In a small saucepan over high heat, combine the sugar and 1/2 cup of water. Bring to a boil and cook until slightly thickened, about 2 minutes. Remove from heat and add the lime juice. Let the lime syrup cool to room temperature.
In a large pitcher, combine 1/2 cup of lime syrup with the cranberry juice, seltzer, and ginger ale. To serve, pour into glasses and top with scoops of raspberry sorbet.
