The use of animals to convert plant proteins and nutrients into meat, milk, eggs and other animal products is hugely inefficient (see (1) Metabolism). High inputs are required (land, water, chemical fertilizers, pesticides) and high outputs are generated (greenhouse gases, acidifying gases, eutrophying substances) (see (1.1), (1.2)). On the one hand, animal agriculture contributes significantly to climate change through its direct greenhouse gas emissions and land lost to carbon sequestration. On the other hand, habitats are minimized, animals and plants are displaced and killed, complete ecosystems are destroyed, and species are extirpated through the clearing of forests, through high water consumption, through acidifying gases, and through eutrophying and toxic substances. This ultimately leads to a loss of biodiversity.
Climate change and biodiversity loss are closely related and mutually reinforcing (positive feedback loop).
The figures and calculations on the share of animal agriculture in climate change differ widely. The most common and at the same time smallest figure (14.5%) comes from the Food and Agriculture Organization of the United Nations (FAO). Here it must be taken into account that the FAO is steered and financed by all global animal industries.
The following slides provide an overview of various analyses, their calculations and main issues.
A tabular one-page overview (PDF) incl. list of references and links to all references can be downloaded here:
Based on the input of customizable parameters, this online calculator allows the calculation of the contributions of livestock and fossil fuel industries to global warming:
Averting the climate catastrophe without a phase-out of animal agriculture is impossible for the mere reason that not only must greenhouse gas emissions be reduced to zero, but the CO2 concentration in the atmosphere must also drop drastically.
A safe CO2 concentration in the atmosphere for the survival of mankind is 350 ppm . In 2020, the concentration was 413 ppm. To get back from there to the level of 350 ppm, a sequestration of 491 Gt CO2 is required . Technical solutions are lacking, so the only option is afforestation. Enormous areas are needed for this. A shift to a purely plant-based food system frees up an area of 31 million square kilometers . Renaturalization (reforestation, conversion back to peatlands, etc.) can sequester up to 547 Gt CO2  within 30 years and 810 Gt CO2  within 100 years (= carbon opportunity cost of animal agriculture).
The necessary sequestration of 491 Gt CO2can be achieved within 27 years.
An additional prerequisite for achieving the safe CO2 concentration is the reduction of CO2 emissions to net zero, which totaled 39 Gt CO2 per year in 2020. This essentially requires a complete phase-out of fossil energies and solutions for cement.
Simply reducing CO2 emissions to net zero will not solve the problem of high CO2 concentrations in the atmosphere, since CO2, unlike methane, for example, is a very long-lived gas over centuries.