In a small saucepan, bring the ginger, sugar and 2 1/2 cups water to a boil, stirring until the sugar dissolves. Reduce to a simmer and cook until the liquid reduces to 2 cups, 15 to 20 minutes. Strain through a fine-mesh sieve set over a medium bowl; discard the solids. Refrigerate the ginger syrup until cold, about 2 hours. Divide the ginger syrup and the seltzer among 8 tall glasses; stir gently to combine. Add 2 scoops of sorbet to each glass.
