Environmental Engineering Reference
In-Depth Information
development of new seeds that can better withstand water and heat stress and better utilize
elevated CO
2
. A wide range of maize varieties are currently sown throughout the country, cus-
tomized to local factors such as latitude, growing season length, and soil, and new varieties are
continually developed by private seed companies. These companies have historically focused
on biotic stresses, but are now releasing the first varieties explicitly targeted for drought re-
sistance. Heat tolerance has not received much investment outside of drought-related traits,
likely because of limited economic incentives in current climate. A comparison of maize yields
in northern and southern states suggests minimal historical adaptation to heat, as varieties that
are more frequently exposed to temperatures above 30°C exhibit similar sensitivities to varieties
grown in the North (Schlenker and Roberts, 2009). A major challenge in developing drought and
heat tolerance is that traits that confer these often reduce yields in good years, and growers and
seed companies have little economic incentive to accept this trade-off given current markets
and insurance programs. Another persistent challenge is the decade or more lag between initial
investments and seed release. In short, adaptation could offer large benefits, but only if formi-
dable technical and institutional barriers are overcome. To put the challenge in context, global
cereal demand is expected to rise by roughly 1.2% per year (FAO, 2006), so that adapting to 1°C
global warming (or avoiding 11% yield loss) is equivalent to keeping pace with roughly 9 years
of demand growth. The corresponding expected impact of 2°C global warming is 25%, or roughly
20 years of demand growth.
Future development of new varieties that perform well in hot and dry
conditions may also promote adaptation, but again the extent to which this
will help remains unclear. Breeders and geneticists must continually weigh
trade-offs between producing ample yield under stressful conditions and
producing high yields under favorable conditions (Campos et al., 2004). At
the higher warming levels considered in this report, it will be increasingly
difficult to generate varieties with a physiology that can withstand extreme
heat and drought while still being economically productive.
Although most studies have focused on crops, effects of climate change
on livestock, aquaculture, and fisheries have also been considered in recent
years. Livestock in parts of the world are raised mainly on grain and oilseed
crops, in which case impacts will largely follow from the prices of these
commodities and the costs of cooling or losing animals during heat waves.
In other cases livestock depend on grazing pasture and rangeland grasses,
which follow a similar pattern to crops in that temperate regions will see
modest gains up to ~2°C local warming, although forage quality may de-
crease with higher CO
2
(Easterling et al., 2007). Although livestock systems
are vulnerable in tropical areas, they may become increasingly relied upon
as a strategy to cope with greater risks of crop failures (Thornton et al.,
