So many want so simple answers. Is meat good or bad for the environment? Does it take away food from poor people? Sorry, there is no such simple answer. It is one thing if you don't want to kill animals, or support a system that kill animals at all. That is a ethical position which obviously can be discussed as such (I am not going to in this posting though). It is quite another story to say that meat is bad for other reasons. I believe it is simply not possible to make such statements. Fish can be produced in a nice way in small scale ponds integrated with farming and it can be produced (rather consumed) from deep-sea fishing, depleting the fish stocks, using huge amounts of energy. Lettuce can be produced in heated green-houses, consuming disastrous amounts of energy compared with its energy content. Grain can be produced in monocultures with irrigation depleting ground water. As little as we refrain from eating grain because some (as a matter of fact most of it) grain production is bad, should be we judge meat as it was one product.
Global meat consumption per capita doubled between 1983 and 2005, which meant that total production increased even more as population also grew with almost 2 billion. Consumption of meat is estimated to double again between 2000 and 2050, the biggest increase in developing countries. China is today both the biggest producer and consumer (FAO 2006). Meat consumption is fairly strongly correlated to income. For an average Brazilian an increase of income with ten percent leads to an increase of meat consumption with 7 percent, but for the average American, who already eats so much meat, the same increase results in only one percent increase in meat consumption (USDA 2008).
At any given time, there are around 4.3 billion animals in our service. Not only has the number of animals increased but their role in farms has changed. In traditional European farming, livestock was an integrated part of farming; they were a source of power and they occupied different ecological niches than the humans; they transformed nutrients, both to the field and directly to humans, from lands that were not suitable for arable farming and they supplied us with meat, milk, eggs, fibre, feather, fats and natural remedies. In many cultures, especially the pastoral cultures, livestock was and is a source of status, pride and social position.
Livestock provides 17 percent of all energy and 33 percent of all protein for human nutrition. Meat consumption ranges from 5 kg per person and year in India to 123 kg in the United States (FAO 2006). The changes in the livestock sector influences also the arable farming as more meat is nowadays grain fed. This is particularly the case for pigs and poultry. Thirty-eight percent of all grain in the in the world is used for animal feed, in high income countries as much as 60 percent. It takes 3 kg of grain to produce 1 kg of chicken and slightly more for pork. Ruminants, cattle, sheep and goats, are increasingly also fed by grain, something they are not well adapted to. Their main diet should be grass, which humans can’t eat.
Things are not so simple though. Globally it is assessed that it takes 1.3 to 1.4 kg of protein in grain and others sources to produce 1 kg of animal protein, but animal proteins have mostly higher quality than vegetable proteins. Increasing consumption of animal protein is one of the easiest (admittedly not the only one) methods to rapidly improve nutrition and avoid long term damage caused by malnutrition of children, who may never develop to their full bodily or intellectual potential. Livestock is an important buffer of food and provides livelihoods for people who otherwise had no income and there is no evidence that livestock production detract food from those who currently go hungry (CAST 1999, FAO 2006). 600 million people today are engaged in small-scale livestock production and 200 million people live on pastoralism. This kind of animal production uses ecological niches that would not be suitable for arable farming. Ruminants graze pastures that are no good for arable farming, e.g. to steep, too dry, to cold. Small-scale pig or poultry rearing is based on waste products from field or kitchen, sometime from food processing (e.g. pigs getting whey or distiller’s wash). That kind of livestock production doesn’t really compete with production of food for humans. In addition, there are 400 million heads of livestock used as draft animals, and their role is mainly to help humans to grow more food for themselves, even if they also, mostly, also end up on our tables. (FAO 2007, Worldwatch Institute 2006, Erb and others 2009). We have seen in the case of the first agriculture revolution in Europe that integration of livestock in the farm production, including the growing of fodder on arable land, actually increased production tremendously, both of feed and food.
Industrial livestock production is, however, the opposite to the traditional systems, it needs a lot of capital and nature resources, water consumption is big. To produce 1 kg of beef 100,000 litres of water is used. Energy use is also high; a calorie of beef requires thirty-three times as much energy to produce than one calorie of potatoes. Large scale ranching might be better in many of these regards, but instead poses threats to other valuable ecosystems e.g. from forests that are cleared for grazing; A big share of the increased meat production in Brazil is from the rainforest zone (FAO 2007, Worldwatch Institute 2006).
What about green house gases?
Methane is emitted from ruminants, from paddy rice production and from manure. Knowledge is still scarce and incomplete for many aspects of methane emissions, e.g. how the feeding influence methane emissions or how one can reduce methane emissions from paddy rice. Rice production with no or less flooding, e.g. the SRI system, release less methane than the normal practices. The methane emission is from the rice plant itself (up to 90%) and there are variety differences that could be exploited (FAO 2003). Intensification of livestock systems will lead to higher emissions from manure handling, but is claimed to lead to less methane per produced unit of meat or milk. But we also need to relate the emissions from the domesticated ruminants with the wild fauna they have replaced. In some areas cattle, sheep and goats have replaced herds of buffalo, deer and bison, animals that also are ruminants and emit methane. On the American prairies there were 30 million bison two hundred years ago (Chapman and Reiss 1999) and the number of cows today are more or less the same. Presumably cows are eating more, despite their smaller size, because of their high production. Research has shown that permanent pastures, such as the Hungarian puszta, can accumulate organic matter for centuries and thereby be substantial carbon sinks and to some extent balance the methane emissions from grazing cows (SvD 2008). The methane emissions of a typical African cow are, according to researchers at the International Livestock Research Institute, normally offset by carbon sequestration in their pastures (Maarse 2010). In addition, if land can not be used for pasture it has three other destinations, converted into forest, left idle or converted into farm land. The last alternative leads inevitably to more carbon emissions; the average loss of soil carbon was 59% according to a meta-analysis of 80 reports on conversion of grassland to cropland (FAO 2009). Therefore, it is doubtful if one can count the metabolism of ruminants like any other factor.
A report from a “meat-negative” organization, Friends of the Earth, says:
“It should also be noted that extensive livestock systems with large input-output ratios are not necessarily inefficient. The efficiency measure (input-output ratio) is based on the assumption that animal protein is the major output of livestock systems, a perspective which fails to account for the utility of livestock in less developed regions where livestock fulfils a huge range of functions besides production of protein-rich food for human consumption. In low input agriculture, livestock is required to provide power for agriculture and transport and indispensable for the management of nutrients. A crucial function of livestock is the ability of ruminants to convert biomass not digestible by humans into food for humans, for example, biomass from waste lands or semi-deserts. Thus, livestock systems that appear to be inefficient due to their input-output ratio may in fact represent well-adapted, highly efficient production systems in their respective local contexts.”(Erb and others 2009).
It is well known that a lot of water is used to produce meat. But many misses a point. Pastoral meat production in dry places does use a lot of water (as rainfall), but there is not enough water per area unit to support farming, instead the animals walk around and eat the scarce grass and thereby make use of it. Again, industrial farming is quite something different.
From this brief overview, it should be clear that one can’t make general statements of the effect of eating meat. Most of the criticism of how meat is produced is targeting two different production systems: grazing of cattle in the rainforests zones and the industrial grain-based feeding in richer countries, but this has almost no relevance to large parts of the livestock production, and certainly not for the pastoralist systems. Still, the total increased pressure on nature resources caused by the combination of growth of population and growth of meat consumption is worrying.
 System of Rice Intensification, see more on http://ciifad.cornell.edu/sri/
 The evidence for this seems weak and few. To measure actual emissions from the two ends of the cow in practical experiments is complicated and the empirical materials is therefore very thin.