The first information that is needed to carry out an impact assessment is the baseline (or control) scenario, i.e. a scenario in which no climate change is included. Such scenarios have been developed by the Intergovernmental Panel on Climate Change (IPCC), in the Special Report on Emission Scenarios (SRES, Nakicenovic and Swart, 2000). The numerous SRES scenarios describe the world evolution in terms of demography, technology and economy, with different assumptions about, for instance, how the world becomes more globalized or remains regionalized or how economic development focuses on industrial production. These scenarios, however, are at a very low spatial resolution, considering only large regions.
From these socio-economic scenarios, global emission scenarios can be derived, for all greenhouse gases (GHG). From GHG emissions, carbon cycle and climate models (or General Circulation Models, GCMs) can produce climate scenarios, i.e., possible evolutions of all meteorological variables (temperature, precipitation, wind, pressure, etc.) at a resolution of about 200 km.
Both the socio-economic and climate scenarios, therefore, are not at a resolution that would allow a local impact analysis to be carried out. Hurricane damages, indeed, depend on population and exposure in coastal areas, on building norms, on wind speed at the scale of a neighbourhood, and other local information. This information, obviously, is not available in the scenarios developed at the global scale.
First, therefore, an impact assessment requires a socio-economic downscaling, i.e. the development of a local scenario that describes the socio-economic conditions at a pertinent scale. This is very difficult, because predicting social and economic trends is very difficult. An alternative, therefore, is to consider the current society and economy, and to assess the impact the future climate would have on current societies and economies. The drawback of such an approach is that vulnerability can change over time, as explained in Section 4.2. This is especially the case in developing countries where (i) the shift from the primary sector (agriculture) to other economic sectors reduces the vulnerability to climate events; (ii) the improvement in the standards of living (especially for housing quality) limits damages due to extreme weather; (iii) governments and local institutions become more able to implement mitigation policies and emergency measures. In developed countries, however, this difficulty in assessing future vulnerability is less serious, because economic changes that are expected in the next decades are less dramatic. The advantage of considering the current economy, moreover, is to control uncertainty and reduce the number of unknown parameters in the analysis.
Then, climate change also needs to be downscaled at the spatial scale that allows for impact analysis. The next section will discuss the methods to do so.
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