Energy can be converted from one type to another, , but the process of moving from one specific type to another is usually inefficient. Sunlight falls on green plants and photosynthesis does some chemistry that stores some of the energy in sugars (among other things) that animals and people can use. The process is usually less than one percent efficient.
We can eat the plants directly or we can eat animals that eat the plants. The conversion process from plant to growing animal is also inefficient. Not only is the animal inefficient at turning plant into animal, but food and animal must be transported. The economics of farming attempts to optimize the growth rate of the animal - grow them fast and slaughter them as soon as possible, use foods that are optimized for growth, make sure they don't exercise too much, etc - but the process is still very inefficient. (optimizing animal growth and lowering cost diminishes the quality of life of the animal too, but most people ignore that. It is strange that the most energy efficient methods may be the least humane).
There are other problems when you make a lot of certain types of animals. Some of them emit large quantities of methane - a green house gas that is much more potent that carbon dioxide. Processing, refrigerating and moving the meat around adds other ocsts. The February, 2009 issue of Scientific American has a short article (online version
here) that quantifies some of the environmental costs of eating different types of meat. Beef is particularly bad (cows emit a lot of methane).
snip
Pound for pound, beef production generates greenhouse gases that contribute more than 13 times as much to global warming as do the gases emitted from producing chicken. For potatoes, the multiplier is 57.
Beef consumption is rising rapidly, both as population increases and as people eat more meat.
Producing the annual beef diet of the average American emits as much greenhouse gas as a car driven more than 1,800 miles.
You can work the numbers, but the average meat consumption of people in the US yields a greenhouse gas footprint by itself that is too large to stabilize greenhouse gases at current levels even if we stop using other forms of greenhouse gas emitting energy, Moving to environmentally more friendly meats and eating less seems reasonable, but beef consumption is rising rapidly worldwide.
The FAO report
Livestock's Long Shadow is referenced. We've mentioned it here before. Fascinating reading. I have serious doubts people will make serious changes to their diets. I have a difficult time dealing with more inhumane treatment of our food supply, but that is more efficient. This may be a case where lower prices from efficiency increase total consumption and erase the benefits of lower emissions.