In a large heavy saucepan, cook chopped scallion, onion, and gingerroot in butter with curry powder and salt and pepper, to taste, over moderately low heat until softened and add carrots and broth. Simmer mixture, covered, 20 minutes, or until carrots are very soft.
In a blender puree mixture in batches with coconut milk until very smooth, transferring as pureed to a bowl. Stir in 1 tablespoon lime juice and chill soup at least 6 hours or overnight.
Thin soup with ice water and season with additional lime juice, salt, and pepper.
Garnish soup with sliced scallions.