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Tuesday, July 24, 2012

Don’t count people, measure biomass



Many calculations about global resources rely on the current and projected world population. However, we may be off by quite a bit by not taking obesity into account. According to Sarah Walpole and her colleagues from the London School of Hygiene and Tropical Medicine, we should be measuring total human biomass rather than counting individuals.

Biomass is the total mass of whatever biological entity you’re interested in. It could be every living thing on Earth, or all plants, or all red pandas. The human biomass is the combined mass of all humans on Earth.

Clearly, fat people have more mass than skinny people. Heavier people not only take up more space than thin people (which is rarely an issue), but they use up more resources, which can be significant when we’re counting up how much of something there is to go around. It simply takes more energy to sustain a larger body than a smaller one. Thus, if you have a population of obese people, they’re going to use up resources much faster than the same number of thin people would have.

The researchers used information from the United Nations, the World Health Organization and national databases to estimate the population and the mean body mass index (BMI) of adults in 190 countries. From this, they extrapolated to the total adult human biomass of the planet.

The scientists estimated that in 2005, the total adult human biomass of the planet was about 287 million metric tonnes. If none of those people had been overweight, the total would have been about 15 million tonnes less, or about a 5% difference. 

The researchers then compared two different scenarios. In the first, the entire planet had BMIs in the same distribution as that seen in Japan, and in the second, the whole world looked like the U.S. These represented normal weight and fat extremes, as it takes about seventeen Asians to make up the same body mass as twelve Americans. If the whole world looked like Japan, it would be the equivalent of losing almost a quarter of a billion people from the world population. On the other hand, if every place looked like the U.S., it would be like gaining close to a billion new inhabitants.

There were some limitations to this study, not least of which was that they did not include children, who make up quite a large fraction of the human biomass, obese or otherwise. And of course, these are only rough estimates. However, it’s important to note that projections about how many people an area can feed or how long a resource is expected to last may be vastly underestimated if people use population counts without adjusting for biomass. As there’s every indication that the rest of the world is emulating the U.S. and not Japan, the discrepancy between predicted resource usage based on population and actual usage will only get greater.