Query: Arctic sea ice extent was lower in the past.

Relevant passages:
Arctic Ocean <=SEP=> The Arctic Ocean is the smallest and shallowest of the world 's five major oceans . The International Hydrographic Organization ( IHO ) recognizes it as an ocean , although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea , classifying it a mediterranean sea or an estuary of the Atlantic Ocean . Alternatively , the Arctic Ocean can be seen as the northernmost part of the all-encompassing World Ocean .   Located mostly in the Arctic north polar region in the middle of the Northern Hemisphere , the Arctic Ocean is almost completely surrounded by Eurasia and North America . It is partly covered by sea ice throughout the year and almost completely in winter . The Arctic Ocean 's surface temperature and salinity vary seasonally as the ice cover melts and freezes ; its salinity is the lowest on average of the five major oceans , due to low evaporation , heavy fresh water inflow from rivers and streams , and limited connection and outflow to surrounding oceanic waters with higher salinities . The summer shrinking of the ice has been quoted at 50 % . The US National Snow and Ice Data Center ( NSIDC ) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years .

Climate change in the Arctic <=SEP=> The effects of global warming in the Arctic , or climate change in the Arctic include rising temperatures , loss of sea ice , and melting of the Greenland ice sheet with a related cold temperature anomaly , observed in recent years . Potential methane release from the region , especially through the thawing of permafrost and methane clathrates , is also a concern . The Arctic warms twice as fast compared to the rest of the world . The pronounced warming signal , the amplified response of the Arctic to global warming , it is often seen as a leading indicator of global warming . The melting of Greenland 's ice sheet is linked to polar amplification . According to a study published in 2016 , about 0.5 ◦ C of the warming in the Arctic has been attributed to reductions in sulfate aerosols in Europe since 1980 .

Polar ice cap <=SEP=> A polar ice cap or polar cap is a high-latitude region of a planet , dwarf planet , or natural satellite that is covered in ice .   There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap , nor any geological requirement for it to be over land ; only that it must be a body of solid phase matter in the polar region . This causes the term `` polar ice cap '' to be something of a misnomer , as the term ice cap itself is applied more narrowly to bodies that are over land , and cover less than 50,000 km ² : larger bodies are referred to as ice sheets .   The composition of the ice will vary . For example , Earth 's polar caps are mainly water ice , whereas Mars 's polar ice caps are a mixture of solid carbon dioxide and water ice .   Polar ice caps form because high-latitude regions receive less energy in the form of solar radiation from the Sun than equatorial regions , resulting in lower surface temperatures .   Earth 's polar caps have changed dramatically over the last 12,000 years . Seasonal variations of the ice caps takes place due to varied solar energy absorption as the planet or moon revolves around the Sun . Additionally , in geologic time scales , the ice caps may grow or shrink due to climate variation .

Sea ice <=SEP=> Sea ice arises as seawater freezes . Because ice is less dense than water , it floats on the ocean 's surface ( as does fresh water ice , which has an even lower density ) . Sea ice covers about 7 % of the Earth 's surface and about 12 % of the world 's oceans . Much of the world 's sea ice is enclosed within the polar ice packs in the Earth 's polar regions : the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean . Polar packs undergo a significant yearly cycling in surface extent , a natural process upon which depends the Arctic ecology , including the ocean 's ecosystems . Due to the action of winds , currents and temperature fluctuations , sea ice is very dynamic , leading to a wide variety of ice types and features . Sea ice may be contrasted with icebergs , which are chunks of ice shelves or glaciers that calve into the ocean . Depending on location , sea ice expanses may also incorporate icebergs .