A rational exergy management model was developed, which establishes a common metric and provides a tool for matching the exergy of energy sources with different applications in the built environment for global sustainability. In order to develop and evaluate this model, first a base case was defined, which involves a building using a natural gas boiler for comfort heating and receiving electricity from a remote thermal power plant. For the base case, the rational exergy management efficiency, which is a measure of how much the supply and demand exergies match, is only 4%. In order to analyze the impact of increasing the rational exergy efficiency on carbon mitigation relative to the base case, three exergy and environment aware cases were considered. These cases were: the same building thermally linked to the power plant through a district energy system (Case One), the same building with a micro CHP using a natural gas internal combustion engine (Case Two), and the same building with a renewable energy driven electric power generator and a ground-source heat pump (Case Three). A detailed analysis of the carbon reduction potential of the three cases revealed that it is possible to realize a new CO2 reducing wedge in addition to the seven wedges that have already been identified by the Carbon Mitigation Initiative (CMI). Calculations show that Case Three may reduce the global emissions to about 4 Gton CO2/year by the year 2055 from the current value of 7 Gton CO2/year Therefore, the eighth wedge may not only help to stabilize the current levels of emissions but also decrease it below the current level. This paper explains the rational exergy management model (REMM) and its emission projections and describes the three cases in detail. Results indicate the importance of rational exergy management for global sustainability and for second generation green buildings and show that this rational exergy metric can manage a robust roadmap to curb CO2 emissions from buildings.