Environmental Engineering Reference
In-Depth Information
5
Impacts in the Next Few Decades
and Coming Centuries
5.1 FOOD PRODUCTION, PRICES, AND HUNGER
Even in the most highly mechanized agricultural systems, food produc-
tion is very dependent on weather. Concern about the potential impacts
of climate change on food production, and associated effects on food
prices and hunger, have existed since the earliest days of climate change
research. Although there is still much to learn, several important findings
have emerged from more than three decades of research.
It is clear, for example, that higher CO
2
levels are beneficial for many
crop and forage yields, for two reasons. In species with a C
3
photosynthetic
pathway, including rice and wheat, higher CO
2
directly stimulates photosyn-
thetic rates, although this mechanism does not affect C
4
crops like maize.
Secondly, higher CO
2
allows leaf pores, called stomata, to shrink, which
results in reduced water stress for all crops. The net effect on yields for C
3
crops has been measured as an average increase of 14% for 580 ppm rela-
tive to 370 ppm (Ainsworth et al., 2008). For C
4
species such as maize and
sorghum, very few experiments have been conducted but the observed effect
is much smaller and often statistically insignificant (Leakey, 2009).
Rivaling the direct CO
2
effects are the impacts of climate changes
caused by CO
2
, in particular changes in air temperature and available soil
moisture. Many mechanisms of temperature response have been identified,
with the relative importance of different mechanisms varying by location,
season, and crop. Among the most critical responses are that crops develop
more quickly under warmer temperatures, leading to shorter growing periods
and lower yields, and that higher temperatures drive faster evaporation of
water from soils and transpiration of water from crops. Exposure to extremely
high temperatures (e.g., > 35°C) can also cause damage in photosynthetic,
reproductive, and other cells, and recent evidence suggests that even short
exposures to high temperatures can be crucial for final yield (Schlenker and
Roberts, 2009; Wassmann et al., 2009).
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