Int. J. Environ. Res. Public Health 2020, 17, 1062 14 of 22
problems at various levels of diﬀerent domains [ 92]. No similar mechanism was observed in the global
warming discourse, suggesting that global warming was less evocative of diverse adjacent sub-themes.
5.2. Evolution of Associations in the Two Discourses
5.2.1. Shrinking of Traditional Political Discussions and Emergence of Discourse Alliance
The temporal analysis revealed the evolution of public discourse from 2009 to 2018. In both
discourses, political discussions gradually faded, indicating a trend in the public perception of climate
issues to seeming slowly return to the scientiﬁc dimension in general. In 2009, political discussion was
the second most important topic in the global warming discourse, but its ranking fell to last in 2018.
In the climate change discourse, as shown in Figure 3, the association to political hashtags disappeared
in the recent two years. These ﬁndings seem to contradict earlier studies suggesting noticeable political
polarization in the climate discussion [ 66,93] and a Pew report saying that the partisan divide had
widened in terms of attitudes toward climate change issues [ 94]. We suggest a probable reason
responsible for this contradiction: Although political clusters in both discourses were shrinking, in the
climate change discourse, the proportion of red associations started to increase in recent years, as
shown in Figure 5a. The red cluster is a topic cluster with blurred edges and is composed of diverse
hashtags, such as “healthcare”, “education”, “poverty”, “innovation”, and “politics”. We examined
the tweets with these co-occurrences and found that although few typical political hashtags such as
“p2” and “tcot” were used, most of these tweets had strong political intentions. For example, when the
public noticed that whitehouse.gov no longer provided speciﬁc pages for climate change as well as
healthcare, LGBT rights, and civil rights in January 2017 [95], the red cluster expanded, as shown in
Figure 5a, and a large amount of co-occurrences among these issues emerged to blame the government’s
wrong practices of neglecting all these issues. We observed that “climatechange” occurred often
with hashtags such as “education”, “poverty”, “economy”, “leadership”, and “innovation” as the
object of the verb “rethink” in a large amount of tweets arguing for the government’s reﬂections on
several domestic policies. Thus, we think that climate politics did not disappear but changed their
form in public discussions in the last two years of the studied decade. A kind of discourse alliance
formed among climate change and several other domestic political issues to show political appeal
together, where a trend of pan-politicization rather than de-politicization might be identiﬁed in climate
change discussion.
5.2.2. Strengthened Associations between Global Warming and Weather Abnormalities
Although global warming is linked to abnormal weather phenomena, temporal analysis suggests
their associations were not innate or changeless. An increase in association strength was observed in
2012 according to Figure 4. Two reasons may have contributed to this change. Firstly, the Hurricane
Irene and Hurricane Sandy, two serious extreme weather events in 2011 and 2012, were proven to
raise the volume of climate-related tweets and evoke the public’s consciousness of global warming
[2] to some extent. However, Sandy may not be the only reason. Studies that analyzed the discourse
of three extreme events on Twitter pointed out that political and ideological debates, rather than
phenomenon-related discussions, dominated during Sandy, evidenced by hashtag “#sandy” not being
even to the temperature and extreme weather cluster (red in Figure 4), but was identiﬁed to the cluster
describing hesitation between climate facts and actions (yellow in Figure 4).
After examining every associated node in the red cluster in 2012, we suggest that the hashtag