IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

9.4.4 Agriculture

Results from various assessments of impacts of climate change on agriculture based on various climate models and SRES emissions scenarios indicate certain agricultural areas that may undergo negative changes. It is estimated that, by 2100, parts of the Sahara are likely to emerge as the most vulnerable, showing likely agricultural losses of between 2 and 7% of GDP. Western and central Africa are also vulnerable, with impacts ranging from 2 to 4%. Northern and southern Africa, however, are expected to have losses of 0.4 to 1.3% (Mendelsohn et al., 2000b).

More recent assessments combining global- and regional-scale analysis, impacts of climate change on growing periods and agricultural systems, and possible livelihood implications, have also been examined (Jones and Thornton, 2003; Huntingford et al., 2005; Thornton et al., 2006). Based on the A1FI scenario, both the HadCM3 and ECHAM4 GCMs agree on areas of change in the coastal systems of southern and eastern Africa (Figure 9.4). Under both the A1 and B1 scenarios, mixed rain-fed semi-arid systems are shown to be affected in the Sahel, as well as mixed rain-fed and highland perennial systems in the Great Lakes region and in other parts of East Africa. In the B1 world, marginal areas (e.g., semi-arid lands) become more marginal, with moderate impacts on coastal systems (Thornton et al., 2006; see Chapter 5, Section 5.4.2). Such changes in the growing period are important, especially when viewed against possible changes in seasonality of rainfall, onset of rain days and intensity of rainfall, as indicated in Sections 9.2.1 and 9.3.1.

Figure 9.4

Figure 9.4. Agricultural areas within the livestock-only systems (LGA) in arid and semi-arid areas, and rain-fed mixed crop/livestock systems (MRA) in semi-arid areas, are projected by the HadCM3 GCM to undergo >20% reduction in length of growing period to 2050, SRES A1 (left) and B1 (right) emissions scenarios, after Thornton et al. (2006).

Other recent assessments using the FAO/IIASA Agro-Ecological Zones model (AEZ) in conjunction with IIASA’s world food system or Basic Linked System (BSL), as well as climate variables from five different GCMs under four SRES emissions scenarios, show further agricultural impacts such as changes in agricultural potential by the 2080s (Fischer et al., 2005). By the 2080s, a significant decrease in suitable rain-fed land extent and production potential for cereals is estimated under climate change. Furthermore, for the same projections, for the same time horizon the area of arid and semi-arid land in Africa could increase by 5-8% (60-90 million hectares). The study shows that wheat production is likely to disappear from Africa by the 2080s. On a more local scale, assessments have shown a range of impacts. Southern Africa would be likely to experience notable reductions in maize production under possible increased ENSO conditions (Stige et al., 2006).

In other countries, additional risks that could be exacerbated by climate change include greater erosion, deficiencies in yields from rain-fed agriculture of up to 50% during the 2000-2020 period, and reductions in crop growth period (Agoumi, 2003). A recent study on South African agricultural impacts, based on three scenarios, indicates that crop net revenues will be likely to fall by as much as 90% by 2100, with small-scale farmers being the most severely affected. However, there is the possibility that adaptation could reduce these negative effects (Benhin, 2006). In Egypt, for example, climate change could decrease national production of many crops (ranging from –11% for rice to –28% for soybeans) by 2050 compared with their production under current climate conditions (Eid et al., 2006). Other agricultural activities could also be affected by climate change and variability, including changes in the onset of rain days and the variability of dry spells (e.g., Reason et al., 2005; see also Chapter 5).

However, not all changes in climate and climate variability will be negative, as agriculture and the growing seasons in certain areas (for example, parts of the Ethiopian highlands and parts of southern Africa such as Mozambique), may lengthen under climate change, due to a combination of increased temperature and rainfall changes (Thornton et al., 2006). Mild climate scenarios project further benefits across African croplands for irrigated and, especially, dryland farms. However, it is worth noting that, even under these favourable scenarios, populated regions of the Mediterranean coastline, central, western and southern Africa are expected to be adversely affected (Kurukulasuriya and Mendelsohn, 2006a).

Fisheries are another important source of revenue, employment and proteins. They contribute over 6% of Namibia’s and Senegal’s GDP (Njaya and Howard, 2006). Climate-change impacts on this sector, however, need to be viewed together with other human activities, including impacts that may arise from governance of fresh and marine waters (AMCEN/UNEP, 2002). Fisheries could be affected by different biophysical impacts of climate change, depending on the resources on which they are based (Niang-Diop, 2005; Clark, 2006). With a rise in annual global temperature (e.g. of the order of 1.5 to 2.0°C) fisheries in North West Africa and the East African lakes are shown to be impacted (see ECF and Potsdam Institute, 2004; Warren et al., 2006). In coastal regions that have major lagoons or lake systems, changes in freshwater flows and a greater intrusion of salt water into lagoons will affect the species that are the basis of inland fisheries or aquaculture (République de Côte d’Ivoire, 2000; République du Congo, 2001; Cury and Shannon, 2004). In South Africa, fisheries could be affected by changes in estuaries, coral reefs and upwelling; with those that are dependent on the first two ecosystems being the most vulnerable (Clark, 2006). Recent simulations based on the NCAR GCM under a doubling of carbon dioxide indicate that extreme wind and turbulence could decrease productivity by 50-60%, while turbulence will probably bring about a 10% decline in productivity in the spawning grounds and an increase of 3% in the main feeding grounds (Clark et al., 2003).

The impact of climate change on livestock farming in Africa was examined by Seo and Mendelsohn (2006a, b). They showed that a warming of 2.5°C could increase the income of small livestock farms by 26% (+US$1.4 billion). This increase is projected to come from stock expansion. Further increases in temperature would then lead to a gradual fall in net revenue per animal. A warming of 5°C would probably increase the income of small livestock farms by about 58% (+US$3.2 billion), largely as a result of stock increases. By contrast, a warming of 2.5°C would be likely to decrease the income of large livestock farms by 22% (–US$13 billion) and a warming of 5°C would probably reduce income by as much as 35% (–US$20 billion). This reduction in income for large livestock farms would probably result both from a decline in the number of stock and a reduction in the net revenue per animal. Increased precipitation of 14% would be likely to reduce the income of small livestock farms by 10% (–US$ 0.6 billion), mostly due to a reduction in the number of animals kept. The same reduction in precipitation would be likely to reduce the income of large livestock farms by about 9% (–US$5 billion), due to a reduction both in stock numbers and in net revenue per animal.

The study by Seo and Mendelsohn (2006a) further shows that higher temperatures are beneficial for small farms that keep goats and sheep because it is easy to substitute animals that are heat-tolerant. By contrast, large farms are more dependent on species such as cattle, which are not heat-tolerant. Increased precipitation is likely to be harmful to grazing animals because it implies a shift from grassland to forests and an increase in harmful disease vectors, and also a shift from livestock to crops.

Assessing future trends in agricultural production in Africa, even without climate change, remains exceedingly difficult (e.g., contributions to GDP and impacts on GDP because of climate variability and other factors - see, for example, Mendelsohn et al., 2000b; Tiffen, 2003; Arrow et al., 2004; Desta and Coppock, 2004; Ferguson, 2006). While agriculture is a key source of livelihood in Africa, there is evidence that off-farm incomes are also increasing in some areas - up to 60 to 80% of total incomes in some cases (Bryceson, 2002). Urbanisation and off-farm increases in income also seem to be contributing to reduced farm sizes. Future scenarios and projections may thus need to include such changes, as well as relevant population estimates, allowing for the impact of HIV/AIDS, especially on farm labour productivity (Thornton et al., 2006).