| 1. Effects of enhanced CO2 on crop growth
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| | irrigation more expensive, particularly
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| Plants grow through the well-known
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| | when with drier conditions more water
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| process of photosynthesis, utilizing the
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| | will be required per acre. Peak
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| energy of sunlight to convert water from
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| | irrigation demands are also predicted to
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| the soil and carbon dioxide from the air
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| | rise due to more severe heat waves.
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| into sugar, starches, and cellulose--the
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| | Additional investment for dams,
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| carbohydrates that are the foundations of
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| | reservoirs, canals, wells, pumps, and
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| the entire food chain. CO2 enters a plant
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| | piping may be needed to develop
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| through its leaves. Greater atmospheric
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| | irrigation networks in new locations.
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| concentrations tend to increase the
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| | Finally, intensified evaporation will
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| difference in partial pressure between
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| | increase the hazard of salt accumulation
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| the air outside and inside the plant
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| | in the soil.
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| leaves, and as a result more CO2 is
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| | 4. Climate variability
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| absorbed and converted to carbohydrates.
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| | Extreme meteorological events, such as
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| Crop species vary in their response to
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| | spells of high temperature, heavy storms,
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| CO2. Wheat, rice, and soybeans belong to
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| | or droughts, disrupt crop production.
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| a physiological class (called C3 plants)
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| | Recent studies have considered possible
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| that responds readily to increased CO2
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| | changes in the variability as well as in
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| levels. Corn, sorghum, sugarcane, and
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| | the mean values of climatic variables.
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| millet are C4 plants that follow a
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| | Where certain varieties of crops are
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| different pathway. The latter, though
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| | grown near their limits of maximum
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| more efficient photosynthetically than C3
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| | temperature tolerance, such as rice in
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| crops at present levels of CO2, tend to
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| | Southern Asia, heat spells can be
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| be less responsive to enriched
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| | particularly detrimental. Similarly,
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| concentrations. Thus far, these effects
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| | frequent droughts not only reduce water
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| have been demonstrated mainly in
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| | supplies but also increase the amount of
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| controlled environments such as growth
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| | water needed for plant transpiration.
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| chambers, greenhouses, and plastic
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| | 5. Soil fertility and erosion
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| enclosures. Experimental studies of the
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| | Higher air temperatures will also be felt
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| long-term effects of CO2 in more
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| | in the soil, where warmer conditions are
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| realistic field settings have not yet
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| | likely to speed the natural decomposition
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| been done on a comprehensive scale.
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| | of organic matter and to increase the
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| Higher levels of atmospheric CO2 also
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| | rates of other soil processes that affect
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| induce plants to close the small leaf
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| | fertility. Additional application of
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| openings known as stomates through which
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| | fertilizer may be needed to counteract
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| CO2 is absorbed and water vapor is
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| | these processes and to take advantage of
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| released. Thus, under CO2 enrichment
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| | the potential for enhanced crop growth
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| crops may use less water even while they
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| | that can result from increased
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| produce more carbohydrates. This dual
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| | atmospheric CO2. This can come at the
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| effect will likely improve water-use
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| | cost of environmental risk, for
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| efficiency, which is the ratio between
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| | additional use of chemicals may impact
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| crop biomass and the amount of water
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| | water and air quality. The continual
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| consumed. At the same time, associated
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| | cycling of plant nutrients--carbon,
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| climatic effects, such as higher
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| | nitrogen, phosphorus, potassium, and
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| temperatures, changes in rainfall and
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| | sulfur--in the soil-plant-atmosphere
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| soil moisture, and increased frequencies
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| | system is also likely to accelerate in
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| of extreme meteorological events, could
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| | warmer conditions, enhancing CO2 and N2O
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| either enhance or negate potentially
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| | greenhouse gas emissions. Nitrogen is
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| beneficial effects of enhanced
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| | made available to plants in a
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| atmospheric CO2 on crop physiology.
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| | biologically usable form through the
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| 2. Effects of higher temperature
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| | action of bacteria in the soil. This
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| In middle and higher latitudes, global
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| | process of nitrogen fixation, associated
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| warming will extend the length of the
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| | with greater root development, is also
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| potential growing season, allowing
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| | predicted to increase in warmer
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| earlier planting of crops in the spring,
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| | conditions and with higher CO2, if soil
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| earlier maturation and harvesting, and
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| | moisture is not limiting. Where they
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| the possibility of completing two or more
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| | occur, drier soil conditions will
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| cropping cycles during the same season.
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| | suppress both root growth and
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| Many crops have become adapted to the
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| | decomposition of organic matter, and will
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| growing-season day lengths of the middle
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| | increase vulnerability to wind erosion,
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| and lower latitudes and may not respond
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| | especially if winds intensify. An
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| well to the much longer days of the high
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| | expected increase in convective
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| latitude summers. In warmer, lower
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| | rainfall--caused by stronger gradients of
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| latitude regions, increased temperatures
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| | temperature and pressure and more
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| may accelerate the rate at which plants
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| | atmospheric moisture--may result in
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| release CO2 in the process of
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| | heavier rainfall when and where it does
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| respiration, resulting in less than
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| | occur. Such "extreme precipitation
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| optimal conditions for net growth. When
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| | events" can cause increased soil erosion.
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| temperatures exceed the optimal for
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| | 6. Pests and diseases
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| biological processes, crops often respond
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| | Conditions are more favorable for the
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| negatively with a steep drop in net
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| | proliferation of insect pests in warmer
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| growth and yield. If nighttime
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| | climates. Longer growing seasons will
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| temperature minima rise more than do
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| | enable insects such as grasshoppers to
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| daytime maxima--as is expected from
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| | complete a greater number of reproductive
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| greenhouse warming projections--heat
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| | cycles during the spring, summer, and
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| stress during the day may be less severe
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| | autumn. Warmer winter temperatures may
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| than otherwise, but increased nighttime
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| | also allow larvae to winter-over in areas
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| respiration may also reduce potential
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| | where they are now limited by cold, thus
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| yields. Another important effect of high
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| | causing greater infestation during the
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| temperature is accelerated physiological
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| | following crop season. Altered wind
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| development, resulting in hastened
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| | patterns may change the spread of both
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| maturation and reduced yield.
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| | wind-borne pests and of the bacteria and
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| 3. Available water
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| | fungi that are the agents of crop
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| Agriculture of any kind is strongly
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| | disease. Crop-pest interactions may shift
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| influenced by the availability of water.
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| | as the timing of development stages in
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| Climate change will modify rainfall,
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| | both hosts and pests is altered.
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| evaporation, runoff, and soil moisture
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| | Livestock diseases may be similarly
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| storage. Changes in total seasonal
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| | affected. The possible increases in pest
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| precipitation or in its pattern of
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| | infestations may bring about greater use
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| variability are both important. The
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| | of chemical pesticides to control them, a
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| occurrence of moisture stress during
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| | situation that will require the further
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| flowering, pollination, and grain-filling
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| | development and application of integrated
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| is harmful to most crops and particularly
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| | pest management techniques.
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| so to corn, soybeans, and wheat.
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| | 7. Sea-level rise
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| Increased evaporation from the soil and
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| | Global warming is predicted to lead to
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| accelerated transpiration in the plants
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| | thermal expansion of sea water, along
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| themselves will cause moisture stress; as
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| | with partial melting of land-based
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| a result there will be a need to develop
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| | glaciers and sea-ice, resulting in a rise
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| crop varieties with greater drought
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| | of sea level which may range from 0.1 to
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| tolerance.
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| | 0.5 meters (4 to 20 inches) by the middle
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| The demand for water for irrigation is
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| | of the next century, according to present
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| projected to rise in a warmer climate,
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| | estimates of the Intergovernmental Panel
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| bringing increased competition between
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| | on Climate Change (IPCC). Such a rise
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| agriculture--already the largest consumer
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| | could pose a threat to agriculture in
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| of water resources in semiarid
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| | low- lying coastal areas, where impeded
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| regions--and urban as well as industrial
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| | drainage of surface water and of
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| users. Falling water tables and the
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| | groundwater, as well as intrusion of sea
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| resulting increase in the energy needed
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| | water into estuaries and aquifers, might
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| to pump water will make the practice of
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| | take place.
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