Putting your head in the sand and saying "eat less meat" is not going to fix the problem. As billions of people come out of poverty, they're going to rightly demand their share of meat.

As has been pointed out, it's easy to feed 9 billion people on grain alone. But feeding 9 billion people meat and a healthy variety of vegetables is a much more difficult challenge.

But luckily, there are ways to fix this problem and still let people eat their meat:

1) cows eat grass. Currently, modern cows eat grain. Historically, cows ate grass. Grass-fed cows are healthier, happier and much more nutritious. Grain fed cows also waste most of the grain they're fed: a cow fed corn needs 12 calories of grain to produce a single calorie of meat. But a chicken needs far fewer pounds of grain since birds evolved to eat grains & insects. Cows eat grass. A grass-fed cow doesn't "steal" any food from humans.

So why do we feed them grain instead of grass? Because it's cheaper. Huge numbers of dollars have been invested in increasing our production of wheat, corn and rice, to great success: yields today on non-virgin farmland are 4-10 times as large as they were 150 years ago. A comparable effort to raise the production of crops such as alfalfa will provide huge benefits. It's insane that it's more efficient to to plant a crop of barley, harvest the seeds to feed to cows and throw away the rest of the plant than it is to plant something else and feed the whole plant to the cow.

The other part of the solution is to eat a wider variety of meats and less beef. Bacon is the only form of pork that Americans eat, yet it's tastier than beef, IMO. Luckily, most Asians already know that.

2) fix our fishing: Current fishing practices are hugely wasteful. We've got a huge number of boats chasing a small number of fish, reducing stocks well below the most efficient breeding population size. Properly managed oceans could produce far more meat calories for humans than it currently does. We'd have to reduce our fishing now, but do we want a few calories now or a large number of calories later? Through the tragedy of the commons, we're saying a "few calories now".