As the number of farm animals is increased to meet the growing population's demands for more meat, egg, and dairy products, greenhouse gas emissions will rise. Farm animal production is expected to double by the year 2050, however, the current practices will be unsustainable for that long.
Impacts of Animals
There are estimates of around 70 billion animals being slaughtered for human consumption every year. With each animal comes the production of manure. Although some of the manure can be used to help provide nutrients to nearby cropland, not all of it can be used. In the case when all of the manure can't be used, it is stored in a small area of land. Storing it this way leads to soil accumulation and runoff of pollutants such as phosphorus. Pasture-raising cattle is more of a common practice in some parts of Asia and Africa, but as cattle production increases in Latin America, they turn to unsustainable production-focused systems.
Impacts of Animal Feed
Producing feed for farm animals is responsible for 9% of total CO2 emissions. All of that CO2 is emitted by producing fertilizer for the feed crops, transporting that feed, creating animal product, transporting animal product, and on-farm energy use. The amount of CO2 created to produce fertilizers from burning fossil fuels alone is 41 million metric tons a year. 90 million metric tons are used just to confine animals into these industrial production-focused operations, which produce a lot more CO2 than smaller operations or pasture-based farms. Close to 1 million metric tons of CO2 are produced just transporting animal products and feed to the animals. Producing just one kg of beef will result in 36.4 kg of CO2 emissions a year.
Impacts on Land
Animal production facilities consume 1/3 of the planet's land surface. Out of all of the land used for agriculture, 2/3 is used to produce feed for farm animals. Converting woodlands into agricultural land or land to be used for animal production is estimated to create around 2.4 billion tons of CO2 emissions every year. Cultivation of that soil in order to produce animal products releases another 28 million metric tons.
Methane Production
Farm animals are a large producer of methane, accounting for 30-40% of methane emissions. Farm animal manure creates 18 million metric tons of methane annually. Although cattle who are grain-fed are able to get to the desired weight quicker when compared to pasture-fed (thus having shorter life-spans), the unnatural high protein diet results in higher methane output. The manure of pasture-fed cattle have been found to have about half the methane-producing capacities
Source
Koneswaran, & Nierenberg, D. (2008). Global Farm Animal Production and Global Warming: Impacting and Mitigating Climate Change. Environmental Health Perspectives, 116(5), 578–582. https://doi.org/10.1289/ehp.11034