15.2.3.1.3. Response to climate variability and extreme events
The effects of changes in the variability of temperatures and precipitation
on crop yields have been evaluated through simulation modeling. Changes in diurnal
and interannual variability of temperature and moisture can result in substantial
changes in the mean and variability of wheat yields. In Kansas, doubling of
temperature variability resulted in greatly reduced average yield and increased
variability of yield, primarily as a result of crop failure by winterkill (Mearns
et al., 1996). The main risk of climate change to some regions may be
primarily from the potential for increased variability. Increased variability
of temperature and precipitation results in substantially lower mean simulated
yields, whereas decreased variability produces only small increases in yield
that were insignificant (Reilly et al., 2000). This asymmetric response
to temperature variability underscores a major reason that the corn belt region
of the United States is so productive: There generally is low variability in
temperature across the region. It should be noted, therefore, that if minimum
temperatures increase more than maximums, two outcomes could be suggested: Temperature
variability may decline, and winterkill should be reduced.
These effects of diurnal and interannual climate variation may have important
implications for farm values. Economic analysis has shown that greater interannual
variation is harmful to farm values, and the marginal effect of temperature
variation is relatively larger than the effect of variations in precipitation
(Mendelsohn et al., 1999).
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Box 15-1. Carbon Sequestration: Adaptation Issues
The Kyoto Protocol commits industrialized nations to take
on binding targets for GHG emissions for the period 2008-2012.
The Protocol mentions human-induced land-use changes and forestry activities
(afforestation, reforestation, deforestation) as sinks of GHGs for which
sequestration credits can be claimed; it also mentions that agricultural
sinks may be considered in the future. As a result, a significant market
is emerging in North America for ways to enhance carbon sequestration
in these sectors. Although it is not within the purview of this section
to deal with mitigation strategies, land management decisions impact
a wide range of factors. There may be several consequent issues that
result or are derived from implementation and adoption of these strategies.
Negative consequences of reduced tillage implemented to enhance soil
carbon sequestration may include (medium confidence):
- Increased use of pesticides for disease, insect, and weed management.
This increased pesticide load may affect adjacent ecosystems and the
quality of water within and outside agroecosystems.
- Capture of carbon in labile forms that are vulnerable to rapid oxidation
if the system is changed. This may require that reduced-till systems
be maintained for an extended period (which also would lengthen the
beneficial aspects of reduced tillage).
- Reduced yields and cropping management options and increased risk
for farmers (Would yield reductions and increased risk be compensated?).
Beneficial consequences of reduced tillage (especially no-till) may include
(high confidence):
- Reduced input costs (e.g., fuel) for farmers, thereby increasing the
economic profit margin
- Increased soil moisture and hence reductions in crop water stress
in dry areas
- Reduction in soil erosion, which inhibits loss of carbon from erosional
forces and preserves the natural land base
- The overall combination of these effects improves soil quality and
the ability of soils to physically and chemically support plant growth,
as well as conserving the continent's natural resources.
The extent to which carbon will be sequestered in agricultural and forest
systems will be related to practical economics and land-use policies.
For example:
- Expansion of agricultural lands for carbon sequestration may increase
competition with use of agricultural lands for traditional food and
fiber production. The effect may be decreased food and fiber production,
with subsequent increases in prices and decreases in exports for agricultural
commodities.
- Land prices may change (e.g., increase) as a consequence of competition
between crops for food and crops for mitigation strategies.
- Reduction of agricultural lands by transferring these lands to forestry
to enhance carbon sequestration also may increase competition for food
and fiber production, but the influx of land into forestry subsequently
may decrease forestry prices.
Thus, a focus on carbon sequestration in ecosystems may result in the
transfer of large quantities of land between agriculture and forestry
and change the management of existing agricultural and forest ecosystems.
These changes may provide opportunities for landowners, but they also
may have implications for food and fiber production and ecosystem functions.
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15.2.3.1.4. Vulnerability of livestock
The effects of climate change on livestock can be direct (e.g., effects of
higher temperature on livestock appetite) or indirect (e.g., effects of changes
in quantity and quality of forage from grasslands and supplies of feed). In
areas where livestock rely on surface water availability, water quality could
have an impact on weight gain. This would be particularly important where fewer
water sources become used by greater numbers of cattle.
Estimates of livestock production efficiency suggest that the negative effects
of hotter weather in summer outweigh the positive effects of warmer winters
(Adams et al., 1999). The largest change occurred under a 5°C increase
in temperature, when livestock yields fell by 10% in cow-calf and dairy operations
in the Appalachia, southeast, Delta, and southern Plains regions of the United
States. The smallest change was 1% under 1.5°C warming in the same regions.
Livestock production also is affected by changes in temperature and extreme
events. For example, an ice storm in eastern Canada and the northeast United
States in the winter of 1998 had severe effects on livestock in the region (see
Section 15.3.2.6).
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