Designing persuasive technology—that is, technology to change people's behaviors and attitudes—is seen as a morally risky venture. Recent work has begun to apply established approaches such as Value Sensitive Design (VSD) and Participatory Design, which guide the designer to deeply and deliberately engage with the needs and values of future users and other stakeholders. But with super-wicked problems such as global climate change, these approaches can mire the designer in analysis paralysis—particularly problematic as harmful effects of climate change are already upon us. Is sustainability, thus, an exception to the emerging consensus that ethical design proceeds slowly and cautiously? We argue not: whether caused by climate change, war, disease or injustice, human suffering always demands the swiftest response that is prudently possible. We argue for a mature balance between deliberation and action—especially when the consequences of action are unforeseeable but the consequences of inaction are unthinkable. We propose a route forward whereby practitioners may ethically realize the full persuasive potential of persuasive technologies: by adopting a VSD approach to cultivating values and fostering deliberation; by engaging in Participatory Design not with end-users, as is traditionally done, but with domain experts who can offer real insight into meaningful persuasive technology goals; by supporting collective action rather than preserving above all users' rights to opt out of persuasion; and by appealing to fear in cases where fear is integral to the narrative driving the persuasive technology goal, while promoting efficacy in the face of fear.