Seasonal variation in the extent of ice and snow cover is greatest in the Northern Hemisphere. Imagine the Earth with white caps on the top and bottom. The top cap increases by a factor of six from summer to winter, while the bottom cap only doubles from summer to winter. This difference is due to snow cover: in the Northern Hemisphere snow cover on land varies from less than 2 million km2 in the summer to 40 to 50 million km2 in the winter. There is little snow cover in the Southern Hemisphere. In Antarctica, land ice covers about 14 million km2 year round, with little change from summer to winter. Sea ice cover in the Arctic varies between approximately 7 and 15 million km2 seasonally, while sea ice cover in the Antarctic, though about the same extent at its peak, varies much more – from around 3 million km2 during summer to 18 million km2 in winter. This means that there is less multi-year sea ice in the Antarctic than in the Arctic, where much of the sea ice is older than one year.
From collection: International Polar Year (IPY) educational posters
Hugo Ahlenius, UNEP/GRID-Arendal