As the global climate changes, rising sea levels, combined with high tides, storms and flooding, put coastal and island communities increasingly at risk. Protection can be achieved by building dikes or seawalls and by maintaining natural features like mangroves or coral reefs. Communities can also adjust by reclaiming land from the sea and adapting buildings to cope with floods. However, all measures have their limits, and once these are reached people may ultimately have to retreat.
Choices made today influence how coastal ecosystems and communities can respond to sea level rise (SLR) in the future. Reducing greenhouse gas (GHG) emissions would not just reduce risks, but also open up more adaptation options. Global Mean Sea Level (GMSL) is rising and it will continue to do so for centuries. Sustainable development aspirations are at risk because many people, assets and vital resources are concentrated along low-lying coasts around the world.
Many coastal communities have started to consider the implications of SLR. Measures are being taken to address coastal hazards exacerbated by rising sea level, such as coastal flooding due to extreme events (e.g. storm surges, tropical cyclones), coastal erosion and salinisation). However, many coastal communities are still not sufficiently adapted to today’s ESLs. Scientific evidence about SLR is clear: GMSL rose by 1.5 mm yr-1 during the period 1901–1990, accelerating to 3.6 mm yr-1 during the period 2005–2015. It is likely to rise 0.61–1.10 m by 2100 if global GHG emissions are not mitigated (RCP8.5). However, a rise of two or more metres cannot be ruled out.
‘Hard protection’, like dikes and seawalls, can effectively reduce risk under two or more metres of SLR but it is inevitable that limits will be reached. Such protection produces benefits that exceed its costs in low-lying coastal areas that are densely populated, as is the case for many coastal cities and some small islands, but in general, poorer regions will not be able to afford hard protection. Maintaining healthy coastal ecosystems, like mangroves, seagrass beds or coral reefs, can provide ‘soft protection’ and other benefits. SLR can also be ‘accommodated’ by raising buildings on the shoreline, for example. Land can be reclaimed from the sea by building outwards and upwards. In coastal locations where the risk is very high and cannot be effectively reduced, ‘retreat’ from the shoreline is the only way to eliminate such risk.
Difficult trade-offs are therefore inevitable when making social choices about rising sea level. Institutionalising processes that lead to fair and just outcomes is challenging, but vitally important. Choices being made now about how to respond to SLR profoundly influence the trajectory of future exposure and vulnerability to SLR. If concerted emissions mitigation is delayed, risks will progressively increase as SLR accelerates. Prospects for global climate-resilience and sustainable development therefore depend in large part on coastal nations, cities and communities taking urgent and sustained locally-appropriate action to mitigate GHG emissions and adapt to SLR.
Governance Context for Sustainable Coastal Architecture
Responding to SLR presents society with profound governance challenges and difficult social choices, which are inherently political and value laden. The large uncertainties about post 2050 SLR, and the substantial impact expected, challenge established planning and decision making practices and introduce the need for coordination within and between governance levels and policy domains. SLR responses also raise equity concerns about marginalising those most vulnerable and could potentially spark or compound social conflict.
Choosing and implementing responses is further challenged through a lack of resources, vexing trade-offs between safety, conservation and economic development, multiple ways of framing the ‘sea level rise problem’, power relations, and various coastal stakeholders having conflicting interests in the future development of heavily used coastal zones. Despite the large uncertainties about post 2050 SLR, adaptation decisions can be made now, facilitated by using decision analysis methods specifically designed to address uncertainty.
Coastal Vulnerability Assessment
Sea level changes in the distant geologic past provide information on the size of the ice sheets in climate states different from today. Past intervals with temperatures comparable to or warmer than today are of particular interest, and since AR5 they have been increasingly used to test and calibrate process-based ice sheet models used in future projections. These intervals include the mid-Pliocene Warm Period (mPWP) around 3.3–3.0 Ma, when atmospheric CO2 concentrations were similar to today (~300–450 ppmv) and global mean temperature was 2ºC–4ºC warmer than pre-industrial, and the Last Interglacial (LIG) around 129–116 ka, when global mean temperature was 0.5ºC–1.0ºC warmer.
Non-climatic anthropogenic drivers, including recent and historical demographic and settlement trends and anthropogenic subsidence, have played an important role in increasing low-lying coastal communities’ exposure and vulnerability to SLR and extreme sea level (ESL) events. In coastal deltas, for example, these drivers have altered freshwater and sediment availability. Coastal ecosystems are already impacted by the combination of SLR, other climate-related ocean changes, and adverse effects from human activities on ocean and land. Coastal risk is dynamic and increased by widely observed changes in coastal infrastructure, community livelihoods, agriculture and habitability.
Adaptive Design Principles
A diversity of adaptation responses to coastal impacts and risks have been implemented around the world, but mostly as a reaction to current coastal risk or experienced disasters. Hard coastal protection measures (dikes, embankments, sea walls and surge barriers) are widespread, providing predictable levels of safety in northwest Europe, East Asia, and around many coastal cities and deltas. Ecosystem-based adaptation (EbA) is continuing to gain traction worldwide, providing multiple co-benefits, but there is still low agreement on its cost and long-term effectiveness. Advance, which refers to the creation of new land by building into the sea (e.g., land reclamation), has a long history in most areas where there are dense coastal populations. Accommodation measures, such as early warning systems (EWS) for ESL events, are widespread. Retreat is observed but largely restricted to small communities or carried out for the purpose of creating new wetland habitat.
Choosing and implementing responses to SLR presents society with profound governance challenges and difficult social choices, which are inherently political and value laden. SLR responses also raise equity concerns about marginalising those most vulnerable and could potentially spark or compound social conflict. Despite the large uncertainties about post 2050 SLR, adaptation decisions can be made now, facilitated by using decision analysis methods specifically designed to address uncertainty.
Sustainable Financing Mechanisms
Effective SLR responses depend on taking a long-term perspective when making short-term decisions, explicitly accounting for uncertainty of locality-specific risks beyond 2050, and building governance capabilities to tackle the complexity of SLR risk. Improved coordination of SLR responses across scales, sectors and policy domains can help to address SLR impacts and risk. Prioritising consideration of social vulnerability and equity underpins efforts to promote fair and just climate resilience and sustainable development.
Public awareness and understanding about SLR risks and responses can be improved by drawing on local, indigenous and scientific knowledge systems, together with social learning about locality-specific SLR risk and response potential. Achieving the United Nations Sustainable Development Goals (SDGs) and charting Climate Resilient Development Pathways depends in part on ambitious and sustained mitigation efforts to contain SLR coupled with effective adaptation actions to reduce SLR impacts and risk.