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In the global warming network, politics was the second-largest discourse cluster (20% of the
network), where “tcot”, short for “Top Conservatives on Twitter”, was the node ranked highest,
and “p2”, short for “Progressives 2.0”, is also included. Several political ﬁgures, such as Obama and Al
Gore, are frequently mentioned. Action toward the global climate issue was the third-largest cluster
(16%), including both domestic eﬀorts, such as “us”, “trump”, “climatechangeisreal”, “climateaction”,
and “epa”, and two international items, like “china” and “india”. The fourth cluster (in blue) referred
to emissions, including hashtags like “co2”, “green”, and “carbon”. The smallest cluster (8%) was
composed of “snow”, “winter”, “heatwave”, and “summer”, referring to the temperature abnormalities
on the earth.
4.3. Temporal Analysis of the Associations in the Two Discourses
The online presentations of the climate change and global warming discourses are dynamic.
As shown in Table 2, for the global warming discourse, 11 key concepts remained in the top 50 central
hashtags each year for all 10 years, with 16 for the climate change”discourse. By comparing the 11
nodes of the global warming discourse and the 16 nodes of the climate change discourse, we found that
the two lists shared nine concepts. We found “pollution” and “earth” were unique to the keyword list
of the global warming discourse, and “economy”, “water”, “china”, “coal”, “solar”, “sustainability”,
and “food” only occurred on the critical list for the climate change discourse.
Table 2. Hashtags that remained on the top 50 list for the climate change or the global warming
discourse from 2009 to 2018.
Unique Shared
#climatechange china, solar, water, food, economy, coal, sustainability co2, news, carbon, green, climate,
#globalwarming pollution, earth us, energy, science, environment
Figures 3 and 4 show the overall evolution of critical hashtags’ associations in the 10-year period,
where the nodes in the 10 graphs are located in the same position but the strength of associations varies
across longitudinal time. Vector graphics with the label of nodes are provided in the Supplementary
Materials. Four themes were identiﬁed in each discourse according to the nodes’ associations. To more
explicitly demonstrate the relative importance of each cluster in each year, we calculated the sum of
the degree centrality of all the nodes belonging to each cluster and their change in centrality over the
10 years, as shown in Figure 5.
Figure 3 depicts the associations of hashtags in the climate change discourse for each year
from 2009 to 2018. The scientiﬁc hashtags cluster (in green) was the most important theme in the
climate change discourse, especially more recently. However, some scientiﬁc hashtags, such as “ghg”
(greenhouse gas), “co2”, and “forests”, were not identiﬁed in the scientiﬁc cluster but in the global
actions cluster (in yellow) because these hashtags were frequently used in the global action context and
identiﬁed with a closer semantic association to global action by Gephi. In addition to these hashtags,
the global action cluster included a series of international activities, such as “ipcc” (Intergovernmental
Panel on Climate Change), “unfccc” (United Nations Framework Convention on Climate Change),
and “cop” (Conferences of the Parties) for almost every year. The blue cluster includes to political
hashtags, such as “uniteblue”, “sgp”, “p2”, and “tcot”. In 2017 and 2018, the associations with political
hashtags disappeared among the top 50 hashtags. The small red cluster had a mixed theme, combining
“technology”, “innovation”, “education”, “africa”, “healthcare”, and “politics”. The centrality sum of
the nodes in the red cluster remained rather low throughout the 10-year period but obviously increased
in the last two years of the period according to Figure 5a.