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Increased emissions of greenhouse gases are chang- ing average climate conditions, locally and across the globe. In order to understand the changes in climate that will result from increased concentrations of these gases, scientists rely on climate model simula- tions that are driven by assumptions about future population growth, socio- economic development, and technology change (Nakicenovic et al. 2000). These assumptions, or scenarios, provide the basis for estimating future greenhouse gas emissions and are used as inputs to run global climate models that simulate changes in temperature, precipitation, and other climate - related conditions. The IPCC (2000) has developed a set of 40 scenar- ios that provide multiple alternative models of how future population growth, changes in wealth, and advances in technology may alter future emissions outcomes (Figure 1 -1). These scenarios are based on four narrative storylines that represent different demographic, social, economic, technological, and environmental developments. For example, the Al storyline describes a future with very rapid economic growth, a global population that peaks in mid- centu- ry and declines thereafter, and the rapid introduc- tion of new and more efficient technology (Figure 1 -1). Scenarios that capture the main driving forces behind greenhouse gas and sulfur emissions are then based on each storyline. For example, from the Al storyline, three scenario groups have been distin- guished based on technological emphasis: fossil intensive (A1F1), non - fossil energy sources (A1T), or a balance across all sources (A1B). Each scenario results in a specific quantitative estimate of emissions based on a quantitative interpretation of each story - line (IPCC 2000). These emission scenarios are not predictions or forecasts, rather an alternative image of how the future might unfold based on a set of transparent assumptions. Figure 1 -2. Climate models are systems of differential equations based on the basic laws of physics, fluid motion, and chemistry. Scientists divide the planet into a 3- dimensional grid, apply the basic equations, solve for the equation of state of the system, and update the results for the next model time step. Atmospheric models calculate winds, heat transfer, radiation, relative humidity, and surface hydrology within each grid and evaluate interactions with neighboring points (Source: NOAA 2008).