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Fluid Journal : Summer 2016
13 The Fluid Journal Summer 2016 cause of yield stagnation. Increased variability of climate during the growing season, creating heat stress during grain-filling and water stress during stem elongation and tillering, were noted as significant factors. They attributed some of the yield stagnation to policy and economic changes, which reduced the use of legumes in crop rotations and N fertilizer inputs. The impacts of climate change are already evident in the winter wheat yields for Kansas and Oklahoma where there has been a negative trend in statewide grain yields since 2000 (Figure 2). Analysis of farming in the tropics by Affholder et al. (2013) found that yield gaps between potential and non-limited water yields were not due to global radiation, temperature, rainfall, or soil water holding capacity, but rather due to poor soil fertility and weed infestation. This observation agrees with the Sinclair and Ruffy (2012) assessment of yield Limitations. Wang et al. (2014) noted that YF had already reached Yp in the North China Plain and found that Yp was declining due to decreasing solar radiation and increasing air temperature. They used the APSIM-maize model with local hybrid parameters under irrigation and non-limiting N supply to estimate Yp. If we take the conclusions of Alexandratos and Bruinsma (2012) in which they estimated the yield (Mgha-1) increase from 2005 needed to meet the 2050 demand, maize would have to increase by 28% to 6.06 Mgha-1 , wheat to 3.82 Mgha-1 (+38%), rice to 5.32 Mgha-1 (+31%) and soybeans to 3.15 Mgha-1 (+36%) to meet projected production requirements. Before we can project the future needs, it is instructive to examine how much yields have increased and are projected to increase. Jaggard et al. (2010) stated that the increased atmospheric CO2 concentrations will increase C3 crop yields by 13%, and C4 crops by a negligible amount with a reduction in water consumption offset by increased atmospheric demand due to increased temperature. They stated that plant breeders are likely to significantly increase crop yields because of increased atmospheric CO2 concentrations, and we can assume that these yields will be realized if pests and disease remain controllable. For their analysis they assumed that only soil-borne pathogens would respond to a warmer climate and would be manageable, and that there would be no policy change that would affect the management of crops with chemicals. Given these assumptions, they felt that crop yield could potentially be increased 50% by 2050 and therefore adequate production goals would be feasible. Using simulation models for China, Erda et al. (2005) summarized that maize, wheat, and rice yields would decrease 37% by 2100 due to increased climate stress with no CO2 increase, and only increase by 5 to 15% with a CO2 increase. These projections for yield increase would be far from adequate to meet population demands. Zheng et al. (2012) found that the increasing mean temperature and occurrence of extreme temperatures will reduce wheat yields and subsequently conducted an experiment to evaluate patterns of frost and heat events across Australia. They Figure 2. Trends in Kansas and Oklahoma state level winter yields since 1960. (Data from USDA-NASS)