Social media is becoming more and more integrated in the distribution and consumption of news. How is news in social media different from mainstream news? % This paper presents a comparative analysis covering a span of 17 months and hundreds of news events, using a method that combines automatic and manual annotations. We focus on climate change, a topic that is frequently present in the news through a number of arguments, from current practices and causes (e.g. fracking, CO2 emissions) to consequences and solutions (e.g. extreme weather, electric cars). The coverage that these different aspects receive is often dependent on how they are framed---typically by mainstream media. Yet, evidence suggests an existing gap between what the news media publishes online and what the general public shares in social media. Through the analysis of a series of events, including awareness campaigns, natural disasters, governmental meetings and publications, among others, we uncover differences in terms of the triggers, actions, and news values that are prevalent in both types of media. This methodology can be extended to other important topics present in the news.