The centripetal force required for the rotation of the earth is due to the gravitational pull of sun. Hence an extra torque is not required for it. Hence earth’s angular momentum is constant.
But if the ice of the ice cap melts, it will cause a rise in the sea level and the earth’s mass will be redistributed. It will cause an increase in the moment of inertia of earth, thereby decreasing the angular velocity of earth.
On, applying law of conservation of angular momentum:
Hence, increase in the MI of the Earth, due to melting of ice, will decrease angular velocity. Due to which the earth will rotate slower.