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.

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.

	Physical Pathway
Socio-economic Pathway	RCP 4.5 (Low Greenhouse Gas Emission)	RCP 8.5 (High Greenhouse Gas Emission)
SSP1 (Sustainable World)	SSP1-RCP4.5 Scenario (Sustainable World)
SSP2 (Middle of the Road)		SSP2-RCP8.5 Scenario (Middle of the Road)
SSP3 (Stalled Development)		SSP3-RCP8.5 Scenario (Stalled Development)
SSP4 (Inequality)	SSP4-RCP4.5 Scenario (Inequality)
SSP5 (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.

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.

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).