Njie et al. (2006) investigated climate change impacts and adaptation costs and benefits for cereal production in the Gambia. Under the SRES A2 scenario the study estimated that for the period 2010 to 2039, millet yield would increase by 2 to 13%. For the period 2070 to 2099 the outcome is highly dependent on projected changes in precipitation as it could range from a 43% increase to a 78% decrease in millet yield. Adaptation measures such as the adoption of improved cultivars, irrigation, and improved crop fertilisation were assessed in a framework accounting for projections of population growth, water demand and availability. These measures were estimated to increase millet yield by 13 to 43%, while reducing interannual variability by 84 to 200% in the near term (2010 to 2039). However, net adaptation benefits (value of higher production minus cost of implementation) were not necessarily positive for all adaptation strategies. In the near term, net adaptation benefits were estimated at US$22.3 to 31.5 million for crop fertilisation and US$81.1 to 88.0 million for irrigation. The authors conclude that irrigation is more effective to improve crop productivity under climate change conditions, but the adoption of improved crop fertilisation is more cost efficient. Meanwhile, much uncertainty remains regarding the cost of developing improved cultivars. In the distant future, potential precipitation decrease would make irrigation an imperative measure.
Some adaptation costs are implicitly included in estimates of global impacts of climate change. Tol et al. (1998) estimate that between 7% and 25% of total climate damage costs included in earlier studies such as Cline (1992), Fankhauser (1995b) and Tol (1995) could be classified as adaptation costs. In addition, recent studies, including Nordhaus and Boyer (2000), Mendelsohn et al. (2000) and Tol (2002), incorporate with greater detail the effects of adaptation on the global estimation of climate change impacts. In these models, adaptation costs and benefits are usually embedded within climate damage functions which are often extrapolated from a limited number of regional studies. Furthermore, the source studies which form the basis for the climate damage functions do not always reflect the most recent findings. As a result, these studies offer a global and integrated perspective but are based on coarsely defined climate change and adaptation impacts and only provide speculative estimates of adaptation costs and benefits.
Mendelsohn et al. (2000) estimate that global energy costs related to heating and cooling would increase by US$2 billion to US$10 billion (1990 values) for a 2°C increase in temperature by 2100 and by US$51 billion to US$89 billion (1990 values) for a 3.5°C increase. For a 1°C increase, Tol (2002) estimates global benefits from reduced heating at around US$120 billion, and global costs resulting from increased cooling at around US$75 billion. The same study estimates the global protection costs at US $1,055 billion for a one-metre sea-level rise. There are preliminary estimates of the global costs of 'climate proofing' development (World Bank, 2006), but the current literature does not provide comprehensive multi-sectoral estimates of global adaptation costs and benefits. The broader macroeconomic and economy-wide implications of adaptations on economic growth and employment remain largely unknown (Aaheim and Schjolden, 2004).
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