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

5.2.2 Sensitivity to multiple stresses

Multiple stresses, such as limited availability of water resources (see Chapter 3), loss of biodiversity (see Chapter 4), and air pollution (see Box 5.2), are increasing sensitivity to climate change and reducing resilience in the agricultural sector (FAO, 2003a). Natural land resources are being degraded through soil erosion, salinisation of irrigated areas, dryland degradation from overgrazing, over-extraction of ground water, growing susceptibility to disease and build-up of pest resistance favoured by the spread of monocultures and the use of pesticides, and loss of biodiversity and erosion of the genetic resource base when modern varieties displace traditional ones (FAO, 2003b). Small-holder agriculturalists are especially vulnerable to a range of social and environmental stressors (see Table 5.2). The total effect of these processes on agricultural productivity is not clear. Additionally, multiple stresses, such as forest fires and insect outbreaks, increase overall sensitivity (see Section 5.4.5). In fisheries, overexploitation of stocks (see Section 5.4.6), loss of biodiversity, water pollution and changes in water resources (see Box 5.3) also increase the current sensitivity to climate.

Box 5.2. Air pollutants and ultraviolet-B radiation (UV-B)

Ozone has significant adverse effects on crop yields, pasture and forest growth, and species composition (Loya et al., 2003; Ashmore, 2005; Vandermeiren, 2005; Volk et al., 2006). While emissions of ozone precursors, chiefly nitrous oxide (NOx) compounds, may be decreasing in North America and Europe due to pollution-control measures, they are increasing in other regions of the world, especially Asia. Additionally, as global ozone exposures increase over this century, direct and indirect interactions with climate change and elevated CO2 will further modify plant dynamics (Booker et al., 2005; Fiscus et al., 2005). Although several studies confirm TAR findings that elevated CO2 may ameliorate otherwise negative impacts from ozone (Kaakinen et al., 2004), the essence of the matter should be viewed the other way around: increasing ozone concentrations in future decades, with or without CO2 increases, with or without climate change, will negatively impact plant production, possibly increasing exposure to pest damage (Ollinger et al., 2002; Karnosky, 2003). Current risk-assessment tools do not sufficiently consider these key interactions. Improved modelling approaches that link the effects of ozone, climate change, and nutrient and water availability on individual plants, species interactions and ecosystem function are needed (Ashmore, 2005): some efforts are under way (Felzer et al., 2004). Finally, impacts of UV-B exposure on plants were previously reviewed by the TAR, which showed contrasting results on the interactions of UV-B exposure with elevated CO2. Recent studies do not narrow the uncertainty: some findings suggest amelioration of negative UV-B effects by elevated CO2 (Qaderi and Reid, 2005); others show no effect (Zhao et al., 2003).

Table 5.2. Multiple stressors of small-holder agriculture.

Stressors: Source: 
Population increase driving fragmentation of landholding Various 
Environmental degradation stemming variously from population, poverty, ill-defined property rights Grimble et al., 2002 
Regionalised and globalised markets, and regulatory regimes, increasingly concerned with issues of food quality and food safety  Reardon et al., 2003 
Market failures interrupt input supply following withdrawal of government intervention  Kherallah et al., 2002 
Continued protectionist agricultural policies in developed countries, and continued declines and unpredictability in the world prices of many major agricultural commodities of developing countries  Lipton, 2004, Various 
Human immunodeficiency virus (HIV) and/or acquired immunodeficiency syndrome (AIDS) pandemic, particularly in Southern Africa, attacking agriculture through mass deaths of prime-age adults, which diverts labour resources to caring, erodes household assets, disrupts intergenerational transmission of agricultural knowledge, and reduces the capacity of agricultural service providers Barnett and Whiteside, 2002 
For pastoralists, encroachment on grazing lands and a failure to maintain traditional natural resource management Blench, 2001 
State fragility and armed conflict in some regions Various 

Box 5.3. Climate change and the fisheries of the lower Mekong – an example of multiple stresses on a megadelta fisheries system due to human activity

Fisheries are central to the lives of the people, particularly the rural poor, who live in the lower Mekong countries. Two-thirds of the basin’s 60 million people are in some way active in fisheries, which represent about 10% of the GDP of Cambodia and Lao People’s Democratic Republic (PDR). There are approximately 1,000 species of fish commonly found in the river, with many more marine vagrants, making it one of the most prolific and diverse faunas in the world (MRC, 2003). Recent estimates of the annual catch from capture fisheries alone exceed 2.5 Mtonnes (Hortle and Bush, 2003), with the delta contributing over 30% of this.

Direct effects of climate will occur due to changing patterns of precipitation, snow melt and rising sea level, which will affect hydrology and water quality. Indirect effects will result from changing vegetation patterns that may alter the food chain and increase soil erosion. It is likely that human impacts on the fisheries (caused by population growth, flood mitigation, increased water abstractions, changes in land use and over-fishing) will be greater than the effects of climate, but the pressures are strongly interrelated.

An analysis of the impact of climate change scenarios on the flow of the Mekong (Hoanh et al., 2004) estimated increased maximum monthly flows of 35 to 41% in the basin and 16 to 19% in the delta (lower value is for years 2010 to 2138 and higher value for years 2070 to 2099, compared with 1961 to 1990 levels). Minimum monthly flows were estimated to decrease by 17 to 24% in the basin and 26 to 29% in the delta. Increased flooding would positively affect fisheries yields, but a reduction in dry season habitat may reduce recruitment of some species. However, planned water-management interventions, primarily dams, are expected to have the opposite effects on hydrology, namely marginally decreasing wet season flows and considerably increasing dry season flows (World Bank, 2004).

Models indicate that even a modest sea level rise of 20 cm would cause contour lines of water levels in the Mekong delta to shift 25 km towards the sea during the flood season and salt water to move further upstream (although confined within canals) during the dry season (Wassmann et al., 2004). Inland movement of salt water would significantly alter the species composition of fisheries, but may not be detrimental for overall fisheries yields.