Rising atmospheric carbon dioxide levelshave been observed to have direct beneficial results on crop yields, C3crops especially (high confidence;IPCC, 2013), and livestock farming (Ghahramani and Moore, 2016). Increased CO2levels boost the rate of photosynthesis, which spurs growth, and increases CropWater Productivity (CWP) due to partial stomatal closure which reducestranspiration (Nature Climate Change, 2016).
However, this causes the ratio ofseed to plant mass to fall thus reducing the crop’s protein content (Swann etal., 2016). Also, the FAO (2012a) have reported that CO2 reacts withO3 gases leading to higher ozone concentrations which is harmful tocrops.
In fact, positive impacts to crop yields are frequently offset by progressivelylarger negative impacts (see Figure 2.1b).According to the IPCC (2014) and the NCA(2014), global average warming has diminished the yield and quality of many cropsin most regions of the world except for some high latitude regions. Warmingcauses a reduction in water availability thus “increasing the risk of growingaridity and land degradation” (Hoffmann, 2013), which is enhanced in regionsexperiencing decreased precipitation, andincreases pest, virus, and disease outbreaks (https://www.livescience.com/4296-global-warming-trigger-insect-population-boom.html).
For example,the spreading of the blue-tongue virus, zoonotic diseases (IPCC, 2014), powderymildew, leaf spot disease, leaf rust, and rhizomania disease (Mendelsohn andDinar, 2009). The increasedfrequency in extreme weather events has led to increased stress on cropproduction, obliged the modification of agricultural infrastructure(Rosenzweig, 2002), and caused price spikes for food and cereals in keyproducing regions (Houser et al., 2015; see figure 2.
2). The IPCC projects increasedfood prices by 3 to 83% by 2050 (mediumconfidence) in models including the changes in temperature andprecipitation, but without considering the effects of CO2. Somemodels include the effects of CO2 changes, but ignore O3 and pest and diseaseimpacts, in which the projected global price increases are less likely, “with arange of projected impacts from –30% to +45% by 2050”. In the US, studies haveshown that average yields of major crops are projected to decline modestly bymid-century and more steeply by 2100 in comparison to late 20th century levels(IPCC, 2014; Schlenker and Roberts, 2009; Lychuk et al., 2017). Disruptions on foodproduction and prices in the USA, a major food exporter that is reliant onfarming profitability (https://www.worldatlas.
com/articles/the-american-food-giant-the-largest-exporter-of-food-in-the-world.html), would have negative implications on global food security and USA’seconomy itself (Houser, et al. 2015). Fisheries and aquaculture has also fallenvictim to climatic change.
The warming and acidification of the ocean hasallowed more favorable conditions for invasive species causing the migration ofmany commercial fish species, cold-water fisheries especially (IPCC, 2007).Along the US East and West coastlines, reductions of -1 to -5% in the maximumcatch potential of ~1000 species of fish and invertebrates are projected by2051-2060 compared to 2001-2010 levels with larger projected reductions of -6to -20% in areas close to the coastlines of Florida and California (high confidence, see Figure 2.1a).