Data: NewsWhip; Chart: Chris Canipe/Axios - Note: Hover over the weekly rank on desktop to see articles and interactions for each candidate and issue.Thanks to Greta Thunberg, climate change stories generated 18 million interactions on social media over the last two weeks, the most for the issue this year by far, according to data from NewsWhip provided exclusively to Axios. Why it matters: Climate change has lagged in generating significant online interest, even as it's taken on a great urgency among Democrats and young voters. The latest findings suggest the messenger matters. Climate change stories generated 5 million more interactions (comments, shares, likes on Facebook and Twitter) than the second-biggest two-week period of the year.That's more than guns, immigration and the economy over this time.If not for the Trump-Ukraine-impeachment story, climate change would have been the No. 1 issue.The big picture: Climate change suffers in the attention economy because of the complexity of the issue and the scientific jargon in the specifics of proposed solutions.The day-to-day news lacks the visceral emotional intensity of immigration and guns.For those not devastated by extreme weather events, it lacks the immediate relevance that economic and social issues like health care and inequality offer.While headlines about Trump administration environmental policies and projections of widespread harm grab attention, interest tends to wane quickly.Between the lines: Greta Thunberg is a big reason for the uptick in climate interest.Of the 50 biggest stories about climate change over the last two weeks, measured by interactions, 27 centered on the 16-year old Swedish activist.Among the top 15 stories: her meetings with Barack Obama and Justin Trudeau as well her trolling by President Trump.By the numbers: The top articles from the last two weeks about climate change:2 striking photos taken just over a year apart show how Greta Thunberg's climate strike inspired millions (Business Insider) — 742k interactionsPersonal attacks on Greta Thunberg prove that adults can't argue with her actual message (Mic) — 651kObama meets with teen climate activist Greta Thunberg: 'You and me, we're a team' (CNN) — 636kSwedish teen activist Greta Thunberg nominated for Nobel Prize (Australia's SBS News) — 452k - Note: The article was published in March.Trump Will End California’s Authority to Set Stricter Auto Emissions Rules — New York Times —403kThe bottom line: For an issue that will impact the younger generation most acutely, Thunberg's message has resonated more this year than the words of Al Gore, Jay Inslee or even Alexandria Ocasio-Cortez.---------Our 2020 attention tracker is based on data from NewsWhip exclusively provided to Axios as part of a project that will regularly update throughout the 2020 campaign.Why this tracker matters: The data on interactions — including likes, comments and shares — highlights an important, but under-appreciated element of an election: the ability to see beyond our own social feeds and understand the broader universe playing out of how candidates and issues are moving the minds of voters.It measures enthusiasm in a way that traditional polling does not.The sample size taken from these social media platforms is massive.Social media is powered by emotion-driven content, and emotional responses are likely to be aligned with a voter's true beliefs in a way that can be masked in polling.While the volume of interactions does not gauge the sentiment of the reactions, the ability to generate reach allows a candidate to expand the universe of potential voters.Bots also cannot be ignored, and we will point out in this space if there are documented instances of bot activity for certain candidates or issues.Methodology: This project measures the number of social media interactions generated on stories published about the 2020 candidates and issues. Interactions are calculated from reactions, comments and shares on those stories on Facebook as well as the number of shares from more than 300,000 influential Twitter accounts and retweets and likes on those posts.Tracked published stories come from a defined universe of more than 450,000 domains.A story registers for a candidate or issue if the keyword is mentioned in the headline, summary or URL of the story.Our search format for candidates looks like: "Joe Biden" OR ("Biden" AND ("President" OR "2020" OR "election" OR "Democrats" OR "primary")).For issues, we use a keyword tree for related terms. See past editions of the tracker here.