4.2. Agriculture and Food Security
Figure TS-4: Ranges of percentage changes in crop yields (expressed
in vertical extent of vertical bars only) spanning selected climate change
scenarios -- with and without agronomic adaptation -- from paired studies
listed in Table
5-4. Each pair of ranges is differentiated by geographic location and
crop. Pairs of vertical bars represent the range of percentage changes with
and without adaptation. Endpoints of each range represent collective high
and low percentage change values derived from all climate scenarios used
in the study. The horizontal extent of the bars is not meaningful. On the
x-axis, the last name of the lead author is listed as it appears in Table
5-4; full source information is provided in the Chapter
5 reference list. |
The response of crop yields to climate change varies
widely, depending on the species, cultivar, soil conditions, treatment of CO2
direct effects, and other locational factors. It is established with medium
confidence that a few degrees of projected warming will lead to general increases
in temperate crop yields, with some regional variation (Table
5-4). At larger amounts of projected warming, most temperate crop yield
responses become generally negative. Autonomous agronomic adaptation ameliorates
temperate crop yield loss and improves gain in most cases (Figure
TS-4). In the tropics, where some crops are near their maximum temperature
tolerance and where dryland agriculture predominates, yields would decrease
generally with even minimal changes in temperature; where there is a large decrease
in rainfall, crop yields would be even more adversely affected (medium confidence).
With autonomous agronomic adaptation, it is established with medium confidence
that crop yields in the tropics tend to be less adversely affected by climate
change than without adaptation, but they still tend to remain below baseline
levels. Extreme events also will affect crop yields. Higher minimum temperatures
will be beneficial to some crops, especially in temperate regions, and detrimental
to other crops, especially in low latitudes (high confidence). Higher maximum
temperatures will be generally detrimental to numerous crops (high confidence).
[5.3.3]
Important advances in research since the SAR on the direct effects of CO2
on crops suggest that beneficial effects may be greater under certain stressful
conditions, including warmer temperatures and drought. Although these effects
are well established for a few crops under experimental conditions, knowledge
of them is incomplete for suboptimal conditions of actual farms. Research on
agricultural adaptation to climate change also has made important advances.
Inexpensive, farm-level (autonomous) agronomic adaptations such as altering
of planting dates and cultivar selections have been simulated in crop models
extensively. More expensive, directed adaptations -- such as changing land-use
allocations and developing and using irrigation infrastructure -- have been
examined in a small but growing number of linked crop-economic models, integrated
assessment models, and econometric models.
Degradation of soil and water resources is one of the major future challenges
for global agriculture. It is established with high confidence that those
processes are likely to be intensified by adverse changes in temperature and
precipitation. Land use and management have been shown to have a greater impact
on soil conditions than the indirect effect of climate change; thus, adaptation
has the potential to significantly mitigate these impacts. A critical research
need is to assess whether resource degradation will significantly increase the
risks faced by vulnerable agricultural and rural populations [5.3.2,
5.3.4, 5.3.6].
In the absence of climate change, most global and regional studies project
declining real prices for agricultural commodities. Confidence in these projections
declines farther into the future. The impacts of climate change on agriculture
are estimated to result in small percentage changes in global income, with positive
changes in more developed regions and smaller or negative changes in developing
regions (low to medium confidence). The effectiveness of adaptation (agronomic
and economic) in ameliorating the impacts of climate change will vary regionally
and depend a great deal on regional resource endowments, including stable and
effective institutions. [5.3.1,
5.3.5]
Most studies indicate that mean annual temperature increases of 2.5ºC
or greater would prompt food prices to increase (low confidence) as a result
of slowing in the expansion of global food capacity relative to growth in global
food demand. At lesser amounts of warming than 2.5ºC, global impact
assessment models cannot distinguish the climate change signal from other sources
of change. Some recent aggregated studies have estimated economic impacts on
vulnerable populations such as smallholder producers and poor urban consumers.
These studies indicate that climate change will lower the incomes of vulnerable
populations and increase the absolute number of people at risk of hunger (low
confidence). [5.3.5,
5.3.6]
Without autonomous adaptation, increases in extreme events are likely to
increase heat stress-related livestock deaths, although winter warming may reduce
neonatal deaths at temperate latitudes (established but incomplete). Strategies
to adapt livestock to physiological stresses of warming are considered effective;
however, adaptation research is hindered by the lack of experimentation and
simulation. [5.3.3]
Confidence in specific numerical estimates of climate change impacts on production,
income, and prices obtained from large, aggregated, integrated assessment models
is considered to be low because there are several remaining uncertainties. The
models are highly sensitive to some parameters that have been subjected to sensitivity
analysis, yet sensitivity to a large number of other parameters has not been
reported. Other uncertainties include the magnitude and persistence of effects
of rising atmospheric CO2 on crop yield under realistic farming conditions;
potential changes in crop and animal pest losses; spatial variability in crop
responses to climate change; and the effects of changes in climate variability
and extreme events on crops and livestock. [Box
5-3]
