As variable renewable energies are set to become a key energy source in electricity sector worldwide, there is concern on the impact of the intermittency of their output when penetration is high. This research aims to investigate possible pathways for future electricity sector decarbonization with a focus on renewable development. This research starts with assessing the global capacity and energy potential for wind and solar power technologies, with a consistent approach. Validation of the developed approaches shows high accuracy of the identified potential areas, and high correlation between the estimated capacity factor and observed output records. When temporal and spatial resolution in a model increases, conventional mathematical programming-based models are limited by computational resources in simulating large systems. To find out the best mix of climate-related energies in a country, this research proposes a novel mathematical models that incorporates objectives of minimizing cost, residual load variation and portfolio output variation. The results suggest that for almost every country, the proposed optimal mix can effectively minimize these objectives simultaneously, promoting the case that a high share of these renewables can be reachable. Finally this research assesses pathways for electricity sector decarbonization. A framework is developed for modelling the electricity systems at regional market scale. This framework mainly consist of an economic dispatch model and a capacity expansion model. It incorporates measures to improve the system flexibility such as storage systems, reserve services and interconnection. Four scenarios are defined in this research. The results show that considerable emissions reduction is present in a base reference scenario, resulting from high penetration of renewables worldwide. Other scenarios can further cut the emissions, using policy instruments such as higher carbon prices, renewable penetration targets and emissions caps. Moreover, nuclear and carbon capture technologies may further drive the annual emissions down to negative values by 2050.