"Population fatness has an environmental impact," said Phil Edwards, from the London School of Hygiene and Tropical Medicine. "We're all being told to stay fit and keep our weight down because it's good for our health. The important thing is that staying slim is good for your health and for the health of the planet."
and for the UK
"due to the increased number of people today who are overweight, the UK consumes 19% more food than it did 40 years ago, which equates to an extra 60 mega tonnes of greenhouse gas emissions a year. It also included the extra transport-related emissions caused by the increased obesity."
In addition, the meat eaters are killing our earth
"Last September the world's leading authority on climate change suggested the people should eat less meat, because meat production causes 20% of global emissions."
In my opinion the whole issue with taxing or discriminating the obese or overweight - including in flights- depends how much of individual choice is involved in becoming obese. How many obese actually do choose to be obese, and when does this choice begin? What if you were "nurtured" into being obese. This is an important issue, since depending on this would we be able to consider the obese either the same as, smokers, most cases individual choice and fault, or on the other hand, disabled people, most cases not through one’s choice nor fault.
On the other hand, I think one of the best way to overcome this problem is to make the fatty foods, that lead directly or indirectly to carbon emission - expensive through taxation. Thus, encourage people to eat healthy not through the use of taxing the people directly, but making incentives to eat healthy. Making organic fresh veggies – cheaper and more accessible than what it is now, where they are the most expensive item in the supermarket.
Whereas make food products that directly produce more carbon emissions in their production process – such as meat, as mentioned in the article - and food that indirectly can lead to more costs may it be carbon emissions or health costs, due to the problems it may cause to the individual’s health – such as sweets and fatty foods -more expensive through taxation on two accounts 1) carbon emission throughout the production of the food item and 2) health implications of the food to individuals, not only in the short-term but also in the longer-term.
Of course governments will be reluctant to do this, because it is regulating the market and increasing taxes for them, which all politicians hate to do. In addition, such measures have to be done in collaboration throughout the world, since we need to regulate the whole process of food production to retail. Since world's food production is also becoming more and more globalized, the world has to work together to tackle this issue to examine/regulate the whole chain of food production, but also to make sure there will not be any competitions between countries in trying to save their own food production groups, while enforcing stricter rules on others.
For more read guardian article and this on the UK and how we should think of taxing the obese for this reason