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