3.3.1. Country Collaboration Network

The collaboration network was analysed at the macroscopic, midscopic and microscopic level, corresponding to the collaborations between countries/regions, institutions and authors respectively. The collaboration between countries was first analysed. We put one year per slice, selected the top 100 levels of the most cited references from each slice with no pruning, and obtained the following country/region collaboration network, where there were 51 countries/regions and 263 connections. We used authors' affiliation to determine countries associated with the analysed papers. For example, a 2014 article, entitled "Emerging approaches, challenges and opportunities in life cycle assessment", is an international multi-authors publication, and has co-authors from Switzerland and France respectively, a link has been established between the two countries in country collaboration network and the geographical scope of analysed paper is determined as both Switzerland and France. If two authors of an analysis paper come from the same country, the occurred frequency of the country is recorded twice in Table 7, which means that the unit of measurement of Table 7 is person–time. USA was exceptionally outstanding in the network in that it not only had the largest number of publications (340), but also had actively participated in the collaborations with various countries/regions, thus becoming the node with the highest centrality in the network (the centrality was 0.44). PRC was the only Asian country among the top 10 with the largest number of publications. Although PRC ranked second in terms of the number of publications, its absolute number was only about half of the USA, and its centrality was only 0.1 without so many partners. Referring to the year–colour correspondence bar in Figure A6 of Appendix A, the circle/figure refers to the colour of circle in Figure A3 of Appendix A and represents the corresponding year in which sample literature appeared. It can be seen in Figure A3 that USA was among the first counties/regions to study the carbon footprint in a LCA perspective, and the lighter colour of PRC's circle indicates that it entered the field later. Australia and Canada ranked sixth and tenth respectively in term of publication number, and were also countries with a large number of publications but low centrality. The rest of the top 10 countries were in Europe, including England, Italy, Spain, Netherlands, Germany, Sweden. This may be because Europe has developed animal husbandry, which makes scholars pay more attention to carbon emissions. Most European countries were similar to the USA in that they had a large number of publications and collaborated

with different nodes in the network and their centrality was higher. For example, England published 137 papers and its centrality was 0.31.


**Table 7.** Top 10 countries based on occurred frequency.
