Although there is huge increase in carbon dioxide production by various
human activities, the atmospheric carbon dioxide level is amounted in small
quantities like around 409 parts per million. Explain the situation and its
consequences.
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Effects of Increasing Carbon Dioxide Levels and Climate Change on Plant Growth, Evapotranspiration, and Water Resources
Leon Hartwell Allen, Jr.
U.S. Department of Agriculture
Gainesville, Florida
The atmospheric carbon dioxide concentration has risen from about 270 parts per million (ppm) before 1700 to about 355 ppm today. Climate changes, including a mean global surface temperature rise of between 2.8 and 5.2°C, have been predicted by five independent general circulation models (GCMs) for a doubling of the carbon dioxide concentration. The objectives of this paper are to examine plant responses to rising carbon dioxide levels and climatic changes and to interpret the consequences of these changes on crop water use and water resources for the United States.
BACKGROUND: PLANT RESPONSES TO ENVIRONMENTAL FACTORS
The main purpose of irrigation is to supply plants with adequate water for transpiration and for incorporating the element hydrogen in plant tissues through photosynthesis and subsequent biosynthesis of various tissues and organs. Transpirational flux requires several hundred times more water than photosynthesis.
In a series of U.S. Department of Agriculture studies beginning in 1910 in Akron, Colorado, Briggs and Shantz (1913a,b; 1914) showed that the water requirement of plants is linearly related to the biomass production of plants. They established this linear relationship by growing plants in metal containers filled with soil. Throughout the period of growth, they monitored water use carefully by weighing and adding measured amounts of water to maintain a desirable soil water content as water lost by plant transpiration was replenished.
The findings of Briggs and Shantz have been confirmed repeatedly (Allison et al., 1958; Arkley, 1963; Chang, 1968; Hanks et al., 1969; Stanhill, 1960). Figure 7.1 shows the linear relationship between biomass produced and rainfall plus irrigation water used by Sart sorghum and Starr millet in Alabama, as adapted from data of Bennett et al. (1964). De Wit (1958) examined the relationships among climatic factors, yield, and water use by crops. He found the following general linear relationship to be true, especially in semiarid climates:

where
Y = yield component (e.g., total above-ground biomass or seed production)
T = cumulative actual transpiration
Tmax = maximum possible cumulative transpiration
m = constant dependent on yield component and species, especially on differences among photosynthetic mechanisms
Pan evaporation was used to represent Tmax, which is proportional to climatic factors, especially air vapor pressure deficit (VPD):

where
es = the saturation vapor pressure at a given air temperature
ea = the actual vapor pressure that exists in the air.
Combining these relationships, we see that yield is proportional to cumulative transpiration divided by vapor pressure deficit:

where k is a constant with units millibars • g (dry matter) • g-1 (water). Like m, k depends on yield component, species, and photosynthetic mechanisms.