Climate Change 2001:
Synthesis Report
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3.21 Models of cereal crops indicate that in some temperate areas potential yields increase for small increases in temperature but decrease with larger temperature changes (medium to low confidence). In most tropical and subtropical regions potential yields are projected to decrease for most projected increases in temperature (medium confidence) (see Table 3-3). In mid-latitudes, crop models indicate that warming of less than a few °C and the associated increase in CO2 concentrations will lead to generally positive responses and generally negative responses with greater warming. In tropical agricultural areas, similar assessments indicate that yields of some crops would decrease with even minimal increases in temperature because they are near their maximum temperature tolerance. Where there is also a large decrease in rainfall in subtropical and tropical dryland/rainfed systems, crop yields would be even more adversely affected. Assessments that include autonomous agronomic adaptation (e.g., changes in planting times and crop varieties) tend to project yields less adversely affected by climate change than without adaptation. These assessments include the effects of CO2 fertilization but not technological innovations or changes in the impacts of pests and diseases, degradation of soil and water resources, or climate extremes. The ability of livestock producers to adapt their herds to the physiological stresses associated with climate change is poorly known. Warming of a few °C or more is projected to increase food prices globally, and may increase the risk of hunger in vulnerable populations (low confidence).

WGII TAR Sections 5.3.4-6, & 9.9
Table 3-3: Agricultural effects of climate change if no climate policy interventions are made.*
  2025 2050 2100
CO2 concentrationa 405-460 ppm 445-640 ppm 540-970 ppm
Global mean temperature change from the year 1990b 0.4-1.1°C 0.8-2.6°C 1.4-5.8°C
Global mean sea-level rise from the year 1990b 3-14 cm 5-32 cm 9-88 cm
Agricultural Effectsc
Average crop yieldsg [WGII TAR Sections 5.3.6, 10.2.2, 11.2.2, 12.5, 13.2.3, 14.2.2, & 15.2.3] Cereal crop yields increase in many mid- and high-latitude regions (low to medium confidenced).
Cereal crop yields decrease in most tropical and subtropical regions (low to medium confidenced).
Mixed effects on cereal yields in mid-latitude regions. More pronounced cereal yield decreases in tropical and subtropical regions (low tomedium confidenced). General reduction in cereal
yields in most mid-latitude
regions for warming of more than a few °C (low to medium confidenced).
Extreme low and high temperatures [WGII TAR Section 5.3.3]

Reduced frost damage to some crops (high confidenced). Increased heat stress damage to some crops (high confidenced). Increased heat stress in livestock (high confidenced).
Effects of changes in extreme temperatures amplified (high confidenced).
Effects of changes in extreme temperatures amplified (high confidenced).
Incomes and prices [WGII TAR Sections 5.3.5-6]

  Incomes of poor farmers in developing countries decrease (low to medium confidenced).
Food prices increase relative to projections that exclude climate change (low to medium confidenced).
* Refer to footnotes a-d accompanying Table 3-1.
g. These estimates are based on the sensitivity of the present agricultural practices to climate change, allowing (in most cases) for
adaptations based on shifting use of only existing technologies.

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