Population growth is a major driver for environmental change. Yet, population change has remained as an exogenous variable in most sustainability assessments and methods, playing only a multiplicative function. This is the case of, for instance, the IPAT identity which has governed our understanding of environmental impact for the last 50 years (Chertow, 2000), and where impact (I) is the product of population (P), affluence (A), and technology (T) - I=PAT. In this sense, we argue that it is not only the number of individuals that drives the impact and consumption, but also their constantly evolving biological, physical and demographic characteristics. Evidence of this evolution are the worldwide increase in height and weight (Finucane et al., 2011; NCD Risk Factor Collaboration, 2016), and the changes in sex and age structure of societies (Lutz et al, 2014) over the past century. In this paper, we investigate the evolution of the qualities of the human population in terms of changes in human mass and calculate the implications for food-energy requirements over the past 40 years (1975-2014). This study covers the adult population of 186 countries and uses sex disaggregated data on height, for 100 birth cohorts, coupled with annual average body mass index. Theoretical total energy expenditure -the food-energy required to sustain a human- was calculated as a function of weight, age, sex, and physical activity level, following the FAO and WHO guidelines. We found that while the world population increased by a factor of 2.16, the total human mass and the total theoretical food energy expenditure increased by 2.46 and 2.29 respectively. The net increase in mass is explained by a 14% increment in the average weight and a 1.3% increase in average height, resulting in a 6.1% increase of the energy requirements per capita. These numbers indicate a path dependent, positive feedback, where increased food energy intake has allowed for heavier bodies that in turn demand increasingly more calories per person. The total food-energy grew less than the total mass due to a shift in the average age of adults, from of 39.7 to 42.2 years. A person with a constant weight would demand less food energy towards the end of his life. While all countries have increased their average weight, the change ranges between 6% and 33%. Some of largest changes (above 25%) are observed in Mexico, Malaysia and South Korea, while the smallest ones (below 10%) occurred in Japan, North Korea and Zimbabwe, to mention few examples. The phenomena of an increasing human mass and size has broader implications for resource use beyond food. Other energy and material implications are foreseen in mobility, shelter, furniture, clothing and waste management, e.g. larger living spaces and vehicles. Consequently, the results are discussed in the light of the dynamic qualities of populations as key factors for understanding the anthropocene, particularly in the context of the sustainable development goals 2, 11 and 13 in relation to food security, resource efficiency and climate change.
keywords: human mass, population growth, food security, stock dynamics
1a Sustainable development science: fundamental concepts (definitions, fundamental concept
