Query: Thick arctic sea ice is in rapid retreat.

Relevant passages:
North Pole <=SEP=> The North Pole , also known as the Geographic North Pole or Terrestrial North Pole , is ( subject to the caveats explained below ) defined as the point in the Northern Hemisphere where the Earth 's axis of rotation meets its surface .   The North Pole is the northernmost point on the Earth , lying diametrically opposite the South Pole . It defines geodetic latitude 90 ° North , as well as the direction of true north . At the North Pole all directions point south ; all lines of longitude converge there , so its longitude can be defined as any degree value . Along tight latitude circles , counterclockwise is east and clockwise is west . The North Pole is at the center of the Northern Hemisphere .   While the South Pole lies on a continental land mass , the North Pole is located in the middle of the Arctic Ocean amid waters that are almost permanently covered with constantly shifting sea ice . This makes it impractical to construct a permanent station at the North Pole ( unlike the South Pole ) . However , the Soviet Union , and later Russia , constructed a number of manned drifting stations on a generally annual basis since 1937 , some of which have passed over or very close to the Pole . Since 2002 , the Russians have also annually established a base , Barneo , close to the Pole . This operates for a few weeks during early spring . Studies in the 2000s predicted that the North Pole may become seasonally ice-free because of Arctic ice shrinkage , with timescales varying from 2016 to the late 21st century or later .   The sea depth at the North Pole has been measured at 4261 m by the Russian Mir submersible in 2007 and at 4,087 m ( 13,410 ft ) by USS Nautilus in 1958 . The nearest land is usually said to be Kaffeklubben Island , off the northern coast of Greenland about 700 km away , though some perhaps non-permanent gravel banks lie slightly closer . The nearest permanently inhabited place is Alert in the Qikiqtaaluk Region , Nunavut , Canada , which is located 817 km from the Pole .

Retreat of glaciers since 1850 <=SEP=> The retreat of glaciers since 1850 affects the availability of fresh water for irrigation and domestic use , mountain recreation , animals and plants that depend on glacier-melt , and , in the longer term , the level of the oceans . Studied by glaciologists , the temporal coincidence of glacier retreat with the measured increase of atmospheric greenhouse gases is often cited as an evidentiary underpinning of global warming . Mid-latitude mountain ranges such as the Himalayas , Alps , Rocky Mountains , Cascade Range , and the southern Andes , as well as isolated tropical summits such as Mount Kilimanjaro in Africa , are showing some of the largest proportionate glacial losses .   Glacier mass balance is the key determinant of the health of a glacier . If the amount of frozen precipitation in the accumulation zone exceeds the quantity of glacial ice lost due to melting or in the ablation zone a glacier will advance ; if the accumulation is less than the ablation , the glacier will retreat . Glaciers in retreat will have negative mass balances , and if they do not find an equilibrium between accumulation and ablation , will eventually disappear .   The Little Ice Age was a period from about 1550 to 1850 when the world experienced relatively cooler temperatures compared to the present . Subsequently , until about 1940 , glaciers around the world retreated as the climate warmed substantially . Glacial retreat slowed and even reversed temporarily , in many cases , between 1950 and 1980 as global temperatures cooled slightly . Since 1980 , a significant global warming has led to glacier retreat becoming increasingly rapid and ubiquitous , so much so that some glaciers have disappeared altogether , and the existences of many of the remaining glaciers are threatened . In locations such as the Andes of South America and Himalayas in Asia , the demise of glaciers in these regions has the potential to affect water supplies in those areas .   The retreat of mountain glaciers , notably in western North America , Asia , the Alps and tropical and subtropical regions of South America , Africa and Indonesia , provide evidence for the rise in global temperatures since the late 19th century . The acceleration of the rate of retreat since 1995 of key outlet glaciers of the Greenland and West Antarctic ice sheets may foreshadow a rise in sea level , which would affect coastal regions .

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 .

Holocene glacial retreat <=SEP=> Holocene glacial retreat had a profound effect on landscapes in many areas that were covered by ice at the Last Glacial Maximum .