Global survey shows planners use widely varying sea-level rise projections for coastal adaptation

The appropriate incorporation of sea-level rise (SLR) projections into coastal planning and adaptation strategies is crucial, yet remains a significant challenge. The scenarios employed and their application will have profound consequences for social, ecological, and economic coastal systems. Hundreds of millions of people in coastal zones face substantial SLR risks, necessitating proactive adaptation. Coastal ecosystems, already stressed by ocean warming, acidification, and human interventions, are expected to experience habitat contraction, translocation, and biodiversity loss. The economic costs of coastal adaptation are substantial, estimated to reach $26–89 billion annually by 2040 for developing nations. The choice of SLR scenarios directly influences resource allocation, with potential for either overinvestment in unnecessary protection or underinvestment leading to increased inundation risks. Sea-level science is a mature field with sophisticated modeling capabilities that allow a better understanding of future sea levels and their uncertainties. The IPCC has published six major assessments on this topic, shifting from median SLR estimates to include high-end scenarios to account for uncertainty. Global emissions, ice sheet sensitivity, and tipping points contribute to projection uncertainty. Assimilating long-term SLR projections into planning requires a clear understanding of this uncertainty. Coastal communities have a long history of adaptation, but formal efforts to integrate SLR science into planning have intensified in recent decades. Successful coastal adaptation hinges on robust science-policy integration and well-designed climate services that ensure scientific information is usable. Little assessment exists on adaptation practices, particularly the SLR scenarios used by practitioners. This study aims to fill this gap by analyzing the first global survey on the use of SLR projections in coastal planning.

The literature review section cites numerous studies on sea-level rise, its impacts, and the challenges of incorporating projections into coastal planning. It highlights the IPCC reports as key sources of SLR projections, noting the evolution from median estimates to the inclusion of high-end scenarios to account for uncertainty. Studies on the economic costs of adaptation and the potential for maladaptation due to inaccurate projections are also referenced. Additionally, the literature discusses the importance of science-policy integration and the need for usable climate services to inform effective adaptation. Previous research on the application of SLR projections in coastal planning is acknowledged as limited, highlighting the novelty of this global survey.

This research employed a global survey distributed to coastal practitioners across every inhabited continent. The survey, available in nine languages (Arabic, Chinese, English, French, Hebrew, Japanese, Korean, Portuguese, and Spanish), used a combination of snowball sampling and targeted outreach to coastal managers working primarily at local and sub-national government levels. The questionnaire gathered information about the specific SLR projections used in planning documents, the underlying scientific basis for these projections, and how these projections are applied in decision-making processes. Two hundred and fifty-three coastal managers completed the survey. The analysis involved both quantitative and qualitative assessments of the data. Quantitative analysis focused on the geographical distribution of SLR projection usage, the types of data structures employed to represent SLR projections (single estimates, low/high estimates, low/intermediate/high estimates, and low/intermediate/high/high-end estimates), and the numerical values of the SLR projections provided. The geographical distribution of respondents is acknowledged to be uneven, potentially impacting the ability to correlate SLR projection usage with country-level variables like GDP, education, and the Human Development Index. The qualitative analysis interpreted the patterns identified in the quantitative analysis and placed them within a broader context.

The survey revealed uneven global adoption of SLR projections in coastal planning. While 72% of respondents reported formally incorporating SLR projections into their planning, significant regional variations were observed. Europe, Australia/Oceania, and North America showed higher adoption rates compared to Asia and South America. Within continents, regional differences were also evident; for example, North and West Europe had much higher adoption than Southern Europe. High adoption was linked to countries with established adaptation practices and strong national guidance, such as New Zealand and the United Kingdom. In contrast, regions like Western Africa and parts of Asia showed low adoption, possibly due to capacity constraints or competing priorities. Regarding the structure of SLR projections, a majority (53%) of respondents using formal data structures employed single estimates, rather than a range of projections to capture uncertainty. A low, intermediate, and high estimate structure was the second most common approach. The use of high-end scenarios was limited to certain regions, most notably the United States and a few locations in other countries. Significant variation existed in the actual numerical values of SLR projections used, indicating a lack of global standardization. Many values were either lower or significantly higher than the ranges suggested by the IPCC AR5 and SROCC reports. The variations may reflect regional guidance, consideration of local factors (subsidence), or different interpretations of the questions in the survey.

The findings indicate a potential overreliance on single SLR estimates and a lack of consistent approaches to incorporating SLR projections into coastal planning. While singular estimates are sometimes necessary for specific applications, best practices emphasize the use of multiple scenarios reflecting uncertainty. The study's limitations concerning sampling bias, which may affect generalizability, are acknowledged. While the survey had a geographically broad reach, the disproportionate representation of North American respondents potentially limits the generalizability of some findings to other global regions. The research highlights the need for improved communication and translation of scientific information to ensure effective uptake by coastal practitioners. Additional research is needed to explore the decision contexts that influence the selection of SLR projections, including consideration of risk tolerance and planning horizons. Investigating the various sources of SLR projections used by practitioners is also crucial, along with improving survey design for greater clarity. The study recommends future research to analyze the relationship between national-level guidance and local-level decision-making.

This global survey offers valuable insights into the varying use of SLR projections in coastal adaptation planning. The inconsistent and often simplistic approaches identified highlight a need for improved integration of sea-level science into decision-making processes. Future research should focus on addressing the identified limitations, particularly improving data collection methods to enhance geographical diversity, better understanding the decision-making contexts of coastal managers, and clarifying the relationship between various sources of SLR projections and their application. Promoting better collaboration between scientists and practitioners is key to bridging the gap between complex scientific projections and practical adaptation strategies.

The study's limitations primarily stem from the sampling methodology. The snowball sampling approach, while effective for reaching diverse geographical locations, introduces potential biases that could affect the generalizability of the findings. The uneven geographical distribution of respondents, particularly the overrepresentation of North American responses, limits the ability to draw definitive conclusions about specific regions. The interpretation of survey questions by respondents may have also introduced some variability in the reported SLR projections. Further, the survey did not directly link use cases to specific projection values.