Rising atmospheric carbon dioxide levels
have been observed to have direct beneficial results on crop yields, C3
crops especially (high confidence;
IPCC, 2013), and livestock farming (Ghahramani and Moore, 2016). Increased CO2
levels boost the rate of photosynthesis, which spurs growth, and increases Crop
Water Productivity (CWP) due to partial stomatal closure which reduces
transpiration (Nature Climate Change, 2016). However, this causes the ratio of
seed to plant mass to fall thus reducing the crop’s protein content (Swann et
al., 2016). Also, the FAO (2012a) have reported that CO2 reacts with
O3 gases leading to higher ozone concentrations which is harmful to
crops. In fact, positive impacts to crop yields are frequently offset by progressively
larger 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 crops
in most regions of the world except for some high latitude regions. Warming
causes a reduction in water availability thus “increasing the risk of growing
aridity and land degradation” (Hoffmann, 2013), which is enhanced in regions
experiencing decreased precipitation, and
increases 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), powdery
mildew, leaf spot disease, leaf rust, and rhizomania disease (Mendelsohn and
Dinar, 2009). The increased
frequency in extreme weather events has led to increased stress on crop
production, obliged the modification of agricultural infrastructure
(Rosenzweig, 2002), and caused price spikes for food and cereals in key
producing regions (Houser et al., 2015; see figure 2.2). The IPCC projects increased
food prices by 3 to 83% by 2050 (medium
confidence) in models including the changes in temperature and
precipitation, but without considering the effects of CO2. Some
models include the effects of CO2 changes, but ignore O3 and pest and disease
impacts, in which the projected global price increases are less likely, “with a
range of projected impacts from –30% to +45% by 2050”. In the US, studies have
shown that average yields of major crops are projected to decline modestly by
mid-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 food
production and prices in the USA, a major food exporter that is reliant on
farming 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’s
economy itself (Houser, et al. 2015).

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Fisheries and aquaculture has also fallen
victim to climatic change. The warming and acidification of the ocean has
allowed more favorable conditions for invasive species causing the migration of
many commercial fish species, cold-water fisheries especially (IPCC, 2007).
Along the US East and West coastlines, reductions of -1 to -5% in the maximum
catch potential of ~1000 species of fish and invertebrates are projected by
2051-2060 compared to 2001-2010 levels with larger projected reductions of -6
to -20% in areas close to the coastlines of Florida and California (high confidence, see Figure 2.1a).

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