Understanding what drives collective climate action has never been more urgent. In the field of collective action research, despite consistent evidence of a gap between people's intentions and actual behaviors, most research continues to rely on self-report scales and intentions instead of measuring real-world behavior. To address this gap, we conducted a 10-week longitudinal classroom-based intervention study with undergraduate students (N = 44). As part of the course, participants learned about the climate crisis and collaborated in small groups with local grassroots organizations on different campaigns. Each week, we assessed their climate-related emotions, what they intended to accomplish with the grassroots organization they were assigned to, and what collective climate action they actually did. We objectively verified the actions participants reported, enabling us to assess the quality of their actions. When analyzing climate emotions, the only variable that significantly related to the quality of collective action was the construct of active hope: the more participants reported a stronger sense of the importance of acting collectively now as preparation for future struggles, the higher the quality of their verified collective climate action. Unlike traditional measures of hope, which often include optimism or wishful thinking, active hope is about agency and commitment regardless of the outcome. Our findings highlight both the promise of active hope as a construct for future research and the value of verified behavioral measures for identifying the psychological factors that matter for real-world collective climate action.