text: **3. Methods**
  
  3.1. Data Source 
  
  As Twitter has been recognized as a popular discussion forum [75] and a social activity platform [76] 
  for climate issues, we followed the literature [5,8,18] and used tweets to investigate distinct perceptions 
  of climate issues and evolution on social media. Although Twitter’s ecosystem has been changing 
  in terms of the number of active users, user demographics, and tweeting conventions in the past 
  years [77,78], the problem is unavoidable for all the information ecosystems on the Internet. As Twitter 
  is one of the most popular social websites, we deﬁned our study as characterizing the perception of 
  climate issues among social media users rather than all the netizens or the whole population. 
  
  3.2. Data 