Factors :
1. Drought http://www.nature.com/nature/outlook/agriculture-drought/
“Climate change means the coming decades are likely to bring more frequent episodes of severe drought, with potentially devastating impact on the world’s ability to feed a growing population.”
2. Ozon http://www.ars.usda.gov/Main/docs.htm?docid=12462
3. CO2 http://www.abc.net.au/science/articles/2014/05/08/3999646.htm
“Experimental results published today in the journal Naturehave found that zinc, iron and protein levels all fall in these crucial crops when CO2 levels rise…By 5-10% with 550 ppm.”
4. Fossil fuel prices http://www.organicconsumers.org/corp/fossil-fuels.cfm
“In 1994, David Pimentel and Mario Giampietro estimated the output/input ratio of agriculture to be around 1.4 For 0.7 Kilogram-Calories (kcal) of fossil energy consumed, U.S. agriculture produced 1 kcal of food. ” “Adding in estimates for these (other associated) energy costs brought the input/output energy ratio down to 1. Yet this does not include the energy expense of packaging, delivery to retail outlets, refrigeration or household cooking.”
5. Heat http://pstustudy.blogspot.nl/2010/08/effect-of-temperature-on-crop.html
“The most influential factors in the climate are temperature and moisture. Plants can grow only within certain limit of temperature. For each species and variety there are not only optimal temperature limits, but also optimal temperatures for different growth stages and functions, as well as lower and upper lethal limits. Temperature determines which species can survive in a particular region.”
“There is a risk that severity of epidemics of some wheat diseases may increase within the next ten to twenty years due to the impacts of climate change according to a study by international researchers led by the University of Hertfordshire.”
7. Soil fertility after effects Link
8. Bee death, they may go extinct in the US http://www.theguardian.com/environment/2014/may/15/honeybee-deaths-too-high-longterm-survival
If intensive farming is stopped the soil is dead and polluted. Zones that need to transition due to fossil fuel supply collapse can’t expect to be fertile for a couple of years. This means such collapse would bring widespread famine and starvation.