Combine the sugar and water in a small saucepan and bring just to a boil, swirling the pot to help dissolve the sugar. As soon as the sugar is dissolved, remove the pot from the heat and add the chopped rosemary. Allow the syrup to steep until cool, then pour through a fine sieve to remove the rosemary solids. Discard the rosemary and chill the syrup until you are ready to make the drinks.

Add enough of the grapefruit juice to come about 1/3 of the way up each glass. Add about 2 tablespoons of the rosemary syrup, stir, add ice to the cup and then top the drink off with the sparkling water. Taste and add more syrup if necessary.

Serve the drinks with a sprig of rosemary stuck through a wedge of sliced grapefruit.

Note:  Any remaining rosemary simple syrup will keep in an airtight container in the refrigerator for up to 1 month.