Much of the focus on the effects of elevated [CO2] on crops has been on harvestable yield quantity. However, yield quality is an important issue as well. The two most studied aspects of quality are protein and nitrogen concentration. A meta-analysis of crops grown in elevated [CO2] found that protein content was reduced in grain (Taub et al. 2008). Barley, wheat, rice, potato and soybean all showed significant decreases; for the non-legumes, the decrease was between 10% and 14%, whereas for soybean, a legume, the decrease was a much smaller 1.5% (Taub et al. 2008). This is likely because legumes are able to fix nitrogen, which would prevent nitrogen dilution. Nitrogen concentration was also decreased in the grains of wheat (Kimball 2006, Manderscheid et al. 1995), barley (Manderscheid et al. 1995) and rice (Kobayashi et al. 2006).
Change in mineral quality of the harvestable portion of crops has been less extensively studied although the data suggest that mineral content is generally reduced by growth in elevated [CO2] (Loladze 2002). In two wheat and two barley cultivars grown at 718 compared to 384 ppm [CO2], several minerals including Ca, Mg, and S were reduced between 2% and 20%, and Fe and Zn were decreased by 15-20%, with larger decreases in wheat than barley (Manderschied et al. 1995). In one wheat cultivar grown at 550 compared to 380 ppm [CO2], Ca, S and Fe decreased by 10%, but P, K, Zn and Mn were unchanged (Fangmeier et al. 1999). In rice grown at 700 compared to 370 ppm [CO2], across a range of P concentrations, Fe was reduced by up to 30% and Zn by up to 14.5%, while Ca was increased by 5% (Seneweera and Conroy 1997).
Changes in protein and mineral content in grains have significant consequences for animal and human health. Livestock that are deficient in certain minerals have decreased fertility and productivity even if the deficiency is not great enough for the animal to present clinical symptoms (Fisher 2008). Therefore, in future atmospheric conditions, if animal feed is not supplemented with minerals (an option not available to all farmers) then production of animal-based food and products (e.g., wool) might be reduced compared to production in current [CO2]. Reductions in Fe and Zn content would also have important direct consequences for humans, considering the large numbers of people that currently suffer from micronutrient deficiency (see Chapter 2). Although the risk of Fe deficiency is greatest in developing counties (56% of pregnant women and 54% of school-age children), it is also a large problem in developed countries (18% of pregnant women and 17% of schoolage children; UN ACC/SCN 2000).
Although the decrease in protein could be reduced by addition of nitrogen fertilizer, the cost and availability of fertilizer prohibits widespread adoption by all farmers, particularly in developing countries. Furthermore, additional fertilizer use has a significant environmental cost. Therefore, the decrease in protein content in grains will likely be a problem in the future (Taub et al. 2008). Similarly, while nutrient deficiency can be avoided by eating a more varied diet, taking dietary supplements, augmenting commercial food products with nutrients (a process termed fortification), or breeding crops with increased nutrient content, not all people have access to more varied foods or nutrient supplements. There has been some success in genetically improving nutrient content in crops. For example, the maize line, Opaque 2, has been bred to produce 32% more lysine, an essential amino acid that is typically found in very low concentrations in maize (Higgins and Chrispeels 2003). However, access to enhanced germplasm is not widely available to farmers in developing countries where these deficiencies pose the greatest risk to the population.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.