3.1. Characteristics of document type
From 1980 to 2019, Ambio published 4,154 documents in 10 Web of Science document types.
Table 1 shows the characteristics of these 10 document types, including 3,263 articles (79% of 4,154 documents) with a number of authors per publication (APP) of 3.3. It is worth noting that the documents in the Web of Science Core Collection can be divided into two document types. Amongst those, 137 documents are classified as conference papers and articles. Therefore, the total percentage is higher than 100%.
Document type of discussions with one document had the highest APP of 7.0 followed by reviews with 5.8. The average number of authors per publication in the journal was 3.2, and the maximum number of authors was 53.
The article entitled “Multi-decadal changes in tundra environments and ecosystems: Synthesis of the international polar year-back to the future project (IPY-BTF)” (Callaghan et al., 2011) was published by 53 authors. The co-authors were affiliated to the Royal Swedish Academy of Sciences in Sweden, the University of Sheffield in the UK, and the University of Texas at El Paso in the USA. The CPP2019 of the proceedings paper was much higher than other document types, at 65. Reviews had similar CPP2019 than articles. A review entitled “Coupled human and natural systems” (Liu et al., 2007) was co-authored by 15 authors from the USA, Sweden, and China. This paper was the most frequently cited review in Ambio with a CPP2019 of 408.
According to the Web of Science’s definition, the journal impact factor (IFyear) is defined as in the Journal Citation Reports (JCR) year – i.e. the average number of citations of a journal article published in the past two years. The denominator includes document types of articles or reviews. Some categories of documents that are not normally cited, for example, letters, editorial materials, and other document types are not included in the denominator of the impact factor. Due to the definition of the journal’s impact factor, only the following document types were considered for further analysis: articles, reviews, and proceedings papers.
3.2. Characteristics of the journal’s impact factor (IF)
According to the 2019 Journal Citation Reports (JCR), JCR used 178 Web of Science categories in SCI-EXPANDED to index 9,381 journals. The journal’s impact factor (IF) is defined as the sum of all the citations coming from articles published by the journal in the previous two years in the selected JCR year, divided by the total number of academic articles published by the journal (including articles, reviews and proceedings papers). Periodicals of the first two years have observed the following: (https://incites.help.clarivate.com/Content/Indicators-Handbook/ih-journal-impact-factor.htm?Highlight=impact%20factor).
The journal’s impact factor has the following formula:
where IFyear is the journal’s impact factor in a specific JCR year, Cyear-2: citations from JCR year to items in “year - 2”, Cyear-1: citations from JCR year to items in “year - 1”, TPyear-2: citable items in “year - 2”, TPyear-1: citable items in “year - 1”. CN: the journal’s impact factor contributing indicator (Cyear-1 + Cyear-2) that means the citation number of Cyear-1 + Cyear-2. The denominator is made of three document types such as reviews, articles, and proceedings papers. A research product pertaining to any other document type is excluded from the denominator.
The top 20 journal’s impact factor (IF) contributing papers were listed in Table 2. 70%, 20%, and 10% of them were published in the 2010s, 2000s, and 1990s, respectively. 75% and 70% of the top 20 most IF contributing papers were ranked top 20 in terms of CN (Cyear-2 + Cyear-1), respectively. However, only 25% and 50% of them were ranked as top 20 in terms of TC2019 and C2019, respectively. Only five of the top 20 most journal impact factor (IF) contributing papers including four articles by Mergler et al. (2007), Scheulhammer et al. (2007), Steffen et al. (2011), and Lindberg et al. (2007) and a review by Madronich et al. (1995), ranked within the top 20 in total citations with TC2019 of 699, 569, 535, 524, and 322, respectively. However, 10 of the top 20 highest IF contributing papers – including two in 2007, one in 2011, three in 2014, one in 2016, two in 2017, and one in 2018 – ranked in the top 20 with a C2019 ³ 43 as the most impactful papers in the most the recent year – 2019. Furthermore, 33% and 52% of the top 100 IF contributing papers in terms of CN were found in the top 100 papers in terms of TC2019 and C2019, respectively; and only 23% of the top 100 papers in terms of CN were found in both the top 100 C2019 and TC2019. It can be concluded that the impact factor of a journal is used to evaluate a journal’s relative importance, especially when compared to others in the same field but it is not an appropriate metric when compared to individual research performance. It was also reported in the Web of Science that the journal’s impact factor (IF) is a papers -level metric. It does not apply to individual papers or subgroups of papers that appeared in the publication. Additionally, it does not apply to authors of papers, research groups, institutions, or universities. Figure 1 shows the citation histories of the top ten highest journal’s impact factor contributing papers. Three papers are still keeping an increased trend of citations: Haase et al. (2014), Tengo et al. (2014), and Steffen et al. (2011).
As previously stated, Ambio has been classified in the Web of Science categories of environmental sciences and environmental engineering from its very first indexing. Fig. 2 shows its IF and ranking within the subject categories from 1997 to 2019. Within the years, the journal’s impact factor (IF) fluctuated and slightly increased in the long run, reaching a minimum of 0.929 in 1999 to a maximum of 4.778 in 2019. The ranking of Ambio in the Web of Science subject categories of environmental sciences and environmental engineering were not improved, especially in the last five years. The journal had a higher ranking in the category of environmental engineering with respect to environmental sciences since 2012. In 2014, notable changes occurred in Ambio’s policies, including the enclosure of six major new features (Söderström, 2014).
It is clear from Fig. 2 that the journal’s impact factor increased after that change. The time required to accumulate the citations affects the impact (Garfield, 1999). The size of the scientific community that a journal serves also affects the journal’s impact (Garfield, 1999). How quickly recent publications are cited is another important factor that can affect IF (Garfield, 1999). In general, citations per publication for papers in a journal would have a sharp increase after publication and would reach a peak in a specific year (Fu and Ho, 2015; Monge-Nájera and Ho, 2016).
Figure 2 shows the citations per publication for each year of paper life. The peak year of citations per publication with 2.5 was found to be in the 3rd full year since its publication. That differed from Revista de Biología Tropical (IF2019 = 0.446) with the peak year in the 7th year (Monge-Nájera and Ho, 2016); the Journal of Membrane Science (IF2019 = 7.183) in the 4th year (Fu and Ho, 2015); and the Journal of Orthopaedic Research (IF2019 = 2.728) in the 5th year (Ho, 2019). A special case not showing a peak was reported – unlike previous findings. However, it shows an increasing trend without a peak after the 9th year for the Polish Journal of Environmental Studies (IF2019 = 1.383) (Chuang et al., 2012). Since the IF only considers citations within two years after publication, the IF of Ambio can be reputed as acceptable. It was pointed out that the IF is not an unbiased criterion for all journals, since peak year citations per publication of each journal can be different from each other (Chuang et al., 2012).
3.3. Trends of publication numbers and citations per publication
The trends of publication numbers show a journal’s development, whereas the citations per publication variable provides information about the impact of a journal in the research world. The trends of the number of annual articles (TP) and their citations per publication (CPP2019 = TC2019/TP) have been proposed to discover the development of a journal (Fu and Ho, 2015; Monge-Nájera and Ho, 2016; Ho, 2019). Amidst the 3,405 papers including 3,263 articles with CPP2019 of 29 and 142 reviews CPP2019 of 33 published in Ambio were analyzed. The trend of citations per publication is sketched in Figure 3.
The number of annual papers fluctuated with an increase in the last decade. An average of 85 papers was found from 38 in 1980 to 138 in 1998 (Fig. 4). In 2007, 94 papers had the highest CPP2019 of 76 followed by 71 in 2002. Based on Fig. 4, it roughly takes a decade to the CPPs for reaching a plateau. Similarly, results for the Revista de Biología Tropical (Monge-Nájera and Ho, 2016), the Journal of Membrane Science (Fu and Ho, 2015), and the Journal of Orthopaedic Research (Ho, 2019) also took about one decade to reach a plateau.
3.4. Countries, institutions, and authors of the published papers
Excluding 144 papers without the author’s affiliation information on SCI-EXPANDED, the remaining 3,261 papers originated from 126 different countries. Among those papers, 2,242 (69% of the 3,261 papers) were single country papers from 87 countries, while 1,019 (31%) were internationally collaborative papers from 117 countries. Six publication indicators: number total of papers (TP), country independent papers (IP), internationally collaborative papers (CP), first-author papers (FP), corresponding-author papers (RP), and single-author papers (SP) have been applied as criteria to compare the publication’s performance of countries and institutes for a research topic (Vega-Arce et al., 2019).
Table 3 shows the top 10 most contributing countries with the six publication indicators presented. Six European countries including four countries in Scandinavia, two in America, one in Oceania, and one in Asia were ranked on the top 10 of total papers. The most productive African country was Tanzania with 56 papers (ranked 19th). Sweden dominated in Ambio, ranking as the first in all six publication indicators with a TP of 1,080 papers (33% of 3,261 papers), an IP of 639 papers (29% of 2,242 country independent papers), a CP of 441 papers (43% of 1,019 internationally collaborative papers), an FP of 879 papers (27% of 3,261 first-author papers), an RP of 872 papers (27% of 3,227 corresponding-author papers), and an SP of 186 papers (22% of 838 single-author papers).
Of the total 3,261 papers with affiliation information in SCI-EXPANDED, 1,447 papers (44% of 3,261 papers) came from independent institutions with a CPP2019 of 25 and 1,814 papers (56%) from inter-institutional collaborations with a CPP2019 of 34. The percentage of the inter-institutional collaboration rate of the Ambio (56%) was found higher than that of J. Membr. Sci. (38%) (Fu and Ho, 2015), and Pol. J. Environ. Stud. (31%) (Chuang et al., 2012), but lower than that of J. Orthop. Res. (63%) (Ho, 2019).
The characteristics of the top 10 productive institutions with the six publication indicators were listed in Table 4 (Vega-Arce et al., 2019). Seven of the top ten institutes were located in Sweden. The University of Helsinki in Finland ranked 4th, Aarhus University in Denmark ranked 8th and the University of Copenhagen in Denmark ranked 10th. Stockholm University in Sweden took the leading position in five of the six publication indicators with a TP of 308 papers (9.4% of 3,261 papers), an IP of 68 papers (4.7% of 1,447 institutional independent papers), a CP of 240 papers (13% of 1,814 inter-institutionally collaborative papers), an FP of 159 papers (4.9% of 3,261 first-author papers), and an RP of 157 papers (4.9% of 3,227 corresponding-author papers) while the Swedish University of Agricultural Sciences in Sweden took the leading position with an SP of 27 papers (3.2% of 838 single-author papers).
With the exception of seven anonymous papers , 3,398 papers were published by 8,431 authors including 2,798 authors who published first-author papers, 2,733 authors who published corresponding-author papers, and 834 authors who published single-author papers. The first author and the corresponding author are the most significant author positions (Gaeta, 1999; Mattsson et al., 2011). Three bibliometric indicators such as total papers (TP), first-author papers (FP), corresponding-author papers (RP), and single-author papers (SP) were applied for the analysis of authors’ characteristics for papers in a research field.
Table 5 lists the top 17 productive authors with 13 papers in Ambio. T.V. Callaghan (CPP2019 = 43; CN = 247), who is a member of the advisory board in Ambio, affiliated to the Royal Swedish Academy of Sciences in Sweden, was the largest contributor with 40 papers including seven reviews and 33 articles. Callaghan also ranked top in first-author papers with 20 items and corresponding-author papers with 19 publications. F. Barnaby (CPP2019 = 2.0; CN = 11) ranked the top in single-author papers with 12. M. Johansson (CPP2019 = 46; CN = 163) with 27 papers ranked second in total papers and T.R. Christensen (CPP2019 = 34; CN = 109) with 22 papers ranked third, both of Johansson and Christensen were from Lund University in Sweden. C. Folke (CPP2019 = 227; CN = 278) with 20 papers from the Stockholm University in Sweden and T.D. Prowse (CPP2019 = 50; CN = 72) with 18 papers from the University of Victoria in Canada were also sitting in the advisory board of Ambio.
E. Andersson (CPP2019 = 424; CN = 100) from Stockholm University in Sweden was the most productive associate editor in Ambio, and published seven articles. Comparing the top 24 most productive authors, an advisory board member – C. Folke – with one review and 19 articles, had the highest CPP2019 of 227, followed by T. Elmqvist (16 papers; CPP2019 = 181) and F. Wulff (13; CPP2019 = 111) from Stockholm University, respectively. T. Elmqvist from Stockholm University was the journal’s impact factor best contributor with the highest CN (Cyear-1 + Cyear-2) of 290, followed by C. Folke from the same university with a CN of 278, and T.V. Callaghan from Royal Swedish Academy of Sciences with a CN of 247. The most productive authors, C. Folke and T. Elmqvist also published the top largest number of papers falling within the 100 journal’s impact factor contributing papers with five contributions, respectively. A.D. Fox, C. Folke, J.N. Galloway, T.V. Callaghan, and W. Steffen published two first-author papers and corresponding-authors papers in the top 100 journal’s impact factor contributing papers.
The article entitled “Sustainable intensification of agriculture for human prosperity and global sustainability” (Rockstrom et al., 2017) was the best contributing article in terms of journal impact factor in Ambio with a CN of 145 followed by the article entitled “Leverage points for sustainability transformation” (Abson et al., 2017) with a CN of 102. Furthermore, only 26% and 25% of the top 100 productive authors were ranked in the top 100 in terms of CN of their first author and corresponding author, respectively. J. Rockstrom from Stockholm University published five papers in Ambio including two first-author papers and two corresponding-author papers. Rockstrom had the highest CN of 150 for his first-author papers and corresponding-author papers, respectively. D.J. Abson from Leuphana University of Luneburg in Germany published two papers in Ambio including one first-authored paper and one corresponding-author paper. Abson had the second-highest CN of 102 for his first-author papers and corresponding-author papers, respectively.
3.5. Highly cited papers
The total number of citations was obtained from the Web of Science Core Collection as the bibliometric indicator, TC2019. The main research fescues in a research topic might be reflected by highly cited articles. Highly cited papers in JAMA-Journal of the American Medical Association (Garfield, 1997), Water Research (Wang et al., 2010), and Polish Journal of Environmental Studies (Chuang et al., 2012) were presented. Articles with a TCyear of 100 or more, were generally called highly cited articles (Ho and Gatto, 2020).
In Ambio, 202 (5.9%) of 3,405 papers were highly cited papers including one classic article with a TC2019 of 1,126. The 202 highly cited papers were published by 836 highly cited authors from 412 institutions in 45 countries. The USA had 86 highly cited papers (43% of 200 highly cited papers with affiliation information in SCI-EXPANDED) followed by Sweden (73 articles; 37%), Canada (28; 14%), the UK (24; 12%), and Germany (20; 10%). Stockholm University in Sweden published 32 of 202 highly cited papers (16% of the 202 Royal Swedish Academy of Sciences) followed by the Swedish University of Agricultural Sciences, Royal Swedish Academy of Sciences, and Lund University in Sweden with 10 papers. C. Folke published the most cited papers in the Ambio with 12 papers including the most four first-author and corresponding-author papers, respectively.
The top 20 most frequently cited papers were listed in Table 6. Five of the top 20 highly cited papers were published in 2002 and 2007, respectively. Six and 17 of the top 20 most frequently cited papers were ranked top 20 in terms of CN (Cyear-2 + Cyear-1), respectively while five and 10 of them were ranked top 20 in terms of CN and C2019, respectively. The five of the top 20 most frequently cited papers including four articles by Lindberg et al. (2007), Mergler et al. (2007), Scheulhammer et al. (2007), and Steffen et al. (2011) and a review by Madronich et al. (1995) ranked in the top 20 most IF contributing papers with a CN of 76, 59, 54, 48, and 44, respectively. Ten of the top 20 most frequently cited papers in Ambio were published by Steffen et al. (2007), Folke et al. (2002), Steffen et al. (2011), Cassman et al. (2002), Mergler et al. (2007), Liu et al. (2007), Scheulhammer et al. (2007), Costanza et al. (2008), Galloway and Cowling (2002), and Gadgil et al. (1993), and were also ranked the top 20 in C2019 as the most impactful papers in 2019. The only classic publication with a TC2019 of 1,000 or more was an article entitled “The Anthropocene: Are humans now overwhelming the great forces of nature” (Steffen et al., 2007) published by Steffen et al., written by scholars whose affiliations were from Australia, Germany, and the USA. This most frequently cited article in Ambio was also the most impactful in most the recent year of 2019 with a C2019 of 144.
Citation histories of the top ten most frequently cited papers in Ambio was shown in Fig. 5. The classic article by Steffen et al. (2007) had a sharply increased trend after its publication for a decade and then decreased in 2019. Similarly, highly cited articles by Folke et al. (2002) and Steffen et al. (2011) also had an increasing trend. The highly cited article by Wania and Mackay (1993) had an increasing trend after publication for two full years and keep in a plateau. Highly cited papers would not always have a high impact or visibility after publication. Furthermore, only 45 of the top 100 papers in terms of C2019 were found in the top 100 in terms of TC2019 in Ambio.
3.6. Words in title and author keywords
Several scholars proposed the distribution of words in article titles, abstracts, author keywords, and KeyWords Plus in different periods as information to evaluate main research focuses and find their development trends in research topics (Ho and Gatto, 2020). Table 7 showed the top 20 most used words in the paper title. The distribution of the words in four decades was also presented in the table. “Baltic” was the most recurrent word, used in 218 paper titles (6.4% of 3,405 papers), followed by management, sea, change, environmental, and climate. “Change” and “climate” were the most popular terms in the last decade. The author’s keywords can be found only in the 2010s in Ambio. The top ten used author’s keywords were “climate change”, that can be found in 118 papers (12% of 989 papers in the 2010s), followed by “ecosystem services” (53 papers; 5.4%), “Baltic sea” (47; 5.4%), “eutrophication” (41; 4.1%), “sustainability” (32; 3.2%), “biodiversity” (31; 3.1%), “governance” (31; 3.1%), “phosphorus” (26; 2.6%), “conservation” (25; 2.5%), and “arctic” (23; 2.3%). The results of the analysis considered the author’s keywords and words in the paper title. Papers having the topics of “Baltic sea” and “climate change” were the most popular in Ambio, with respective scores of 231 and 345.