In qualitative terms, many indirect effects of climate change on agriculture can be conjectured. Most of them are estimated to be negative and they catch most of the attention of the media. These effects include:

·         The overall predictability of weather and climate would decrease, making the day-to-day and medium-term planning of farm operations more difficult

·         Loss of biodiversity from some of the most fragile environments, such as tropical forests and mangroves

·         sea-level rise (40 cm in the coming 100 years) would submerge some valuable coastal agricultural land

·         the incidence of diseases and pests, especially alien ones, could increase

·         present (agro) ecological zones could shift in some cases over hundreds of kilometres horizontally, and hundreds of metres altitudinally, with the hazard that some plants, especially trees, and animal species cannot follow in time, and that farming systems cannot adjust themselves in time

·         higher temperatures would allow seasonally longer plant growth and crop growing in cool and mountainous areas, allowing in some cases increased cropping and production. In contrast, in already warm areas climate change can cause reduced productivity

·         The current imbalance of food production between cool and temperate regions and tropical and subtropical regions could worsen.

·         there are damaging effects of increasing UV-B on crops, animals and plankton growth. It has been reported that UV-B affects the ability of plankton organisms to control their vertical movements and to adjust to light levels

·         reductions in yield of up to 10% have been measured at experimentally very high UV-B values, and would be particularly effective in plants where the CO₂ fertilization effect is strongest. On the other hand, UV-B increase could increase the amount of plant internal compounds that act against pests.

·         rising temperatures - now estimated to be 0.2°C per decade, or 1 °C by 2040 (Mitchell et al., 1995) with smallest increases in the tropics (IPCC, 1992) - would diminish the yields of some crops, especially if night temperatures are increased (the temperature increase since the mid-1940s is mainly due to increasing night-time temperatures, while CO₂-induced warming would result in an almost equally large rise in minimum and maximum temperatures (Kukla and Karl, 1993)

·         higher temperatures could have a positive effect on growth of plants of the CAM type. They would also strengthen the CO₂ fertilization effect and the CO₂ anti-transpirant effect of C₃ and C₄ plants unless plants get overheated

·         higher night temperature may increase dark respiration of plants, diminishing net biomass production

·         Higher cold-season temperatures may lead to earlier ripening of annual crops, diminishing yield per crop, but would allow locally for the growth of more crops per year due to lengthening of the growing season. Winter kill of pests is likely to be reduced at high latitudes, resulting in greater crop losses and higher need for pest control

·         higher temperatures will allow for more plant growth at high latitudes and altitudes.

·         the extra precipitation on land, if indeed including present subhumid to semi-arid areas, will increase plant growth in these areas, leading to an improved protection of the land surface and increased rainfed agricultural production in already humid areas the extra rainfall may, however, impair adequate crop drying and storage

·         the extra precipitation predicted to occur in some regions provides possibilities for off-site extra storage in rivers, lakes and artificial reservoirs (on-farm or at subcatchment level) for the benefit of improved rural water supply and expanded or more intensive irrigated agriculture and inland fisheries:

·         The effects on water resources and water apportioning of international river and lake basins can be very substantial, with political overtones.

·         The CO₂ fertilization effect: - Higher concentrations of atmospheric CO₂ due to increased use of fossil fuels, deforestation and biomass burning, can have a positive influence on photosynthesis  under optimal growing conditions of light, temperature, nutrient and moisture supply, biomass production can increase, especially of plants with C3 photo-synthetic metabolism , above and even more below ground (Allen et al.). A total of 10 to 20% of the approximate doubling of crop productivity over the past 100 years could be due to this effect (Tans et al., 1990) and forest growth or regrowth may have been stimulated as well

·         The CO₂ anti-transpirant effect: - With increased atmospheric CO₂ the consumptive use of water becomes more efficient because of reduced transpiration. This is induced by a contraction of plant stomata and/or a decrease in the number of stomata per unit leaf area. This restricts the escape of water vapour from the leaf more than it restricts photosynthesis (Wolfe and Erickson, 1993)