text: Though big data allowed us to decrease the bias by dealing with the whole set of social media 
  data rather than samples, discrepancies still exist between social media users and the public. As most 
  Twitter users do not disclose their age, education, income, and gender in users’ proﬁle, demographics 
  were not introduced as moderator factors in this study. Previous studies noted that in 1970s, global 
  cooling was a prominent climate concern amongst the public [105]. While in the 1980s, ozone layer 
  depletion, species extinction and rainforest destruction became salient on the mass media agenda [106]. 
  Considering the historical background of climate issues, age might inﬂuence how individuals perceive 
  climate issues. According to the statistics in 2017 [107], only 16 % of older people (older than 60) 
  in America use Twitter, while the proportion is 39% for people between 30–59 years old and 47% 
  for people younger than 30 years old (Stastista, 2017). Our results reﬂect the climate perception of 
  older people who use Twitter, as well as younger people amongst whom Twitter is more popular. 
  Although some scholars reported that it is statistically reliable to take data on Twitter as a substitute 
  and supplement for polling [108], we thought our results should be further examined before being 
  generalized to the whole population. 