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

As sea-level rise (SLR) accelerates due to climate change, its multidisciplinary �eld of science has similarly expanded, from about 50 documents in 1990 to nearly 15,000 documents from 1990 to 2021. Here, big data, bibliometric techniques are adopted to systematically analyse this growing, large-scale literature. Four main research clusters (themes) emerge: (I) geological dimensions and biogeochemical cycles, (II) impacts, risks, and adaptation, (III) physical components of sea-level change, and (IV) coastal ecosystems and habitats, with 16 associated sub-themes. This analysis provides insights into the prioritisation of research agendas, the challenges and opportunities of future integrative, global scale assessment processes (e.g., next IPCC report), and how effectively this discipline is achieving societal impact. For example, the relative importance of sub-themes evolves consistently with a decline in pure science analysis towards solution-focused topics associated with SLR risks such as surface elevation change, �ooding, ice sheets dynamics, coastal erosion and squeeze, and engineered shorelines.

These approaches have been recently employed to synthesise important research areas such as the science of sciences (i.e., physical, technical, life, and health sciences) 38 , climate change 39,40,41 , coastal ooding 42 , seawater intrusion 43 , and adaptation and vulnerability plans 37,44,45,46 .However, a comprehensive analysis of the totality of SLR science in academic scholarship is absent.Such an analysis will promote a deeper understanding of the spatial and temporal trends in SLR research, its broad and speci c research streams, the extent of research activities employed by different disciplines, and research and management requirements.The results can help identify the evolution of scienti c themes over time and the recognition of understudied SLR impacts.This information could inform both research and policy, such as effective risk management efforts in coastal areas, developing long-term adaptation and mitigation opportunities, protecting and restoring valuable ecosystems, designing collective environmental strategies, ranking global fund-based climate change initiatives, and identifying knowledge and administration requirements for the way forward.
The aim of this study is to attempt such a valuable analysis of SLR science.The evolving landscape of SLR literature is assessed through a term-based search strategy and a bibliometric approach that scrutinises titles, abstracts, keywords, and references of nearly 15,000 SLR-related documents published between 1990 and 2021 (Methods).These documents (publications) included "sea-level rise" or "sea level rise" in either their titles, abstracts, or keywords (Methods).
This broad perspective provides illuminating insights into (i) the development of SLR science since 1990, (ii) the relative contribution of scholars by country, (iii) the major research clusters (themes) and subthemes, (iv) the relationship between these research themes and their geographic and temporal evolution, (v) the multidisciplinary and in uential references most widely used by SLR researchers, (vi) research, funding, policy, and equity agendas required for the way forward, and (vii) plausible projections of the number of SLR-related documents likely to be available for the next IPCC assessment (expected 2026-2030).The comprehensive and large-scale overview approach developed herein can serve as a framework for scholars and policymakers to utilise similar techniques when evaluating other integrative, versatile, and expanding research elds.
A keyword analysis was performed for six continents (based on authors' a liation data) to highlight the developmental trends and shifts in their SLR research direction across different timeframes and regions (Fig. 1b and Methods).During 1990-2021, the top 10 most frequently used keywords included "sea-level rise", "climate change", "sea-level", "Holocene", "adaptation", storm surge", "saltmarsh", "sea-level change", "coastal erosion", and "sequence stratigraphy".During 2010-2021, "sea-level rise" and "climate change" remained the top 2 keywords.However, the research focus shifted from geology-ecology-oriented and pure science research topics (e.g., "sequence stratigraphy", "subsidence") and localised studies (e.g., "Nile delta", "New Zealand") during 1990-2009, to more multidisciplinary and cross-border subjects such as environmental sciences, oceanography, water resources (management), bio-geomorphology, remote sensing, economics, and political science.The research topics also broadened to include the major SLR hazards and their risks (e.g., "coastal erosion", "coastal ooding"), the ability to prepare for and respond to induced threats (e.g., "adaptation", "vulnerability", "resilience", "coastal management"), importance and susceptibility of coastal estuaries and wetlands (e.g., "mangroves", "ecosystem services"), relevant processes (e.g., "storm surge"), and the application of new approaches in assessing the impacts of climate change (e.g., "remote sensing") (Fig. 1b).Therefore, SLR science has evolved from problem recognition to solution identi cation over the last 30 years.

Major Research Clusters (Themes) Of Sea-level Rise Science
Based on the frequency of occurrence of terms in the titles and abstracts of the SLR documents analysed, four major themes were evident: (I) geological dimensions and biogeochemical cycles, (II) impacts, risks, and adaptation, (III) physical components of sea-level change, and (IV) coastal ecosystems and habitats (Fig. 2a).Terms that mutually appear in multiple articles are visualised adjacently and create a theme (cluster) of research at an aggregate level.Nodes represent terms, with their size being proportional to the number of occurrences within each research theme.The top 5 most frequently used terms, namely "sediment", "record", "basin", "formation", and "deposition" characterise Cluster (I); "climate change", "risk", "assessment", SLR", and "vulnerability" characterise Cluster (II); "contribution", "level change", "warming", "uncertainty", and "projection" characterise Cluster (III); and "marsh", "saltmarsh", "plant", "soil", and "coastal wetland" characterise Cluster (IV).
In general, documents in Cluster (I) are focused on the mechanisms of sea-level change (e.g., eustatic, isostatic, and tectonic) 47,48 and the application of sea-level records preserved in geological or sedimentary data to identify how high and how fast GMSL may rise over time and space 49,50 .Cluster (II) documents consider the risks that low-lying areas and coastal ecosystems face due to SLR-related hazards (e.g., ooding, salinisation, coastal squeeze) and exposure/vulnerability (linked to socioeconomic factors) 5 , assessment methods 51 , and adaptative responses to manage these risks for people, assets, biodiversity, and ecosystem services 52,53 .These documents also present collective environmental risk management schemes via public engagement, effective national and international governance activities, and technological advancements 2 .
Cluster (III) documents consider the physical basis for sea-level change including gain or loss of ice sheets and glaciers 54,55 , thermal expansion and variations in global water storage 1 , signi cance of atmosphere-ocean models and data requirements (e.g., from tidal gauges and altimetry satellites) 56 , and studies about past, present, and future sea-level trajectories 57,58 .Cluster (IV) documents are focused on the value of coastal ecosystems (e.g., saltmarshes, mangroves, seagrasses), their exposure to SLR 59,60,61 , ability to respond to SLR (e.g., via sediment accretion and organic matter accumulation) 62,63 , and wide-ranging services such as bio-sequestration of blue carbon 64,65 , as well as efforts, initiatives, and options for preserving and/or restoring these ecosystems worldwide 66,67 .It is worth noting that Clusters (I) and (II) are positioned furthermost from each other, indicating signi cant thematic differences between their studies (Fig. 2a).This re ects a distinct geologic and humancentric perspective in these two themes, respectively, as they are often drawing on fundamentally distinct literature.
A map of the average publication year (i.e., the average publication year of the documents within the dataset that have mentioned that term in their titles or abstracts) indicates that earlier research focused on Cluster (I), whereas recent research predominantly focused on Clusters (II) and (III), and particularly on Cluster (IV) (Fig. 2b).This is consistent with the identi ed trend of moving from problem identi cation to solutions.It suggests a shift in the direction of SLR science from understanding the factors that govern sea-level change (e.g., variations in the shape of ocean basins and land-sea distribution) towards reliable SLR projections, far-reaching implications and impacts, conservation and restoration of ecosystems, prevention of saltwater intrusion, raising public awareness, and adopting climate mitigation worldwide.Overall, the documents within Clusters (III) and (IV) received the largest average number of citations (i.e., citation counts recorded by the Web of Science divided by documents that mentioned those terms) (Fig. 2c), highlighting their broad research signi cance.

Research Sub-themes Of Sea-level Rise Science
During 1990-2021, 16 sub-themes for SLR science were identi ed using document co-citation analysis (Fig. 3a and Methods).This analysis is based on the observation that articles of a research eld often cite references (from inside and outside of that domain) that are thematically similar and thereby represent a distinct research sub-theme 68 (Methods).The sub-themes of SLR science are ranked from 1 to 16 by number of cited and in uential references (sub-theme 1 is highest while sub-theme 16 is lowest) (Supplementary Table 2).These sub-themes comprise (1) coastal wetlands and estuaries, (2) last glacial maximum, (3) sea-level projections, (4) ice sheets and glaciers dynamics, (5) extreme sea-levels and ooding, (6) sediment dynamics, (7) vulnerability and adaptation, (8) oceanic anoxic events and carbonate platforms, (9) glacial isostatic adjustments and geoid subsidence, (10) coastal erosion and shoreline changes, (11) great earthquakes and stratigraphy, (12) surface elevation change, (13) intertidal benthic communities, ( 14) mega deltas, (15) engineered shorelines and coastal squeeze, and (16) coral reefs and atoll islands (Fig. 3a).Some central sub-themes (e.g., #3, #10) have a multidisciplinary role as they have been studied exclusively or jointly with several surrounding research streams.In contrast, peripheral sub-themes (e.g., #8, #11) are typically explored in isolation or in conjunction with a limited number of research streams.To illustrate this point, research topic "oceanic anoxic events and carbonate platforms" is only strongly interconnected to a single sub-theme of "sediment dynamics", whereas "coastal erosion and shoreline changes" interacts with several sub-themes comprising "sea-level projections", "extreme sea-levels and ooding", "coral reefs and atoll islands", "vulnerability and adaptation", "engineered shorelines and coastal squeeze", "coastal wetlands and estuaries", "surface elevation change", and "mega deltas" (Fig. 3a), highlighting this sub-theme's strong linkages across SLR science.
Understanding the absence or presence of citations to a set of cited references of a speci c sub-theme can provide insights into its level of activities over time 69 (Fig. 3b-e).The links between sub-themes demonstrate how and when a research topic has been inactive/active or had interactions with other topics, emphasising the evolving network and temporal trends in SLR science.As detailed in Fig. 3b-e, the 10-year snapshots of SLR research activities indicate that some sub-themes persistently grew during the entire period, whereas other sub-themes declined or emerged.For instance, in 1990, "coastal wetlands and estuaries" and "last glacial maximum" were the most active research disciplines followed by "oceanic anoxic events and carbonate platforms", "great earthquakes and stratigraphy", and "sediment dynamics" (Fig. 3b).In 2000, interdisciplinary research formed between the sub-themes "last glacial maximum", "glacial isostatic adjustments and geoid subsidence", "surface elevation change", "coastal wetlands and estuaries", "intertidal benthic communities", and "sediment dynamics" (Fig. 3c).In 2010, alternative research topics emerged including "ice sheets and glaciers dynamics", "coral reefs and atoll islands", and "vulnerability and adaptation", whereas direct research activities in "oceanic anoxic events and carbonate platforms", "great earthquakes and stratigraphy", and "intertidal benthic communities" declined substantially (Fig. 3d).In 2020, new research topics emerged as "extreme sea-levels and ooding", "mega deltas", and "engineered shorelines and coastal squeeze", with the least active research sub-themes remaining similar to 2010 (Fig. 3e).This highlights the emergence and recognition of solution focused topics associated with SLR risks.
The temporal trends of SLR science are further highlighted by analysing the maturity of the knowledge foundation (i.e., age and frequency of the fundamental references within each research sub-theme) (Methods and Fig. 3f) and the degree of persistence, growth, emergence, and disappearance of each subtheme (Fig. 3g).Cited references of all sub-themes are plotted against a timeline based on their publication year, and the 'concentration of references' indicates the age and duration of the knowledge basis for each sub-theme (Fig. 3f).Red circles represent references for which citation bursts (i.e., the temporal increase in the citation counts of a document) have been recorded, with circle size indicating burst duration (i.e., a period of time over which a document received citations) (Fig. 3f).Across all subthemes, knowledge of "coastal wetlands and estuaries", "last glacial maximum", "sediment dynamics", "coastal erosion and shoreline changes", and "surface elevation change" has been established since the 1960s with in uential references distributed throughout the entire period to date.In contrast, knowledge of "oceanic anoxic events and carbonate platforms", "glacial isostatic adjustments and geoid subsidence", "great earthquakes and stratigraphy", and "intertidal benthic communities" was primarily founded during the 1960s to 1990s with limited in uential references recognised after the 2000s.Finally, knowledge of "sea-level projections", "ice sheets and glaciers dynamics", "extreme sea-levels and ooding", "vulnerability and adaptation", "mega deltas", "engineered shorelines and coastal squeeze", and "coral reefs and atoll islands" was established during the 1980s and has been growing ever since (Fig. 3f).
Based on the number of citing articles and their corresponding citations from 1990 to 2021, the research streams "coastal wetlands and estuaries", "last glacial maximum", and "coastal erosion and shoreline changes" were generally, consistently active and rising; "oceanic anoxic events and carbonate platforms", "glacial isostatic adjustments and geoid subsidence", "great earthquakes and stratigraphy", and "intertidal benthic communities" were declining; "sediment dynamics" was uctuating; and "sea-level projections", "ice sheets and glaciers dynamics", "extreme sea-levels and ooding", "vulnerability and adaptation", "surface elevation change", "mega deltas", "engineered shorelines and coastal squeeze", and "coral reefs and atoll islands" were emerging and attracting growing attention in recent years (Fig. 3g).
Further details regarding each research sub-theme are presented in the Supplementary Table 2.It should be noted that the observed trends reported are in the context of SLR of science and different trajectories and level of activities may be observed for these sub-themes in other elds of science.
The temporal trends discussed above highlight the evolution of SLR scienti c research from a predominately pure academic discipline towards a more applied research eld due to the widespread, well-documented, and observed short-term and long-term compounding and cumulative impacts of SLR 34 .The emerging disciplines often focus on applying the knowledge gained at designing effective climate policy.These policies integrate climate change adaptation strategies into broader coastal management and policy.It is also apparent that SLR science is getting more speci c with interest in distinct coastal environments (e.g., "mega deltas" or "coastal reefs and atoll islands") and detailed processes (e.g., "coastal erosion and shoreline change" and "extreme sea-levels and ooding") continuing to emerge.

Discussion
SLR science is a multidisciplinary eld with geological dimensions and biogeochemical cycles, impacts, risks, and adaptation, physical components of sea-level change, and coastal ecosystems and habitats emerging as its major research clusters (themes), and 16 detailed research streams as its key sub-themes (Fig. 4a).Analysis of 30 + years of SLR literature indicates that these prevalent research topics have evolved over time.Understanding this scienti c evolution (Fig. 4b, c) provides insights into the funding and prioritisation of research agendas, the challenges and opportunities of periodic integrative assessment of the literature, and how science is informing societal responses and policies 39 .

Research agendas
Research agendas continue to evolve as SLR impacts emerge and gaps are addressed through ongoing research and the prioritisation in research funding.For instance, the rapid loss of estuarine and coastal ecosystems in recent years has raised concerns over declining critical ecosystem services 59 (valued at up to US$194,000 per hectare, per year 70 ) including ood, storm, and erosion protection 53,71,72,73 , carbon sequestration 65,74 , and water quality improvement 75,76 , including the potential intensi cation of hazards through feedback loops 77 .In recent times, these studies have increased substantially (Fig. 4b,  c), and additional studies continue to be undertaken due to the wide variety of ecosystems.In this regard, a recent review found that worldwide geographical coverage remains limited, with most valuation studies considering United States and tropical Asian coastal ecosystems.Further, these studies have largely focused on saltmarsh, mangroves, and near-shore reefs and do not consider sea grass meadows, dunes, and barrier islands 78 .

Equity
The recent literature on SLR highlights the growing imbalance in both geographical and distributional impacts 6,9,22,27,28 .Almost all of the world's 85 million poor people living in rural low-elevation coastal zones reside in 20 developing countries, including Cambodia, Pakistan, Indonesia, Mozambique, Senegal, and the Philippines 79 .The same concern exists for people inhabiting Small Island Developing States (SIDS) 29 and deltaic areas 80,81 .These and other vulnerable coastal populations should be the priority of long-term global planning strategies to adapt coasts and populations to damaging SLR impacts.
Assessing the growing literature SLR science must be responsive to areas of ongoing research, geared towards societal and policy relevance.Advances in assessment have thereby prioritised combinations of multi-model comparisons, meta-analysis, systematic review, and multi-criteria methods of determining the strengths and limitations of current understanding 82,83 .These challenges are particularly relevant to SLR science given its growing, multidisciplinary topics and methods.To illustrate this challenge, a large and growing body of academic literature has been developed to address these expanding topics.In 1990, only 41 SLR-related documents were recorded, and this number increased to 1,369 by 2000, 4,195 by 2010, and 13,476 by 2020 (Fig. 4d).From 1990 to 2021, the growth and doubling rates of SLR science were 15.4% and 4.8 years (Methods), respectively, which are higher compared to other elds of science (e.g., a doubling time of over 17 years for the entire modern science 38 ).If these trends continue, the next IPCC assessment report (expected from 2026 to 2030), will face the daunting challenge of exploring nearly 9,000 to 18,500 new SLR-related documents (Fig. 4d).
Policy SLR literature has increasingly focused on challenges associated with adaptation responses, as re ected in the growing and emerging sub-themes within Cluster (II) (Fig. 4b).In this context, greater attention should be given to the appropriate 'mix' of protection from SLR and climate change from both grey infrastructure, such as seawalls, dikes, barrages, and diversions as well as raising the elevation of buildings and ood-proo ng structures, and green infrastructure, such as saltmarshes, oyster and coral reefs, mangroves, seagrass beds, barrier islands, and beaches 53,84 .The main adaptive response to SLR in coastal areas is to either protect, manage, accommodate, or retreat 9,85 .In many coastal areas, managed retreat and associated migration may be unavoidable, which could encompass a variety of strategies, including the controlled ooding of low-lying coastal areas or the abandonment/relocation of assets and people allowing the shoreline to move inland 86 .To this end, the literature has increasingly focused not only on adaptation options, but also on the implementation, monitoring, evaluation, sideeffects, and co-bene ts.
After 30 + years of increasing SLR knowledge, this analysis, as a rst attempt, allows a new perspective on how different SLR science disciplines have developed and evolved, which is not possible using conventional methods.Given the explosion in published literature, this study provides a repeatable method that can support and complement large-scale assessment processes such as future IPCC reports, and hence, guide and inform the global community in their responses to the dynamic challenge of rising sea-levels.While we are still learning about the best use of these methods, this research can assist global scale reports by identifying emerging or trending research topics and highlighting geographic discrepancies.The ndings could also serve as a guideline for designing evidence-based management strategies and directing future research funding.

Data acquisition strategy
Synthesising the growing volume of sea-level rise (SLR) science in 2022 using manual assessment and review methods is nearly impossible.Hence, the bibliometric approach is becoming a popular technique to overcome such literature assessment challenges 40,87 .This method allows for a rapid classi cation of thousands of documents and reliably and consistently captures the breadth of literature related to a speci c eld 41,45 .Adopting a bibliometric technique enables identifying term co-occurrence patterns, similarity of references between articles, and patterns of co-referencing, providing a comprehensive understanding and evolution of a eld under investigation 69 .
In this study, the global SLR literature was systematically assessed utilising a term-based search strategy in the Web of Science.This search assessed academic literature, such as research articles, review papers, data papers, letters, book chapters, and proceeding papers.Grey literature and indigenous knowledge, although valuable, were not considered in the search.A suitable search strategy was implemented to ensure the majority of key and relevant SLR publications are captured while minimising false positives.Here, false positives are de ned as studies that are not directly related to SLR, although they have used the term "SLR" or similar close terms.
The following methodological approach was applied while constructing the term-based element of the search strategy.Individual terms relevant to SLR (e.g., search units) were used to create a query string.Further, to make the search term more speci c and relevant, terms were combined using Boolean operators (e.g., OR) to form search units or a combination of terms.A search unit is a component of the query string that can generate search results on its own.For example, in the queries used, a search unit is "sea level rise" OR "sea-level rise".Each search unit was then assessed separately in the Web of Science using the titles (TI), abstracts (AB), and authors' keywords (AK) as the search domain.A sanity check was carried out on the rst and the last fteen pages of results (in the Web of Science) from each search unit to ensure no (or very limited) false positives were captured or created.The overall aim of the term-based search approach is to generate many particular search units rather than a few broad search units in order to reduce false positives.This is particularly important for a large, diverse multidisciplinary literature such as SLR science.The following search query was used: TI = ("sea level rise" OR "sea-level rise") OR AB = ("sea level rise" OR "sea-level rise") OR AK = ("sea level rise" OR "sea-level rise").
The developed query string was inserted into the Web of Science Core Collection in January 2022, with the search time span restricted to 1st of January 1990 to 31st of December 2021, returning N = 14,951 unique documents.The bibliographic information (e.g., list of authors, keywords, journals, publication year, etc) for these publications was obtained from the Web of Science and was stored as text les.Bibliometric methods were then applied to the dataset to obtain high level insights on the development of SLR literature and its research streams.The method of Visualisation of Similarities (VOS) and the VOSviewer software 88 which is a popular tool for mapping clustered networks of authors, journals, and citations was used to illustrate the similarities 89 .Additionally, VOSviewer uses text mining algorithms to identify speci c phrases from a publication's title and abstract from which major clusters and networks could be developed.In this context, VOS provides a visualisation such that the similarity between two objects and is depicted by their Euclidean distance .This similarity, which is also referred to as Proximity Index or Association Length, is measured as 90 : 1 where is the number of co-occurrences of objects and , and and denote their respective number of total occurrences.The VOS mapping method creates a two-dimensional network by minimising the weighted sum of the squared Euclidean distances between all pairs of elements (Eq.2).A higher degree of similarity would be indicated by heavier weights as discussed in 90,91 : This is subject to the requirement that the average distance between pairs of objects be equal to unity 90,91 : 3 As a result, Equations (2, 3) are used to nd the spatial locations of items where is the vector of position for item in a two-dimensional map, and signi es the Euclidean norm.VOSviewer uses a majorization approach to derive the answer numerically.

Data analysis
The occurrence and co-occurrence of terms in the title and abstract of articles were used to determine the broad scienti c structure and major research themes of SLR science.When the VOS method was applied to the key terms of the titles and abstracts of this eld, four major themes were identi ed including (I) geological dimensions and biogeochemical cycles, (II) impacts, risks, and adaptation, (III) physical components of sea-level change, and (IV) coastal ecosystems and habitats (Fig. 2a).In Fig. 2, the frequency of co-mention and similarity of terms determine their placement in the gure, and the size of a node represents the frequency of its occurrence.
Authors' keywords were also analysed with a focus on three set of periods including 1990-2009, 2010-2015, and 2016-2021 (due to the disproportionate distribution of SLR publications from 1990 to 2021), and for six different continents of Africa, Asia, Europe, North and Central America, Oceania, and South America identi ed based on authors' a liation (i.e., countries of all authors are considered) (Fig. 1b).In each period, the top 10 keywords most widely used were identi ed and presented to provide an overview on the trends of change in SLR research topics.
A more detailed analysis on the dataset, referred to as Document Co-citation Analysis 68 , was also conducted.This analysis determines key sub-themes of SLR science, provides a greater level of resolution, and identi es temporal and spatial trends of those sub-themes and their respective fundamental references (Fig. 3).This analysis makes the approach robust to possible missing items in the original dataset.To illustrate this point, if a fundamental reference has been cited enough times within a large dataset, it will be detected, regardless of whether it itself exists in the dataset or not.This refers to a set (cluster) of references that have been jointly cited by researchers publishing in the SLR science space.The principal logic behind this approach is that references which are routinely co-cited (cited together) by articles that have a SLR theme can be grouped together as they would be thematically similar.Each cluster of co-references form the knowledge foundation of certain research themes/streams within the SLR literature.In other words, the formed clusters (that have a similar theme) represent distinct streams of research in SLR science.It should be noted that research sub-themes containing fewer than 50 documents were not considered nor presented.
The document co-citation methodology explained in 68 and the CiteSpace software 92 are used to classify the references, categorise its spatio-temporal trends during the development of its disciplines, and identify the most in uential references in the SLR science domain (Fig. 3).In Fig. 3a, each node represents an individual reference, the node size indicates the local citation count (i.e., the frequency of being referenced by SLR publications solely), and links represent instances of co-citation.A K-means clustering algorithm was used with a g-index criterion for the selection of references and the number of look-back years of 30 and time slices of one year.Clusters were ranked based on the number of references within each of them.The degree of activities of each cluster over time was quanti ed based on the number of citing articles of each cluster during each year as well as the total number of citations (i.e., total coverage) from the citing articles to the references of each cluster (Fig. 3b-e).In uential references of each cluster were determined based on three primary metrics, namely, local citation counts, bursts of (sudden spikes in) local citation count to the references, and the extent to which a reference receives citations across various clusters of SLR literature, known as reference centrality (Supplementary Table 2).

Average growth rate and doubling time of SLR
In this study, average yearly growth rate (i.e., growth over a series of equally spaced time periods) and doubling time (i.e., the duration of time it takes for a population -here, SLR publications -to double), which are valuable indicators for evaluating long-term trends in science, were evaluated (Fig. 4).The annual growth rate percentage was rst computed as: 4 where and are ending and beginning values (here documents), respectively.was calculated between two successive years to compute the annual growth rate.The average growth rate for the entire period was then evaluated by averaging the values of for all the periods considered (i.e., 1990-2021).
The doubling time was then estimated as: ln(1 + β)