Conventionally farmed meat is one of the worst things for the environment. A kilogram of beef is responsible for more greenhouse gas emissions and other pollution than driving for 3 hours while leaving all the lights on back home, says the Akifumi Ogino of the National Institute of Livestock and Grassland Science.  The energy required to feed, raise, butcher, transport and generally make meat edible is pretty substantial amount.

Also, livestock farmers in some places are deforesting areas to get more grazing land.

The following information is taken from The Food Revolution.

 Waste, Water, and Rainforests

• In traditional farming, the animals ate grass, crop wastes, and kitchen scraps that people could not eat, and turned them into food that people could eat. Their manure provided the soil with needed nutrients as a natural and biodegradable fertilizer, and it was a cycle that sustained the environment, the animals, and the people.

• With the expansion and mechanization of animal farming, world meat production has quadrupled in the last 50 years. —"Taking Stock: Animal Farming and the Environment"

• There are now 20 billion livestock on Earth—more than triple the number of human beings. —World Watch Paper

• In years past, most of the manure from livestock returned to enrich the soil. But today, when huge numbers of animals are concentrated in feedlots and confinement buildings, there is no economically feasible way to return the animals' wastes to the land.

• Deprived of the manure and continually doused with chemicals, our nation's soils are losing their texture and ability to retain topsoil, the rich layer without which food production becomes seriously endangered.

• The production of every quarter pound hamburger in the US causes the loss of five times the burger's weight in topsoil. —Stuff: The Secret Lives of Everyday Things

• In the U.S., livestock now produces 130 times as much waste as people do. —"Will We Still Eat Meat?" Time Magazine

• We have strict laws governing the disposal of human waste, but the regulars are lax, or often nonexistent for animal waste. —Union of Concerned Scientists

• Instead of being returned to the soil and helping to rebuild topsoil, the waste form today's livestock often end up in our water.

• Water required to produce 1 pound of U.S. beef, according to the National Cattlemen's Beef Association: 441 gallons —"Myths and Facts about Beef Production: Water Use" National Cattlemen's Beef Association

• Water required to produce 1 pound of U.S. beef , according to Dr. Georg Borgstrom, Chairman of the Food, Science and Human Nutrition Department of the College of Agriculture and Natural Resources at Michigan State University: 2,500 gallons — "Impacts on Demand for and Quality of Land and Water"

• Water required to produce 1 pound of California beef, according to the Water Education Foundation: 2,464 gallons —"Water Inputs in California Food Production"

• Water required to produce 1 pound of California beef , according to the Soil and Water specialists, University of California Agricultural Extension, working with livestock farm advisors: 5,214 gallons —Soil and Water

• In California today, you save more water by not eating a pound of beef than you would by not showering 6 months to a year (depending on which figure you want to use).

• In Central America, cattle typically graze on land that was rainforest before being cut down and burned to be use fro rangeland. According to the Rainforest Action Network, 55 square feet of tropical rainforest, an area the size of a small kitchen, is destroyed for the production of every fast food hamburger made from rainforest beef. —People of the Tropical Rainforest

• In both 1993 and 1994, the U.S. imported over 200,000,000 pounds of fresh and frozen beef from Central American countries. Two thirds of these countries' rainforests have been cleared, primarily to raise cattle whose cheap meat is exported the U.S. food industry. When it enters the U.S., the beef is not labeled with its country of origin, so there is no way to trace it to its sources. —"Seven Things You Can Do to Save the Rainforest" Rainforest Action Network

• Imports of beef by the U.S. from southern Mexico and Central America during the past 25 years has been the major factor in the loss of about half of the tropical forests there—all for the sake of keeping the price of hamburger in the U.S. about a nickel less than it would have been otherwise. —We're Killing Our World: The Global Ecosystem in Crisis MacArthur Foundation Report

Here are a few suggestions:

-Avoid fast food meat .
-If you eat meat, try and make it local/organic. (or at least cut down on amount)

Related:

The Food Revolution: How Your Diet Can Help Save Your Life and Our World