Urgent warnings about the existential threat of climate change are coming from leaders in nearly every sector of society, including virtually all climate scientists, notable heads of civil governments around the globe, the world’s top religious leaders, prestigious medical journals, as well as principals of the largest financial firms. Surveys show that the majority of U.S. physicians in several specialties are caring for patients who are experiencing direct health harms due to climate change. In public platforms, physicians are expressing their awareness that this public health crisis places everyone at risk, but many people are at greater risk, including children, pregnant women, people with chronic health conditions, elders, and those who experience environmental injustice or live in harm’s way. Physicians should respond to this crisis with meaningful activities performed within the context of their current roles. The role of medical care provider is the best known role. But, throughout their careers, physicians have ongoing responsibilities as educators of colleagues, trainees, and patients. They are influential employees of medical institutions, trusted experts who exercise civic responsibility, and sources of guidance for public policymakers. Physicians and other health professionals, individually and through their organizations, also work to influence our societal response to the challenge of climate change. The first annual Lancet Journal Countdown Report in 2016 tracking health indicators of climate change stated that climate change had the potential to wipe out all public health gains of the last half century, but it also presents a tremendous opportunity to save lives and improve health. All physicians should work toward the latter outcome.