Positive feedback loops that reinforce global warming are scary. Here’s an example of such a feedback loop: warmer temperatures melt Arctic sea ice earlier in the spring and reduce the size of the summer ice pack. Since the dark ocean has less reflectance than ice, a smaller ice pack means that more solar radiation is absorbed by the ocean every summer, further warming the planet.
Another worrisome positive feedback loop arises when warming temperatures melt long-frozen permafrost. The melting permafrost releases methane gas bubbles that have been captive for thousands of years. The release of methane, a powerful greenhouse gas, warms the planet further.
Now researchers are focusing on another feedback loop affecting the global climate: obesity. Here’s how the loop works: when people walk less and drive more, they gain weight. Once they’ve gained weight, they are even less inclined to walk. (Heavier people are more likely to drive and less likely to walk than people who aren’t overweight.) Once this positive feedback loop starts, the number of miles driven continues to increase — and so does the amount of carbon dioxide released into the atmosphere.
The issue is discussed in a paper by Phil Edwards and Ian Roberts, two researchers from the Department of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine. Their paper, “Population Adiposity and Climate Change”, was published in The International Journal of Epidemiology. “When it comes to food consumption, moving about in a heavy body is like driving around in a gas guzzler,” Edwards and Roberts wrote. “The heavier our bodies become the harder and more unpleasant it is to move about in them and the more dependent we become on our cars.”
Why obesity contributes to global climate change
It turns out that the driving-versus-walking feedback loop is only one of many ways by which obesity worsens global climate change. Among the other factors:
- In hot climates, overweight people require more energy to cool.
- Overweight people require more food every day, and the extra food requires fossil-fuel energy for its production, packaging, transport, and refrigeration. Among the reasons that overweight people require extra calories: it takes more caloric energy to move the extra weight. A paper quantifying our population’s excess weight and its climate-change effects (“The Weight of Nations: An Estimation of Adult Human Biomass”) was recently published in BMC Public Health. The study’s lead author, Dr. Sarah Walpole, was quoted in a Climatewire article: “Energy use is a function of the basal metabolic rate. A heavier body needs more food to be sustained.”
- Overweight people require more fossil-fuel energy per mile to transport, whether by car, bus, train, or plane. In an article called “Worried About Climate Change? Fix Obesity,” Bryan Young quotes Dr. Sheldon Jacobsen, a professor at the University of Illinois, who said, “For every pound the average American is overweight, we use an additional 938 million gallons of gasoline per year. That’s enough to fill 2 million cars with gasoline every year.” Young noted, “It’s a simple matter of physics: the heavier you are, the more energy it takes to move you in your vehicle.”
12 Americans weigh as much as 17 Asians
The average Asian weighs 112 pounds. On the other end of the spectrum, the average North American weighs 178 pounds. Although North America has only 6% of the world’s population, this continent has 34% of the world’s “obesity-related human biomass.” By contrast, Asia has 60% of the world’s population but only 13% of the world’s “obesity-related human biomass.”
Dr. Walpole and her co-authors (David Prieto-Merino, Phil Edwards, John Cleland, Gretchen Stevens and Ian Roberts) used data from the World Health Organization and the United Nations to compare the average body mass index (BMI) in various countries. They wrote, “In 2005, global adult human biomass was approximately 287 million [metric] tonnes, of which 15 million tonnes were due to overweight (BMI > 25), a mass equivalent to that of 242 million people of average body mass (5% of global human biomass). Biomass due to obesity was 3.5 million tonnes, the mass equivalent of 56 million people of average body mass (1.2% of human biomass).”
Summing up, they wrote, “One tonne of human biomass corresponds to approximately 12 adults in North America and 17 adults in Asia.”
Large people have a big carbon footprint
In a Climatewire article discussing Dr. Walpole’s paper, Umair Irfan writes, “If the rest of the world ate, worked and lived like Americans, humanity’s spare tire would swell by the equivalent mass of 935 million average-sized people and soak up the food, water, and electricity of an additional 473 million adults, according to the report. This would mean the average adult would eat an additional 261 calories per day.”
Irfan also imagined a better future: “If Japan were the model for the entire world, global human biomass would shrink by 14.6 million metric tons, equal to 235 million people of average mass. Energy requirements would decrease as if there were 107 million fewer adults on the planet.”
Irfan quoted Ian Roberts, one of Dr. Walpole’s co-authors. “We don’t feed mouths, we feed bodies. We feed flesh,” said Roberts. According to Roberts, “the blame for resource scarcity and ecological degradation should be placed less on poor people having more children and more on the relatively wealthy living unhealthy and unsustainable lives.”
Wealthy and unhealthy
In their The International Journal of Epidemiology paper, Edwards and Roberts calculated that an overweight population requires 19% more food energy than a population with a healthy BMI. They also calculated that lean population of 1 billion people would emit 1,000 million metric tons less carbon dioxide per year than a fat population of the same size.
Global climate change is an unfortunate side-effect of affluence. Compared to the average human, North Americans are wealthier and live in larger houses; we’re also fatter. The trends that have brought us to this point in history are clearly unsustainable. After all, if our houses, chairs, and sofas continue to get bigger every year — presumably, the better to accommodate our increasing biomass — the end of our species isn’t far off.
Last week’s blog: “New Green Building Products.”