The IPCC Sixth Assessment Report (IPCC, 2021) projects that if greenhouse gas emissions continue to rise unabated (i.e., a SSP1-8.5 8.5 emission pathway), global mean sea level will likely rise 0.6–1.0 metres by 2100 (relative to 1995–2014) and, with less confidence, range from 1.7–6.8 m (perhaps more) by 2300 (Figure 1), with continued large changes beyond. The large increases in sea level by 2300 would be mostly attributable to significant inputs from the melting of the large ice sheets of Antarctica and Greenland. A substantial reduction in future greenhouse gas emissions would substantially reduce global sealevel rise. If emissions are reduced to meet the Paris Agreement goal of limiting global warming to “well below 2 °C” (i.e., an SSP1- 2.6 emissions pathway), global mean sea level would still likely rise 0.3–0.6 m by 2100 (relative to 1995–2014) and with less confidence range from 0.3–3.1 m by 2300. This is a lower rise; but since the future emissions pathway is unknown at this point, and the ice sheet response to a given temperature rise is also highly uncertain, future sea-level rise might be substantially higher (IPCC, 2021).
|Figure 1. Panels d) and e) from Figure SPM.8 in IPCC AR6 Working Group 1 Summary for Policy Makers (IPCC, 2021). Panel d) Global mean sea level change in meters relative to 1900. The historical changes are observed (from tide gauges before 1992 and altimeters afterwards), and the future changes are assessed consistently with observational constraints based on emulation of CMIP, ice sheet, and glacier models. Likely ranges are shown for SSP1-2.6 and SSP3-7.0. Only likely ranges are assessed for sea level changes due to difficulties in estimating the distribution of deeply uncertain processes. The dashed curve indicates the potential impact of these deeply uncertain processes. It shows the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact ice sheet processes that cannot be ruled out; because of low confidence in projections of these processes, this curve does not constitute part of a likely range. Changes relative to 1900 are calculated by adding 0.158 m (observed global mean sea level rise from 1900 to 1995–2014) to simulated and observed changes relative to 1995–2014. Projected sea-level rise 1900 to 2100 for a range of emission scenarios. Panel e): Global mean sea level change at 2300 in meters relative to 1900. Only SSP1- 2.6 and SSP5-8.5 are projected at 2300, as simulations that extend beyond 2100 for the other scenarios are too few for robust results. The 17th–83rd percentile ranges are shaded. The dashed arrow illustrates the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact ice sheet year processes that cannot be ruled out.
Sea-level rise threatens the world’s coastal areas through a range of biophysical impacts and changes (Oppenheimer et al., 2019) which include:
- permanent submergence of land by rising mean sea levels and high tides • more frequent and deeper coastal flooding
- enhanced coastal erosion
- degradation, change and loss of coastal ecosystems
- salinization of soils and of ground and surface water
- impeded drainage and waterlogging.
These biophysical impacts will in turn have socioeconomic impacts on coastal residents and their livelihoods, such as direct damage to buildings and infrastructure and disruption of economic activities. In 2020, an estimated 267 million people (or about 4% of the world’s population) were living within 2 m above sea level (Hooijer and Vernimmen, 2021). This number is growing due to both sealevel rise and demographic trends. It has long been recognized that small islands, deltas and coastal cities are especially threatened due to their high exposure and/or vulnerability.
|Figure 2. Scheme of the climate and non-climate driven processes that can influence global, regional (green colours), relative and extreme sea-level events (red colours) along coasts. Major ice processes are shown in purple and general terms in black. SLE stands for Sea Level Equivalent and reflects the increase in GMSL if the mentioned ice mass is melted completely and added to the ocean. [reproduction of Figure 4.4 in Oppenheimer et al., 2019] (GMSL – Global Mean Sea Level; GIA – Glacial Isostatic Adjustment.)
When considering the impacts of sea-level rise, it is important to note that most of these occur because of increases in extreme sea-level events produced by combinations of tides, storm surges and waves that rise with mean sea level. Further impacts are due to local relative sea-level change rather than global mean sea-level rise because of both regional and local climatic – oceanic circulation changes, local hydrology, gravitational changes linked to ice melting, etc – and non-climatic components – land uplift/subsidence – which also contribute to local sea levels (Figure 2). Hence, global changes need to be downscaled when evaluating future impacts and adaptation needs. In coastal areas where land is rising significantly today, for example, Alaska and northern Scandinavia, relative sea-level rise is reduced or may even be falling. In contrast, human-induced land subsidence in densely populated sedimentary coastal plains due to groundwater withdrawal and related processes is causing local substantial relative sea-level rise – for example, sometimes exceeding 1 cm/yr and up to 10 cm/yr in Jakarta. Due to the concentration of people in subsiding coastal areas, such subsidence is of global significance (Nicholls et al., 2021). The Asian coast is particularly prone to this process reflecting its geological heritage (often comprising deltas and alluvial plains) and associated high and growing urban populations (e.g., Jakarta, Bangkok, Shanghai). Adaptation to sea-level rise can be conducted using a range of contrasting methods, (Oppenheimer et al., 2019) including:
- Protection which reduces the likelihood of coastal impacts and can be implemented with (i) hard engineered structures such as dikes, seawalls, breakwaters and surge barriers, and (ii) sediment-based (or soft) protection such as beach and shore nourishment and dunes. It is also important to note that protection always leaves a residual risk—due to extreme events that exceed protection standards – and hence flood damage cannot necessarily be completely prevented.
- Advance creates new land by building seaward and upward or raises existing floodprone land. It can be achieved through land reclamation above sea levels and polderization, the gain of new low land enclosed by dikes. Advance is widely practiced around coastal cities where land is scarce and valuable and needs to take full account of sea-level rise in the future.
- Accommodation involves floodproofing and elevating buildings and infrastructure and is supported by early warning systems for floods. It does not entirely prevent coastal impacts but reduces the vulnerability of coastal residents, infrastructure, and associated activities.
- Planned or managed retreat reduces exposure to coastal impacts by moving people, infrastructure and human activities out of the exposed area – or by avoiding development of the coastal floodplain in the first place.
- Forced migration due to sea-level rise and/or extreme events may also occur.
Ecosystem-based or nature-based adaptation is of growing interest as these solutions recognizes the natural protection provided by coastal ecosystems, an advantage that was often ignored in the past. Coral and oyster reefs, mangroves, marshes and seagrass meadows act as protective buffers that attenuate extreme water levels (surges, waves), reduce rates of erosion and can raise elevation or create new land by trapping sediments and building up organic matter and detritus.
Effective use of these physical adaptation responses requires planning and institutional arrangements. Such plans might define standards for dike heights, building codes and/or setbacks for the flood plain, incentives for risk management, and disaster preparedness and early warning systems. Given the high uncertainties about future sea levels, adaptation pathways are being increasingly explored in coastal areas as an effective way of making adaptation decisions today (Haasnoot et al., 2019). However, these natural systems are poorly understood compared to engineered approaches so further research, development and learning is required to support wider and more confident application and promulgation for the future.
The challenge posed by climate-induced sea-level rise is massive and deeply uncertain, especially beyond 2100. To meet this challenge, the World Climate Research Programme (WCRP), which is jointly sponsored by WMO, the Intergovernmental Oceanographic Commission (IOC) of UNESCO and the International Science Council (ISC), leads an integrated sea level research agenda reaching from the global down to the regional and coastal scales.
Strong mitigation efforts are needed now to avoid the multiple metres of sea-level rise over the next centuries that threatens all the coastal regions of the world. But even with such efforts, sea levels will continue to slowly rise for decades and centuries to come. Thus, coastal adaptation is essential in any future, but it will be easier and more likely to be successful when combined with stringent mitigation. There is a need to start exploring long-term adaptive strategies now if they are not already initiated. Such efforts should be linked to wider coastal management and development objectives. Small islands, deltas and coastal cities are key targets for such action. In addition, the establishment of coastal early warning systems, especially multi-hazard ones, is crucial, considering the multiple sources of coastal flooding in addition to sea-level rise. The WMO Coastal Inundation Forecasting Initiative (CIFI), which establishes early warning systems to enable vulnerable communities to respond and act fast when hazards threaten, is one of many UN activities aimed at coastal adaptation.
Haasnoot, M. et al. 2019: Generic adaptation pathways for coastal archetypes under uncertain sea-level rise. Environ. Res. Comm. 1 071006.
Hooijer, A., and Vernimmen, R. 2021: Global LiDAR land elevation data reveal greatest sea-level rise vulnerability in the tropics. Nat. Comm. 12, 3592, https://doi.org/10.1038/s41467- 021-23810-9
IPCC (2019) Technical Summary [Pörtner, H.-O. et al., (eds.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.- O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)].
Nicholls, R.J., et al. 2021: A global analysis of subsidence, relative sea-level change and coastal flood exposure. Nat. Clim. Chang 11, 338–342, https://doi.org/10.1038/s41558-021- 00993-z
Oppenheimer, M. et al. 2019: Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.- O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)].
IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, et al. (eds.)]. Cambridge University Press. In Press