Sea levels are continuing to rise, driven by the retreat of glaciers, the melting of sea ice sheets and the thermal expansion of warming water (IPCC, 2019a). According to NASA, sea levels have risen 83.5 mm in the last 25 years (see Figure 2.4). The IPCC states that it is “virtually certain” that the global mean sea level is rising and accelerating (high confidence), and that glacier and ice sheet contributions are now “the dominant source” of this (very high confidence) (IPCC, 2019a). It warns that, due to the projected global mean sea level rise, historically rare extreme sea levels – the so-called ‘hundred year events’ – will become common by 2100 (high confidence), and communities living in low-lying cities and small islands will experience extreme sea levels every year by 2050. And there is very high confidence that coastal ecosystems and low-lying areas will experience more coastal flooding events due to sea level rise.

With more and more people and assets concentrated in coastal areas, the IPCC expects increasing exposure to coastal risks such as flooding, erosion, sea level rise and submergence (IPCC, 2014b). Regionally, the IPCC indicates that flood hazards are likely to increase in parts of Asia (in particular in south and Southeast Asia), in Africa (mostly in tropical areas), in Europe (in particular in far north eastern countries) and in the Americas (in particular in South America), while they are likely to decrease in other parts of the world (IPCC, 2014b).

The Blue Guide to Coastal Resilience does not directly address sea level rise since reducing sea level rise is related to stopping retreat of glaciers, the melting of sea ice sheets and the thermal expansion of warming water, which NbS are not equipped to address. Instead, the Blue Guide focuses on the impacts of sea level rise, such as coastal flooding and erosion. Closer to the shoreline, mangroves do their part to reduce hazard exposure by reducing wave heights and retaining sediments, thereby decreasing the impacts of flooding and erosion (Losada et al 2018: 5). Reefs also stabilize shorelines by promoting sediment deposition and buffering wave energy, thereby allowing other habitats, such as sea grass beds and marsh areas, to form, while simultaneously decreasing erosion of the shoreline. Seagrass support wave attenuation and sediment trapping, thus reducing the risk of soil and coastal erosion, particularly after heavy rainfall. Coastal dunes often represent the last line of defense from ocean erosion. Dunes act as a barrier against inundation and deflect wind and salt spray. The presence of a stable dune system provides a natural defense mechanism against wave attack and erosion. Marsh and swampland can preserve lives and property by abating the impacts of tropical storms, cyclones and large storm surges while reducing erosion as plants hold the soil and reduce the strength of waves or the speed of the water flow.