A model of the argument in Michael Powell's Atlantic essay 'The Cities That Said Yes to Drugs' (https://www.theatlantic.com/magazine/2026/09/city-drug-addiction-harm-reduction-policy/687967/). Radical harm reduction (injection sites, 'safer supply', paraphernalia hand-outs, non-enforcement) makes each act of drug use safer, but it removes every consequence and creates open-air drug markets that pull in users and dealers. The street-using population grows until TOTAL overdose deaths and community disorder rise even though the death risk PER USER has fallen. Enforcement paired with a firm push into treatment does the opposite. One stock of street drug users in a Seattle-sized city (~750k residents); one reinforcing loop (the open drug market attracts more users); two balancing loops (deaths and exits to treatment). The drug supply turns from heroin to fentanyl over 2015-2018 in every scenario, so deaths rise everywhere at first, as they did in reality; policy decides whether the city ends up with a 2x or an 8x increase. Time runs 2015-2035: the decade of the fentanyl era plus a decade of policy consequences.

Brief Summary of Albert O Hirschman's Book with 2026 update

Developed by Maurice Karsten (Bielefeld University) as part of a doctoral research in chemistry education.  

World4 is a predictive model for world population. Population has grown hyper-exponentially in the last millenium, with the doubling time decreasing from 900 years  in 1000 CE to a minimum of ~35 years in 1963 CE. Technology is defined as that which decreases the death rate and/or increases the effective birth rate (i.e. by decreasing infant mortality). Technology grows exponentially, therefore population fits a hyper-exponential (exponent within an exponent). Models for the end of growth are explored using equations that express the ways humans are depleting Earth's biocapacity, the nature of resource depletion, and the relationship between natural resources and human carrying capacity. This simple model, containing just two closed systems, captures the subtle shifts in the population trajectory of the last 50 years. Specifically, hyperexponential growth has given way to subexponential growth. A peak is predicted for the time around 2028.  [Bystroff, C. (2021). Footprints to singularity: A global population model explains late 20th century slow-down and predicts peak within ten years. PloS one, 16(5), e0247214.]

Time Settings are used to define the simulation start time, duration, and the step size used from one calculation to another.