The argument that we should eat as vegetarians for environmental reasons is very popular. One of the ones that resonated most with me was Graham Hill, the founder of treehugger.com who gave a TED talk on why he's a weekday vegetarian. It's compelling. Meat is an industry and one that produces carbon dioxide emissions both from the transport of its goods, and from the goods itself in the form of methane. Also the land, water and transport required to produce the food to sustain these goods is problematic. Although you don't see Graham championing a call for a weekday 'refraining from metal'.
On the subject of water in meat production, see a recent twitter thread by Dr. Sarah Taber, a crop scientist, ex-farmer and the host of the Farm to Taber podcast. She looks at why historically, the people of dry grasslands have been meat eaters. The environment is such that what grows there is food fit for cattle consumption, not human consumption. Our stomachs cannot digest those high cellulose plants - but a cow's 4 chambered stomach and cud-chewing eating process can.
The consideration in dry regions is not about land, it is about water. And crops for human consumption use significantly more water than crops for cattle consumption, taking into account the drinking that the cows need to do! The highlight? "It takes a thousand times more water to grow an acre of crops for human consumption, than it takes to grow an acre of cow on wild range." She goes through the math to arrive at this. But in the end it is about location. There are certainly better places that are situated for growing crops for human consumption. But in arid climates which are in fact significantly water-stressed, it might make sense to consider cattle crops as a more sustainable form of food delivery. That is of course if the cattle is 'free range', as industrial meat production does apparently utilize far more water per kilogram of protein.
The point here is that water usage is a critical factor when considering how we will arrange our food in the world of the future accounting for 10 billion people. The vegetarian diet alone can possibly be better if one is getting their protein from lentils grown in non-arid climates. But consider this infographic from the Water Footprint Network.
By this account, those who don't consume beef based on its water use should also stop consuming coffee and chocolate. This doesn't even mention tree nuts like almonds and cashews which have water usage through the roof per kilogram of protein. And there are tons of milk and meat substitutes out there composed of these water-intensive tree nuts.
It is extremely hard to change global habits particularly as new economies grow and develop and want the wealthier lifestyle that is afforded much of the developed world. Rather than limiting how people continue to consume their food, there need to be technological leaps and planning that account for a world where people can continue to eat meat, drink coffee, eat chocolate and eat vegetables and continue to have the water they require.
We need need technological innovations to make taking planes a much cleaner way to travel, just as we need technological innovations to allow everyone to eat meat if they want to.