"Our [IPCC] assessment shows that failure to mitigate greenhouse gas emissions or to adapt to sea level rise will cause major disruptions to many low-lying coastal communities and jeopardize achievement of all UN sustainable development goals and other societal aspirations. Immediate and ambitious Greenhouse gas emissions reduction is necessary (Hoegh-Guldberg et al., 2018) to contain the rate and magnitude of sea level rise, and consequently adaptation prospects. Under unmitigated emissions, coastal societies, especially poorer, rural and small islands societies, will struggle to maintain their livelihoods and settlements during the 21st century. Without mitigation, sea levels will continue to rise for centuries, reaching 2.3–5.4m by 2300 and much more beyond, making adaptation extremely challenging, if not impossible, for all low-lying coasts, including more intensively developed urbanized coasts.

But even with ambitious mitigation, sea levels will continue to rise, reaching 0.6–1.1m by 2300. Hence, adaptation will continue to be imperative irrespective of the uncertainties about future greenhouse gas emissions and key physical processes such as those determining the Antarctic contribution to sea level rise. Our assessment also shows that all types of responses, from hard protection to ecosystem-based adaptation, advance and retreat, have important and synergistic roles to play in an integrated and sequenced response to sea level rise. The merits of a particular type of response, at a particular point in time, critically depends on the biophysical, cultural, economic, technical, institutional and political context."