Snow and ice have a very large albedo, i.e. they reflect the majority of the incoming solar radiation. They thus play a major role in the global heat balance of the Earth (see section 2.1.1). By storing and releasing latent heat, they affect the seasonal cycle of the surface temperature (see section 2.1.6). They are also good insulators that reduce the heat loss from the underlying surface (land or ocean) towards the cold atmosphere in winter. More generally, the presence of sea ice restricts the exchanges of heat and gases between the ocean and the atmosphere. When sea ice forms, only a fraction of the salt present in the ocean is trapped in the ice, the remainder being ejected towards the ocean (this is called brine rejection). The resulting sea ice salinity is between 10 psu in relatively young ice and less than 2 psu in very old ice (compared to around 35 psu for the ocean, see section 1.3.3). Because of this brine rejection, sea ice formation increases the salinity at the ocean surface while, melting sea ice is associated with surface freshening. Sea ice drift is also associated with a horizontal freshwater transport. If there is a net convergence of the sea-ice transport and intense ice melting in a region, this will decrease the salinity of surface water there. On the other hand, in regions such as coastal polynyas, the strong winds in winter pushes continually off shore the newly formed ice, leading to a strong divergence of the sea ice transport. This implies very high ice formation rates in these polynyas (Up to 10 m per year at some locations) and thus large amounts of brine rejection which can lead to very high ocean salinities in those regions.
Ice sheets store large amounts of water on land. Any change in their volume thus has a considerable effect on the sea level. It is estimated that, if all the ice sheets melted completely, taking into account the fact that some ice sheets are grounded below sea level, the sea level would rise by more than 60 m. On the other hand, if we neglect the effect of dilution on sea water density and volume, the melting of sea ice and ice shelves melting does not influence sea level. Indeed, because of Archimede's law, floating ice displaces its own weight of sea water and the melt water thus simply replaces the volume of ice previously below sea level. Ice sheets are also big mountains that, because of their height, help to maintain cold conditions on the surface. The presence of cold air on the ice sheet also has a regional influence, cooling the surrounding areas.