 2020)
The MAR totals hide the substantial variation in the annual amounts of rainfall actually received. Over much of eastern and southern Africa, the typical range of variation is from less than half to nearly twice the MAR, generating a coefficient of variation, or CV (standard deviation divided by the mean) of around 25 percent. The CV widens as the mean annual total diminishes, and vice versa. Arid savanna regions may go for a year or longer without rainfall.
The timing of the first rains of the wet season and when the dry season sets in also varies from year to year, with important ecological consequences. Hence the blocks of months representing wet and dry season conditions can differ from year to year. Rain received during the normally dry season months alleviates the dryness of the grass cover, but has little or no effect on trees. Dry spells may also interrupt the normally rainy season months. During my field study in the Hluhluwe-iMfolozi Park, the state of the grassland in January 1970, normally the peak wet month, was as dry as normally experienced during July in the mid dry season. Herbivores must cope somehow with the resultant variability in food availability. Storm fronts also vary in their spatial tracks. Even during severe droughts, some regions may receive adequate rainfall and alleviate the shortfall in grass growth for animals able to move towards the greener regions.1
The effectiveness of rainfall for plant growth depends further on how temperature and wind conditions control evaporative losses. Within High Africa, temperatures do not rise very high even near the equator, being tempered by the elevation coupled with persistent cloud cover. The maximum midday temperature generally ranges between 25 and 30°C year-round, and seldom much exceeds 30°C. Temperature conditions are less pleasant in coastal regions, where they are coupled with high humidity. In more southern and northern latitudes, midday temperatures frequently rise above 40°C in the period preceding the rains. The highest temperature recorded in the north of Kruger NP, just within the tropics is 48°C, while in subtropical Hluhluwe-iMfolozi Park the highest temperature I experienced was 44°C. The range in temperature between day and night can also be quite wide in the interior, typically spanning ~20°C during the dry season months.
Along the eastern coast of South Africa, the south-flowing Agulhas current brings tropical conditions as far as 30°S. In the west, the cold Benguela current produces coastal fog and promotes cloudy conditions in the dry season, reaching as far north as Gabon in west-central Africa. Overnight frosts, as well as occasional snow, occur during winter in the southern interior of South Africa, and at any time of the year in the highest mountains of eastern Africa and Ethiopia. Sub-zero temperatures may also develop overnight in interior Botswana and adjoining parts of Zimbabwe, producing ‘black’ frosts because of the low humidity at that time of the year.
Over most of Africa, more water potentially evaporates than falls as rain. Climatograms relating monthly rainfall to prevailing temperatures indicate periods with negative water balance, if scaled appropriately (Figure 2.2). Potential evapotranspiration (including water lost through plants) reaches around 1600 mm annually at a latitude of 25°S, diminishing to 1300 mm nearer the equator because of the greater cloud cover.2 Positive water balances enabling plant growth are restricted to the wet season months, and perhaps to only a portion of this period. Wind accelerates water losses through evaporation and further restricts plant growth. Windy conditions feature especially in the plateau and escarpment regions of southern Africa. During intense storms, the deluge of water can exceed the infiltration capacity of soils, so that much water runs off into gullies, streams and rivers, accentuating erosion.
Topographic variation in elevation and surface terrain generates quite wide variation in annual rainfall amounts over short distances.3 For instance, while MAR on the rim of the Ngorongoro Crater in Tanzania reaches up to 1700 mm, it falls to ~500 mm on the plains to the west where Olduvai Gorge is situated. The MAR within the Serengeti–Mara ecosystem spans an overall range from ~500 mm in the south-east, in the shadow of the Crater Highlands, to 1100 mm in the far north over a distance of 250 km. Tsavo East NP, situated a few degrees south of the equator and 200–300 km inland from the Kenyan coast, receives a MAR of only 530 mm, while Mombasa on the coast has a MAR of 1200 mm. The Hluhluwe-iMfolozi Park, situated in the escarpment foot-slopes of eastern South Africa, encloses a rainfall gradient from under 600 mm in the low-lying south-west to nearly 1000 mm in the hilly north-east over a distance of 30 km.
Past Climates
Indicators of past climates are obtained from hydrogen and oxygen isotope ratios preserved in air pockets retained in glacial ice, sediments accumulated in the adjoining oceans, local soil features and records of lake levels. Global climates since the end of the Cretaceous period 66 Ma have shifted extremely from conditions so warm that both the South and North Poles were free of ice, and times so cold that massive ice sheets covered much of the northern continents.4 Cyclic alternations in prevailing temperatures became manifested particularly over the last two million years. These cycles are generated by changing features of the Earth’s orbit, tilt and spin relative to the sun, named the Milankovitch cycles (Box 2.1). The climatic oscillations and trends had important implications for the evolution of humans and their hominin ancestors.
Box 2.1Milankovitch Cycles
A Serbian astronomer named Milankovitch recognised how orbital variations in the Earth’s movements generate cyclic oscillations in climatic conditions (Figure B2.1). Three contributions interact. (1) The eccentricity in the Earth’s orbit, affecting its nearness to the sun, oscillates with a period of around 100 kyr affecting the intensity of the solar radiation impinging on the Earth. (2) The tilt, or obliquity, of the Earth’s axis of rotation, which varies between 21.8° and 24.4° with a period of 41 kyr, affects which hemisphere receives most sunshine in particular seasons. This modifies how extreme the seasonal contrasts are, amplified when the tilt is greatest. (3) The precession or wobble in the Earth’s axis of rotation, in conjunction with the orbital eccentricity, determines when during the Earth’s orbit the sun is vertically overhead at noon. This increases the seasonal contrast in one hemisphere and decreases it in the other, affecting regional rainfall particularly in the tropics and subtropics with a 19–23 kyr period.
Figure 2B.1
Aspects of orbital geometry generating the Milankovitch cycles in global climate. Eccentricity changes the shape of the orbit on a 100,000-year cycle from a circular to a more elliptical shape. Obliquity is the change of the angle of the Earth’s axis of rotation, which ranges from 22° to 24° from normal, and occurs on a 41,000-year cycle. Precession or ‘wobble’ in the axis affects the Earth’s aspect relative to the sun in particular seasons, with a period varying between 19,000 and 23,000 years.
(from Ross et al. (2010) Climate Change Past, Present & Future: A Very Short Guide. Paleontological Research Institution Special Publication No. 38; download)
Carbon dioxide levels in the atmosphere affect how much of the heat radiation received from the sun is retained (the greenhouse effect) and hence contributes to warmth. During the Eocene epoch ~53 Ma, atmospheric CO2 concentrations were projected to lie between 1000 and 2000 ppm, dropping to around 400 ppm by the commencement of the Pliocene 5 Ma. Around the Last Glacial Maximum (LGM) 20 ka, CO2 levels dropped as low as 180 ppm. Following industrialisation and the burning of fossil fuels, atmospheric CO2 levels have recently risen above 400 ppm again. Local temperature regimes depend additionally on how ocean currents redistribute heat. When temperatures are colder globally, conditions are generally drier, because less moisture evaporates from the oceans.
Following the breakup of Gondwana ~180 Ma, most of the world remained particularly hot and wet from the mid-Cretaceous period into the Eocene.5 Temperature conditions at the start of the following Oligocene epoch remained almost 10°C warmer than those prevailing today (Figure 2.4).6 In Africa the climatic equator, i.e. where movements of the ITCZ are centred, lay further north than at present because of the latitudinal location of Africa at that time. Consequentially, wet conditions prevailed through much of what is now the Sahara Desert. A cooling trend occurred during the Oligocene, by up to 8°C.7 Radical changes in the barriers represented by the continents caused the drift of major sea currents to shift from north-to-south to east-to-west. The Southern Ocean circulation passing between Australia and Antarctica became operational around 35 Ma, when South America finally split off from Antarctica, opening the Drake Passage. As a consequence, an ice sheet developed in the eastern Antarctic.6 The Miocene until 15 Ma remained 5°C warmer than present-day conditions, but cooled by 2–3°C between 15 and 12.5 Ma, shortly preceding the spread of savanna grasslands. The uplift of the Tibetan plateau, which commenced rising ~50 Ma following the collision of drifting India with the rest of Asia, contributed further to drying in eastern Africa by deflecting moisture brought to eastern Africa by the Indian Ocean monsoon.8 Freshly exposed rock surfaces in the Himalayas sequestrated carbon in limestone deposits on ocean floors, lowering atmospheric carbon dioxide levels and contributing further to global cooling.
Figure 2.4
Global temperature variation through time inferred from various sources, with the scale increasingly finely partitioned within five periods: (i) through the Palaeozoic and Mesozoic eras from 520 to 65 Ma (red); (ii) through the Cenozoic era from 65 to 5.3 Ma (green); (iii) through the Pliocene and early Pleistocene from 5.3 to 1.0 Ma (black), (iv) through the late Pleistocene from 1.0 to 0.2 Ma (blue), and (v) through the period following the Last Glacial Maximum into the Holocene. Note how global mean temperature has declined progressively while the amplitude of temperature fluctuations has widened.
(from Wikipedia, assembled using various data sources)
The Tethys Sea in the north lost its connection with the Indian Ocean between 11 and 7 Ma, accentuating aridity in the Sahara region. In the south-west, the cold Benguela current developed, suppressing moisture drawn from the Atlantic Ocean and producing the Namib Desert by ~7 Ma. Around 5.6 Ma, the Mediterranean Sea dried up completely, following closure of its connection with the North Atlantic Ocean, promoting extreme aridity in the north of Africa. Temperature conditions then stabilised during the Pliocene from 5 Ma to 3.5 Ma, after the Mediterranean Sea had filled again. Between 2.8 and 2.6 Ma, the gap between South and North America in the region of Panama, which had connected the Atlantic and Pacific Oceans, closed. This initiated the formation of glacial ice around the North Pole and produced a sharp drop in global temperatures, initiating the transition from the Pliocene into the Pleistocene epoch.
The climatic oscillations between glacial advances and interglacial interludes, characterising the Pleistocene from a northern hemisphere perspective, then set in. Initially this coupled cycle periods of 41 kyr, governed by changes in the obliquity of the Earth’s axis of rotation, and ~21 kyr, related to the precession wobble in this axis (see Box 2.1). The progressive cooling and drying trend continued (Figure 2.4).9 Tectonic movements associated with rift valley formation further influenced local precipitation in eastern Africa. After 0.8 Ma beginning in the mid-Pleistocene, the period between glacial peaks lengthened to 100 kyr, and global temperature fluctuations became widened to as much as 5°C between full glacial and interglacial conditions (Figure 2.4). Cold glacial conditions lasted around 100 kyr, while warmer interglacial interludes persisted only ~10–15 kyr. Rapid deglaciation took place within merely a few thousand years approaching the warm peaks. Through the course of the Pleistocene, the cold extremes, and hence inferred dryness in Africa, reached unprecedented levels, while little change took place in interglacial temperature conditions.10 The most extreme lows in glacial cold occurred around 650 ka, 450 ka, 150 ka and during the LGM 20 ka. Temperature conditions during the peak glacial advances around 550 ka, 350 ka and 250 ka were not as extreme. Global temperature regimes additionally affected the Hadley cells governing where air rising over the climatic equator settled in the subtropics. This circulation contracts towards the equator during cold extremes and expands towards higher latitudes during interglacial interludes, affecting local rainfall governed by the ITCZ.
Finer-scale reconstructions of past temperature and moisture conditions are available for Africa after 130 ka, during the late Pleistocene.11 The region of south-central Africa from Lake Malawi to Lake Tanganyika experienced a ‘mega-drought’ from 135 to 75 ka, as conditions cooled after the preceding interglacial, perhaps exacerbated by a northward shift in the ITCZ.12,13,14,15 MAR could have been reduced to as little as 60 percent of current levels through this region. Lake temperatures in eastern Africa spanned a 4°C range between glacial and interglacial extremes during the late Pleistocene, compared with 6–7°C for terrestrial temperatures. At Olduvai Gorge, local rainfall possibly varied as widely as 200–700 mm.16 These extremely variable conditions shortly preceded the major exodus of humans from Africa.
The wobble in the Earth’s rotation varying in period between 19 and 23 kyr generates oscillations between wet and dry extremes taking place within as little as 2000 years, separated by more stable periods lasting 8000 years.17 Projected annual rainfall in the vicinity of the Tswaing Crater, situated near the northern margin of the South African Highveld, varied between 500 and 870 mm with a 23 kyr period over the past 250 kyr.18 During the LGM, small local glaciers developed in the Lesotho highlands because winter rainfall penetrated further inland than at present, even as far as the Free State and Gauteng provinces.19
In the Kalahari region of Botswana, rainfall dropped to as little as 40 percent of the current mean during the LGM.20 Rainfall rose but fluctuated widely during the cool conditions that ensued there between 16.6 and 12.5 ka after the LGM.21 A brief cool interval between 12.9 ka and 11.7 ka, known in Europe as the Younger Dryas, was associated with intensified aridity over most of southern Africa. Between 7000 and 4500 years ago, conditions became ~2°C warmer than at present during a period known as the Holocene altithermal. It was associated with lowered rainfall through South Africa and southern Zimbabwe, while the Kalahari region and further north remained wetter than at present.18 During the ‘Little Ice Age’ in Europe, which extended from 1300 to 1810 CE, temperatures in South Africa fell by about 1–2°C.
Although most of Africa was drier when northern continents were colder during glacial advances, exceptions occurred in parts of southern Africa. Marine sediments show that the coastal hinterland in the south-east was subject to greater erosion, indicating wetter conditions, during glacial periods, possibly due to strengthening of the Agulhas current.22 The flow of the Zambezi River surged between 16 and 12 ka, indicating higher rainfall in its catchment, although the northern hemisphere still remained cool at that time.23
In the north, the Sahara Desert became extremely dry during the late Miocene 7–11 Ma, following closure of the Tethys Sea.24 The drying trend there intensified further after 2.8 Ma.5 North Africa was relatively humid for a period between 133 and 117 ka during interglacial conditions, then again from 100 to 75 ka, when south-central Africa was especially dry.15 The Sahara supported a lush savanna thronged with animals (even crocodiles) from 9000 until 4500 years ago, after which desert conditions took hold again.
Overview
Africa is relatively dry compared with other continents spanning tropical latitudes. Most of Africa’s eastern region receives under 1000 mm of rainfall annually, and parts even less than 650 mm, the functional threshold between moist and dry savannas (see Chapter 7). Dry seasons span several months with little or no rainfall and occur during the cooler period of the year away from the equator. Mean annual rainfall in South America tends to be twice that recorded at similar latitudes within Africa.2 Easterly trade winds convey heavy rain deep into the low-lying interior of tropical South America, while Africa’s regional rainfall totals get attenuated to the west of its uplifted rim, especially in rift valley depressions and in the southern subtropics. In Australia, rainfall remains low through most of the interior away from the eastern seaboard and fluctuates even more erratically between years than in tropical and subtropical Africa. In temperate latitudes of Eurasia and North America, precipitation in the form of winter snow moistens soils in spring. Even where there is no snow, rainfall is more effective for promoting plant growth because of the prevalently cooler conditions.
Past climatic conditions, particularly in rainfall, would have been more widely variable than during the interglacial interlude that we are currently experiencing, despite the developing disruption by global warming. Global temperature regimes became progressively colder and hence drier from the late Miocene through the Pliocene and Pleistocene, influenced by declining atmospheric CO2 levels and oceanic circulation patterns. Major climatic transitions occurred around 2.63 Ma when cyclic variation with a 41 kyr period took hold, associated with the onset of Arctic glaciation, and around 0.8 Ma, when the period of the glacial–interglacial alternations lengthened to ~100 kyr. Within these regimes, shorter-term oscillations with a ~21 kyr period further affected rainfall variability in Africa. As a consequence of the reduced rainfall, dry seasons became more intensely dry and wet season totals fluctuated more extremely from one year to the next than at present. Major droughts would formerly have been both more extreme and more frequent. Somehow our hominin ancestors coped through the dry seasons when plant growth ceased.
Rainfall amounts and seasonal variation therein had ramifying effects on river flows and lake levels. Rainfall also strongly influences soil properties, in interaction with the geological substrate. These interrelationships will be described in the following chapters.
SUGGESTED FURTHER READING
Burke, K; Gunnell, Y. (2008) The African erosion surface: a continental-scale synthesis of geomorphology, tectonics, and environmental change over the past 180 million years. Geological Society of America Memoirs 201:1–66.
Feakins, SJ; deMenocal, PB. (2010) Global and African regional climate during the Cenozoic. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 45–55.
Levin, NE. (2015) Environment and climate of early human evolution. Annual Review of Earth and Planetary Sciences 43:405–429.
Sepulchre, P, et al. (2006) Tectonic uplift and Eastern Africa aridification. Science 313:1419–1423.
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