*3.3. Trends of Research Themes*

A strategic diagram can generally represent the structure of a research field, in which all of the research hotspots are placed in the four quadrants of the coordinate graph, based on the values of the centrality and density, thus describing the research status and trend of each hotspot. The density is used to determine the closeness of the terms within each cluster of hotspots. It represents the self-sustainability of each cluster of hotspot, i.e., stability [41]. The centrality spot measure the closeness between the terms of each cluster and those in the other cluster, indicating the degree of mutual influence of one cluster of a hotspot and the other clusters. The greater the centrality of one cluster of hotspot, the more central it tends to be in the entire research field [42]. Therefore, the clusters in Quadrant 1 are the relative core and stable themes (strongly connected with other clusters and having intense internal relationships). The clusters located within Quadrant 2 represent peripheral but already well-developed themes. The clusters in Quadrant 3 are both peripheral and unstable. The clusters in Quadrant 4 are central but not stable, yet they are becoming mature or are vanishing to some extent [39]. Typically, analyses are the most interested in the new and exciting topics in Quadrant 4.

Based on the results of the co-word biclustering analysis and the co-occurrence matrix of high-frequency terms, we calculated the centrality and density of each cluster and drew two strategic diagrams on research hotspots for the two decades. Then, we analyzed the basic framework, alteration, and trend of research hotspots on climate change and infectious diseases in the world (Figure 5).

Four clusters from 1999 to 2008 are scattered in the four quadrants, i.e., Cluster 1 in Quadrant 1, Cluster 2 in Quadrant 2, Cluster 0 in Quadrant 3, and Cluster 3 in Quadrant 4. Cluster 1, global and public health, is considered as the motor theme (of which centrality and density are both high). In the decade from 2009 to 2018, Cluster 2 lies in Quadrant 1, Cluster 0 in Quadrant 2, and the rest of three clusters (Clusters 1, 3, and 4) lie in Quadrant 3, while no cluster is in Quadrant 4. The contrast of the clusters and their positions in the strategic diagrams between the two 10-year periods can also be visualized in Figure 5, showing the trend and alteration of the hot research themes.

**Figure 5.** Strategic diagrams of hotspots in research output on climate change and infectious diseases from 1999 to 2008 and from 2009 to 2018. (**a**) Strategic diagrams of hotspot from 1999 to 2008; (**b**) strategic diagrams of hotspots from 2009 to 2018. € Q stands for quadrant. Clusters inside each strategic diagram refer to the clustering results shown in Table 2. The descriptions indicated by arrows are the hotspots of each cluster. The words in the red font represent the exclusive hotspot research topics in the first decade or the second decade.
