In addition, at the level of plants and units, there is no evidence of any differences in the speed of energy technology change across a wide range of technologies since the nineteenth century (Wilson, forthcoming). Policy has typically focused on technology as the main lever for reducing emissions and it is indeed a powerful driver. Future energy technology change will be as important for determining future greenhouse gas emissions levels as long-term demographic and economic developments over the course of the twenty-first century (Roehrl and Riahi, 2000). Figure 11.4 illustrates the contributions of energy technologies to greenhouse gas emissions reductions, under a high-emissions baseline scenario, required for stabilization at a concentration of 550 ppmv CO e by 2100.