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The agriculture sector highlights perhaps more clearly than any other the extent and severity of potential impacts of climate change on food production, food security, lost livelihoods, environmental damage and environmental migration. A “Green PlanetRevolution” in crops and agricultural technology can help reduce emissions, limit damage and increase our adaptability to change.

One of the great achievements of the 20th century was the successful expansion of food production to keep pace with growing demand caused by growing populations and rising incomes. The Food and Agriculture Organization (FAO) estimates that as these two factors continue to push demand upwards, the world will require about 50 per cent more food by 2030, compared to 1998 (FAO 2005a). Climate change will be an important factor in determining whether this can be achieved.

The most recent assessment by the Intergovernmental Panel on Climate Change (IPCC) projected that global average surface
temperature would increase by between 1.4 to 5.8°C over the period 1990 to 2100, while sea-levels could rise by between nine and 88 centimetres (IPCC 2001a). Temperatures have already increased by 0.6°C over the 20th century, and most of this warming is attributable to human activities (IPCC 2001a).

The rise in temperatures will influence crop yields by

● shifting optimal crop growing zones;
● changing patterns of precipitation (quantity and variability) and potential evapotranspiration;
● reducing winter storage of moisture in snow and glacier areas;
● shifting the habitats of crop pests and diseases;
● affecting crop yields through the effects of carbon dioxide and temperature; and
● reducing cropland through sea-level rise and vulnerability to flooding.

The overall impact of these effects will vary by elevation, soil type, crop and other local factors. This variability, along with the
uncertainties of very long-term climate forecasting, especially at the regional level, makes discussion of the effects of climate
change on crop production tentative at best. Generalizations can usually only indicate ranges of possible scenarios. Overall, there may be benefits for agriculture in many temperate zones, where the length of the growing period will increase, costs of overwintering livestock will fall, crop yields may improve and forests may grow faster.

For many tropical zones, the overall picture looks more negative: there may be increased rainfall variability, increased incidence of extreme weather events, and reduced crop yields. Improvements in crops, techniques of cultivation, and soil and water management may be able to compensate, but increasing food production in these zones will be made that much harder (FAO 2002). Shifting growth zones: As a result of rising temperatures, the zones where individual crops do better due to certain climactic conditions are likely to shift polewards and to higher elevations. This will result in a loss of food production and export revenue for some countries in the tropics. For example, coffee is the first, second or third largest export crop for 26 mostly poor countries in Africa and Central America. Yet coffee is sensitive to changes in average temperatures.

In Uganda, a warming of only 2°C would massively cut back the land area that is suitable for coffee (GRID 2002) (Figure 1). In northerly latitudes, global warming may expand the potential production areas northwards – most extensively in Canada and Russia (Epstein and Mills 2005). However, soil types in the new climactic zones may not always be suitable for intensive agriculture as currently practiced in the main producer countries (STOA 1998). Precipitation changes: Precipitation patterns are likely to change in many parts of the world as a result of global warming.

According to the IPCC, globally averaged annual precipitation is projected to increase during the 21st century, though some regions will experience a decline. In areas where an increase is projected, larger year-to-year variations are likely (IPCC 2001a). Water stress during flowering, pollination and grain-filling stages is known to depress yields in maize, soybean, wheat and sorghum (Epstein and Mills 2005).

Changes in precipitation and increased evapotranspiration could lead to further water shortages and affect water quality in some regions of the world. Access to water is a key factor in ensuring food security (Figure 2). Agriculture, which accounts for almost 70 per cent of global water use and up to 95 per cent in Asia and West Asia, will be severely impacted by water stress (FAO 2005a). Precipitation changes will also affect soil moisture. A recent analysis of 15 global climate models found certain predictions consistently across all the models.

Drier soils caused by increased evaporation due to warming were expected in Southwest USA, Mexico, Central America, the Mediterranean, Australia and Southern Africa in every season. Much of the Amazon and West Africa would experience drier soils in June, July and August, while the Asian monsoon region would experience drier soils in December, January and February. Predictions of wetter soils were highly consistent across the models only for Northern middle and high latitudes, and only during non-growing seasons.

The study concluded that global warming could cause an overall reduction in global food production potential due to lack of soil moisture (Wang 2005). Precipitation changes will also affect streamflow and the availability of irrigation.In regions where irrigation depends on melting snow, as in much of South Asia, the retreat of glaciers and reduction in snowfall could have grave consequences for water availability in summer (Barnett and others 2005). Effects of rainfall variability: Climate change will further exacerbate the frequency and magnitude of droughts in Central Asia, Northern and Southern Africa, the Middle East, the Mediterranean region, and Australia (IPPC 2001b).

The increase in the frequency and intensity of extreme weather events – including droughts, storms and floods – could result in crop damage and land degradation (IPCC 2001a). Droughts and floods already rank as the single most common cause of severe food shortages in developing countries (FAO 2005a) (Figure 3). Climate-driven changes in the abundance and distribution of insects, weeds, and pathogens could also affect food productivity (Box 1).

Effects of temperature changes: Crop yields vary considerably according to temperature (Figure 4). Rice yields can decline with even moderate warming, because rice is grown under conditions close to maximum temperature tolerances (Fischer and others 2002a, IPCC 2001b).

A recent study on the effects of global warming on rice yields in the Philippines found that yields declined by ten per cent for each 1°C increase in mean daily minimum (night-time) temperature in the growing season (Peng and others 2004). Carbon dioxide effects: The increase in carbon dioxide concentrations in the atmosphere could increase the net productivity of many crops as a result of carbon ‘fertilization’, which results in increased photosynthesis. This effect varies between crops. It has a positive fertilizing effect on some crops, known as C3 crops.

These include the major cereals of Europe and Asia – wheat and rice. On the other hand, C4 crops such as maize, sorghum,
sugarcane, and millet do not respond well to carbon dioxide. Since some C3 weeds may respond well, the effect would be to depress C4 yields. C4 crops are the major food staples of tropical African and Latin American agriculture (IPCC 2001a).

Recent research on test crops of maize, wheat, soybeans and rice grown under realistic conditions in China, Japan and the US reveals that the fertilization boost from rising carbon dioxide levels in the field may only be about half its theoretical optimum, blunted by environmental stresses such as high ground-level ozone concentrations (Ainsworth and Long 2005, Rogers and others 2004).

Loss of land to sea-level rise: Some of the world’s most densely populated areas could lose fertile arable land, especially in low-lying delta areas such as those of the Nile, the Mekong and the Ganges-Brahmaputra. A one metre sea-level rise, for example, could result in the loss of 5 800 km2 in the lower Nile delta, affecting 15 per cent of Egypt’s habitable land (Nicholls 1994).

In Bangladesh, a one metre rise could flood almost 30 000 km2, affecting over 13 per cent of the population, while in Vietnam 40 000 km2 could be lost affecting 23 per cent of the population (Table 1) (IPCC 2001b). Even where land is not flooded, soil quality may decline due to salinization of soils and groundwater sources and increased risk of tidal surges.

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