Climatic modification induced by the enrichment of the atmosphere with carbon dioxide has direct effects on agriculture. Photosynthesis, the mechanism through which plants convert inorganic substances such as carbon dioxide sulphates, nitrate and water into organic compounds, is the basis not only of all present life and current biomass production by also of fossil fuels, the accumulated products of past photosynthesis. Therefore, any factor which would influence the rate of photosynthesis will have a strong effect of foods, fibres and fuels.
It was reported in 1993 that the impacts of increased carbon dioxide concentration, temperature and UV-b radiation on vegetation were studied in an attempt to determine their combined effects. Increased carbon dioxide tends to reduce transpiration, increase photosynthetic water use efficiency; increased temperature caused increased water use and accelerated plant development; greater exposure to UV-b radiation decreased photosynthesis and growth in sensitive species. However, the effect of all three variable is very species-specific and thus hard to quantify on a broad scale.
Researchers searching for evidence of a carbon dioxide fertilization effect in tree ring records has generally been inconclusive. This suggests that climate warming could have a greater effect on growth than via the soil and roots. In a study of 2.5 year old beech trees, stem diameter and height and leaf areas increased in a doubled carbon dioxide atmosphere. Sour orange trees grown in a double carbon dioxide atmosphere in open changers were 2.8 times larger than controls at the 18 month point and have maintained this difference for 5 years. In experiments, the carbon dioxide effect in early successional species (North American gray birch, ash and red maple) were more sensitive to nutrient levels while in late successional species (red oak, yellow birch and striped maple) the effect was more sensitive to the level of light.
Certain crops (known as C3 crops) prosper in greenhouse (higher carbon dioxide) conditions. These include wheat, barley and rye, and tend to be grown predominantly in the developed world. Other crops (C4 crops) decrease their yield in increasing carbon dioxide conditions; the crops affected include sorghum, maize and millet, which form the staple diet of some developing countries.
A global assessment of the portential impacts of climatic changes on the world food (cereal) supply found that a double atmospheric carbon dioxide concentration and its associated climate would lead to only a small decrease in global crop production. But developing countries are likely to bear the brunt of the problem as climate change increased the disparities in cereal production. Changes in crop yields were mostly negative in low altitude (largely developing) countries and positive in higher latitude countries. Adaptations at the farm level were found to have the greatest impact in developed countries. Despite the mitigating effects of the adaptations, cereal prices and thus the population at risk from hunger increased.
A study of South Carolina peach production found that a daily increase of minimum and maximum temperatures of 2 and 4øC would decrease the number of essential chilling hours during the normal state, to the point that the number of chilling hours may often not be sufficient under a 4øC scenario. However, the temperature increases would reduce losses due to later"killer frosts". In rice growing areas of Japan, an increase in the temperature of shallow-ponded water of 2-4øC could induce a remarkable shift northward of the 15øC isotherm that delineates the zone of safe transplantation of seedlings. A Netherlands study with simulated spring wheat yields found that when a temperature increase was considered alone, yields were predicted to decrease. However, in combination with increased carbon dioxide, small yield increase were predicted for years in which water did not limit growth, and large yield increases were expected for dry years due to increases water-use efficiency. In a comprehensive US MINK regional study on the production of corn, sorghum, soybeans and wheat, it was found that farm level adjustments and the carbon dioxide effect could eliminate 80 percent of the negative impact of temperature increases. The crop insurance industry would have to establish new rates based on shifts in crop varieties, extension of crop zones and changes in yields. There may be reduced coverage due to greater risks of unknowns, which may lead to more self insurance and thus more diversification.