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 impacts 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 impact of global climate changes on agriculture might help to properly anticipate and adapt farming to limit potential food shortage.

Table of contents
1 Situation at the beginning of the XXI century
2 Models and scenarios used to estimate global climate change consequences
3 Potential global climate changes consequences on agricultural production
4 See also
5 External links

Situation at the beginning of the XXI century

Assessment on global scale or local scale ?

Despite technological advances, such as improved varieties, genetically modificd organisms, or irrigation systems, weather is still a key factor in agricultural productivity, as well as soil properties and natural communities. The effect of climate on agriculture is related to variabilities in climate rather than in global climate patterns. Consequently, agronomists consider it has to be individually considered for each local area.

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 confortable income to exporting ones. The international aspect of food trade and food security implies the need to also consider the impacts of climate change on a global scale.

Shortage in grain production

Between 1996 and 2003, grain production has stabilized slightly over 1800 milions of tons. In 2000, 2001, 2002 and 2003, grain stocks have been dropping, resulting in a global grain harvest short of consumption by 93 milions of tons in 2003.

The earth's average temperature has been rising since the late 1970s, with the three warmest years on record coming in the last five years. In 2002, India and the United States suffered sharp harvest reductions because of record temperatures and drought. In 2003 Europe suffered very low rainfall throughout spring and summer, and a record heat damaged most crops from the United Kingdom and France in the west through Ukraine in the east. Bread prices have been rising in several countries in the region. (see w:fr:canicule 2003).

Models and scenarios used to estimate global climate change consequences

Climate models limitations

Some major limitations to climate changes consequences estimates are related to the models that are being used.

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 impacts 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.

Crop development models

The study of the impacts is using other types of models, such as crop development models, yield prediction, quantities of water or fertilizer consumed. The models condense the knowledge accumulated in influence of the climate, soil, and agricultural practices. They can make it possible to test strategies of adaptation to the modifications of the environment.

Because these models are necessarily simplifying natural conditions (often based on the assumption that weeds, disease and insect pest are controlled), it is not clear whether the results they give will have an in-field reality. However, some results are partly validated with an increasing number of experimental results.

Other types of biological models used

Other models, such as insect and disease development models based on climate projections are also used (for example simulation of aphid reproduction or septoria (cereal fungal disease) development).

Scenarios in biological models

Scenarios are used in order to estimate climate changes effects on crop development and yield. Each scenario is defined as a set of meteorological variables, based on generally accepted projections.

For example, many models are running simulations based on doubled CO2 projections, temperatures raise ranging from 1C up to 5C, rainfall -/+20%. Other parameters may include, humidity, wind, and global radiation.
Scenarios of crop models are testing farm-level adaptation, such as sowing date shift, climate adapted species (vernalisation need, heat and cold resistance), irrigation and fertilizer adaptation, disease resistance.
Most developed models are about wheat, maize, rice and soybean.

Potential global climate changes consequences on agricultural production

Many scientists position is that agricultural shifts are likely.

Several types of changing parameters can have an impact on agriculture

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.

Temperature potential impact on growing period

Duration of crop growth cycles are above all related to temperature.
An increase in temperature will speed up development. In the case of an annual crop, the duration between sowing and harvesting will shorten (for example, corn duration cycle could shorten between 1 and 4 weeks). The shortening of the cycle would rather has adverse effect on productivity because of
senescence occurring sooner.
Temperature changes could also have serious implications for crops and trees that need vernalisation.

Atmospheric CO2 potential impact on yield

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 impact 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 +2C scenario on average. However, these results mask great differences among countries.

Water availability impact on productivity

Water is a major limiting factor in the growth and production of crops worldwide.
In spite of better water efficiency use, higher summer temperature and lower summer rainfall is likely to have adverse impact. The intensification of the
hydrological global cycle will have consequences such as more frequent drought in northern sub-tropical areas or desertification extension in arid areas. In developed areas of the world agriculture (and competing industry and municipal users) are mining fossil water supplies. In coastal areas deep water wells also reverse normal ground water flow toward the ocean, leading to saline water intrusion into aquifers. Further increases in usage will accelerate the day of reckoning when societies must conform ground water usage to actual recharge rates.

Erosion and fertility

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.

Global climate change potential impact on pests, diseases and weeds

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.

Agricultural surfaces and climate changes

Climate change is likely to increase agricultural land surface near the poles by reduction of frozen lands. Sea levels are expected to get up to one meter higher by 2100, though this projection is disputed. Rise in sea level should result in agricultural land loss in particular in South East Asia. Erosion, submergence of shorelines, salinity of water table, could mainly affect agriculture through inundation of low-lying lands.

Ozone and UV-B

Some scientists think agriculture could be affected by any decrease in stratospheric ozone, which could increase biologically dangerous ultraviolet radiation. Excess UV radiation can directly impact plant physiology, and indirectly through changed pollinator behavior, though such changes are difficult to quantify.

Temporal variability and forecasting of the climate

Many believe the general foreseeability of the climate will decrease, making it more difficult to plan agricultural practices. They also think likely that extrem climatic conditions become more frequent, particularly in terms of intense rainfall, droughts and heat spells.

Conclusions

In the long run, the climatic change could affect agriculture in several ways :

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.

See also

External links