A stable climate is the most fundamental determinant of human health. Therefore, the goal of the Paris Agreement—limiting global warming to no more than 2 degrees Celsius—is arguably humanity's most important public health goal. To accomplish this goal, nearly all nations must greatly increase the ambition of their Nationally Determined Contributions at the upcoming United Nations COP26 meeting in 2021. We argue that health professionals and health organizations can and must join the growing global community of science-based advocates working to achieve the goal of the Paris Agreement. Doing so can be our greatest contribution to the health and wellbeing of all people, especially the world's most vulnerable, marginalized and disempowered people who tend to be harmed first and worst.