Sea Ice
Sea ice forms in the winter, when the ocean freezes over. The ice cover on the polar ocean comes and goes with the passing of the seasons. This does not influence sea level, since the same amount of water remains, whether as ice or as water. However, when sea ice melts it is replaced by dark ocean, which does not reflect sunlight but absorbs it, converting it into heat. This is probably the major reason why temperatures have increased twice as fast in the Arctic as in the Northern Hemisphere in general.
And what happens in the Arctic doesn’t stay in the Arctic – Arctic warming also affects the climate at lower latitudes. Extreme weather events at lower latitudes are among the consequences of the loss of sea ice from the Arctic Ocean.
Glacial Ice
Glacial ice is a different matter. In Greenland we find a continent-wide ice sheet, produced by falling snow over millions of years. This glacial ice flows slowly towards the ocean, where it either melts or breaks apart to form icebergs. The amount of ice lost at the edges used to equal to the accumulation of new snow every year, but the warmer climate has thrown the Greenland ice sheet out of balance. Currently, the amount lost each year is 200–300 billion tonnes, a rate that is expected to increase dramatically.
Unlike melting sea ice, melting glacial ice does affect global sea levels. Water that evaporated from the ocean in the past has been stored for millennia as ice. When it returns to the ocean, this water causes the sea level to rise.
Global Consequences
Water from Greenland’s ice sheet raises sea level approximately 0.3 mm each year, and this amount is dramatically increasing. In 2015, for example, the sea level rose 0.7 mm. Before the end of the 21st century, Arctic temperatures may well have risen by more than 3 degrees Celsius, making the Greenland ice sheet unstable. If all of this ice melts, sea level will rise 7 metres.
Water from glacial ice has already changed the salinity of the ocean around Greenland, which could potentially cause a reduction of the Gulf Stream, the basis for north-western Europe’s relatively warm climate. The first signs of a weakening of the Gulf Stream in the North Atlantic have already been detected.