Int. J. Environ. Res. Public Health 2020, 17, 1062 7 of 22
column name to create two matrices. One matrix was created for the climate change discourse, and we
ﬁlled the cell whose column name and row name were among the top 50 list in the climate change
discourse with the frequency at which the two hashtags were associated in this discourse, and the
other cells were ﬁlled with 0. This was repeated for the global warming matrix. We thus obtained
two matrices with the same row and column names but diﬀerent values in the cells. Then, the two
matrices were input to the quadratic assignment procedure (QAP) [85] analysis provided by UCINET
software [86] to assess their correlation for each year.
4. Results
4.1. General Descriptions
Association networks surrounding #climatechange and #globalwarming showed di ﬀerent
properties. The climate change discourse included 38,821 hashtags, whereas the global warming
discourse only contained 8788 hashtags. Table 1 displays the 50 most signiﬁcant hashtags in the
two discourses based on centrality. As some hashtags were used in the form of an abbreviation or
phrase, explanations are provided in the table. Two networks shared 32 out of the 50 most signiﬁcant
words. Hashtags “canada”, “cdnpoli”, “sdgs”, “biodiversity”, “education”, “environmental”, “cop24”,
“sustainable”, “auspol”, “food”, “agriculture”, “cleanenergy”, “renewableenergy”, “renewables”,
“emissions”, “coal”, “fossilfuels”, and “cop21” only showed up on the top 50 list of the “climate change”
network. Hashtags “tcot”, “california”, “p2”, “nyc”, “snow”, “agw”, “summer”, “global”, “winter”,
“india”, “planet”, “heatwave”, “hoax”, “nasa”, “algore”, “world”, “oil”, and “eco” were unique on the
top 50 list of the global warming network. The two lists only shared three out of the top ﬁve hashtags.
In the #climatechange network, “climateaction” was ranked third place and “sustainability” was
ranked fourth place, whereas they were ranked signiﬁcantly lower, 17th and 22nd, respectxively, in the
#globalwarming network. In the #globalwarming network, “earth” and “weather” were among the
top ﬁve nodes, whereas they were ranked 14th and 24th in the #climatechange network, respectively.
Table 1. The top 50 central hashtags on Twitter surrounding #climatechange and #globalwarming from
2009 to 2018. The hashtag with * is explained in Appendix A in ascending alphabetical order.
No. #Climatechange #Globalwarming
Hashtag Centrality Hashtag Centrality
1 climate 0.466 climate 0.530
2 environment 0.465 environment 0.446
3 climateaction 0.391 science 0.319
4 sustainability 0.316 earth 0.296
5 science 0.314 weather 0.280
6 energy 0.283 us * 0.280
7 trump 0.257 trump 0.263
8 us * 0.247 pollution 0.256
9 cop21 * 0.232 co2 0.244
10 parisagreement * 0.232 green 0.239
11 actonclimate * 0.225 tcot * 0.229
12 water 0.221 nature 0.213
13 pollution 0.210 news 0.198
14 earth 0.207 energy 0.192
15 green 0.200 climatechangeisreal 0.187
16 climatechangeisreal 0.195 obama 0.181
17 renewableenergy * 0.194 climateaction 0.175
18 health 0.193 algore * 0.174
19 nature 0.187 water 0.171
20 renewables 0.186 agw * 0.164
21 cleanenergy 0.176 carbon 0.164
22 carbon 0.175 sustainability 0.163