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The Global Livestock Environmental Assessment Model (GLEAM 2.0) is a program under the Food and Agricultural Organization of the United Nations. Scientists use a Geographic Information Systems (GIS) framework that simulates the biophysical processes and activities along livestock supply chains under a life cycle assessment approach to calculate the impacts of livestock on the environment.
According to GLEAM’s 2019 report, total GHG emissions from livestock supply chains are estimated at 8.1 gigatonnes CO2-eq (GT CO2-eq) per year for the 2010 reference period, not including land-use change. 1,2
Methane (CH4) accounts for 50 percent of the total. Nitrous oxide (N2O) and carbon dioxide (CO2) represent almost equal shares with 24 and 26 percent, respectively. The report used 298 as the Global Warming Potential (GWP) for N2O, and 34 as GWP for CH4.3 So the livestock supply chain total is 8.1 GT, composed of:
The total GHG budget for 2010 according to the Climate Analysis Indicators Tool (CAIT) historical climate data as graphed by Our World in Data is 46.55 GT CO-eq.5
Calculating the Percentage of Global Emissions from the Livestock Sector
Because the GLEAM Model uses different GWPs for N20 and CH4, it is necessary to convert their units to the same used by CAIT. The following equation accomplishes this:
To equate the total percentage emissions from the livestock sector globally, we therefore divide the total livestock emissions by the total emissions globally:
7.17 / 46.55 = 15.4% of global GHGs
1 Carbon dioxide equivalent (CO2-eq) is calculated from GWP. It can be measured in weight or concentration. For any amount of any gas, it is the amount of CO2 that would warm the Earth as much as that amount of that gas. It provides a common scale for measuring the climate effects of different gases, and is calculated as GWP times the amount of the other gas. For example if a gas has GWP of 100, two tons of the gas have CO2e of 200 tons, and 1 part per million of the gas in the atmosphere has CO2e of 100 parts per million.
2Including land use change--the clearing of rainforests in the Amazon to grow soy and corn to feed livestock, as well as for livestock to graze, for example—would increase the total emissions attributable to raising animals for food.
3Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (CO2).
4Primary carbon dioxide sources are fertilizer, equipment, transport, storage, and processing; primary methane sources are animal belches and manure; primary nitrous oxide sources are manure and fertilizer.
5The Climate Analysis Indicators Tool (CAIT) is an information and analysis tool on global climate change developed by the World Resources Institute. CAIT provides a comprehensive and comparable database of greenhouse gas emissions data (including all major sources and sinks) and other climate-relevant indicators. You can read their FAQ’s here.