Environmental degradation and loss of ecosystem services will directly affect pests (weeds, insects and pathogens), soil erosion and nutrient depletion, growing conditions through climate and weather, as well as available water for irrigation through impacts on rainfall and ground and surface water. These are factors that individually could account for over 50% in loss of the yield in a given “bad” year. The interactions among these variables, compounded by management systems and society, are highly complex. A changing climate will affect evapo-transpiration, rainfall, river flow, resilience to grazing, insects, pathogens and risk of invasions, to mention a few. In the following section we attempt to provide for each variable, rough estimates of how much environmental degradation and loss of some ecosystem services could contribute to reducing yields by 2050. This is based on peer reviewed studies, models and expert judgment, and with the understanding that conditions and estimates vary considerably and relationships are highly complex.
IMPACTS OF LAND DEGRADATION ON CROP YIELDS
Unsustainable practices in irrigation and production may lead to increased salinization of soil, nutrient depletion and erosion. An estimated 950 million ha of salt-affected lands occur in arid and semi-arid regions, nearly 33% of the potentially arable land area of the world. Globally, some 20% of irrigated land (450,000 km2) is salt-affected, with 2,500–5,000 km2 of lost production every year as a result of salinity (UNEP, 2008). In South Asia, annual economic loss is estimated at US$1,500 million due to salinization (UNEP, 1994).
|Figure 16: Losses in land productivity due to land degradation. (Source: Bai et al., 2008).
Nutrient depletion as a form of land degradation has a severe economic impact at the global scale, especially in Sub-Saharan Africa. Stoorvogel et al. (1993) estimated nutrient balances for 38 countries in Sub-Saharan Africa. Annual depletion rates of soil fertility were estimated at 22 kg nitrogen (N), 3 kg phosphorus (P), and 15 kg potassium (K) per ha. In Zimbabwe, soil erosion alone results in an annual loss of N and P totalling US$1.5 billion. In South Asia, the annual economic loss is estimated at US$600 million for nutrient loss by erosion, and US$1,200 million from soil fertility depletion (Stocking, 1986; UNEP, 1994).
Kenya land use and rain-use efficiency
Sub-Saharan Africa is particularly impacted by land degradation. In Kenya, over the period 1981–2003, despite improvements in woodland and grassland, productivity declined across 40% of cropland – a critical situation in the context of a doubling of the human population over the same period (Bai and Dent, 2006). In South Africa, production decreased overall; 29% of the country suffered land degradation, including 41% of all cropland (Bai and Dent, 2007a); about 17 million people, or 38% of the South African population, depend on these degrading areas. (Source: Bai and Dent, 2007).
Trend in biomass in 1981–2003 (left) and in rain-use efficiency (RUE) in 1981–2002 (right). Decreases in RUE could be due to various factors, including degradation and run-off, soil evaporation, increasing depleted soils, overgrazing by livestock or other forms of range degradation.
Red major decline
Yellow moderate decline
Erosion is very significant in land degradation. On a global scale, the annual loss of 75 billion tonnes of soil costs the world about US$400 billion/year (at US$3/tonne of soil for nutrients and US$2/tonne of soil for water), or approximately US$70/person/year (Lal, 1998). It is estimated that the total annual cost of erosion from agriculture in the US is about US$44 billion/year or about US$247/ha of cropland and pasture (Lal, 1998). In Sub-Saharan Africa it is much larger; in some countries productivity has declined in over 40% of the cropland area in two decades while population has doubled. Overgrazing of vegetation by livestock and subsequent land degradation is a widespread problem in these regions.
The productivity of some lands has declined by 50% due to soil erosion and desertification (Figure 16). Yield reduction in Africa due to past soil erosion may range from 2–40%, with a mean loss of 8.2% for the continent. Africa is perhaps the continent most severely impacted by land degradation (den Biggelaar et al., 2004; Henao and Baanante, 2006), with the global average being lower, possibly in the range of 1–8%. With increasing pressures of climate change, water scarcity, population growth and increasing livestock densities, these ranges will be probably conservative by 2050.
IMPACTS OF CLIMATE CHANGE ON YIELD
Global climate change may impact food production across a range of pathways (Figure 17): 1) By changing overall growing conditions (general rainfall distribution, temperature regime and carbon); 2) By inducing more extreme weather such as floods, drought and storms; and 3) By increasing extent, type and frequency of infestations, including that of invasive alien species (dealt with in a separate section).
|Figure 17: Projected impacts of climate change. (Source: Stern Review, 2008).
The estimated impacts of changes in the general climate regime vary with the different models in the short to mid-term (2030–2050), but after 2050 an increasing number of models agree on rising negative impacts (IPCC, 2007; Schmidhuber and Tubiello, 2007). Many models have projected that the potential for global food production may rise with increases in local average temperature over a range of 1–3ºC (before 2050), but above this range (after 2050) may decrease (IPCC, 2007; Meehl et al., 2007). Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration (CO2) will offset these losses. The CO2 fertilization factors used in models to project future yields were derived from enclosure studies conducted about 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated CO2 levels under full open-air field conditions. In those trials, elevated CO2 enhanced yield by about 50% less than in the enclosure studies. Hence, previous projections of no impact or even a slight positive impact of increasing CO2 on global agricultural production by 2030 and 2050 may be too optimistic (Long et al., 2006). Current research results conclude that while crops would respond positively to elevated CO2 in the absence of climate change, the associated impacts of high temperatures, altered patterns of precipitation, and possible increased frequency of extreme events such as droughts and floods, will likely combine to depress yields and increase production risks in many world regions (Tubiello and Fischer, 2006).
Furthermore, projected changes in the frequency and severity of extreme climate events are predicted to have more serious consequences for food and food security than changes in projected mean temperatures and precipitation (IPCC, 2007). Also, regional differences will grow stronger with time (Parry et al., 2005), with potentially large negative impacts in developing regions but only small changes in developed regions (IPCC 2007; Slater et al. 2007). Developing countries are more vulnerable because of the dominance of agriculture in their economies, the scarcity of capital for adaptation measures, their warmer baseline climates and heightened exposure to extreme events (Tubiello and Fischer, 2006; Brown and Funk, 2008).This will aggravate inequalities in food production among regions (Parry et al., 2005).
|Figure 18: Projected losses in food production due to climate change by 2080. (Source: Cline, 2007).
Regional impacts will be strongest across Africa and Western Asia where yields of the dominant regional crops may fall by 15–35% once temperatures rise by 3 or 4º C (Stern Review, 2006). Sub-Saharan Africa is expected to be worst affected, meaning the poorest and most food insecure region is also expected to suffer the largest contraction of agricultural production and income. Despite the uncertainties regarding short-term effects, models do point to many cases where food security is clearly threatened by climate change by 2030, with losses in major crops by this time (Lobell et al., 2008).
There is wide variation in how individual species in different regions respond to a warming climate and Lobell et al. (2008) identified 3 general classes of crop responses to climate change projections: 1) Consistently negative, for example, Southern African maize; 2) Large uncertainties ranging from substantially positive to substantially negative, for example, South Asian groundnut; and 3) Relatively unchanged, for example, West African wheat. Adaptation to climate change by switching from highly vulnerable to less vulnerable crops may be viable, and is recommended particularly for South Asia and South Africa where the case for adaptation is particularly robust (Lobell et al., 2008).
The impacts on crops are also highly variable in different regions and on different types of crops. For example, in Southern Africa, declines in production of 15% for wheat and 27% for maize in the absence of any agricultural adaptation to climate change have been projected by Lobell et al. (2008). The effects of extreme weather are not included in these estimates. In addition, these effects are projected to 2030 only, when the impacts of climate change would be only just emerging. Increasing our understanding how crops may be impacted under climate change conditions may provide alternatives for adaptive strategies in the most vulnerable regions of the world (Lobell et al., 2008).
|Figure 19: Impacts of climate change on cereal output in Africa. (Source: Fischer et al., 2005).
Based on a consensus estimate of 6 climate models and two crop modelling methods, Cline (2007) concluded that by 2080, assuming a 4.4° C increase in temperature and a 2.9% increase in precipitation, global agricultural output potential is likely to decrease by about 6%, or 16% without carbon fertilization. Cline suggested a range of output potential decline between 10 and 25% among regions. As climate change increases, projections have been made that by 2080 agricultural output potential may be reduced by up to 60% for several African countries, on average 16–27%, dependent upon the effect of carbon fertilization (Figures 18 and 19). These effects are in addition to general water scarcity as a result of melting glaciers, change in rainfall patterns, or overuse.