Climate change and agriculture

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Global warming is projected to have significant impacts on conditions affecting agriculture, including temperature, precipitation and glacial run-off. These conditions determine the carrying capacity of the biosphere to produce enough food for the human population and domesticated animals. Any short-term fluctuations of the climate can have dramatic effects on the agricultural productivity. Thus, the climate has a direct influence on food supply. Rising carbon dioxide levels would also have effects, both detrimental and beneficial, on crop yields.

A positive effect of rising carbon dioxide concentrations would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis. The overall effect on agriculture will depend on the balance of the beneficial and detrimental effects.

Assessment of the effects of global climate changes on agriculture might help to properly anticipate and adapt farming to maximize agricultural production.

[edit] Background

[edit] Assessment: global vs local

Despite technological advances, such as improved varieties, genetically modified organisms, and 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 local climates rather than in global climate patterns. Consequently, agronomists consider any assessment has to be individually consider each local area.

On the other hand, agricultural trade has grown in recent years, and now provides significant amounts of food, on a national level to major importing countries, as well as comfortable income to exporting ones. The international aspect of trade and security in terms of food implies the need to also consider the effects of climate change on a global scale.

[edit] IPCC

The 2001 IPCC Third Assessment Report concluded that the poorest countries would be hardest hit, with reductions in crop yields in most tropical and sub-tropical regions due to decreased water availability, and new or changed insect pest incidence. In Africa and Latin America many rainfed crops are near their maximum temperature tolerance, so that yields are likely to fall sharply for even small climate changes; falls in agricultural productivity of up to 30% over the 21st century are projected. Marine life and the fishing industry will also be severely affected in some places.

[edit] Shortage in grain production

Crops such as these sunflowers, can be affected by severe drought conditions in Australia

Between 1996 and 2003, grain production has stabilized slightly over 1800 millions of tons. In 2000, 2001, 2002 and 2003, grain stocks have been dropping, resulting in a global grain harvest that was short of consumption by 93 millions 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 level of heat damaged most crops from the United Kingdom and France in the Western Europe through Ukraine in the East. Bread prices have been rising in several countries in the region. (see w:fr:canicule 2003).

[edit] Increases in agricultural production

Between [2003] and [2004] worldwide wheat production increased 13%, oilseed production increased 16%, rice production increased 3%, and cotton production increased 23%. [1] although this may not be directly related to global warming.

[edit] Models and scenarios used to estimate global climate change consequences

Some major limitations to climate changes consequences estimates are related to the models that are being used. The climate models do not have a true ability to give accurate projections because of inadequate understanding of natural processes and the limitation of computing power, and the sheer amount of variables which bring in the need to invoke chaos theory. As a consequence, the assessment of possible effects of climate changes are based on estimations. Most models are also 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 tends to be a lack of consensus among experts in prediction of regional soil moisture changes.

[edit] Crop development models

In order to further study effects of global warming on agriculture, other types of models, such as crop development models, yield prediction, quantities of water or fertilizer consumed, can be used. Such models condense the knowledge accumulated of the climate, soil, and effects observed of the results of various agricultural practices. They thus could make it possible to test strategies of adaptation to modifications of the environment.

Because these models are necessarily simplifying natural conditions (often based on the assumption that weeds, disease and insect pests 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.

[edit] Other types of biological models

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 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 carbon dioxide projections, temperatures raise ranging from 1°C up to 5°C, and with rainfall levels an increase or decrease of 20%. Other parameters may include humidity, wind, and solar activity. 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, resistance to disease. Most developed models are about wheat, maize, rice and soybean.

[edit] Consequences of potential global climate changes on agricultural production

Many scientists hold the position that agricultural shifts are likely. The possible effects proposed are listed below:

The first direct effect is the composition of the earth atmosphere, such the amount of carbon dioxide and ozone. Gases such as methane, nitrogen dioxide and chlorofluorocarbons, however, are commonly believed not to have any effect on physiological processes. Some indirect effects are climate parameters resulting from climate change, such as temperature, insolation, rainfall, and humidity. Other indirect effects include the side effects due to the climatic changes, such as the increase of the sea level, changes in ocean currents, or tornadoes.

All these influences may combine negatively or positively - the assessment of these effects depends on whether one considers annuals crops (cereals and legumes) 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. Effects also depend on altitude, for example, places at higher altitudes tend to benefit 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 hydrologic conditions.

More favourable effects on yield tend to depend to a large extent on realization of the potentially beneficial effects of carbon dioxide 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.

[edit] Temperature potential effect 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, the duration in order to harvest corn could shorten between one and four weeks). The shortening of such a cycle would have an adverse effect on productivity because senescence would occur sooner.

[edit] Potential effect of atmospheric carbon dioxide on yield

Carbon dioxide is essential to plant growth. Rising CO₂ concentration in the atmosphere can have both positive and negative consequences.

Increased CO₂ is expected to have positive physiological effects by increasing the rate of photosynthesis. Currently, the amount of carbon dioxide in the atmosphere is 380 parts per million. In comparison, the amount of oxygen is 210,000 ppm. This means that often plants may be starved of carbon dioxide, being outnumbered by the photosynthetic pollutant oxygen. The effects of an increase in carbon dioxide would be higher on C3 crops (such as wheat) than on C4 crops (such as maize), because the former is more susceptible to carbon dioxide shortage. Under optimum conditions of temperature and humidity, the yield increase could reach 36%, if the levels of carbon dioxide are doubled.^{[citation needed]}

Plants take in CO₂ through multiple tiny pores in their leaves called stomata. These pores can open or close in response to the plant's requirements. A higher ambient level of carbon dioxide allows plants to respire CO₂ more readily with less stomatal opening. Plants lose water due to transpiration through open stomata, so they are adapted to conserve water by opening the stomata only as necessary.

The stomata also serve to allow the plant to eliminate oxygen which is the by-product of photosynthesis. Higher carbon dioxide levels within cells can allow the stomata to be closed longer without suffering photorespiration, an effect of too much oxygen in ratio to carbon dioxide in the plant cell's chloroplasts. For more details see Calvin Cycle.

However, other studies also show a change in harvest quality. The growth improvement in C3 plants could favor vegetative biomass on grain biomass; thus leading to a decrease in grain production yield.

Carbon dioxide is believed by many scientists to be potentially responsible of increase in agricultural production: a 10-15 % increase for wheat and soybean, 8% for corn and rice for a +2°C scenario on average. However, these results disregard great differences among countries, not least between varying climates.^{[citation needed]}

[edit] Effect on quality

According to the IPCC's TAR, "The importance of climate change impacts on grain and forage quality emerges from new research. For rice, the amylose content of the grain--a major determinant of cooking quality--is increased under elevated CO₂" (Conroy et al., 1994). Cooked rice grain from plants grown in high-CO2 environments would be firmer than that from today's plants. However, concentrations of iron and zinc, which are important for human nutrition, would be lower (Seneweera and Conroy, 1997). Moreover, the protein content of the grain decreases under combined increases of temperature and CO₂ (Ziska et al., 1997)."[2]

Studies have shown that higher CO₂ levels lead to reduced plant uptake of nitrogen (and a smaller number showing the same for trace elements such as zinc) resulting in crops with lower nutritional value.[3] [4] This would primarily impact on populations in poorer countries less able to compensate by eating more food, more varied diets, or possibly taking supplements.

Reduced nitrogen content in grazing plants has also been shown to reduce animal productivity in sheep, which depend on microbes in their gut to digest plants, which in turn depend on nitrogen intake.[5]

[edit] Global warming and water distribution

Global warming would be able to modify the global distribution of water, possibly leading to several effects, both detrimental and beneficial.

[edit] Temporal variability and forecasting of the climate

If global warming happens, many believe the general ability to predict weather patterns will decrease, due to more extreme weather. This would make it more difficult to plan agricultural actions. If extreme climatic conditions become more frequent, there would be more intense rainfall, droughts and heat spells in different parts of the globe or the year.

[edit] Agricultural surfaces and climate changes

Climate change is likely to increase the amount of arable land near the poles by reduction of the amount 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 would result in agricultural land loss, in particular in areas such as South East Asia. Erosion, submergence of shorelines, salinity of the water table due to the increased sea levels, could mainly affect agriculture through inundation of low-lying lands.

[edit] Erosion and fertility

With global warming, soil degradation is more likely to occur, and soil fertility would probably be affected by global warming. However, due to the fact that the ratio of carbon to nitrogen is a constant, a doubling of carbon is likely to imply a higher storage of nitrogen in soils as nitrates, thus providing higher fertilizing elements for plants, providing better yields. The average needs for nitrogen could decrease, and give the opportunity of changing often costly fertilisation strategies.

Due to the extremes of climate that would result, the increase in precipitations would probably result in greater risks of erosion, according to the intensity of the rain. The possible evolution of the organic matter in the soil is a highly contested issue: while the increase in the temperature would induce a greater rate in the production of minerals, lessening the soil organic matter content, the atmospheric CO₂ concentration would tend to increase it.

[edit] Potential effects of global climate change on pests, diseases and weeds

A very important point to consider is that weeds would undergo the same acceleration of cycle as cultivated crops, and would also benefit from carbonaceous fertilization. Since most weeds are C3 plants, they are likely to compete even more than now against C4 crops such as corn. However, on the other hand, some results make it possible to think that weedkillers could gain in effectiveness with the temperature increase.

Global warming would cause an increase in rainfall in some areas, which would lead to an increase of atmospheric humidity and the duration of the wet seasons. 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.

[edit] Ozone and UV-B

Some scientists think agriculture could be affected by any decrease in stratospheric ozone, which could increase biologically dangerous ultraviolet radiation B. Excess ultraviolet radiation B can directly effect plant physiology and cause massive amounts of mutations, and indirectly through changed pollinator behavior, though such changes are difficult to quantify. [6] However, it has not yet been ascertained whether an increase in greenhouse gases would decrease stratospheric ozone levels.

In addition, a possible effect of rising temperatures is significantly higher levels of ground-level ozone, which would substantially lower yields.[7]

[edit] Conclusions

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

productivity, in terms of quantity and quality of crops
agricultural practices, through changes of water use (irrigation) and agricultural inputs such as herbicides, insecticides and fertilizers
environmental effects, 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, and hydraulic amenities.

They are large uncertainties to uncover, particularly because there is lack of information on many specific local regions, and include 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, because there is less time for optimum natural selection and adaption. The adoption of efficient new techniques tends to be far from obvious. Some believe developed nations are too well-adapted to the present climate. developing nations also would often have extensive social or technical constraints that prevent them from achieving sustainable production.

[edit] See also

Land Allocation Decision Support System - a research tool that is used to test how climate change may affect agriculture (eg. yield and quality).

[edit] References

Fischer G., Shah M. and van Velthuizen H. (2002) "Climate Change and Agricultural Vulnerability". International Institute for Applied Systems Analysis. Report prepared under UN Institutional Contract Agreement 1113 for World Summit on Sustainable Development. Laxenburg, Austria

[edit] External links

LADSS - Climate Change and Agriculture - Are we asking the right questions?
John Vidal and Tim Radford, The Guardian, June 30, 2005, One in six countries facing food shortage

Part of a series on Global Warming

Subtopics

Scientific opinion • Attribution of causes • Effects • Mitigation • Adaptation • Controversy • Politics • Economics

Related articles

Climate change • Deforestation • Global climate modelling • Global cooling • Global dimming • Greenhouse effect • Greenhouse gases
Intergovernmental Panel on Climate Change • Kyoto Protocol • Peak Oil • Renewable energy • Temperature data

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