The pathway from scientific knowledge, based on data, models and forecasts, to societal implications and policy advice is a perilous one. The shift from is to ought may be slippery, in terms of climate, biodiversity, regulations and business. Yet, what is to be done if your research discloses that your fellow humans are unwittingly carrying out destructive actions on a large scale? If they are unaware of the dynamics within which they are - or are in danger of becoming - imprisoned, is there then not an ethical obligation to respond, to endeavor to act beyond the scope of your given role? This dilemma lies at the core of the issue of science based activism, which the life and work of Jorgen Randers in many ways epitomizes. The best means of understanding what science based activism is may be found in exploring his steps, as this Festschrift does. Jorgen Randers co-authored The Limits to Growth in 1972 and its sequels in 1992 and 2004. He is a Professor of Climate Strategy at the Norweg