Sea Level Rise and Climate Change - Impacts on African Coastal Systems and Cities

Abstract

Coastal zones are more densely populated and of great ecological and economic significance. The potential implications of climate change are greatest in areas with dense populations and associated economic activities, such as low-lying coastal cities. Few, if any, African coastal cities are prepared to deal with the impacts of climate change, particularly sea level rise and storm events. African coastal cities are characterized by rapid and unplanned growth, high population concentrations, and overburdened infrastructure, all of which will influence the extent of any possible consequences caused by changes in extreme water levels in the twenty-first century. Most of the African coastal cities currently face significant threats from direct and indirect impacts of climate variability and change. Despite these threats, few coastal cities have been assessed for potential coastal impacts. Sea level rise threatens infrastructure, coastal agriculture, key ecosystems and fisheries at risk. Concern about all these effects of the changing climate and rising sea levels is apparent. This chapter, therefore, provides a broader understanding of the potential hazards and anticipated impacts on most African coastal systems and cities based on physical exposure and socio-economic vulnerability to climate extremes and sea level rise.

1. Introduction

The coastal zone encompasses some of the planet’s largest and most heavily populated megacities, along with some of the fastest-growing urban regions. It is important to note that the population density in many coastal regions is quite low. The majority of individuals reside in smaller towns and settlements that are situated along subsiding coastlines and in river deltas [1, 2, 3, 4, 5]. It is essential to acknowledge the undeniable fact that the impact of climate change on the coastline is far-reaching and affects numerous people, economic activity, and critical infrastructure. One must not forget that around 23–37% of the world’s population resides within 100 km of the shore, implying that they are susceptible to these impacts, whether directly or indirectly [1, 6, 7, 8]. The coastal zone features valuable ecosystems and larger population concentrations than inland areas [9, 10, 11]. Moreover, it contributes significantly to national wealth [12, 13, 14, 15].

Sea-level rise (SLR) has far-reaching consequences for low-lying coastal communities and beyond. The direct impacts of SLR can be quite severe, including flooding of low-lying areas, erosion of coastlines, destruction of coastal wetlands, infiltration of saltwater into freshwater sources, higher groundwater levels, and elevated water levels that can cause coastal flooding [16, 17, 18]. Human-induced pressures on the coastal zone (such as population growth, subsurface fluid abstraction, and changes in the hydrological regime, including damming) will exacerbate the effects of sea-level rise [9, 19].

The West, Central, East, and Mediterranean coastal regions make up the majority of Africa’s coastline zone, which is also quite low-lying (Figure 1). Several cities like Dakar, Abidjan, Accra, Lomé, Cotonou, Lagos, Douala, Dar es Salaam, Maputo, Durban, Cape Town, Alexandria, Tripoli, and Tunis, are located within these coastal zones. These coastal cities are characterized by a thriving population and industry, an extensive range of coast-based tourist destinations, and a dense network of transport and communication links [23].

Figure 1.

Map of Africa showing the estimated population density of five regions of Africa in 2019 (source: [20]). Africa’s population is 1.312 billion, about 17% of the world’s population, as estimated in 2020. It’s projected to reach 40% by 2100. Africa is the world’s fastest-urbanizing continent, expected to shift to a majority urban population in the 2030s and reach 60% urban population by 2050 [21, 22].

Widespread erosion and flooding are currently ravaging large areas along the African coast, resulting in significant ecological issues as well as a high level of human misery. According to Ref. [23], a 1-m sea level rise could exacerbate the already existing ecological issues. This would lead to a rise in coastal erosion rates, prolonged flooding, wetland depletion, heightened salinization of groundwater and soil, and an influx of different pollutants. In many places, the phenomenon of subsidence and coastal erosion may also exacerbate the effects of the SLR [24]. Other socioeconomic effects include the destruction of human settlements, the displacement of port and navigational infrastructure, and the disruption of the coastal fishing and tourism-based industries. The already struggling African economy would be subjected to intolerable pressure from these negative consequences [25].

2. The African coast: Current status

The Atlantic Ocean, the Indian Ocean, the Mediterranean, and the Red Seas all encircle Africa’s coastline in its western, eastern, and northern halves, respectively. Thus, the African coastal zone can be divided into the coastal zones of West and Central Africa, East Africa, and the Mediterranean [23]. The coastal region of West and Central Africa, which runs from Mauritania to Namibia, makes up 29.5% of the entire African continent [23, 26, 27]. This coastal zone is divided into four main basins, which are surrounded on the ocean side by low-lying, occasionally muddy, sandy coasts. Beach altitudes typically range from 2 to 3 m above sea level [23].

The state capitals of Dakar, Banjul, Conakry, Abidjan, Lomé, Accra, Lagos, and Brazzaville are all located in the West and Central African region, several of which are port cities. Over the past five decades, the industrial growth in these cities has accelerated. Within these significant coastal cities is where the majority of the region’s population is concentrated [23, 27].

The Eastern African coastal zone stretches 4600 km from southern Somalia to the Natal shores of South Africa. Important cities and ports such as Mombasa, Dar es Salaam, Maputo, Beira, Nacala, Quelimane and Durban are located in the East African region. Many sections are characterized by extensive low-lying flat coastal zones, river basins, coastal lagoons and dunes [23]. The northern and southern parts of the Eastern African coastal zone are slightly less vulnerable to sea level rise. It can be attributed to the protective function provided by a variety of coral reef systems along the coastline and both northern and southern parts experience fewer tropical cyclones compared to the central. The central stretch of the region is on the track of tropical cyclones of the Southwest Indian Ocean basin from November to April. The main cyclogenesis hotspot is between 10° S and 15° S, and more generally between 5° S and 20° S [23, 28, 29, 30, 31].

The coastal regions of Madagascar, Mauritius, Reunion, and Seychelles in the Southwest Indian Ocean, have a wide range of widths but are typically low-lying. Due to the lack of significant rivers, the seashore remains mostly undisturbed. The island states typically have volcanic origins and are distinguished by extremely condensed coastal lowlands. Some regions, like the Seychelles, have essentially no coastal plains. Large rivers in Madagascar are connected to huge coastal plains, which were formed when sediment from the plateau uplands was deposited [23, 31].

Because of its expanding industrial infrastructure and increasing growth of coastal activities including fishing, seaports for imports and exports, coastal tourism, and businesses, the coastline region of Eastern Africa is densely populous [23, 32]. About 13% of the East African population lives near the ocean [32]. Despite being small, the Mediterranean coastal region has significant economic value as well as many major cities, including Cairo, Alexandria, Tripoli, Benghazi, Tunis, and Algiers [23, 26].

Recent sea-level variations and trends along the coast of Africa are primarily estimated with tide gauge records. However, the significant spatial and temporal gaps in the African tide gauge records limit the ability to monitor and understand sea-level observations as well as the driving mechanisms that underspin sea-level variations in the coastal zone [33, 34, 35, 36].

Satellite altimetry can help overcome this issue by significantly increasing the spatial and temporal coverage of the sea-level observing system along the African coast (Figure 2). Indeed, recent advances in the field of coastal altimetry (e.g. [24, 38, 39, 40]) have highlighted the potential of satellite altimetry within up to a few kilometers from the coast (e.g. [39, 41, 42], a region where satellite altimetry observations have historically been deemed as unreliable.

Figure 2.

Sea-level trends of 12 African coastal regions from Jan 1993 to Aug 2021. SLR rates in each region and globally. Adapted from the [37].

The contribution of satellite altimetry is however limited. First, a dedicated validation of coastal altimetry datasets is needed before their use, but this is hindered by the lack of tide gauge records (e.g. [43, 44, 45, 46]). Furthermore, satellite altimetry observations date back at most to the early 1990s, when the first satellite altimeters were launched into space.

The combination of Geographical Information System (GIS) and Remote Sensing (RS) technology has proven to be a valuable resource in studying shoreline changes. This powerful tool can generate information, monitor changes, conduct analysis, and predict future shoreline alterations. Despite the availability of such facilities, a limited number of studies applying GIS and RS analyzing the shoreline changes in African coasts are found in the literature [47].

3. Projected sea level rise and impacts along African coasts

The impacts of sea level rise along the African coasts are projected to be high due to a considerable population increase in the low-lying coastal zone, more than any other region of the globe from 2000 to 2060 (Figure 3; [10]). According to [10] (Figure 3a), in 2000, the population of the low-lying coastal zone in Sub-Saharan Africa accounted for 45% of the population of African coastal nations. This number could potentially increase from 24 million to a staggering 66 million by the year 2030. This is due to an average coastal growth rate of up to 3.3% (2000–2030) and a population growth rate of up to 174 million by 2060. This demographic pressure is being driven by the economic significance of Africa’s coastal cities and settlements.

Figure 3.

Current and future population exposed to sea level rise in low elevation coastal zones in Africa. (a) Population at risk from sea level rise in low-elevation African coastal zones (LECZ). (b) African coastal countries in the top 25 global analysis with the highest populations in LECZ based on 100-year floodplains under growth scenario C (2023–2060). Adapted from [20].

By 2100, even if the population growth is not taken into account, the projected sea level rise rates would pose a threat to existing Africa’s coastal cities and settlements (Figure 4). This is under low (RCP2.6) and intermediate (RCP4.5) and high (RCP8.5) scenarios [48]. Within this timespan, many coastal lands are projected to be lost or SLR flood-prone.

Figure 4.

Sea level rise will affect major African coastal cities including (a) Dar es Salaam, Bagamoyo and stone town; (b) Lagos, Cotonou and Porto-Novo; (c) Cairo and Alexandria. Permanent flooding is expected by 2050 and 2100 (blueish shades) under low, intermediate, and high emissions scenarios. Adapted from [20].

3.1 Impact on African coastal cities

Future rapid coastal development is predicted to exacerbate already high vulnerability to SLR and coastal hazards, notably in East Africa (Figure 4; [32, 49]). In a 2°C warming scenario, sea levels are anticipated to increase at least 40 cm above those in 2000 by 2100, and possibly up to 1 m by the end of the century in a 4°C warming scenario [48]. Under a 4°C warming scenario, Egypt, Mozambique, and Nigeria will bear the brunt of SLR in terms of the number of people who are at risk of flooding annually. If no adaptation measures are taken, the consequences will be severe [32]. Recent estimates of the potential damages caused by SLR and coastal extreme events in 12 major African cities show that the total average damage to these cities in 2050 will be USD 65 billion for RCP4.5; USD 86.5 billion and 137.5 billion for RCP8.5 and high-end scenario, respectively (Table 1; [20, 50]). In the light of low-probability, high-damage occurrences, aggregate damage risks under RCP4.5 and RCP8.5 scenarios may exceed USD 187 billion and USD 206 billion, respectively, and USD 397 billion under the high-end scenario. In North Africa, the city Alexandria tops the list, with a staggering aggregate damage estimated at USD 36 billion and USD 50 billion under RCP4.5 and RCP8.5 scenarios, respectively, and a whopping USD 79.4 billion in a high-end scenario (Table 1).

(a) Regional relative sea level rise (cm)
City	year	RCP2.6			RCP4.5			RCP8.5			High-end
		Median		P95	Median		P95	Median		P95	Median	P95
Abidjan	2050	21		30	22		32	24		34	28	48
	2100	44		69	53		86	75		114	86	206
Alexandria	2050	18		26	18		28	21		30	25	43
	2100	36		58	46		73	67		102	7B	186
Algiers	2050	19		27	19		29	22		31	25	45
	2100	39		£2	47		76	66		98	7B	192
Capetown	2050	20		30	21		31	23		33	27	48
	2100	44		69	53		B7	75		117	86	199
Casablanca	2050	19		27	20		29	22		31	26	46
	2100	39		S3	47		78	65		99	77	19B
Dakar	2050	21		31	21		31	23		33	27	48
	2100	43		69	S3		86	73		111	85	209
Dares	2050	20		29	21		31	24		33	27	47
Salaam	2100	45		70	54		86	76		117	87	206
Durbar	2050	20		30	22		32	25		34	28	49
	2100	46		72	55		90	78		119	89	207
Lagos	2050	21		30	22		32	24		34	28	48
	2100	44		69	54		86	75		113	86	205
Lora	2050	21		30	22		32	24		34	2B	48
	2100	44		69	53		86	76		115	87	205
Luanda	2050	21		30	23		32	25		35	29	49
	2100	45		JO	55		88	78		119	90	205
Maputo	2050	21		31	22		32	24		34	28	49
	2100	45		71	55		89	78		120	89	209
(b) Expected average damages and risk measures (USD millons)
City	RCP2.6			RCP45			RCP8.5			High end scenario
	EAD	VaR(95%)	ES(95%)	EAD	VaR(95%)	ES(95%)	EAD	VaR(95%)	ES(95%)	EAD	VaR(9S%)	ES(95%)
Abidjan	14,290	33,910	41,690	16,730	38,230	46,390	20,910	42,140	49,550	32,670	77,750	96,570
Alexandria	32,840	74,100	92,470	36,220	83,700	104,270	49,990	99,500	117,580	79,360	180,090	221,390
Algiers	270	620	760	300	700	870	390	810	960	640	1540	1920
Capetown	110	310	400	130	360	450	170	410	490	300	800	1010
Casablanca	350	1150	1520	420	1340	1740	610	1570	1930	1230	3590	4630
Dakar	590	1310	1590	620	1390	1690	760	1530	1800	1180	7880	3610
Dares Salaam	880	2100	2600	1050	2440	2970	1360	2760	3250	2140	5120	6360
Durban	110	370	470	150	420	530	210	490	590	370	970	1230
Lagos	3680	6790	7950	4200	7660	8930	4920	8270	9420	6750	13,820	16,730
Lome	3230	10,480	13,460	4280	12,580	15,780	5980	14,430	17,380	10,720	28,580	36,010
Luanda	160	380	470	200	440	530	260	510	600	400	910	1130
Maputo	650	1990	2530	700	2080	2620	980	2410	2910	1790	4830	6110
Aggregate damage and risk	57,160	133,510	165,910	65,000	151,340	186,770	86,540	174,830	206,460	137,550	320,880	396,700

Table 1.

Projected sea level rise and damage risks for 12 African coastal cities until 2050. (a) Regional SLR between 2050 and 2100, with median and 95th percentile is shown. (b) Probabilistic damage estimates for 2050, such as expected average damages, losses at the 95th percentile, and the expected shortfall. Adapted from [20].

As indicated by the IPCC report [51], sea level rise and associated episodic floods will result in net migration of 750,000 people out of the East African coastal zone between 2020 and 2050. These trends, in conjunction with the creation of climate “hotspots” that trigger both in-migration and out-migration, will have a significant impact on climate-sensitive sectors as well as the viability of human settlements, including urban infrastructure and social support systems [20].

3.2 Impact on African coastal heritage sites

Sea-level rise and its related hazards will pose an increasing climatic risk to African heritage (Figures 5 and 6) in the coming decades. Ref. [55] indicates that 27 African countries will face the impact of SLR by 2100 (RCP8.5) on their natural heritage sites. Among these sites, eight countries need immediate proactive management actions due to their high levels of biodiversity, international conservation relevance, and vulnerability to SLR. In total, 15 sites fall under this category (Figure 5). Several studies (e.g., [52, 53, 55]) have pointed out the potential severity of risk, loss, and damage from climate change to African heritage, in addition to knowledge gaps regarding climate risk to African culture and nature, particularly biocultural heritage. Ref. [54] provide more insights into the impacts of rising seas under different scenarios and timespans for comparison purpose (Figure 6). Climate change has a cumulative effect on cultural heritage sites, making them more vulnerable to other threats such as conflict, terrorism, poverty, invasive species, competition for natural resources, and pollution [56]. These challenges may have an impact on tourism such as beach vacations, cultural tourism and historic city visits [57]. Climate change impacts have the potential to raise tourist safety issues, particularly in areas where increasing severity of extreme weather events or vulnerability to floods are predicted [56].

Figure 5.

The risk of SLR and erosion to Africa’s cultural and natural coastal heritage sites by 2100 (adapted from [20]): (a) world heritage sites that are expected to be flooded by SLR by 2100 under a high-emission scenario (RCP8.5) [52, 53]. Multiple sites in North Africa have already been identified as being at medium or high risk of erosion under both present and future SLR circumstances [53, 54]. (b) the 15 African natural sites (coastal protected areas) that are expected to be most vulnerable to the negative impacts of SLR are top priority for adaptation [55].

Figure 6.

Maps of African heritage sites impacted by coastal extreme events. Dots indicate location, and colors indicate the fraction of the site’s area exposed to hazards. (adapted from [54]).

3.3 Impact on African mangrove

Coastal wetlands include seagrass meadows, intertidal flats, tidal salt marshes, mangrove forests, and tidal freshwater wetlands [58]. Mangrove ecosystems play a crucial role in safeguarding coastal areas and their inhabitants from the perils of natural disasters such as floods, storms, and erosion [59, 60, 61, 62]. Additionally, they significantly contribute to enhancing the quality of coastal water and conserving biodiversity by providing essential habitats for coastal flora and fauna [59, 63, 64, 65]. Moreover, mangroves have always played a role of resource provisioning including timber and fuelwood [66] along generations and currently play a critical role in climate change mitigation by sequestering carbon [67, 68] and facilitating coastal accretion in response to SLR [69, 70].

Africa accounted for over 20% of the world’s mangrove extent in 2001 [59] and can be found in both the southern and northern regions of the continent. Angola and South Africa have the southernmost mangroves, while Mauritania and Egypt have the northernmost ones on the west and east coasts, respectively (Figure 7). The geographical features of African mangroves differ greatly but are mostly influenced by powerful water movements caused by the vast size of the Atlantic and Indian Oceans (Figure 7).

Figure 7.

Distribution of mangroves in Africa. Data derived from [71].

The exposure of African mangroves to SLR is little known because this relationship has been poorly explored. The lack of a baseline might be preventing such progress [72]. Studies suggest that the quantity of mangroves in West Africa has undergone significant fluctuations over time owing to sea level changes during glacial and interglacial periods [73]. Nonetheless, it is worth noting that only a handful of locations in East Africa have directly measured the current change in surface elevation [74].

Quantifying vertical crustal movements, mainly assessed through the glacial isostatic adjustment (GIA), in the past has been a challenging task due to the utilization of different models (e.g. [75, 76, 77]). Global Positioning System (GPS) observations can confirm the accuracy of GIA models by addressing elastic signals caused by mass change [77]. The utilization of satellites has enabled the precise measurement of the Earth’s land and water surface against an imaginary geoid. This facilitates the identification of horizontal and vertical variations in the Earth’s crust [78, 79]. Accurate location-specific rates of surface elevation change are of crucial importance, and the lack of them hampers assessments of sea level rise vulnerability all over African coasts [72].

4. Conclusions

Africa’s coastal cities are dealing with multiple crises at once, including rising sea levels, fast-pacing populations, land pressure, and a shortage of affordable housing. As people are forced to move deeper into the marshes and shorelines that surround cities, they are undermining the protection that these natural barriers and ecosystems offer the entire city, exposing these settlements to storm surges and flooding. On the other hand, the lack of drainage systems favors the emergence of wetlands in addition to groundwater rising, thus turning habitable areas into wetlands. Thus, addressing other sources of vulnerability at the same time as mitigating the effects of sea level rise would be necessary. However, the problem of sea level rise cannot be solved by a single panacea. There is, therefore, a need for multi-layered and mutually reinforcing policy measures. As the region prepares for the threat posed by sea level rise, policymakers and decision-makers at regional, national and local levels should work towards increasing the resiliency of Africa’s coastal cities by balancing ecological, economic, and political initiatives.

Acknowledgments

The first author would like to thank Fabio Mangini for the valuable discussion on the latest developments aimed at improving satellite altimeter observations. Authors acknowledge the financial support by the Nansen Scientific Society (NANSI) to conduct this research.

Conflict of interest

The authors declare no conflict of interest.