Solar Energy, Temperature and Rainfall

Abstract

The climatic forces that determine Angola’s biodiversity and ecosystem patterns (and all life forms on Earth) are based on the energy that comes from the Sun. This chapter examines the concepts and functioning of solar radiation and photosynthesis, the basis of primary production. Strong seasonality of temperature and rainfall and their variation across latitude and altitude result in the diversity of Angola biomes, including Guineo-congolian lowland rain forest, Afromontane forest, mesic savanna, arid savanna, desert and mangrove biomes. Driving the climatic gradients are the oceanic and atmospheric systems, dominated in the north by the Intertropical Convergence Zone and in the south by the South Atlantic and Botswana Anticyclones. The physical processes driving these systems are described, and the local influences of maritime, continental, altitudinal and aspect factors that account for subtle changes from one ecoregion to the next are illustrated graphically and statistically. The recent impacts of El Niño events are described and the anticipated impacts of climate change on Angola are outlined.

FormalPara Key Concepts and Questions: This Chapter Explains

• The nature and source of solar energy and how it is transferred to the planet.

• How the Sun’s energy, and the Earth’s rotation on its axis, create the climate that we experience.

• Why seasons are important to the rhythms of life and what drives seasonal patterns.

• How solar energy is captured, retained and converted to sustain living organisms through photosynthesis.

• How the oceans, atmosphere and land interact to create the climates of Angola.

• How the patterns of climate vary across Angola, and what processes account for them.

• Why Angola has both rain forest and desert in a single country.

• What the El Niño phenomenon is, and how it affects Angolan ecosystems and society.

Context: Solar Radiation, Weather and Climate

Life on Earth is possible because of the energy it receives from the Sun. Climate, geology, topography, hydrology, soils, fire and herbivory determine the distribution and abundance of plant and animal species and of ecosystem patterns. Of overriding importance driving these interlinked factors is solar radiation which shapes the global patterns of temperature, air pressure and moisture, and of atmospheric and oceanic dynamics. Solar radiation determines the weather and climate experienced at the scale of individual organisms to continental biomes. Most importantly, solar radiation drives photosynthesis, the source of life on Earth.

Angola offers excellent opportunities to examine the influence of climate on ecosystems that range from hyper-arid desert to tropical rain forest. Before focusing on Angola, it is helpful to understand the driving forces of climate at global and regional scales. Solar radiation, the rotation of the Earth on its axis and its annual path around the Sun, result in both long-term cycles and annual seasonality of climate, and associated processes of oceanic and atmospheric circulation. These drivers are modulated by latitude, altitude and distance from the ocean to produce the patterns of temperature and rainfall experienced over Angola.

A distinction must be made between the concepts of weather and climate.

• Weather is the temperature, humidity, rainfall and wind experienced at a particular place and at a specific time, such as a hot, humid summer afternoon in Luanda. Weather refers to local, short-term phenomena.

• Climate refers to the long-term average pattern of weather at local, regional and global scales, such as the cool desert climate of Tômbua

• or the wet tropical climate of Buco Zau. Climate describes the overall summary of regional weather features. For example, spells of wet weather can be experienced in a dominantly arid climate.

1 Where Life Begins: Solar Radiation and Photosynthesis

Solar energy drives the functioning and dynamics of all ecosystems. The first step in understanding the ecology of living organisms is to become familiar with the key life-giving mechanisms, the transfer of solar energy from Sun to Earth, and the process of photosynthesis.

Energy is emitted from objects as electromagnetic radiation. In brief, hot objects emit shortwave and cool objects emit longwave radiation. The Sun has a surface temperature of 5800 °C and emits shortwave solar radiation to the Earth, some of which is reflected back into the atmosphere. The Earth, with an average temperature of 15 °C emits longwave radiation. The Earth’s energy budget is the balance between radiant energy that reaches the Earth from the Sun, and that which flows back from Earth into space (Fig. 5.1). The proportion of sunlight that is reflected back into the atmosphere by different land surfaces is called their albedo. Light surfaces, like the Namib Desert, have a high albedo, reflecting energy. Dark surfaces, such as the rain forests of Cabinda, have a low albedo, absorbing energy.

Fig. 5.1

The Earth’s energy budget illustrating the main flows of solar radiation and how it is absorbed and reflected by Earth. From Trenberth (2020) Journal of the Royal Society of New Zealand, 50:2, 331–347. c The Royal Society of New Zealand

The solar energy (shortwave radiation) received at the Earth's surface as sunlight is harnessed through the process of photosynthesis—a series of biochemical reactions by which atmospheric carbon dioxide and water are transformed by light energy into carbohydrates (simple sugars), with oxygen released as a by-product. The process occurs within specialised organelles (chloroplasts) in the mesophyll cells of plant leaves. Basically, six molecules of carbon dioxide and six molecules of water are transformed into one molecule of sugar and six molecules of oxygen (6CO₂ + 6H₂O = C₆H₁₂O₆ + 6O₂). The energy received from the Sun and captured in plants by photosynthesis is stored as carbohydrates and as high-energy molecules (adenosine triphosphate—ATP) which carries energy within and across cells.

Gas transfer takes place via the opening and closing of small pores (stomata) on the surface of plant leaves. The process is catalysed by a nitrogen-based enzyme, rubisco (ribulose biphosphate carboxylase-oxygenase), the most abundant protein on Earth and a major source of nitrogen for herbivores. Photosynthesis requires sunlight (photosynthetically active radiation—PAR) to function. In the absence of PAR (at night) the carbohydrates are used during cellular respiration, a chemical reaction which breaks down carbohydrates to form ATP, while releasing carbon dioxide. Respiration is the opposite process to that of photosynthesis, requiring oxygen and releasing carbon dioxide and water (C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O). During these processes, while the stomata are open for the exchange of carbon dioxide and oxygen, water is lost from the leaf via transpiration. One of the key adaptations to survival in plants is the regulation of gas exchange (and water loss) via the stomata.

The net gain of carbohydrates (sugars and cellulose) formed through photosynthesis, minus the losses through respiration, provide the energy and structural components of plants, which are then available to the rest of the food chain that progresses from primary producers (plants), to consumers (herbivores) to predators (carnivores) and ultimately to decomposers. The importance of solar energy, water, nutrients (such as nitrogen and phosphorus), rubisco and ATP will become evident in later discussions in Part lll Chap. 10 on ecosystem processes, and more specifically in Sect. 10.2, which describes the three photosynthetic pathways that have developed in the long history of plant evolution. These different pathways (abbreviated as C₃, C₄, and CAM) account for the success and diversity of Angola's forest, savanna and desert ecosystems which function under very different climatic regimes.

2 Seasonality of Day Length and Temperature

The major climatic patterns of the globe result from a sequence of physical processes triggered by the Earth’s rotation on its axis as it progresses on its annual elliptical orbit around the Sun. The Earth’s tilt is 23.5° from the perpendicular. Each season results from the relative position of the equator (and any other point on the Earth’s surface) relative to the incoming Sun’s rays (Fig. 5.2).

Fig. 5.2

Changes in the concentration of the Sun’s rays during the Earth’s annual orbit around the Sun. Left: winter solstice 22 June, Sun overhead the Tropic of Cancer. Centre: spring and autumn equinoxes, Sun overhead the Equator. Right: Summer solstice, 22 December, Sun overhead Tropic of Capricorn. From Smith and Smith (2015) Elements of Ecology. (9th Edition). Pearson, Boston

The Sun is directly overhead the equator twice per year at midday on the spring and autumnal equinoxes (22 September and 21 March) when day and night are of equal length at all points on the planet. As the Earth moves on its orbit around the Sun, the Southern Hemisphere receives an increasing exposure to solar radiation until the Tropic of Capricorn falls directly under the Sun at midday on 22 December, which marks the summer solstice (and winter solstice in the Northern Hemisphere). The tilt of the Earth on its axis then accounts for the migration of the overhead Sun northwards, until it is overhead the Tropic of Cancer on 22 June, when the Southern Hemisphere experiences its shortest day of the year on the southern winter solstice. On the Equator, seasonal changes in incoming solar radiation are not great, but at high latitudes, the acute angle at which the Sun’s rays reach the Earth’s surface, after passing through a broad band of atmosphere, explains the decrease in solar radiation (and hence temperatures) experienced at the poles, and why the poles are covered in ice, while the equatorial regions are mostly covered by forest (Fig. 5.3).

Fig. 5.3

The amount of radiation received at the Earth’s surface depends on season and latitude. When the Sun is overhead the Equator, the Sun’s rays are concentrated over a small area, while at the high latitudes, it is spread out over a much wider area, with much less radiation and heat being received. From Smith and Smith (2015) Elements of Ecology. (9th Edition). Pearson, Boston

In tropical countries, such as Angola, the changes in day length and mean daily temperature are not nearly as noticeable as they are in high latitudes such as northern Europe. In Angola, seasonality of rainfall is much more important for ecological processes than are temperature conditions, where warm wet summers are followed by mild dry winters, regulated by large-scale atmospheric dynamics. The duration of the dry season varies from a few months in the north to eight months in the south, and is a primary factor influencing the structure and functioning of Angola’s rain forests, mesic and arid savannas, floodplain grasslands and deserts.

3 Large-Scale Drivers of Climate: Oceanic and Atmospheric Circulation

The seasonal differences in the amount of solar radiation received at the equator and the high latitudes are the major drivers of energy, air and moisture circulation around the globe. There is a net surplus of heat from the Equator to the mid-latitudes, and a net deficit over the mid-latitudes to the poles (Fig. 5.4). The reason why the tropics do not become increasingly hotter and the poles increasingly colder is because the atmosphere and ocean currents transfer the surplus heat from the tropics to the poles. The transfer of heat from the Equator to the poles is via convection—where there is a transfer of warm equatorial oceanic water and atmospheric air moving towards colder regions.

Fig. 5.4

Variation of mean annual incoming shortwave radiation, outgoing longwave radiation, and net radiation as a function of latitude. Note the surplus of incoming radiation over outgoing radiation between the equator and 30°–35° north and south, and the radiation deficit between 35° and the poles. This gradient of net radiation drives the transport of heat from the tropics to the poles through circulation within the atmosphere and the oceans. From (Smith & Smith, 2015) Elements of Ecology (9th Edition). Pearson, Boston

These mass movements of energy result in three circulating air systems (cells) over the northern and southern hemispheres, driven by low and high pressure zones of the equatorial and the mid-latitudes (Fig. 5.5). The three large-scale atmospheric circulation ‘cells’ are known as the Hadley, Ferrel (or Mid-latitude) and Polar cells. The Hadley cells are of special importance to Angola and are illustrated in cross-section in Fig. 5.6. The mechanisms of atmospheric circulation are depicted in Figs. 5.4, 5.5 and 5.6. The converging air mass that is warmed over the equatorial regions rises into the lower atmosphere, thereby establishing an area of low pressure at the Earth’s surface—the equatorial low. This area of low pressure is called the Intertropical Convergence Zone (ITCZ). As the altitudinal position of the Sun changes relative to the Equator, the ITCZ moves latitudinally, northwards or southwards, between equinoxes and solstices. The shift in position of the ITCZ produces seasonality in precipitation, reflected in rainy seasons and dry seasons. In Angola, with increasing distance from the Equator, the tropical dry season tends to be longer and the rainfall lower. Understanding the role of the ITCZ and of the Hadley cells is helpful to understanding the climate of Angola. Several features deserve emphasis:

Fig. 5.5

Source Wikipedia Commons

A simplified depiction of large-scale atmospheric circulation on Earth, as at the equinoxes. Note the position of the Hadley cells, the ITCZ and of the Southeasterly Trades.

Fig. 5.6

A cross-section of the Hadley, Ferrel and Polar cells in the Southern Hemisphere. Note the position of the ITCZ, Tropopause, Stratosphere, Subtropical and Polar jets, and the descending dry air between the Hadley and Ferrel cells, that cause the desert belt and the Trade winds. From Lovegrove (2021) The Living Deserts of Southern Africa (2nd Edition). Penguin Random House. Artwork John Warren

• The rising, warm and wet equatorial air along the ITCZ is unstable and cools as it rises, causing condensation, saturation, cloud formation and precipitation. This accounts for the high rainfall received across northern Angola.

• From space, the ITCZ appears as a band of clouds which produce extensive thunderstorms and intense rainfall encircling the globe near the equator, supporting the equatorial rain forests of South America, Africa and Southeast Asia.

• The strong precipitation along the ITCZ releases latent heat, driving the air upwards to the tropopause at about 15 km above sea level.

• At this altitude, the air is no longer buoyant, and this sub-stratospheric air is forced poleward by the continuing thrust of the rising air below it.

• At about 30°S of the equator, the cool, dry, high altitude air begins to sink and is warmed adiabatically by compression from the overlying air. The subsiding air forms subtropical high pressure zones (also known as anticyclones, such as the South Atlantic Anticyclone). The subsiding air is dry and stable, not conducive to forming rainfall.

• At the arid mid-latitudes (ca 30°–35° north and south of the equator), the Sahara and Namib deserts and their specialised animals and plants have evolved over many millions of years.

• A key feature of global atmospheric and oceanic circulation patterns is that of the Coriolis Effect. As the Earth spins on its axis, the relative ground speed at the Earth’s surface increases from the poles to the equator (because of the great differences in the Earth’s circumference between low and high latitudes). This phenomenon is known as the Coriolis Effect. As a result, circulating air and water currents are deflected to the right (clockwise) in the Northern Hemisphere and to the left (anticlockwise) in the Southern Hemisphere). These patterns of circulation are best illustrated in global ocean currents (Fig. 5.7).

  Fig. 5.7

  

  Ocean currents of the world. Blue arrows indicate cold currents, red arrows indicate warm currents. Note how the oceans are connected by currents, how the circulation of currents—clockwise in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere—is influenced by the Coriolis force and by the positions of the continental landmasses. From http://www.coastalwiki.org/wiki/Ocean_circulation

• Finally, as the summer season approaches, the position of the ITCZ moves southwards across central to southern Africa, moving northwards after the summer equinox (Fig. 5.8).

  Fig. 5.8

  

  The southwards and northwards movement pattern of rainfall seasonality (highlighted by the green tones) across Angola during 2009/2010, in tandem with the migration of the Intertropical Convergence Zone. From Huntley (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature

The pressure differences between the low pressure belt of the wet ITCZ and the high pressure of the dry subtropics results in winds from the south and north converging along the ITCZ. The winds from the northern and southern high pressure zones thus converge towards the low pressure of the ITCZ. These Trade winds follow a regular pattern of seasonality. Blowing across the Atlantic and Indian oceans, for instance, they provided the early Portuguese navigators with a reliable guide to crossing the oceans. The mariners had to learn to avoid the notorious region known as the doldrums, the low pressure belt of the ITCZ, where the rising hot air results in little surface wind, a serious challenge to sailing ships.

The winds blowing across the oceans generate the major ocean currents. Following the principles of the Coriolis effect, these currents move clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere, creating gyres (like massive whirlpools) bringing cold waters to the equatorial regions and taking warm equatorial waters to the polar regions (see Fig. 5.7). The south-westerlies bring cold Antarctic waters to the southwest African coast and, coupled with the influence of offshore southeast trade winds, cause upwelling (the Benguela Current) bringing rich nutrients to the surface, and cool foggy air to the coast of Angola. As we will see in Part IV, Sect. 16.3, the Benguela Current has a profound influence on Angolan ecology.

In Angola, the seasonal migration of the Intertropical Convergence Zone, in response to Earth-Sun dynamics resulting from the tilt of the Earth’s axis, can be followed in rainfall patterns and pulses, illustrated in Fig. 5.8. In winter, the high pressure system, lying over central Angola, blocks out the moist tropical air. In summer, the southward movement of the ITCZ brings rain across the country.

4 Local Influences: Maritime, Continental, Altitudinal and Aspect

The temperatures experienced on the ground are primarily determined by latitude, seasonality, altitude, aspect and proximity to the ocean. We have already discussed the influence of latitude and seasonality at global scales. At more regional and local scales, land and water surfaces heat and cool at different rates because of differences in their specific heat. Water bodies, such as the Atlantic Ocean, require four times more solar energy to raise the temperature by 1 °C than do land surfaces. This difference in the heat balance of water and land has two outcomes:

• The coastal belt of Angola is cooled by its proximity to the ocean (maritime influence). Coastal cities experience much less variation in diurnal and seasonal temperatures than do cities in the interior. The mean daily temperature range in Luanda is 7 °C in July, compared with a range of 24 °C in July at Menongue. This is due to the effect of Menongue’s distance from the ocean, known as continentality.

• In addition, hot air over the coastal plains rises through the day, creating a vacuum that is filled by cooler air drawn off the ocean—experienced as strong sea breezes on many summer afternoons along the Angolan coast. This inflow of cool oceanic air as it moves across the Benguela Current also accounts for the presence of the stratus clouds (cacimbo) that characterise coastal Angola, and that penetrate many km inland, especially during winter.

The highest mountains of Angola such as Mount Moco (2620 m above sea level) experience a much cooler and wetter climate than the coastal lowlands below them, due partially to the altitudinal decrease in temperature of about 0.6 °C per 100 m. The rainfall gradients are locally accentuated by the orographic influence of the sea-facing slopes of the escarpment and the highland mountain massifs. The folded mountain ridges of Cabinda, Zaire and Uíge are effective in catching the rain-bearing winds, with marked differences in annual precipitation being received by the west-facing compared to the east-facing slopes that lie in the rain-shadow. The direction a slope faces (aspect) is very important in temperate latitudes, and even in the tropics of Angola, the temperature and moisture microclimate of south-facing mountain slopes in the highlands of Huambo and Huíla support larger patches of montane forest than do warmer, drier north-facing slopes, where evaporation rates can be up to 50% higher than south-facing sites. Microclimates can also be important for the survival of plants sensitive to frost, which is infrequent over most of Angola, but has been recorded where associated with pockets of cold air drainage in the valleys of Cunene and Bié. Finckh et al. (2021) propose that frost is the primary causal factor in shaping the treelines of Angolan miombo woodlands and driving the evolution of geoxyles. The hypothesis deserves testing over a wider range of miombo landscapes than the two study sites on which it is based.

5 Water Cycle Processes: Precipitation and Evapotranspiration

Precipitation, like temperature, is a key determinant of the distribution and abundance of animal and plant species. Mean Annual Precipitation (MAP) is influenced by the same mix of factors as is air temperature. Air masses, whether from over the sea or land, carry water vapour with them. Two processes describe the state of water within a given body of air. Evaporation is the transformation of water to a gaseous state—water vapour. Conversely, condensation is the transformation of water vapour to a liquid state—water—or precipitation as rain. Each transformation requires equivalent amounts of energy. A simple measure of the amount of water vapour in the air is relative humidity, which is scaled from zero to 100%, at which point the air is saturated. The capacity of air to hold water vapour increases with increasing temperature. On a cool morning, dew might be found on plants, but this rapidly evaporates as the Sun rises and the air temperature increases, simultaneously increasing the water-holding capacity of the air.

Most large-scale studies of climate/vegetation correlations refer to mean annual temperature (MAT) and mean annual precipitation (MAP). However, plants grow in response to effective precipitation, not mean annual rainfall, and to potential evapotranspiration, not to relative humidity. Evapotranspiration is the combined water loss both from plant surfaces and from the soil. Potential evapotranspiration (PET) is a measure of the energy available to evaporate water, defined as the amount of water that would evaporate or be transpired from a saturated surface. Effective precipitation is MAP minus PET. Effective precipitation is the amount of rainfall that is actually added to the soil, and is hence available for plant growth. Ecological modellers therefore use effective precipitation and potential evapotranspiration as parameters that more closely relate to plant growth than do measurements of rainfall. Over much of southwest Angola, PET exceeds MAP for up to eight months of the year, with effective rainfall ranging from minus 250 to minus 1000 mm per year, accounting for the presence of arid savannas. In Angola, mesic savannas occur where PET exceeds MAP for five months; and moist tallgrass savannas and forest occur where PET exceeds MAP for two months, and where effective annual rainfall can reach 1200 mm.

6 Regional Climates of Angola

The basic principles of global climatology, and how they explain regional climates and ecosystem patterns and processes, are nowhere better illustrated than in Angola. Here we illustrate the general principles relative to the climate experienced on the ground.

Latitude

The geographic position of Angola, stretching from near the Equator to close to the Tropic of Capricorn, across 14° of latitude, accounts for the overall decrease in solar radiation received and thus influencing the mean annual temperature experienced from north to south. The latitudinal decrease in mean annual temperature is illustrated by data from stations in the hot northwest and northeast (Cabinda: 24.7 °C; Dundo: 24.6 °C), compared with stations in the milder southwest and southeast (Moçâmedes: 20.0 °C; Cuangar: 20.7 °C).

Altitude

Both temperature and precipitation are influenced by altitude. The altitudinal decrease in mean annual temperature can be illustrated from sites on the same latitude, but at different altitudes above sea level. From the base of the Chela Escarpment to the highest points at its summit, the mean annual temperature drops as follows:

• Bruco: altitude 699 m, mean annual temperature 23.8 °C;

• Jau: altitude 1700 m, mean annual temperature 18.0 °C; and finally

• Humpata-Zootécnica: altitude 2300 m, mean annual temperature 14.6 °C.

Atmospheric Systems

Of the greatest importance to the rainfall patterns that determine vegetation and habitat structure are the influences of the atmospheric systems which dominate central and southern Africa. Angola's climate is strongly seasonal, with hot wet summers (October to May) and mild to cool dry winters (June to September). As described in Sect. 5.3, the Intertropical Convergence Zone (ITCZ) moves southwards over Angola during summer, and then returns northwards to the Equator as winter approaches. The rainfall season that is triggered when the ITCZ passes across northern Angola from early summer reaches southern Angola in late summer (Fig. 5.8). The changing position of the ITCZ results in two rainfall peaks in northern Angola, and one over the rest of the country as illustrated in the climate diagrams in Fig. 5.9.

Fig. 5.9

Climate diagrams illustrating rainfall and temperature seasonality and other climatic parameters. Note the weak bimodal rainfall maxima for stations in northern Angola and unimodal maxima in central and southern Angola. The diagrams follow the system of Walter and Lieth (1967). Stipples represent dry periods; vertical stripes represent rainy periods; solid blue represents months with rainfall exceeding 100 mm. From Huntley (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature

Moving in tandem with the low pressure system of the ITCZ are two subtropical high-pressure systems over the Atlantic and over southern Africa. These are the South Atlantic Anticyclone and the Botswana Anticyclone. These two anticyclones block the southward movement of moist air from the ITCZ during winter (preventing cloud formation). As the high-pressure cells move southwards in summer, the conditions required for cloud formation return. This migration of rainfall systems is clearly illustrated in the series of rainfall maps prepared from weather satellite imagery and presented in Fig. 5.8.

During winter and early summer, the Botswana Anticyclone generates strong winds that blow across Angola from east to west. In the southwest, the winds pick up dust from the arid lands and create hot, choking dust storms that feed and sculpture the sand dunes of the Namib. The winter winds are also notorious in Lunda-Norte, where they desiccate the grasslands and promote the spread of extensive fires. In the east, the winds and their sand deposits account for dune formation across the Bulozi Floodplain (Mendelsohn & Weber, 2015).

Benguela Current

While altitude and seasonality determine temperature conditions, an anomaly to this general rule occurs in the coastal belt of Angola, especially in the far south, where the cold, upwelling Benguela Current creates a temperature inversion (see Chap. 16, Fig. 16.6). The cold Benguela Current has a stabilizing effect on the lower atmosphere and inhibits the evaporation and upward movement of moist, rain-forming air off the ocean. Its impact extends as far north as Cabinda, where a narrow belt of arid savanna and dry forest, of acacias, sterculias and baobabs, flanks the rain forests of the Maiombe (Chap. 15.3).

Despite the aridity of the coastal zone, the cooling effect of the Benguela Current results in low stratus cloud and fog (cacimbo) through much of winter, with heavy dew condensing on vegetation along the coast, even during the driest months of winter. This contributes significantly to the moisture balance of the region. The fog belt is most pronounced between Moçâmedes and Benguela, where epiphytic lichens reach great abundance in an otherwise desert environment. The Benguela Current also results in a gradient of increasing precipitation from south to north and from west to east. The rainfall gradients are locally accentuated by the orographic influence of the escarpment and the highland mountain massifs. The sharp relief of the Angolan Escarpment creates conditions for orographic rainfall along most of this zone, supporting the ‘coffee forests’ of Seles, Gabela, Cuanza-Norte and Uíge. The distribution of mean annual precipitation across Angola is summarised in Fig. 5.10. Table 5.1 provides summarised climatic data for selected Ecoregions.

Fig. 5.10

Mean annual precipitation over Angola. From Huntley (2019) Biodiversity of Angola. Science & Conservation: a Modern Synthesis. Springer Nature

Table 5.1 Summary of climatic data for selected Ecoregions

Variability

Average measures of climatic parameters, such as mean annual temperature (MAT) and mean annual precipitation (MAP) are often too simplistic to reflect ecological drivers, where extreme but infrequent events, such as frost, might be of great significance. In 1947, for instance, the cotton crop of the Baixa de Cassange, a perennially hot basin in northern Angola, dropped by 70% because of frost damage (Gouveia, 1956). Finckh et al. (2016) recorded 49 frost nights during the winter of 2013, at a highland site subject to cold air drainage near Cusseque. However, Angola’s long-term meteorological records report the absence or very low incidence of frost activity over most of the country. Based on further research, Finckh et al. (2021) propose that frost is the primary causal factor in determining the treelines of Angolan miombo woodlands and of driving the evolution of geoxyles. This interesting hypothesis deserves testing over a wider range of miombo landscapes than the two study sites (Cusseque and Bicuar) on which it was based.

As described in Sect. 5.8, episodic droughts (such as El Niño events) have had devastating impacts on livestock and wildlife populations across southwest Angola. The distribution of mean annual precipitation across Angola is summarised in Fig. 5.10. Table 5.1 provides summarised climatic data for selected Ecoregions.

Like temperature, precipitation in general increases from the poles to the equator, with a dip in the mid-latitudes as described earlier. Rainfall variation is not only spatial, but also temporal. Temporal variations are not only intra-annual, but are also inter-annual, with some years or periods experiencing floods or droughts with exceptionally high or low rainfall and temperature conditions, as illustrated in Fig. 5.25, in Sect. 5.8.

7 Climate as a Determinant of Vegetation Patterns

Global Patterns

Early attempts to understand the global patterns of vegetation (Humboldt & Bonpland, 1805; Schimper, 1903) found a strong correlation between vegetation formations and climate—especially mean annual precipitation (MAP) and mean annual temperature (MAT). Elaborate systems of classifying climate and vegetation were published by the German-Russian climatologist Wladimir Köppen (1900) and American geographers Leslie Holdridge (1947) and Charles Thornthwaite (1948). These scientists mapped the world into regions that were as much a reflection of vegetation pattern as they were of climate. In fact, for many regions where very few climate records were available, vegetation maps were used as a surrogate for climate. Although widely used for many decades, these classifications are less popular today.

In 1975, American ecologist Robert Whittaker plotted biome types (as defined by ecologists) against gradients of MAP and MAT (Fig. 5.11). The results give a generalised model of the relationship, although they omit the importance of disturbance factors such as fire and herbivory, and resource factors such as soils and topography. The Angolan biomes and ecoregions fit well within the patterns presented by Whittaker (1975).

Fig. 5.11

The distribution of global terrestrial biomes in relation to mean annual temperature and precipitation. Numbers within boxes correspond to the MAP and MAT of weather stations within the numbered Angolan ecoregions. The close fit of Angolan biomes and ecoregions with the global model is evident. Redrawn after Whittaker (1975) Communities and Ecosystems. McMillan, New York

Continental Patterns

At a continental scale, Angola fits into broader patterns of climate-vegetation relationships. Analyses of climate and tree cover of savanna ecosystems across the globe have revealed that savannas and forests may be alternative stable states in the African tropics (Bond, 2019; Staver et al., 2011) (see Sect. 10.7). Rainfall deterministically results in arid savanna in low rainfall, and rain forest in high rainfall regions. At intermediate rainfall levels, such as between 1200 and 1800 mm in northern Angola, both savanna and forest occur in a mosaic. Tree cover does not increase continuously with rainfall, but is bimodal, with savannas having less than 40–50% and forests more than 75% projected canopy cover (Fig. 5.12). Intermediate levels of tree cover rarely occur.

Fig. 5.12

Frequency distributions (y-axis) of the percentage of tree canopy cover (x-axis) in areas of intermediate rainfall (1000–2500 mm MAR) and mild seasonality (dry season <7 months), with fire present (grey bars) and with fire absent (hatched black bars). The canopy cover (0–55%) of fire-tolerant savannas is distinct from fire-intolerant forests (50–90% canopy cover). From Staver et al. (2011) Science, 334(6053): 230–2

Above 40–50% canopy cover, shading by tree canopies prevents sufficient photosynthetically active radiation from reaching the ground for C₄ grasses to establish. As a consequence, insufficient grass biomass accumulates to support the hot fires needed to prevent tree saplings from growing to maturity. The canopy cover increases and the feedback enhances canopy density until a new stable state (forest) may be established.

Patterns Across Angola

The strong gradients of mean annual rainfall across Angola are reflected in the distribution of biomes. Nowhere in Angola are radiation and temperature limiting to tree growth. Rainfall seasonality, however, is a key feature of tropical climates, and in Angola seasonality is strongest in the south and weakest in the north. For most of the country, sufficient rainfall is received for tree growth. Sankaran et al. (2005) used data from over 800 sites across Africa (Fig. 5.13) to demonstrate that the cover of woody plants of savannas is determined by resources (water and nutrients) and disturbance regimes (fire, herbivory). Two patterns were demonstrated:

Fig. 5.13

Change in woody cover of African savannas as a function of mean annual rainfall. Below 650 mm MAP, tree cover is limited by rainfall. Above 650 mm MAP, disturbance factors (primarily fire) limit tree cover. Fire frequency indicated by open circles (fire return interval less than 3 yrs); closed circles (return intervals more than 3 yrs). Redrawn from Sankaran et al. (2005) Nature, 438: 846–9

• At sites with less than 650 mm MAP, maximum tree growth and cover is related linearly to rainfall. Vegetation structure (woody cover) is limited by climate: higher rainfall supports denser maximum tree cover.

• At sites receiving more than 650 mm MAP, structure is determined not by rainfall but by disturbance (fire) that maintains a balance between trees and grasses in a system that is buffered against transition to a closed forest canopy.

These findings, together with similar data-rich studies by Lehmann et al. (2011) of savannas of the southern continents, confirm earlier descriptions of drivers of mesic and arid savannas (Huntley, 1982).

In Angola, in areas receiving more than 1200 mm annual rainfall, closed canopy forests can establish if fire is excluded. However, even in the highest rainfall region, the Maiombe of Cabinda, two to three months without rainfall prevents the total dominance of evergreen trees except along water courses. Semi-deciduous tree species are therefore important components of Angola’s moist closed-canopy forests.

Vegetationless dunes, desert grasslands and low shrublands occupy the extremely arid southwest, where trees are rare or absent in areas receiving less than 150 mm MAP, other than along the ephemeral riverbeds of the Curoca, Bero and Bentiaba, where deep sands retain moisture throughout the year and tall acacia woodlands line the river course. For the remainder of the country, where rainfall ranges from 150 to 1800 mm per annum, three patterns emerge.

• Below ca. 650 mm MAP, arid savannas dominate.

• Above 650 mm MAP, mesic savannas dominate, gradually transitioning into moist savanna/forest mosaics above 1200 mm MAP.

• Above 1400 mm MAP, in the absence of regular fire, moist closed forest will dominate.

Patterns Across Steep Landscape Gradients

One of the world’s most striking demonstrations of the influence of macroclimatic factors on vegetation is to be found in southwest Angola. A transect from Humpata in Huíla to Moçâmedes in Namibe, 140 km in distance, provides a classic example of vegetation zonation according to climate (Cardoso et al., 2006; Van Jaarsveld, 2010). The vegetation is best illustrated by a series of photos, from above the Angolan Escarpment at Leba Pass, to the Atlantic Coast (Figs. 5.14, 5.15, 5.16, 5.17, 5.18, 5.19, 5.20).

Fig. 5.14

The zig-zag Leba Pass leading down from the summit of the Chela Escarpment clothed with a dense thicket of Acacia, Albizia, Commiphora, Spirostachys and Terminalia. Photo Ernst van Jaarsveld

Fig. 5.15

Photo Ernst van Jaarsveld

Montane Grassland on the Humpata Plateau at 2300 m

Fig. 5.16

Photo Ernst van Jaarsveld

Burkea, Brachystegia Mesic Savanna mixed with Afromontane elements in sheltered valley

Fig. 5.17

Photo Ernst van Jaarsveld

Succulent caudiciform Cyphostemma uter.

Fig. 5.18

‘Dwarf’ baobab Adansonia digitata on calcrete pavement of coastal plains, 2 km from the Atlantic Ocean

Fig. 5.19

Photo Ernst van Jaarsveld

Pachycaul trees of Pachypodium lealii

Fig. 5.20

Photo Ernst van Jaarsveld

Arid savanna on the margin of the Namib Desert, Welwitschia in foreground.

• The transect begins at 2300 m, at the Zootechnical Research Institute, Humpata, where MAT is 14.6 °C and MAP is 805 mm. Here the vegetation is an open grassland with fragments of Afromontane forest, with species such as Podocarpus milanjianus, Buxus macowanii, Erythroxylon emarginatum and Maytenus acuminata in ravines protected from frequent fires. Adjoining the grasslands are savannas and woodlands of Brachystegia spiciformis, Julbernardia paniculata, Burkea africana and Ochna pulchra—a typical mesic/dystrophic savanna.

• At the crest of the Leba Pass, at 1700 m, drier mixed woodlands and thickets, with Albizia antunesii, Brachylaena huillensis, Combretum apiculatum, Pteleopsis myrtifolia and Terminalia sericea, are dominant.

• On descending the Escarpment, at 1200 m, one passes into drier woodlands and thickets of Combretum zeyheri, Tarchonanthus camphoratus and Ptaeroxylon obliquum.

• At 850 m Adansonia digitata and Spirostachys africanus appear.

• At the base of the Escarpment, at 700 m, the MAT increases to 23.8 °C and MAP decreases to 466 mm. The vegetation changes to a Colophospermum mopane and Terminalia prunioides arid savanna.

• At Caraculo one is already at the margin of the Namib Desert with low Acacia tortilis. Here, 50 km from the coast and at 440 m, the MAT is 22.9 °C and the MAP is 123 mm.

• Within 30 km of the Atlantic Ocean, the MAP has dropped to 50 mm. The vegetation transitions into a dwarf shrubland, with ephemeral grasses, tufts of lichen and short succulent (caduciform) tree trunks of Adenium boehmianum, Sterculia africana, Catophractes alexandri, Pachypodium lealii, Cyphostemma currorii, C. uter and even dwarf Adansonia digitata trees.

• At the coast, 15 m, Moçâmedes has a MAT of 20.0 °C, and a MAP of 37 mm. This is the habitat of Welwitschia mirabilis, a true desert ‘miracle plant’ (Box 16.1).

The transect provides the ornithologist with opportunities to see many Angolan bird endemics, from rocky outcrops and scrub at the summit of the Escarpment, where Angola Cave Chat, Swierstra’s Francolin, Finsch’s Francolin, Angola White-headed Barbet, Angola Slaty Flycatcher, Ludwig’s Double-collared Sunbird, Oustalet’s Sunbird, Angola Swee Waxbill and Ansorge’s Firefinch may appear. As one descends to the base of the Escarpment, Bocage’s Akalat, Benguela Long-tailed Starling and Cinderella Waxbill may be found. The altitudinal and climate gradient, reflected by habitat, determines species distribution.

8 El Niño, La Niña, Droughts and Floods

Weather patterns are notoriously changeable. Of global importance are the major climatic variations known as El Niño or El Niño-Southern Oscillation (ENSO), associated with the periodic abnormal warming of the surface waters of the east-central equatorial Pacific and stronger than usual westerly winds, causing a decrease in upwelling of cool water off Peru. This in turn results in the collapse of the normally rich fisheries of the Humboldt Current, and in flooding in Peru and Ecuador, as well as in droughts in Indonesia and Australia. A reverse, cool ENSO phase, known as the La Niña has opposite effects.

ENSO events occur on average once every four years. In southern Africa, ENSO brings higher temperatures and less rainfall to the region, with negative impacts on agricultural crops and hence on socio-economic conditions. During the 2015/2016 ENSO event, Angola suffered the worst drought in 60 years across the entire country, with the coastal region receiving less than 30% of its normal rainfall (Fig. 5.21).

Fig. 5.21

The impact of El Niño on rainfall in southern Africa in the period 1 October 2015 to 11 February 2016. The rainfall for the Angolan coastal region was less than 30% of the long-term average. From Rembold et al. (2016)

Droughts and floods are not only the result of the El Niño-Southern Oscillation. Change is a constant feature of southern African weather and climate. Over long time scales, exceptional droughts and floods are experienced over southern Africa. Examples include the droughts of 1966, 1971 and more recently, the prolonged drought of 2015–17. Floods occur at frequent intervals. In the Cuvelai Basin, over a period of 64 years, 11 major floods were recorded, while in 21 of the 64 years, there was no surface flow due to abnomally low rainfall (Mendelsohn & Mendelsohn, 2019). The Cunene ran dry at its mouth in 2010, but reached its highest recorded flow in 2011. The Bero and Giraul river floods of 2001 and 2011, which caused severe damage to property and the loss of many lives in Namibe, were triggered by the so-called Benguela Niño, when the South Atlantic is warmer than normal and the ITCZ progresses further south, bringing higher than normal rains to the Escarpment and causing flash floods to the lowlands.

Droughts are not only associated with El Niño events. Over southern Africa, drought occurrence is to a large degree associated with multiyear climate variability and follows a cycle of about 18 years in southern Africa (Tyson & Preston-Whyte, 2000). The perceived severity of a drought is also influenced by public opinion. For much of Africa, rangelands tend to be placed under excessive pressure by pastoralists, and even large national parks are often subject to very high densities of herbivores. The conflation of grazing pressure impacts and perceived drought events is common in media reports. Arid regions are naturally subject to high coefficients of variation in inter-annual rainfall. It is often management practices rather than weather that is the driving force. Droughts are less dramatic in their impacts in the mesic savannas of Angola than they are in the arid savannas. Arid savannas reflect event-driven (stochastic) rather than deterministic ecosystem dynamics (Scholes & Walker, 1993).

In Tsavo National Park, Kenya, and Kruger National Park, South Africa, which have ecosystems very similar to Quiçama National Park and parts of southwest Angola, droughts combined with high herbivore densities have resulted in the massive mortality of both animals and woody plants. In Iona, during 1973, competition between cattle and gemsbok resulted in high mortalities of both. In 2009, in sites where boreholes had been provided to supplement water in areas far beyond the normal grazing areas of domestic stock, starving cattle resorted to eating the tough leaves of Welwitschia mirabilis, destroying the local populations of these unique desert plants, Fig. 5.22.

Fig. 5.22

Cattle resort to eating the tough leaves of Welwitschia mirabilis during a drought in Iona National Park, 2009. Photo Bill Branch

In order to distinguish between a perceived drought and an actual extreme climatic event, meteorologists use the Extreme Climate Index (ECI). Two statistical components are used to calculate ECI—the Standardised Precipitation Index (SPI) and the Standardised Heatwave Index (SHI). Using the ECI, Malherbe et al. (2020) identified four major drought events in Kruger National Park in the period 1980–2018 (Fig. 5.23). These coincided with El Niño events, and had severe impacts on both vegetation and fauna, influencing forage availability for both grazers and browsers, fuel available for fires, and increased mortalities in buffalo, hippo and other large mammal species. Droughts are sometimes broken by high rainfall episodes, with rapid recovery of annual grasses. The spasm of new plant growth triggers post-drought population eruptions (population explosions) such as recorded in rodent and certain bird species such as the nomadic Red-billed Quelea Quelea quelea. Coincident with these droughts, Namibia and Angola also suffered severe impacts. However, although dry years were experienced at frequent intervals, only four major extreme droughts were identified within the 40 year period.

Fig. 5.23

Accumulated number of days within a moving 12-month window, when the Extreme Climate Index exceeded the 90th percentile value for three sites in Kruger National Park. From Malherbe et al. (2020) African Journal of Range & Forage Science, 37:1, 1–17

Box 5.1: Human-Environment Interactions: Climate Change and the Impacts of Increasing Atmospheric Carbon Dioxide

The most important and challenging ecological problem of the twenty-first century is that of accelerated global climate change. It is important to recognise that climate change is not a new phenomenon. Major changes in the concentrations of atmospheric oxygen, carbon dioxide and other gases, and the temperature at the Earth's surface, have occurred throughout the hundreds of millions of years since life first appeared on the planet. An example of the extent of climate changes over what is today Angola, is visible in the Cunene valley. During the Permian/Carboniferous period, 300 million years ago, Gondwana was located over the South Pole, and glaciers cut the deep valley of what is now the lower Cunene (Figs. 4.7 and 4.8). Glacial striations can be seen on the rock floor and cliff faces of the river.

Over many millions of years, slight changes in the Earth's tilt, or in its elliptical path around the Sun, result in cycles of climatic conditions that recur at approximately 22,000, 41,000, 100,000 and 413,000 year intervals (Milankovitch cycles). These cycles were prominent through the Pleistocene Ice Ages, which will be discussed in Chap. 12 in relation to the biogeography of Angola's rain forests. Global climate change in the twenty-first century, however, is unequivocally due to human activity, in particular the burning of fossil fuels, which release carbon dioxide (CO₂) into the atmosphere (Fig. 5.24). Successive reports by the Intergovernmental Panel on Climate Change (IPCC) confirm global trends, which document a human-induced warming of 1.5 °C since pre-industrial times. Projections suggest that warming will grow to 3 °C by 2100. Impacts will differ from one region to another, with the polar regions warming more than the tropics (IPCC, 2019). The speed of change is as alarming as its magnitude. Current rates are 100 times the speed of temperature changes that occurred between the cool and warm periods of the Pleistocene Ice Ages. This has implications for the ability of plants and animals to adapt to rapidly-changing climates and environments.

Fig. 5.24

Atmospheric carbon dioxide levels over the past 800,000 years, based on ice core data. From NOAA

Fig. 5.25

Projected spatial patterns of change in annual average temperature (°C) and annual average precipitation (expressed as a percentage change) for the period 2081–2100 relative to the period 1979–2014, as assessed from the ensemble average of 30 global climate models that contributed to the Coupled Model Intercomparison Project Phase Six under the low mitigation scenario SSP5-8.5. From Engelbrecht and Monteiro (2021). South African Journal of Science: 117

The driver of the current pace of climate change is popularly called the greenhouse effect. The Earth reflects some of the energy it receives from the Sun back into the lower atmosphere. Here some of the energy is absorbed by water vapour and carbon dioxide (termed greenhouse gases) and some of this energy is once more reflected back to Earth. The greenhouse gases provide a thin envelope around the planet which maintains the Earth’s temperature. As a result of this thin but effective shield, the Earth is a much cooler body than the Sun, with an average temperature of only 15 °C. Without the protective envelope of greenhouse gases, the Earth would lose energy and become an icy planet like Mars.

The term ‘greenhouse effect’ needs explanation. It comes from the similarity in the processes of temperature increase experienced within a nursery greenhouse—or a parked car—on a sunny day. In an analogy with the gases that form an envelope around the globe, incoming shortwave radiation passes through the glass windows of a greenhouse or a car, but cannot escape because outgoing longwave radiation cannot pass through glass. The air temperature within the greenhouse, car, or the Earth's lower atmosphere, rises rapidly. The ecological consequences of global warming, and of increasing atmospheric carbon dioxide and other greenhouse gas concentrations which are driving the warming, are of serious global concern.

A recent study (Carvalho et al., 2017) provides the first analysis and comparison of a set of four Regional Climate Models (RCMs) that include Angola. Scenarios of future temperature and precipitation anomaly trends, and the frequency and intensity of droughts, are presented for the twenty-first century. Consistent results were found for temperature projections, with an increase of up to 4.9 °C by 2100. The temperature increases are lowest for the northern coastal areas and highest for the southeast of Angola. In contrast to temperature rises, precipitation was projected to fall over the century, with an average of −2% across the country. The strongest change was projected for the southeast, with decreases of as much as −4%. The central coastal region is expected to have a slight increase in precipitation. More recent models for southern Africa (Engelbrecht and Monteiro (2021), Fig. 5.25, suggest even more extreme warming and drying than the projections described by Carvalho et al. (2017).

The IPCC and RCM projections focus on changes in temperature and precipitation. Of ecological interest is the influence that increases in atmospheric carbon dioxide concentrations will have on photosynthesis and soil water balance. CO₂ concentrations were at 285 parts per million (ppm) in 1850, rising to 300 ppm by 1900 and 410 ppm by 2018. They are projected to reach 700 ppm by 2100. As we will see in Chap. 14, African savannas are dominated by C₄ grasses, and have been so since the C₄ photosynthetic pathway evolved about 30 million years ago, in response to an increasingly arid and low CO₂ environment. Trees, which are C₃ species, will benefit from increasing CO₂ concentrations, and might out-compete C₄ grasses. Some researchers predict a major expansion of forests at the expense of grassy savannas due to the increase in woody C₃ species. Other models suggest that an increase in fires due to global warming will maintain savannas. However, other factors apart from climate also determine the balance between C₃ and C₄ plants and ecosystem composition, including rangeland management practices and agriculture. The future remains uncertain, emphasising the urgency for increased ecological research.