"WE KNOW the world is warming and we know humans are causing it, but the headaches really start when we try to predict how hot it will get. This is partly because of the many subtle environmental and lifestyle factors driving climate change.
One of these is the feedback that could amplify warming. Once the powerful greenhouse gas methane starts to be released from warming oceans or permafrost, it could rapidly ratchet up the global thermostat. The same goes for decreasing snow and ice cover. Because of this, researchers are keeping a close eye on the oceans, ice caps, forests and tundra for signs of such changes.
Now consider what may seem a more unlikely driver of climate change: the global obesity epidemic. We tend to think of obesity only as a public-health problem, but many of its causes overlap with those of global warming. Car dependence and labour-saving devices have cut the energy people expend as they go about their lives, at the same time increasing the amount of fossil fuel they burn. It's no coincidence that obesity is most prevalent in the US, where per capita carbon emissions exceed those of any other major nation, and it is becoming clear that obese people are having a direct impact on the climate. This is happening through their lifestyles and the amount and type of food they eat, and the worse the obesity epidemic gets the greater its impact on global warming.
One reason is the extra food that obese people eat - about 40 per cent more in terms of calories than their lean counterparts. Since food production accounts for over 20 per cent of greenhouse gas emissions - more than transport or industry - an obese population leaves a significantly heavier carbon footprint than a thin one. Fats and refined sugars, which tend to dominate the diets of obese people, are particularly carbon intensive. Greater food consumption means more organic waste, which produces methane as it decomposes.
The wide availability of food is a relatively modern phenomenon, born partly of the revolution in agriculture that kicked off in the 1940s. Before then, the amount of food that could be harvested from a piece of land was limited by the nutrients and water available naturally in the soil. Seemingly limitless supplies of oil changed everything. Over the second half of the 20th century, fertilisers, pesticides, irrigation and mechanisation - all dependent on fossil fuels - dramatically increased food yields. As the physicist Albert Bartlett observed: "Modern agriculture is the use of land to turn petroleum into food." Coupled with sedentary lifestyles, the increased availability of food has helped the number of obese adults worldwide to quadruple over the past 25 years.
Demand for food is not the only way in which obesity has dire consequences for the climate. Consider what happens to someone on the path to obesity. It might start when he (or she, of course) decides to drive rather than walk the half mile to the office, just to get there a few minutes earlier. A year on he might have gained a kilogram of fat, and as the weight continues to pile on he eventually finds it harder to move around and is loath to walk or cycle anywhere. Then the medical problems start: back pain, arthritis and shortness of breath, or worse. Obesity increases the risk of heart disease, stroke, diabetes, osteoarthritis, infertility, gallstones and several types of cancer.
By now he'll be suffering low self-esteem, which leads to comfort eating and perhaps heavier drinking too. He'll even notice a load on his household energy bills: his greater bulk and higher metabolic rate will cause him to feel the heat more in the globally warmed summers, and he will be the first to turn on the energy-intensive air conditioning.
The result is more food intake, more vehicle use and greater reliance on medical services, all of which use prodigious amounts of energy. And as the number of obese people increases, a kind of positive feedback kicks in. Obese people in the US are already throwing their political weight around. The American Obesity Association is dedicated to "changing public policy and perceptions about obesity". How long before there are calls for energy-guzzling escalators, moving sidewalks and motorised mobility aids? The social stigma attached to obesity is one of the few forces slowing the epidemic - even though obesity is not a personal failing but a problem of society. We live in an environment that serves primarily the financial interests of the corporations that sell food, cars and petroleum.
The Stern review on the economics of climate change, published by the British government in October 2006, warned of the dire ecological consequences of continuing a "business as usual" approach to greenhouse gas emissions. That applies equally to the obesity epidemic, which is spiralling out of control. Pandemic obesity is an energy vortex. It is time to treat it as the potential global environmental catastrophe that it is."
Ian Roberts is professor of public health at the London School of Hygiene and Tropical Medicine