IMPACTS OF WATER SCARCITY ON YIELD
Water is essential not only to survival but is also equally or even more important than nutrients in food production. Agriculture accounts for nearly 70% of the water consumption, with some estimates as high as 85% (Hanasaki et al., 2008a,b). Water scarcity will affect over 1.8 billion people by 2025 (WHO, 2007). This could have major impacts on health, particularly in rural areas, and thus also major impacts on farmer productivity. Although of great significance, such indirect effects are not considered here. Current projections suggest that water demand is likely to double by 2050 (Figure 20). Estimates project water withdrawals to increase by 22–32% by 2025 (De Fraiture et al., 2003) and nearly double by 2050, for all SRES scenarios (Shen et al., 2008). For poor countries with rapid population growth and depletion of groundwater, water-deficit induced food insecurity is a growing problem (Rosegrant and Cai, 2002; Yang et al., 2003). One major factor beyond agricultural, industrial and urban consumption of water is the destruction of watersheds and natural water towers, such as forests in watersheds and wetlands, which also serve as flood buffers (UNEP, 2005).
|Figure 20: Historic and projected changes in water consumption for food production, 1960-2050. (Source: ).
Studies of 128 major river basins and drainage regions show that approximately 20 to 50% of the mean annual river flow in different basins needs to be allocated to freshwater-dependent ecosystems in order to maintain them in good ecological condition. In large parts of Asia and North Africa and some parts of Australia, North America and Europe, current total direct water withdrawals (primarily for irrigation) already tap into the estimated environmental water requirements (Smakhtin et al., 2004). The global consumption of both “blue’’ water (withdrawn for irrigation from rivers, lakes and aquifers) and “green’’ water (precipitation) by rainfed and irrigated agriculture and other terrestrial ecosystems is steadily rising (Rost et al., 2008).
Water is probably one of the most limiting factors in increasing food production. Yields on irrigated croplands are, on average, 2–3 times higher than those on rainfed lands. Irrigated land currently produces 40% of the world’s food on 17% of its land (FAO, 1999), most of it downstream and dependent upon glacial and snowmelt from the Hindu Kush Himalayas. It is evident that in regions where snow and glacial mass are the primary sources of water for irrigation, such as in Central Asia, parts of the Himalayas Hindu Kush, China, India, Pakistan and parts of the Andes, melting will eventually lead to dramatic declines in the water available for irrigation, and hence, food production (Figure 21).
The melting glaciers will impact certain countries more than others, and also substantially impact hydropower production. The Indus River and its tributaries, for example, in addition to providing nearly 60% of the water utilized for irrigation, also provide 45% of the electrical energy in Pakistan.
Of great importance, therefore, is the effect of climate change on the extent of snow and glacial mass (UNEP, 2007) and on the subsequent supply of water for irrigation. Climate change could seriously endanger the current food production potential, such as in the Greater Himalayas Hindu Kush region and in Central Asia (Figure 21). Currently, nearly 35% of the crop production in Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan is based on irrigation, sustaining over 2.5 billion people. Here, water demand is projected to increase by at least 70–90% by 2050. This also includes supply to regions of Central Asia, China and Pakistan, which are under direct water stress today.
|Figure 21: Many of the largest rivers in the Himalayas Hindu Kush region are strongly dependent upon snow and glacial melt for waterflow. Indeed, some scenarios suggest a 20–90% increase in annual flow due to glacial reduction, followed by a 10–40% decline, as glaciers and snow fall below critical thresholds for functions as water towers in 2050–2100. Combined with possible extreme precipitation events, this may result in greater seasonal droughts, and damage from floods. (Source: Rees and Collins, 2004; UNEP, 2007).
A decline of 10–30% in irrigated yields in the basins originating from the mountains of the Himalayas and Central Asia corresponds to 1.7–5.0% of the world cereal production (see box). A 10–30% yield loss due to lower availability of water for irrigation (without increased water efficiency) on the world’s irrigated croplands would equate to losses in the range of 4–12% of world cereal production. In many regions, greater losses have already been observed due to over-extraction of water resources from aquifers and rivers. Studies suggest that almost half of the irrigation water comes from non-renewable and non-local sources (Rost et al., 2008). Indeed, river discharge is decreasing in many areas mainly as a result of anthropogenic use, particularly irrigation (Gerten et al., 2008). Currently, an estimated 24% of the world river basin area has a withdrawal/availability ratio greater than 0.4, which some experts consider to be a rough indication of “severe water stress”. Under a “business-as-usual” scenario of continuing demographic, economic and technological trends up to 2025, water withdrawals are expected to increase in 59% of world river basin area, outweighing the assumed improvements in water-use efficiency, although with great geographic variation (Alcamo et al., 2003). On the assumption that the melting glaciers would cause reduced production by 2050, as indicated, and that a similar estimate for the remaining irrigated lands is considered an upper estimate, then the range of reduced yields due to water scarcity is in the region of 1.7–12% of the projected yield by 2050. Given the high dependence on many of the world’s rivers for irrigation, this estimate could be quite optimistic.
Except for the fact that glaciers are melting rapidly in many places, we do not have adequate data to more accurately project when and where water scarcity will affect irrigation schemes in full. Making accurate projections is also difficult because of variations in the effects on ground and surface water, as well as on water for urban needs and industrial purposes Furthermore, the cost of water may also increase, seriously complicating the water scarcity question. Recent studies show that cost of water has increased by about 400–500% since 1990 in the Indo-Gangetic Basin of India.
Extreme weather events are also very hard to predict. Floods and particularly drought can offset production gains and create great fluxes in crop production, as well as in the survival of livestock. Indeed, a higher frequency of extreme weather events can have severe impacts on crop and livestock production, particularly in Africa that appears especially vulnerable to such events. For example, nine major droughts in selected African countries between 1981 and 2000 resulted in an average livestock loss of 40%, with a range of 22–90% (Figure 22). Similar effects may be observed on crop production. Based on the extent of irrigated cropland impacted in Asia and increasing water scarcity as a result of extreme weather, an annual reduction in the future from climate-induced water scarcity and decreasing water tables may account for an estimated reduction of the world food production by 1.5% by 2030 and at least 5% by 2050.
Melting glaciers jeopardize Asian and world food production
Irrigated croplands, mainly rice, in the watersheds of the Indus, Ganges, Brahmaputra, Yangtze, Huang He (Yellow River), Tarim, Syr Darya and Amu Darya are all, to varying extents, dependent on glacial water and snowmelt from the mountains (Winiger et al., 2005). With rising temperatures, combined with changes in the monsoon, up to 80% of the glaciated area may be lost within this century (Böhner and Lehmkuhl, 2005; UNEP, 2007).
While data are sparse in this region, actual observations from Nepal indicate that current warming at high altitudes is occurring much faster than the global average, up to 0.03º C per year (Shrestha, 1999), and even faster at higher altitudes, up to 0.06º C per year (Liu and Chen, 2000; Eriksson et al., 2008). Scenarios suggest that the effects on the rivers are highly variable, ranging from a major increase in annual flow until around 2050 followed by a relatively rapid decline in flow for the Indus , to a gradual decline in flow in rivers such as the Brahmaputra. If temperatures rise quickly, such as >0.06º C per year, the annual flow of the rivers will invariably decline over time, particularly for those dependent on the mountains, but less so for those more dependent on the monsoons (UNEP, 2004; 2007).
|Figure 22: Impacts on drought on livestock numbers in selected African countries. (Source: IPCC, 2007).
The irrigated cropland in these basins, which are the most dependent upon the mountains for water flow, comprises approximately 857,830,000 ha (UNEP, 2005; 2008). If average production on irrigated rice is projected at 6 tonnes/ha (range 2–10 tonnes/ha), compared to 2–3 tonnes/ha for non-irrigated land (combined, average about 3.3 tonnes/ha in Asia), the water from the melting Himalayas annually supports the production of over 514 million tonnes of cereals, equivalent to nearly 55.5% of Asia’s cereal production and 25% of the world production today. A reduction of, for example, 10–30% due to increased flood damage to irrigated lands combined with reduced water flow and seasonal drought, would thus lower world cereal production of 3,000 million tonnes (by 2050) by 1.7–5%, even if we assume no other yield increases in this period (in which case losses would be larger).
Water scarcity in terms of drought or depleted groundwater could therefore have great impacts on livestock and rangelands. These interactions are also complex. While drought can directly threaten livestock, other factors that influence water availability for livestock are seasonal droughts and socio-economic changes, such as permanent settlement and occupation of seasonal pastures by people other than pastoralists, availability and quality of rangelands, livestock numbers and management approaches.
The combined effects of melting of glaciers, seasonal floods and overuse of ground and surface water for industry, settlements and irrigation, combined with poor water-use efficiency are difficult to estimate. However, given that 40% of the world’s crop yields are based on irrigation, and almost half of this from the basins of rivers originating in the Himalayas alone, the effect of water scarcity can be substantial.
IMPACTS OF SPECIES INFESTATIONS ON YIELD
Invasive alien species (IAS) are now thought to be the second gravest threat to global biodiversity and ecosystems, after habitat destruction and degradation (Mooney et al., 2000; CBD, 2001; Kenis et al., 2009). The steady rise in the number of invasive alien species is predicted to continue under many future global biodiversity scenarios (Sala et al., 2000; Gaston et al, 2003; MA, 2005), although environmental change could also cause non-alien species to become invasive. Environmental change (e.g., rising atmospheric CO2, increased nitrogen deposition, habitat fragmentation and climate change) could promote further invasions (Macdonald, 1994; Malcolm et al., 2002; Le Maitre et al., 2004; Vilà et al., 2006; Song et al., 2008). As invasive or alien species comprise over 70% of all weeds in agriculture (estimated in the US) (Pimentel et al., 2005), increases in invasive species pose a major threat to food production (Mack et al., 2000; MA, 2005; Pimentel et al., 2005; Chenje and Katerere, 2006; van Wilgen et al., 2007).
Worldwide 67,000 pest species attack crops: 9,000 insects and mites, 50,000 pathogens and 8,000 weeds. Up to 70% of them are introduced, with major impacts on global food production.
Across Africa, IAS of the genus Striga have a direct impact on local livelihoods: it affects more than 100 million people and as much as 40% of arable land in the savannahs. These invasive species stunt maize plant growth by attacking the roots and sucking nutrients and water, and thus in addition to the direct financial costs, have implications for food security (Chenje and Katerere, 2006).
Invasive alien species such as pests and diseases have been estimated to cause an annual loss of US$12.8 billion in yield of eight of Africa’s principal crops (Oerke et al., 1994).
In West Africa the larger grain borer (Prostephanus truncates), is responsible for cassava losses of approximately US$ 800 million per year thereby jeopardizing food security (Farrell and Schulten, 2002).
In Tanzania the larger grain borer (Prostephanus truncates) causes some US$ 91 million in maize losses per year (GISP, 2008).
Pimentel et al. (2001) estimated that crop losses due to introduced arthropods in South Africa amount to US$ 1.25 billion per year.
In Australia, the varroa mite, a serious pest in honeybee hives, may result in the loss of $30 million a year in free pollination services from feral bees (CSIRO, 2008). The varroa mite has recently invaded New Zealand and is expected to have an economic cost of US$267–US$602 million, forcing beekeepers to alter the way they manage their hives (GISP, 2008). Invasive alien species such as pests and diseases also impose major constraints on world crop and livestock production (Oerke et al., 1994). Pests and pathogens have had particularly severe effects on crop yields in the world’s poorest and most food insecure region of Sub-Saharan Africa. They have been estimated to cause an annual loss of US$12.8 billion in yield of eight of Africa’s principal crops, and may reduce yields in developing countries overall by around 50% (Oerke et al., 1994).
Importantly, increased climate extremes may promote the spread of invasive species, plant diseases and pest outbreaks (Alig et al., 2004; Anderson et al., 2004; Gan, 2004; FAO, 2008). For instance, there is clear evidence that climate change is altering the distribution, incidence and intensity of animal and plant pests and diseases such as Bluetongue, a sheep disease that is moving north into more temperate zones of Europe (van Wuijckhuise et al., 2006; FAO, 2008). According to FAO (2008), climate scenarios with more winter rain in the Sahel may provide better breeding conditions for migratory plant pests such as desert locust (Schistocerca gregaria) that are totally dependent on rain, temperature and vegetation, with catastrophic impacts on crop and livestock production.
Current and future global food crises may also facilitate the spread of invasive species
The spread of invasive species frequently occurs in the provision of humanitarian emergency food aid. Lower sanitary and phytosanitary standards apply to food aid, particularly emergency food aid, so it may not be surprising that the introduction and spread of potentially invasive species would follow the distribution of emergency relief. For example:
- The grey leaf spot (Circosporda zeae-maydis) is thought to have been introduced into Africa via US food aid shipments of maize during the 1980’s (Ward et al., 1999). It has subsequently spread into all the main maize-growing areas of Africa, and its effect on yields has been such that it is now argued to pose a serious threat to food security (Rangi, 2004).
- The parthenium weed (Parthenium hysterophorus) from Mexico arrived in Africa through grain shipments for famine relief to Ethiopia, where it has earned a local indigenous name which translates to “no crop” (Chenje and Katerere, 2006).
Therefore, the spread of plant pests, weeds and animal diseases across physical and political boundaries threatens food security and represents a global public “bad” that links all countries and all regions.
People relying most directly on ecosystem services, such as small and subsistence farmers, the rural poor and traditional societies, face the most serious and immediate risks from IAS. These people depend on the safety net provided by natural ecosystems in terms of food security and sustained access to fuel, medicinal products, construction materials and protection from natural hazards such as storms and floods (MA, 2005). With the number of IAS in terrestrial ecosystems expected to increase, these impacts are likely to worsen and hamper efforts to meet the growing demands for food (FAO, 2008). In addition, they will likely be exacerbated further by climate change (Pyke et al., 2008).
Using crop genetic diversity to combat pests and diseases in agriculture
Each year farmers experience significant crop losses as a result of disease and pest infestation. These losses can be intensified by changes in climatic conditions. To cope with pest and disease problems, modern agriculture depends to a great extent on the use of pesticides and the continuing production of new crop varieties with specific resistance genes, although the value of integrated pest management techniques and biological control are increasingly recognized. Other ways of increasing productivity while reducing dependence on pesticides are essential for increasing productivity in sustainable ways.
Traditional crop varieties are a primary source of new resistant germplasm for both farmers and breeders. These crop varieties often contain a number of different resistance genes and resistance mechanisms against a range of pests and diseases. In many regions of the world, farmers have local preferences for growing mixtures of varieties, which they understand provide resistance to local pests and diseases and enhance yield stability. Within-crop diversity through the use of variety mixtures, multilines or the use of different varieties in the same production environment has been found to reduce disease incidence and increase productivity without the need for pesticides.
Small-scale farmers in developing countries continue to depend on local genetic diversity to maintain sustainable production and meet their livelihood needs. Loss of genetic choices, reflected as the loss of traditional crop varieties, therefore diminishes farmers’ capacities to cope with changes in pest and disease infection, and leads to yield instability and loss. Intra-specific diversity can be used to reduce crop damage from pest and diseases today and for maintaining levels of diversity against future crop loss, that is, crop populations that have less probability that migrations of new pathogens or mutations of existing pathogens will damage the crop in the future.
In China, interplanting 2 varieties of rice has been found to have significant effects on disease incidence and productivity (Zhu et al., 2000) and is now being used in 3 different provinces on thousands of hectares. A global project supported by UNEP and the Global Environment Facility is under way in China, Ecuador, Morocco and Uganda to develop ways in which farmers can use this approach to combat diseases in crops such as bananas, barley, faba beans and rice.
Alien invasive weeds and pathogens are estimated to be responsible for about 8.5% and 7.5% in yield reduction, respectively, equivalent to US$24 billion and US$21 billion of a crop value of US$267 billion (USBC, 2001; Pimentel et al., 2004; Rossman, 2009). Different estimates range from US$1.1–US$55 billion in losses every year, corresponding to annual losses of 0.4% (OTA, 1993) to 17% (Pimentel et al., 2004; 2005; Rossman, 2009). This does not include increased expenses for more mechanical or pesticide weed control or losses from invasive insects (about 5%) or diseases of livestock.
IMPACTS OF ENVIRONMENTAL DEGRADATION ON FISHERIES
|Figure 23: A shift in desert locust (Schistocerca gregaria) host range due to climate change might have catastrophic impacts on food and livestock production. According to UNICEF (2005) it is estimated that two-thirds of the 2004 loss in food production and pasture in Niger is rooted in the impact of drought at national level, while desert locusts, which infested the country afterwards, caused one-third of the overall damages. In certain areas, swarms of desert locusts consumed nearly 100% of the crops. The desert locust, like other locusts, can change its behaviour and physiology from solitary individuals to gregarious stages that form swarms. Solitary desert locusts occur at low density in the recession area, which covers North Africa, the Sahel, the Red Sea countries and parts of Afghanistan, India, Iran and Pakistan. The outbreak area stretches from Mauritania to India and from southern Europe to Cameroon and Tanzania. Outbreaks and plagues originate in the recession areas when there are several cycles of good breeding conditions. Although the effects of climate change on this system are difficult to judge, climate scenarios with more winter rain in the Sahel may provide better breeding conditions. Large amounts of chemicals are being used to stem this plague, at considerable risk to the environment and public health. A hazard is that locusts depend on the wind and rain to travel. (Source: CIRAD/GRID-Arendal 2005).
At present capture fisheries yield 110–130 million tonnes of seafood annually. Of this, 70 million tonnes are directly consumed by humans, 30 million tonnes are discarded and 30 million tonnes converted to fishmeal. Aquaculture, freshwater and marine fisheries supply about 10% of world human calorie intake – but this is likely to decline or at best stabilize in the future, and might have already reached the maximum.
The primary and most important fishing grounds are found along the continental shelves within less than 200 nautical miles of the shores. The distribution of these fishing grounds is patchy and very localized. Indeed, more than half of the 2004 marine landings were caught within 100 km of the coast in depths generally less than 200 m covering an area of less than 7.5% of the world’s oceans, while 92% was caught in less than half of the total ocean area.
Climate change and increased CO2 assimilation in the oceans will result in increasing ocean acidification, die-back of up to 80% of the world’s coral reefs and disruption of thermohaline circulation and other processes. It will particularly impact dense-shelf water cascading, a “flushing” mechanism that helps to clean polluted coastal waters and carry nutrients to deeper areas. Coastal development is increasing rapidly and is projected to impact 91% of all inhabited coasts by 2050 and contribute to more than 80% of all marine pollution. Increased development, coastal pollution and climate change impacts on currents will accelerate the spreading of marine dead zones, many in or around primary fishing grounds (Diaz and Rosenberg, 2008).
Overfishing and bottom trawling are reducing fish stocks and degrading fish habitats, and threatening the entire productivity of ocean biodiversity hotspots, making them more vulnerable to climate change. Up to 80% of the world’s primary fisheries stocks are exploited close to or beyond their optimum harvest capacity and large areas of productive seabeds on some fishing grounds have been partly or extensively damaged. For example, over 95% of the damage and change to seamounts has been caused by bottom trawling, which has been estimated to be as damaging to the seabed as all other fishing gear combined. Damaged from overfishing , bottom trawling and pollution, the worlds fishing grounds are increasingly becoming infested by invasive species mainly through ballast water, with the pattern closely following the major shipping routes.
The result of unsustainable fishing practices are that we might no longer able to increase the landings from conventional fisheries, and might, in fact, be facing a substantial decline in the world’s fisheries in the coming decade. This will also have severe impacts on aquaculture production, which relies on fish for feed.
Aquaculture production has increased more than seven-fold in weight (from 5 to 36 million tonnes) between 1980 and 2000. The value generated has grown from US$9 billion in 1984 to US$52 billion in 2000 (Deutsch et al., 2007). In 2006, the world consumed 110.4 million tonnes of fish, of which 51.7 million tonnes originated from aquaculture. In order to meet the growing fish demand, aquaculture will have to produce an additional 28.8 million tonnes – 80.5 million tonnes overall – each year, just to maintain per capita fish consumption at current levels. Aquaculture growth rate is declining, however: a yearly growth rate of 11.8% from 1985 to 1995 slowed to 7.1% during the following decade, and to 6.1% for the 2004–2006 period. In October 2008, FAO cautioned that a series of emerging challenges need to be addressed if aquaculture is to meet increasing demand for fish.
THE FEED BOTTLENECK
Almost 40% of all aquaculture production is now directly dependent on commercial feed. Most farmed fish that are consumed in the developing world, such as carps and tilapia, are herbivores or omnivores. But other species like salmon or shrimp – often raised in developing countries – are fed other fish in the form of fishmeal or oil. Salmon, shrimp and trout aquaculture alone accounts for almost 50% of all fishmeal used in aquaculture, but provides less than 10% of the production volumes (Deutsch et al., 2007). In 2006, aquaculture consumed 3.06 million tonnes (56%) of world fishmeal production and 780,000 tonnes (87%) of total fish oil production. Over 50% of the sector’s use of fish oil occurs on salmon farms. Fishmeal and fish oil production has remained stagnant over the last decade and significant increases in their production are not anticipated, according to FAO. At the same time, the volume of fishmeal and fish oil used in formulated aquaculture feeds tripled between 1996 and 2006. This was made possible by a significant reduction of the poultry sector’s reliance on fishmeal for poultry feeds. As formulated feeds are increasingly being used for non-filter feeding omnivorous fish like carps, the demand for fishmeal is increasing.
|Figure 24: Possible individual ranges of yield and cropland area losses by 2050 with climate change (A2 scenario), non-food crops incl. biofuels (six OECD scenarios), land degradation (on yield and area, respectively, see text), water scarcity (including gradual melt of Himalayas glaciers, see box and text) and pests (invasive species of weeds, pathogens and invertebrates such as insects, see text). Although these effects may be considerable, cumulative and indirect effects or interactions are not considered here, nor are the cumulative loss of ecosystems services endangering the entire functioning of food production systems. Notice that the climate impact bar only relates to changes in general growing conditions incl. temperature, evapotranspiration and rainfall, not the indirect impacts of climate change such as on glacial melt (water scarcity) and increases in invasive species. The other bars in part incorporate some of these important climate change impacts. Effects of extreme weather is not included, but could be substantial (Source: Compiled by UNEP for this report).
As for meat production, feed is a major bottleneck. It is extremely difficult to project the future role of fisheries and aquaculture, but it is evident that the growth in aquaculture may be limited by access to feed, which, in turn is partly dependent on capture fisheries. There is no indication that today’s marine fisheries could sustain the 23% increase in landings needed to sustain the 56% growth in aquaculture production required to maintain per capita fish consumption at current levels. Given the grave nature of the trends and scenarios on overfishing and ocean degradation, a future collapse of ocean fisheries would immediately affect aquaculture production and the prices of aquaculture products. Even assuming that marine fisheries landings can be maintained at current levels, the proportion of fish in the diet (in terms of calorie intake) may go down from the current 2% of world human calorie intake to 1.5% by 2030 and to only 1% by 2050. This loss will have to be compensated for by either meat or crops.
RANGES OF IMPACTS ON CROPLAND AREA AND YIELDS
The combined impact of reductions in yield and in the area available for food production will have to be compensated either by even further yield increases, cropland expansion, or by increasing food energy efficiency.
The extent of the impact of each individual factor on food production is likely to exhibit great regional variation. This probably also applies to the possible socio-economic responses, including that of policy changes as well as the responses and incentives for change of the individual farmer. It also applies to the financial and institutional capacity of the country, region and individual farm to cope with increasing stressors.
As the extent of interaction, synergistic or cumulative effects are not known, the projections should be interpreted cautiously, reflecting mainly a risk assessment and indication of the possible magnitude and relevance of environmental degradation for future food supply.
Uncertainties in future scenarios
As defined by the FAO, food security exists when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life. Future food security depends on developments in both supply and demand, but projections for these variables are cursed with uncertainty. On the demand side, population and economic growth are particularly subject to a high degree of uncertainty. Key uncertainties for future supply have to go with agricultural productivity and energy markets. In addition, developments are contingent on new policies being put in place.
More specific causes of uncertainty in predicting future trends are:
- Climate change: While mean temperature changes are quite well modelled, rainfall changes and extreme weather events are much less so, particularly at smaller scales, neither are extreme weather conditions predictable today.
- Energy supply: If peak oil supply is reached within the period under consideration, this will have major consequences for the economics of virtually all aspects of food production as well as on likely demands for biofuels.
- Technological advances in food production, such as by the use of genetically modified crops may also influence yield projections.
- Availability of freshwater (linked with climate change and with technology).
- Human behaviour: Food preferences, ability to adapt to changing conditions for food supply, commitment to more equitable distribution of resources or increased tendency to defend local resource base. (Economic factors as major proximate driver of food production decisions: supply/demand curves, input costs, extent of exposure to international markets, government policy as expressed in subsidies, tariffs, etc).
- Impacts of pests and diseases (including alien invasive species) on food supply.
- Actual versus predicted population growth.
- Major disease outbreaks in humans.
- Other catastrophic events (war, major earthquakes, volcanic events, etc).
The future impact of some of these is so unpredictable that it is difficult to see how they can realistically be incorporated into any quantitative models, other than through including some essentially arbitrary tolerance limits in calculating necessary food supplies.
Overall, no fully integrated model currently exists that assesses agriculture in a holistic way. Current models and scenarios focus on one or very few of these areas, e.g., land use change (IMAGE model), global climate models (e.g., UK Met office model), or are add-ons to these models (e.g., GLOBIO biodiversity model) with feedbacks and interconnections not fully integrated.