Due to the chaotic nature of weather systems, no single hurricane can be proven to be the direct result of global warming. What is clear, however, is that an ever-warming climate produces more hurricane events on average than a cooler climate would, and the hurricanes in a warm climate are generally more severe than hurricanes in a cooler climate.
Hurricane Sandy began, as all hurricanes do, as a tropical storm over the Atlantic Ocean. The prevailing winds at those latitudes pushed it west toward North America. As it traveled over the ocean, the storm that would become Hurricane Sandy gained strength from water evaporating off the ocean surface. When it grew large enough, Sandy organized as a cyclonic storm and was assigned a name.
The role global warming plays in hurricane formation is that of an accelerant. In cool climates, less energy is available to drive the evaporation of water. Tropical weather patterns have less energy available to them to build up hurricane-force winds and rain in a cool climate, and are therefore less common and less severe than in a warm climate.
Earth's climate is known to have been warming since at least the 19th century. As it has warmed, the number and severity of Atlantic hurricanes have both increased, validating the models of climate scientists.