This chapter focuses on the second law of thermodynamics concerning sustainable instruments of effective decarbonization with a primary emphasis on buildings, which are responsible for about 42% of global carbon dioxide emissions. To address the current global warming issue, the Rational Exergy Management Model was used to establish a direct relationship between exergy destructions in energy systems and equipment in the built environment and the so-called nearly avoidable carbon dioxide emissions. It is shown that these emissions are more than direct emissions measured on-site, yet they remain unrecognized in the open literature. These emission responsibilities are described as nearly avoidable because they are related to exergy destructions, which may be minimized by correct solutions within the scope of the second law. It is further shown that as a result of this ignorance, the major root causes of global warming are unknown; thus, true mitigation measures cannot be taken and the Paris Agreement goals may not be satisfied on time. Sample cases regarding conventional energy systems and renewable energy systems in power, heat, and cold generation and utilization are given, and new decarbonization metrics are presented.