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Exploring regional coastal populations at risk of sea level rise using future socioeconomic pathways under high and low emission scenarios

Rising sea levels

One prediction of where rising sea levels will end up at Cottesloe Beach, Perth Western Australia.

Photo Credit: "Julie G." CC-BY-ND 2.0, some rights reserved.
Lead author: Liana Talaue McManus
Other contributors:
Maria Estevanez

In a warming earth, sea level will most likely rise for over 95% of the global ocean with areas near glaciers and ice sheets very likely to experience sea level fall (because land rises with the reduced weight of melting ice) by 2100. Greenhouse gases in the last 200 years have committed us to millennia of sea level rise. The pace and magnitude at which seas will permanently flood and reconfigure present-day coastal ecosystems will have profound consequences on human societies. Those that live close to shore, and whose livelihoods depend on coastal and marine ecosystem services are at risk of sea level rise in a direct way. Populations living farther inland may not be directly threatened by permanent flooding events or by coastal storm surges, but the tight linkages between inland and coastal economies are bound to influence inland areas as well.

Based on total land area within the 10m- elevation × 50km zone, Northern America (480 049 km2), Southeastern Asia (397 495 km2), Eastern Europe (330 654 km2), and Southern America (260 002 km2) have the highest land exposure in decreasing order

We estimate the future risk of sea level rise (SLR) in 2100 within a framework of hazard, exposure and vulnerability using internally consistent future development scenarios (or pathways) for 139 coastal countries over the time period 2010-2100. We combine socioeconomic and greenhouse gas concentration pathways into five future reference development pathways or scenarios as shown on the table below: Shared Socioeconomic Pathway (SSP)1 (sustainable development) and SSP4 (inequality) with Representative [greenhouse gases and pollutants] Concentration Pathway (RCP) 4.5 to form low emission scenarios; and SSP2 (middle of the road), SSP3 (stalled development), and SSP5 (fossil fuel led development) with RCP8.5 to form high emission scenarios. The emission scenarios as elaborated by the Fifth Assessment Report of the IPCC provide regional estimates of sea level rise for the period 2010 to 2100, and which we use as hazard measures for estimating risk of sea level rise. The SSPs provide projected human development measures (life expectancy at birth, mean years at school and females with tertiary education, and income) that we combine into a Human Development Index (HDI) at country-scale annually over the same period. The difference between the theoretical highest level of development and the HDI projections, called the HDI Gap, is a measure of vulnerability. For exposure, we estimate the total land area and population living in the coastal zone up to 10 m in elevation and within 50 km from shore in 2100. Risk to sea level rise is the geometric mean of hazard, exposure and vulnerability of a coastal country.

Population increase to 2100 is most pronounced for SSP3. Major population centers in Southern Asia (22%), Southeastern Asia (15%), Western Africa (11%), Eastern Asia (8%) and Northern Africa (8%) are projected to account for 64% of the most susceptible coastal inhabitants by 2100
Physical Pathway
Socio-economic Pathway RCP 4.5
(Low Greenhouse Gas Emission)
RCP 8.5
(High Greenhouse Gas Emission)
(Sustainable World)
SSP1-RCP4.5 Scenario
(Sustainable World)
(Middle of the Road)
SSP2-RCP8.5 Scenario
(Middle of the Road)
(Stalled Development)
SSP3-RCP8.5 Scenario
(Stalled Development)
SSP4-RCP4.5 Scenario
(Fossil-Fuel Led Development)
SSP5-RCP8.5 Scenario
(Fossil-Fuel Led Development)
Our findings show that:
  1. While sea levels (hazard), total land area and people living in the 50 km coast (exposure) and vulnerability (HDI gap) contribute equally to risk, vulnerability influences risk very significantly with 80% correlation across development pathways. Human wellbeing determines risk and its mitigation.
  2. Countries that will experience highest sea levels (highest degree of hazard), on average across scenarios are: the USA, Canada, Russia, South Africa and Mozambique (tied), Japan, Australia and New Zealand (tied), Madagascar and Mauritius, in decreasing order.
  3. Averaged across the five future scenarios, the countries with the highest degree of exposure are: USA, Indonesia, China, Brazil, Viet Nam, Nigeria, Bangladesh, Egypt and Australia, in decreasing order.
  4. Using the HDI Gap as vulnerability metric, the most vulnerable countries, on average across the five scenarios are: Somalia, Mozambique, Sierra Leone, Liberia, Madagascar, Guinea-Bissau, Solomon Islands, Eritrea, Papua New Guinea and Benin, in decreasing order.
  5. The ten countries most threatened by SLR indicated by the SLR Risk Index, on average and across the five reference projection pathways (in decreasing order) are: Somalia, Mozambique, Madagascar, Angola, Liberia, Sierra Leone, Papua New Guinea, Senegal, Guinea-Bissau and Mauritania. Seven of these coastal states are identified among the most vulnerable.
  6. Despite an overall projected trend of increasing levels of human development among all countries and regions across all reference futures, there are scenarios, for example high emission SSP3 (stalled development) where gender-sensitive education indicators are projected to decrease, and which contribute to increasing vulnerability and risk by 2100.
  7. Societal choices reflected in the five reference development pathways underpin hazard, exposure and vulnerability metrics of risk to SLR. They also indicate strategic ways to mitigate risk. Reducing emissions and population growth are fundamental. Reducing vulnerability by nurturing human development appears the most prudent with long-term generational impacts. The reduction of risk through improved education, health and income are key to enhancing ability of households and societies to adapt to climate change, including rising sea levels. However, human development must be pursued within a sustainability framework where clear limits in greenhouse gas emissions are capped for both developing and developed economies.
In terms of human development in the Americas, Europe and Oceania, SSP3 and SSP1 give the lowest and highest human development indices (HDI) across regions, resp. Among the regions of Asia and Africa, SSP3 and SSP5 showed the lowest and highest human development indices, resp.

Sea level rise in 2100 for High (RCP8.5) Emission Scenarios

RCP 8.5 total ensemble mean sea surface height for 2100 (University of Hamburg Integrated Climate Data Center Live Access Server at

Despite the overall projected trend in increasing levels of human development among all countries and regions across all SSPs, there are scenarios where gender-sensitive education indicators are projected to decrease, and which are projected to contribute to increasing risk by 2100.

All raw data for population, land use, and SSP metrics are at country scale. However, because of the 0.5° grid size for land use data, and the 1° grid size for regional sea level rise, we report all values at regional scale so that both sea level rise values and land use values are represented at scales appropriate to the coarse native resolution of these input data. For consistency, the country-scale population and SSP wellbeing metrics are aggregated to regional scales.

Under a low emission scenario, SSP4 has significantly increased risk levels, being second highest in magnitude across all SSPs. Thus, even if emissions are low (RCP4.5), low human development levels in SSP4 are projected to impede the abilities of coastal societies to adapt to climate change, thus increasing overall risk.
The Sea Level Rise (SLR) Index by region, sorted from highest to lowest for each of the five development scenarios. The regions are grouped into five relative risk levels, with the top 4 classified as Very High Risk; the next 4 regions as High Risk; the next 4 as Moderate Risk; the next 3 as Low Risk; and the last 3 as Very Low Risk. Eastern, Western and Middle Africa consistently rank as the top three most vulnerable regions across all 5 scenarios. The scenario where countries are least at risk is RCP4.5 – SSP1 (Sustainability Pathway) because it features the lower emission scenario. The highest risk is projected to occur under RCP8.5 –SSP3 (Stalled Development Pathway).
Under a high emission scenario, SSP3 consistently showed the highest risk levels across all regions.