Ice can melt as a result of an increase in energy, either by radiation or heat, or it can melt as a result of a dramatic decrease in pressure. Ice in nature, found on the tops of bodies of water and as icebergs can only melt from solar radiation or heat gain.
Solar radiation involves energy from sunlight, in the form of radiation, adding energy to ice. When the water molecules in water are excited, they can overcome the intermolecular bonding forces and start to move; becoming liquid water. Heat can come from the atmosphere, or even man-made heat sources like heaters and fire.