Global warming and agriculture
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As global warming causes climate change, the issue of effects of global warming on agriculture due to the change in weather conditions is often invoked in arguments on the course of action involving prediction of climate events. These conditions, including 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. Also, the often thought anthropogenic cause of global warming, an increase in the amount of carbon dioxide levels, would also have effects, both detrimental and beneficial, on crop yields.
For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis. This would be true in some regions (such as Siberia), but recent evidence often portrays global yields would be negatively affected. "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades, large-scale experiments have shown."[1] (http://news.independent.co.uk/world/environment/story.jsp?story=633349)
Thus, assessment of the effects of global climate changes on agriculture might help to properly anticipate and adapt farming to limit potential food shortage.
The situation at the beginning of the 21st century
Assessment differentiation between global scale or local scale
Despite technological advances, such as improved varieties, genetically modificd 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 climate 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 the recents 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.
Shortage in grain production
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).
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 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.
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.
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 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.
Consequences of potential global climate changes on agricultural production
Many scientists hold the position is 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 chloroflourocarbon 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 hydric 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.
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. Temperature changes could also have serious implications for crops and trees that need vernalisation.
Potential effect of atmospheric carbon dioxide on yield
Carbon dioxide could have both positive and negative consequences.
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 to oxygen, which is 21,000. 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 suspectible 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.
A higher level of carbon dioxide would also allow plants to close their stomata, or make the opening smaller, reducing the loss of water through transpiration. This is because higher carbon dioxide levels would allow the stomata to be closed without suffering photorespiration, which due to too much oxygen in the plant cell's chloroplasts in ratio to carbon dioxide. Due to the carbon dioxide starvation mentioned above, the carbon dioxide molecules are outnumbered by oxygen molecules, oxygen often replaces carbon dioxide in the Calvin Cycle first. This not only halts sugar production but destroys existing sugars, badly stunting growth and crop output. Higher levels of carbon dioxide would reduce this likelihood, allowing sugar production to take place without destructive setbacks due to oxygen. This would mean the plant would be able to be able to allow the waste product of photosynthesis, oxygen to remain longer inside the chloroplasts, which would normally exit through the stomata, which is the normal solution to excess carbon dioxide. Allowing the stomata to be closed, and thus the reduction of loss of water decreases the plants need for water.
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 mask great differences among countries.
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.
Effects of water availability on productivity
Water is one of the major limiting factors in the growth and production of crops worldwide. In spite of better water efficiency use, higher summer temperature and lower summer rainfall caused by global warming is likely to have adverse effects. The intensification of the extremes of the hydrological global cycle, will have consequences such as more frequent drought in northern sub-tropical areas or desertification extension in arid areas, while causing devastating flood in other areas.
In developed areas of the world agriculture, and competing industry and municipal users mine 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 would force societies to conform ground water usage to actual recharge rates.
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.
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.
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.
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.
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 [[mutation]s, and indirectly through changed pollinator behavior, though such changes are difficult to quantify. [2] (http://www.guardian.co.uk/uk_news/story/0,,1470944,00.html)
In addition, a side effect of rising temperatures is higher levels of ground-level ozone, which substantially lowers yields.[3] (http://news.independent.co.uk/world/environment/story.jsp?story=633349)
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 techiques 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.