The evolving landscape of sea-level rise science from 1990 to 2021

Global mean sea-level (GMSL) has risen by about 20 cm from 1901 to 2018 due to ocean thermal expansion, loss of land ice, and changes in land-water storage, and is projected to accelerate, potentially exceeding 2 m by 2100. Rising seas threaten nearly one billion inhabitants of low-lying coastal and estuarine zones by 2030 through more frequent/intense flooding, ecosystem degradation and loss (e.g., islands and coral reefs), shoreline erosion, and salinisation of surface and groundwater. These impacts jeopardize infrastructure, economies and tourism, can drive migration, increase risks to vulnerable populations, threaten heritage sites, and affect renewable energy plans. In response, adaptation requires integrated, cross-disciplinary efforts across natural and social sciences and engineering. The literature on SLR has grown exponentially, making it challenging to assess trends and synthesize knowledge across disciplines. This study aims to provide a systematic, timely evaluation of the SLR research field using bibliometric techniques to assess global trends, disciplinary breadth, thematic clusters, and evolution of research agendas from 1990–2021.

Recent bibliometric approaches have synthesized major research areas in climate and environmental sciences, including climate change, coastal flooding, sea-level rise, and adaptation/vulnerability. However, a comprehensive analysis encompassing the full breadth of SLR science has been lacking. Prior work has typically focused on subdomains (e.g., flooding, intrusion, vulnerability, adaptation) or specific time frames, leaving gaps in understanding cross-disciplinary interactions, temporal evolution of themes, and identification of understudied regions and topics. This study addresses this gap by applying large-scale bibliometric analysis to titles, abstracts, keywords, and references of SLR-related publications to map themes, sub-themes, and their evolution.

- Data source and scope: Web of Science database. Articles included if “sea-level rise” or “sea level rise” appeared in titles, abstracts, or keywords. Time period: 1990–2021. Document types included research articles, reviews, data papers, reports, and proceedings papers. Grey literature and Indigenous knowledge were excluded. - Term co-occurrence analysis: Extracted terms from titles and abstracts; visualized co-occurrence networks to identify major research themes (clusters). Nodes represent terms, with node size proportional to occurrence frequency; clusters formed by frequently co-occurring terms. - Document co-citation analysis: Identified sub-themes by clustering references co-cited by SLR articles. Used a K-means clustering algorithm with a 30-year look-back and 1-year time slices. Sub-themes with fewer than 50 cited references were excluded. Clusters were ranked by number of cited and influential references and labeled based on the content of citing articles. - Temporal and impact analyses: Mapped average publication year for terms to assess thematic evolution; calculated average citation counts associated with terms to gauge impact. Assessed activity of sub-themes over time (persistence, growth, emergence, decline) via temporal co-citation networks and citation burst detection (burst duration indicated by symbol size in visualizations). - Growth metrics: Computed annual growth rates between successive years and averaged over 1990–2021. Doubling time estimated from average annual growth rate. Projected article counts for 2022–2030 using a third-degree polynomial fit. - Geographic contribution: Assigned articles to countries by author affiliations; summarized contributions by nation and global regions. - Data availability: Underlying data and bibliographic information available at https://doi.org/10.5281/zenodo.8013630.

- Corpus size and growth: Identified 14,951 unique SLR-related articles (1990–2021). Annual output expanded from 41 (1990) to 1,475 (2021); cumulative counts rose from 1,369 (by 2000) to 4,195 (by 2010) and 13,476 (by 2020). Average growth rate corresponds to a doubling time of about 4.8 years (growth rate ~15.4% per year). Projections suggest the next IPCC cycle (2026–2030) may need to assess an additional ~1,000–18,500 SLR-related articles. - Geographic distribution: Article production by region: Europe 42.2%, North and Central America 29.6%, Asia 15.7%, Oceania 7.8%, Africa 2.4%, South America 2.3%. Top contributing countries (counts): USA 5,909; England 1,875; Australia 1,388; Germany 1,334; China 1,160; Netherlands 1,035; France 1,030; Canada 1,060; Italy 748; Spain 558. - Thematic clusters (term co-occurrence): Four major themes identified: 1) Geological dynamics and sea-level interactions: characterized by terms such as sediment, record, basin, formation, deposition. 2) Risks and adaptation: climate change, risk, assessment, SLR, vulnerability. 3) Physical components of sea-level change: contribution, level change, warming, uncertainty, projection. 4) Coastal ecosystems and habitats: marsh, saltmarsh, plant, soil, coastal wetland. Clusters III and IV exhibited the highest average citation rates. - Evolution of focus: Early research emphasized geological records and long-term controls (Cluster I). Recent work shifted toward physical projections (Cluster III), ecosystems and habitats (Cluster IV), and risks/adaptation (Cluster II), reflecting a transition from problem identification to solution-oriented topics. - Sub-themes (co-citation; ranked 1–16 by influence): (1) Coastal wetlands and estuaries; (2) Quaternary sea-level; (3) Global sea-level reconstruction and projection; (4) High-end sea-level rise; (5) Extreme sea-levels and flood hazards; (6) Sedimentation and stratigraphy; (7) Vulnerability and adaptation; (8) Pre-Quaternary sea-level; (9) Glacial isostatic adjustments and eustatic changes; (10) Coastal erosion and shorelines; (11) Earthquakes and abrupt changes; (12) Contemporary saltmarsh–mangrove dynamics; (13) Geophysical variables dynamics; (14) Mega deltas; (15) Coastal adaptation and resilience; (16) Coral reefs and atoll islands. - Temporal dynamics of sub-themes: Persistent/consistently active: coastal wetlands and estuaries; Quaternary sea-level; coastal erosion and shoreline change. Declining: pre-Quaternary sea-level; glacial isostatic adjustments and estuarine sea-level changes; earthquakes and abrupt changes; geological wetlands dynamics. Fluctuating: sedimentation and stratigraphy. Growing/emerging: global sea-level reconstruction and projection; high-end sea-level and ice sheet dynamics; extreme sea-levels and flood hazards; vulnerability and adaptation; contemporary saltmarsh–mangrove dynamics; mega deltas; coastal adaptation and resilience; coral reefs and atoll islands. - Keyword evolution: Shift from geology-focused and localized topics (e.g., sequence stratigraphy, subsidence, Nile delta, New Zealand) toward multidisciplinary topics including environmental sciences, oceanography, water resources, biomorphology, economics, political science, and cross-border studies, with increased emphasis on adaptation, vulnerability, resilience, coastal management, ecosystem services, storm surge, and remote sensing.

Findings show SLR science has evolved from a predominantly geological and physical science discipline toward a broader, solutions-oriented, multidisciplinary field focused on risk, adaptation, and ecosystem management. The identification of four major themes and 16 sub-themes, along with their temporal trajectories, clarifies how knowledge production has shifted in response to observed and anticipated impacts, data/modeling advances, and policy needs. - Research agendas: Rapid loss of coastal and estuarine ecosystems and associated declines in critical ecosystem services (e.g., flood storage, nursery habitats, carbon sequestration) have elevated research on conservation, restoration, and nature-based solutions. Despite growth, geographic coverage remains limited, with concentrations in the United States and parts of tropical Asia, and emphasis on certain habitats (saltmarshes, mangroves, reefs) over others (e.g., seagrass meadows, dunes, barrier islands). - Equity: There is a growing imbalance between regions most affected by SLR and those contributing to research. Many of the world’s rural poor in low-elevation coastal zones reside in developing countries whose contributions to the SLR literature are <1%. Small Island Developing States and other vulnerable locations are underrepresented, underscoring the need to engage local experts and build context-specific knowledge for effective adaptation. - Assessing the growing literature: The rapid expansion and disciplinary breadth of SLR science challenge assessment efforts (e.g., IPCC). The study’s bibliometric mapping supports more systematic synthesis through multi-modal comparisons, meta-analysis, and systematic reviews to identify strengths, limitations, and knowledge gaps across sub-themes. - Policy relevance: Increasing attention to adaptation within the literature reflects practical needs for a mix of grey and green measures, as well as process-oriented adaptation (evaluation, implementation, monitoring, co-benefits). The evolution from problem detection to solution development indicates that SLR science is increasingly informing policy and practice, though disciplinary silos and regional imbalances hinder fully effective adaptation.

This study provides a comprehensive bibliometric assessment of sea-level rise science from 1990–2021, mapping four major themes and 16 sub-themes, their relative importance, and temporal evolution. The literature has expanded rapidly, shifted toward solution-focused research, and diversified across disciplines, with high-impact work concentrated in physical projections and coastal ecosystems. The results offer actionable insights for prioritizing research agendas, designing integrative assessments (e.g., IPCC), and guiding adaptation strategies. Future directions include: expanding geographic coverage and equity by engaging researchers in highly exposed but underrepresented regions; fostering interdisciplinary integration to bridge disciplinary silos; advancing systematic reviews and meta-analyses across sub-themes; and developing policy-relevant evidence on effective mixes of grey and green adaptation, managed retreat strategies, and ecosystem restoration to address compounding hazards and uncertainties associated with high-end sea-level rise.

- Data source limitations: Analysis restricted to Web of Science-indexed literature; grey literature and Indigenous knowledge were excluded. - Search constraints: Inclusion required the terms “sea-level rise” or “sea level rise” in titles, abstracts, or keywords, potentially omitting relevant studies that use alternate terminology. - Methodological choices: Co-citation clustering excluded sub-themes with fewer than 50 cited references; clustering, time-slicing, and labeling decisions may influence sub-theme delineation and interpretation. - Field specificity: Reported temporal trends are specific to SLR science; different trajectories may occur in other fields. - Affiliation/geography assignment: Country contributions are based on author affiliations, which may not perfectly capture the locus of study or fieldwork.