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AGRICULTURE AND A CLIMATE CHANGED BY MORE CARBON DIOXIDE
Pages 383-418

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From page 383...
... Except for l7 pounds of fish in the l400 pounds each American eats yearly, all the food for us and the feed for our animals, too, grows on a third of a billion acres of cropland and vast rangelands and pastures, exposed to the annual lottery of the weather. Although about 50 million acres of American crops are protected from drought by irrigation, even these depend on precipitation in the long run, and sheltering crops in greenhouses from temperature extremes is too expensive for staples.
From page 384...
... American crops growing from 35 to 49 degrees latitude are within the zone that meteorologists predict will experience a change in the weather as CO2 increases, and thus these critical and susceptible crops are also exemplary. Often we shall concentrate further, examining wheat, corn, and soybeans, which outdistance in value any other American crop.
From page 385...
... Reports spread of people eating their own children, of the poor in Poland feeding on hanged bodies taken down from the gibbet. In this ancient experiment were demonstrations that colder as well as warmer weather can damage; also the length of the season as well as the mean temperature is critical, transportation can alleviate hunger, and the impact of changed climate is sharp at the poleward margin of farming.
From page 386...
... Like a chapter in Exodus, the report of l936 was of a loss of $l06 million, more than the total gross income from all farm products in Arizona, Nevada, New Mexico, Utah, and Wyoming combined. The hoppers made such a clean sweep in South Dakota that jackrabbits, faced with starvation, escaped into Nebraska (Schlebecker, l953)
From page 387...
... 2000, the agriculturalist may be uncertain how the climate of a precise place will change, but he can reasonably consider how crop production would be changed by an increase to about 400 ppmv, a mean warming of about l°C in the northern United States with a growing season about l0 days longer, and more frequent drought in the United States caused by somewhat less rain and slightly more evaporation.
From page 388...
... 6.2.l Photosynthesis About 90% of the dry weight of plants derives from the reduction of CO2 to carbohydrates by photosynthesis. In single leaves in bright light, net photosynthesis increases with CO2 above the current atmospheric level of 340 ppmv, and this is confirmed in whole plants by greater crop yields (see Section 6.2.6)
From page 389...
... Net photosynthesis of wheat is about 70 mg of CO2 dm"2 ITl compared with maize (about 55 mg of CO2 dm"2 h-l) for equivalent light intensity (0.4 cal cm"2 min-l)
From page 390...
... 6.2.l.2 Duration of Photosynthesis Prolonged and faster photosynthesis caused by increased CO2 in bright light produces more sucrose, sometimes increasing starch accumulation in the chloroplasts, and excessive starch can deform chloroplasts and decrease photosynthesis (Guinn and Mauney, l980)
From page 391...
... As photosynthesis increases with increasing C02, the carbohydrate available for plant growth will increase and, in turn, impose demands for increased fertilizer or available soil nutrients. Since plant biomass typically has a minimum nitrogen content of between
From page 392...
... Doubling atmospheric CO2 would likely have little direct effect on roots and soil microbes. If CO2 increased plant growth it could increase the plant remains incorporated as soil organic matter and influence the cycling of minerals in the soil and other soil properties.
From page 393...
... Thus, increased photosynthesis and degradable biomass are likely to increase soil nitrogen levels, perhaps by 5 to l0%, and may slightly decrease the phosphorus and soil nutrients not tied up in living biomass (Lemon, l983)
From page 394...
... 6.2.6 Direct Effects of CO? on Yield The integration and the practical outcome of all the effects enumerated above is yield.
From page 395...
... Proceeding to the yield of grain or cotton lint, one sees increases of 0.07 to 0.34% per ppmv increase in C02 in optimum growing environments. There is no clear evidence that this relative change is less in wheat that lacks water, but the wheat deprived of nutrients did respond little to C02.
From page 396...
... More must be said of the climatic change to be caused by an increasing concentration of atmospheric C02 than simply "l°C warmer and a bit drier." The direct effects of C02 on plant growth have already been dealt with in the preceding section and are not incorporated here. To approximate the CO2-induced weather change for the year 2000, we shall employ the actual weather data for a location, which includes the natural correlations between temperature and rain, and then increase the temperature by l°C and subtract l0% of the precipitation.
From page 397...
... In Kansas, heading occurs in winter wheat around May; therefore hot spells in Kansas in May decrease yields there. South Dakota shows the effect of weather in the transition region between winter and spring wheat where spring wheat is relegated to unfavorable sites where
From page 398...
... of Winter Crop Region, State Red River North South Valley Dakota Dakota Nebraska Kansas Oklahoma (SW)
From page 399...
... A l0% decrease in July rain and correspondingly an increase in potential evapotranspiration would decrease July Prec minus PET by about ll mm and yield by 0.28 quintal/ha. A decrease in precipitation during planting in May, on the other hand, increases yield.
From page 400...
... Variable Yield, Average l978-l980 Temperature, °C July Aug Oct July to Aug Average Precipitation, mm May Sept to June Sept to June SDFN^ July Combined Variables Apr and May PET2 May Prec/PET July Prec minus PET 72.7 68.8 -l.56 -0.64 0.57 0.0l3 -0.000l -0.l2 0.076 (June ET/ET^ + July ET/ET)
From page 401...
... For example, July precipitation and temperature are correlated, and both affect Iowa corn. The two were combined in a single variable, July Prec minus PET, by calculating PET from temperature and subtracting it from rainfall, making a single factor expressing the harm to corn from drought in July or the benefits from timely rain during tasseling.
From page 402...
... For example, there are substantial differences in the average yield between spring and winter wheat, therefore, a decrease of l.36 quintals/ha in South Dakota is an ll% decrease in yield, whereas a decrease of l.04 quintals/ha is a 5% drop in yield for Kansas. Also, the season of sensitivity to weather is earlier in Kansas than northward in South Dakota.
From page 403...
... To perform our simulations we must have some weather observations. He began with the actual observations of nine crop reporting districts in North Dakota, each district with l5 to 20 stations.
From page 404...
... LU o til rr LL LU U oc a 206.5 10.5 14.5 18.5 YIELD (q/ha) FIGURE 6.2 North Dakota simulated spring wheat yield (l949-l980)
From page 405...
... Despite the fog created by the foregoing list of qualifications and warnings, a clear conclusion shines through. The warmer and drier climate assumed to accompany the increased CO2 will decrease yields of the three great American food crops over the entire grain belt by 5 to l0%, tempering any direct advantage of CO2 enhancement of photosynthesis.
From page 406...
... Because all the Helminthospor ium diseases were accused of decreasing corn yield only about 2%, Americans naturally paid little attention when Philippine scientists reported in l96l an especial susceptibility of T-cytoplasm corn to Helminthosporium maydis. During l969, however, greater-than-normal susceptibility to Helminthosporium was observed in seed and test fields, and over winter the association between T-cytoplasm corn and Southern corn leaf blight, the disease caused by Helminthosporium maydis, was confirmed (Tatum, l97l)
From page 407...
... Instead, agriculture must expect novel pests, keep its research system in trim, and control new pests as they arise or the direct and indirect effects discussed above will be rendered trivial by a shifty and aggressive pest. 6.5 IRRIGATION IN A WARMER AND DRIER CLIMATE Irrigation is both important and susceptible.
From page 408...
... is irrigated and about 2 x l05 ha of this irrigated land is removed from production annually because of buildup of salt concentrations that inhibit crop production (Hodges et al., l98l)
From page 409...
... These two states have a third of the irrigated cropland, and a decrease in water and irrigated area in those states could reduce yields to zero on many acres and thus decrease the fresh vegetables in the produce section of the supermarket, especially in the winter. 6.6 ADAPTING TO THE CHANGE TO A WARMER, DRIER CLIMATE The predictions in the preceding sections tell from a foundation of physiology and history how a change in climate would change the yields of crops if farmers persisted in planting the same varieties of the same species in the same way in the same place, ignoring the weather.
From page 410...
... Winter wheat is another example (Rosenberg, l982)
From page 411...
... Tillage during the fallow year controls weeds and decreases the loss of soil moisture while stubble captures more TABLE 6.5 Progress in Fallow Systems and Wheat Yields, Akron, Colorado (Greb, l979) Fallow Efficiency Storage/ Water-Use Precipitation Yield Efficiency Years Fallow System (%)
From page 412...
... Other means of increasing storage include barriers that catch snow, leveling and terracing to decrease runoff, and harvesting water from nearby acreage. Controlling nonbeneficial plants along western rivers could increase water supply by 3l billion nr or about the same as the overdraft of groundwater in America (Jensen, l982a)
From page 413...
... The indirect effects of warmer and drier, on the other hand, are slightly harmful in the American grain belt as calculations from both past statistics and physiological simulators show. Although pests will change, the direction that they will change is imponderable.
From page 414...
... . Plant breeding under increasing C02 concentration and in a changing climate.
From page 415...
... In Plant Breeding II, K
From page 416...
... . Impacts of different types of temperature change on the growing season for maize.
From page 417...
... . The Southern corn leaf blight epidemic.
From page 418...
... . The close relationship between net photosynthesis and crop yield and the critical role of oxygen stress.


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