There are several estimates for the food system's contribution to total global greenhouse gas emissions: 44-57% -- GRAIN: "The industrial food system is responsible for 44 to 57% of all global GHG emissions" "If the world gets serious about putting these four shifts into action, it is quite possible that we can cut global GHG emissions in half within a few decades" The four shifts: 1. "shift towards an agriculture that builds up organic matter in the soil" 2. "shift to local markets and shorter circuits of food distribution" 3. "reintegration of crop and animal production" 4. "stopping of land clearing and deforestation" "25 to 40% of the current excess of CO2 in the atmosphere comes from the destruction of soils and its organic matter." "GRAIN research has shown that, if the right policies and incentives were in place worldwide, soil organic matter contents could be restored to pre-industrial agriculture levels within a period of 50 years – which is roughly the same time frame that industrial agriculture took to reduce it. The continuing use of these practices would allow the offset of between 24-30% of current global annual GHG emissions" "The expansion of the agricultural frontier is the dominant contributor to deforestation, accounting for between 70-90% of global deforestation. This means that some 15-18% of global GHG emissions are produced by land-use change and deforestation caused by agriculture." "Food and climate change: the forgotten link," GRAIN, Sep. 28, 2011. http://www.grain.org/article/entries/4357-food-and-climate-change-the-fo... ----------------------- 19-29% and 47-61% -- Annual Review "Food systems contribute 19%–29% of global anthropogenic greenhouse gas (GHG) emissions, releasing 9,800–16,900 megatonnes of carbon dioxide equivalent (MtCO2e) in 2008." "The caveat with these figures is that they depend on extrapolation from single-country data; using the UK data rather than the China data for the postproduction stages of the food chain gives total global GHG emissions for the year 2008 in the range of 16,800 to 23,900 MtCO2e, with agricultural production contributing 47%–61% of all food-related emissions. This difference may be indicative of the future trajectory of global food system emissions, toward a higher proportion associated with postproduction stages of the food chain." They claim that China is a good proxy for global extrapolation based "on the assumption that as a large middle-income country it is suitably representative of the global level."
Sonja J. Vermeulen, Bruce M. Campbell, and John S.I. Ingram, "Climate change and food systems," Annual Review of Environment and Resources 37, (2012): 195-222, DOI: 10.1146/annurev-environ-020411-130608. (found here: http://www.annualreviews.org/doi/full/10.1146/annurev-environ-020411-130608)