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.]

A simple agent based foraging model. Consumer agents will move between fertile patches consuming them.

In the early 1950s, there was a serious outbreak of malaria amongst the Dayak people in Borneo. The World Health Organization (WHO) decided to help and massively sprayed the area with DDT. The mosquitoes died in large numbers and the Dayak were happy - for a while. However, in complex situations the likely systemic impacts of actions, even if well intended, need to be taken into acount. In this particular case the Dayak got a very unpleasant surprise. The DDT did not only kill mosquitoes but also wasps. But these wasps used to feed on caterpillars that feed on thatched roofs. Without the wasps the caterpillars rapidly increased in number with disastrous effect on the structure of thatched roofs, which began to collapse. But that was not all. Gecko lizards ate the poisoned insects, and  as cats often catch and eat geckoes they  began to die in large numbers. Soon the number of rats increased. Not only were the rats a nuisance but they carried sylvate plague and typhus. This simple Sign Graph illustrates that they can be very effective in identifying unintended negative consequences, and on many occasions they can help to identify potential side effects before they become a serious problem.