The weather conditions - temperature, radiation and water - determine the carrying capacity of the biosphere to produce enough food for the human propulation and domesticated animals. Any short-term fluctuations of the climate can have dramatic effects on the agricultural productivity. Thus, the climate has a direct incidence on food supply.
demographic studies indicate that world population growth is expected to slow markedly in the next century, increasing of nearly 3 billion people by 2050. Hence, in the coming years, unless population size is stabilized as soon as possible, agriculture will have to face an increasing challenge in feeding the growing population of the world.
Many people believe it will also have to face the perspective of global climate changes. A forecasted climate change is global warming, induced by an increasing concentration of radiatively active greenhouse gases.
The appreciation of the effects of potential climatic changes is essential. Many believe it is not until a certain threshold of gravity of the modifications observed, that it will be convenient or pressing to deal with these issues. Agriculture is one of these fields that are carefully monitored.
Besides, assessment of the effect of global climate changes on agriculture might help to properly anticipate and adapt farming to limit potential food shortage.
On the other hand, agricultural trade has grown in the recents years, and now provides significant national food amounts to major importing countries, as well as comfortable income to exporting ones. The international aspect of food trade and food security implies the need to also consider the effects of climate change on a global scale.
The climate models are not really able to give accurate projections because of inadequate understanding of natural processes and computer power limitation. As a consequence, the assessment of possible effects of climate changes are based on estimations.
Moreover, most models are not able yet to provide reliable projections of changes in climate variability on a local scale, or in frequency of exceptional events such as storms and drought. For example, there is a lack of consensus among experts in prediction of regional soil moisture changes.
other indirect effects are the side effects due to the climatic changes : increase of the sea level, changes in ocean currents, tornadoes...
All these influences combine negatively or positively :
The assessment of these effects is different whether one considers annuals crops (cereals, leguminous) or herbaceous perennial cultures (fodder, meadows) or other cultures such as vine or fruit trees...
The effects are also different depending on the latitude : in temperate countries, effects are found less negative or even rather beneficial, while in tropical and desertic countries they tend to be adverse.
Finally, effects depend on altitude, mid and high altitude places rather benefiting from a warmer temperature.
Climate change induced by increasing greenhouse gases is likely to affect crops differently from region to region. For example, average crop yield is expected to drop down to 50% in Pakistan according to the UKMO scenario whereas corn production in Europe is expected to grow up to 25% in optimum hydric conditions.
However, the more favourable effects on yield depend to a large extent on realization of the potentially benefiting effects of CO2 on crop growth and increase of efficiency in water use. Decrease in potential yields is likely to be caused by shortening of the growing period, decrease in water availability and poor vernalization.
carbon dioxide is a perfect example of a change that could have both positive and negative consequences.
CO2 is expected to have positive physiological effects through photosynthesis increase. This effect should be higher on C3 crops (such as wheat) than on C4 crops (such as corn). Under optimum conditions of temperature and humidity, the yield increase could reach 36 % (for a doubling of CO2).
Higher amounts in CO2 will also reduce the loss of water through transpiration, hence decreasing the plants need in water.
On the other hand, other studies also show a change in harvest quality. The growth improvment in C3 plants could favor vegetative biomass on grain biomass; thus leading to a decrease in grain production yield.
CO2 is believed by many scientists to be potentially responsible of productivity increase : 10-15 % for wheat and soybean, 8% for corn and rice for a +2°C scenario on average. However, these results mask great differences among countries.
Soil degradation is more likely to occur, and soil fertility would probably be modified.
A soil constant is its carbon/nitrogen ratio. A doubling of carbon is likely to imply a higher storage of nitrogen in soils, thus providing higher fertilizing elements for plants, hence better yields. The average needs for nitrogen could decrease, and give the opportunity of changing the fertilisation strategies.
The increase in precipitations would probably result in greater risks of erosion, according to the intensity of the rain.
The possible evolution of the soil organic matter is a very debated point though : while the increase in the temperature would induce a greater mineralisation (hence lessen the soil organic matter content), the atmospheric CO2 concentration would tend to increase it.
A very important point to consider is that weeds would undergo the same acceleration of cycle than cultivated crops, and would also benefit of carbonaceous fertilization.
Most weeds being C3 plants, they are likely to compete even more than now against crops such as corn. However some results make it possible to think that weedkillers could gain in effectiveness with the temperature increase.
The increase in rainfall is likely to lead to an increase of atmospheric humidity and maybe to the duration of moisturing. Combined with higher temperatures, these could favor the development of fungal diseases.
Similarly, because of higher temperatures and humidity, there could be an increased pressure from insects and disease vectors.
environmental level, in particular in relation of frequency and intensity of soil drainage (leading to nitrogen leaching), soil erosion, reduction of crop diversity
rural space, through the loss of previously cultivated lands, land speculation, land renunciation, hydraulic amenities.
They are large uncertainties to uncover, particularly the lack of information on the local scale, the uncertainties on magnitude of climate change, the effects of technological changes on productivity, global food demands, and the numerous possibilities of adaptation.
Most agronomists believe that agricultural production will be mostly affected by the severity and pace of climate change, not so much by gradual trends in climate.
If change is gradual, there will be enough time for biota adjustement. Rapid climate change, however, could harm agriculture in many countries, especially those that are already suffering from rather poor soil and climate conditions.
The adoption of efficient new techiques (varieties, planting date, irrigation...) is far from obvious. Some believe developed nations are too well-adapted to nowadays climate. As for developing nations, there may be social or technical constraints that could prevent them from achieving sustainable production.
