This is a partial answer, based on some research. Many web searches on this topic are difficult, as many sites have a bias one way or another. This answer will include names of researchers and links to articles when I can.

Note: a lot of articles are based on the American situation; however, these studies can be used as a 'measuring stick' for much of the meat-eating world.

Some statistics are presented in the article Sustainability of meat-based and plant-based diets and the environment. (Pimentel and Pimentel, 2003) stating that globally, as of about 2003:

• About 2 billion people have a meat based diet, the remaining having a plant based diet, due to

shortages of cropland, fresh water, and energy

• In a comparison (for the US), all food production was considered by the authors as being unsustainable - utilising 50% of the land, 80% of the fresh water and critically (according to the authors), 17% of the fossil fuels. However, having said that, when they compared

The use of land and energy resources devoted to an average meat-based diet compared with a lactoovovegetarian (plant-based) diet

comparing the resources required for the same amount of calorie intake, they found that

the lactoovovegetarian diet is more sustainable than the average American meat-based diet.

in terms ofland and water use and fossil fuel requirements.

Further data is presented in the article How Does Meat in the Diet Take an Environmental Toll?, stating that studies by the Environmental Working Group indicated that

red meat such as beef and lamb is responsible for 10 to 40 times as many greenhouse gas emissions as common vegetables and grains.

Another critical point the article reported states is that

“If all the grain currently fed to livestock in the United States were consumed directly by people, the number of people who could be fed would be nearly 800 million,” reports ecologist David Pimentel of Cornell University’s College of Agriculture and Life Sciences.

I know that is a single country example, but it does provide an indication of statistically how a change to a vegetarian/vegan diet could be of benefit.

The article Diet and the environment: does what you eat matter? (Marlow et al. 2009), describes a key statisical measure, using California as an example:

or the combined differential production of 11 food items for which consumption differs among vegetarians and nonvegetarians, the nonvegetarian diet required 2.9 times more water, 2.5 times more primary energy, 13 times more fertilizer, and 1.4 times more pesticides than did the vegetarian diet.

This is a partial answer, based on some research. Many web searches on this topic are difficult, as many sites have a bias one way or another. This answer will include names of researchers and links to articles when I can.

Note: a lot of articles are based on the American situation; however, these studies can be used as a 'measuring stick' for much of the meat-eating world.

Some statistics are presented in the article Sustainability of meat-based and plant-based diets and the environment. (Pimentel and Pimentel, 2003) stating that globally, as of about 2003:

• About 2 billion people have a meat based diet, the remaining having a plant based diet, partly due to

shortages of cropland, fresh water, and energy

• In a comparison (for the US), all food production was considered by the authors as being unsustainable - utilising 50% of the land, 80% of the fresh water and critically (according to the authors), 17% of the fossil fuels. However, having said that, when they compared

The use of land and energy resources devoted to an average meat-based diet compared with a lactoovovegetarian (plant-based) diet

comparing the resources required for the same amount of calorie intake, they found that

the lactoovovegetarian diet is more sustainable than the average American meat-based diet.

in terms of land and water use and fossil fuel requirements.

Further data is presented in the article How Does Meat in the Diet Take an Environmental Toll?, stating that studies by the Environmental Working Group indicated that

red meat such as beef and lamb is responsible for 10 to 40 times as many greenhouse gas emissions as common vegetables and grains.

Another critical point the article reported states is that

“If all the grain currently fed to livestock in the United States were consumed directly by people, the number of people who could be fed would be nearly 800 million,” reports ecologist David Pimentel of Cornell University’s College of Agriculture and Life Sciences.

I know that is a single country example, but it does provide an indication of statistically how a change to a vegetarian/vegan diet could be of benefit.

The article Diet and the environment: does what you eat matter? (Marlow et al. 2009), describes a key statisical measure, using California as an example:

or the combined differential production of 11 food items for which consumption differs among vegetarians and nonvegetarians, the nonvegetarian diet required 2.9 times more water, 2.5 times more primary energy, 13 times more fertilizer, and 1.4 times more pesticides than did the vegetarian diet.

A final global statistic (estimate) reported in the article UN urges global move to meat and dairy-free diet (Carus, 2010), states that a UN report determined that

Agriculture, particularly meat and dairy products, accounts for 70% of global freshwater consumption, 38% of the total land use and 19% of the world's greenhouse gas emissions.

TL:DR Given that statistically, many studies at local, state, national and global levels have shown that meat product agriculture is a major consumer of not only water and land, and not only are a significant contributor to environmental problems, they are also are a great consumer of grains that could be used as food.

However, many studies indicate that as modern agriculture is not all that sustainable, changes would be needed to be made to agricultural practices to make a global shift towards vegetarianism truly sustainable.