The polar regions

At the top of the Earth is the Arctic. At the bottom is the Antarctic.

Both places are very cold and icy. In winter, the Sun hardly shines there at all. In summer, it can shine all day and all night!

The two polar regions are the coldest places on Earth: the Arctic around the North Pole and the Antarctic around the South Pole. They are cold because of sunlight. In winter, the Sun hardly rises there, so it stays dark and freezing for weeks or months. In summer, the Sun shines for many hours or even around the clock, but it stays low in the sky, so its light is weak.

Scientists sometimes call the polar regions Earth's iceboxes, because they help cool the whole planet. They may look alike, with lots of ice and snow, but they are built in opposite ways, and that makes them very different places.

The National Snow and Ice Data Center (NSIDC) calls the Arctic and Antarctic Earth's iceboxes: they help cool the planet and shape the jet streams and so weather patterns far away. Sunlight is the most important factor in their climates. Above the Arctic Circle, which lies at about 66° 33' N, there is little or no solar energy in winter, and the sunlight that does arrive comes in at a low angle; in summer the Sun shines for many hours or around the clock. At the North Pole the Sun rises and sets only once a year, giving about six months of daylight and six of night.

The two regions are geographic mirror images. In the British Antarctic Survey's words, unlike the Arctic, Antarctica is a continent surrounded by ocean; NSIDC describes the Arctic as an ocean largely surrounded by land. That one difference, ocean in the middle or land in the middle, shapes their ice, temperatures, wildlife and people.

In the Arctic, the middle is a sea. In winter, the top of the sea freezes into ice.

Around the sea there is land. Snow covers it in winter. In summer, the snow melts and little plants grow and flower.

In the middle of the Arctic is the Arctic Ocean, with the lands of North America, Europe, Asia and Greenland almost all the way around it. In autumn and winter, the top of the ocean freezes into sea ice. In spring and summer, much of it melts again.

One way to mark the edge of the Arctic is the Arctic Circle, an imaginary line around the globe. Another is the tree line: north of it, the ground is too cold for trees, and the land is covered with low shrubs and lichens. This treeless land is called tundra. When the snow melts in summer, the tundra turns green and blooms with flowers.

NSIDC describes the north polar region as largely an ocean surrounded by land. On the ocean, sea ice grows through autumn and winter and melts through spring and summer; on land and ice, snow builds up in winter and melts in summer, exposing tundra that greens and blooms, then giving way southward to forest. Two definitions of the Arctic are common: the area north of the Arctic Circle, and the area north of the Arctic tree line, which broadly follows the line where the average July temperature does not rise above 10 °C. In some places, trees grow well north of the Arctic Circle.

The Arctic has two main climate types. Near the ocean, a maritime climate brings stormy, wet winters, 60 to 125 cm of snow and rain a year in coastal Alaska, Iceland and Scandinavia, and cool, cloudy summers averaging around 10 °C. Inland, a continental climate is drier, with severe winters: in parts of Siberia, average January temperatures are below −40 °C. Yet in summer, some interior weather stations record 30 °C or more.

In the Antarctic, the middle is land. It is called Antarctica.

Antarctica is covered with thick ice. The Southern Ocean goes all the way around it. In winter, the sea around it freezes too.

In the far south is Antarctica, a whole continent covered by a giant ice sheet. The British Antarctic Survey says the ice is about 2 kilometers thick on average, and less than 1 percent of Antarctica is free of ice. The British Antarctic Survey says the ice sheet holds about 60 percent of the world's fresh water. Other scientists count a little differently: the National Snow and Ice Data Center says Antarctica's and Greenland's ice sheets together hold more than 68 percent.

The Southern Ocean flows all the way around Antarctica. A strong current circles the continent and helps keep warm water and weather away. In winter, the ocean around Antarctica freezes into sea ice, and the continent seems to almost double in size. A long chain of mountains, the Transantarctic Mountains, divides it into East and West Antarctica.

Antarctica is a continent beneath the largest single mass of ice on Earth. The British Antarctic Survey (BAS) gives the Antarctic Ice Sheet's area as nearly 14 million km² and its average thickness as about 2 km, and says it holds 60 percent of the world's fresh water (NSIDC, counting differently, puts the Antarctic and Greenland ice sheets together at more than 68 percent of Earth's fresh water and more than 99 percent of its land ice); less than one percent of the continent is ice-free. Ice streams such as Thwaites Glacier make up about 10 percent of the sheet but carry most of the ice leaving the continent, some at more than 1,000 m a year.

The Transantarctic Mountains divide East Antarctica, where the ice is thicker and rests mostly on bedrock above sea level, from West Antarctica, where much of the bedrock lies below sea level, making it more vulnerable to warming ocean water. Because Antarctica is unconnected to any other continent, the Antarctic Circumpolar Current circles it, isolating it from warmer waters and weather. Each winter, freezing of the surrounding ocean makes the continent appear almost to double in size.

The Antarctic is even colder than the Arctic.

In the middle of Antarctica, it is freezing even in summer. Very little snow falls there. So it is a kind of desert!

Antarctica is much colder than the Arctic. Along its coasts, summer is near freezing, but deep inside the continent, summer temperatures rarely rise above −20 °C, and winter drops below −60 °C. The lowest temperature ever recorded on Earth, −89.2 °C, was measured at Vostok Station in Antarctica.

One reason is its location. Winds and ocean currents around Antarctica cut it off from the rest of the world's weather, which keeps it cold. The Arctic Ocean, by contrast, is closely linked to the weather around it. Antarctica is also very dry: inland, only a few centimeters of snow fall in a year, so much of Antarctica counts as a desert.

BAS summarizes Antarctic temperatures: coastal summers near freezing, coastal winter monthly averages of −10 to −30 °C, interior summers rarely above −20 °C and interior winters below −60 °C; Vostok Station holds Earth's lowest recorded temperature, −89.2 °C. NSIDC explains the contrast with the Arctic in terms of geography: wind and ocean currents around Antarctica isolate the continent from global weather patterns, keeping it cold, whereas the Arctic Ocean is intimately linked with the climate systems around it.

The cold interior is also extremely dry. Snowfall reaches several meters a year near the coasts but only centimeters inland, which classifies much of Antarctica as a desert. Coastal areas meet frequent gales from katabatic winds, cold, dense air flowing down from the high plateau. Sea ice behaves differently too: around Antarctica it spreads more freely, is often much thinner than Arctic ice and nearly melts out in the southern summer, while much more Arctic sea ice survives through summer.

Many people live in the Arctic. They live in towns and villages. Polar bears walk on the sea ice.

No people lived in Antarctica long ago. Today, scientists visit to study it. Penguins, seals and whales live in the sea around it.

The Arctic is home to almost four million people, the Arctic Council says. They include Indigenous Peoples, who are about one in ten of them, as well as newer arrivals, hunters and herders, and people living in towns and cities. Polar bears live in the Arctic too, walking out on the sea ice.

Antarctica has no native people. Today, people such as scientists from the British Antarctic Survey work there at research stations. In 1959, twelve countries signed the Antarctic Treaty to keep Antarctica open for science and to share what scientists find. Antarctica's land is almost bare, with mostly mosses, lichens and liverworts, but the Southern Ocean around it is full of life: whales, seals, seabirds, penguins and swarms of krill so huge they can be seen from space.

According to the Arctic Council, the Arctic is home to almost four million people, roughly 10 percent of them Indigenous, including many peoples distinct to the Arctic; six Indigenous Peoples' organizations are Permanent Participants in the Council. Their communities range from hunters and herders living on the land to city dwellers.

Antarctica has no native population. The Antarctic Treaty, signed in Washington on 1 December 1959 by the twelve countries whose scientists had been active there during the International Geophysical Year of 1957–58, entered into force in 1961 and now has 58 Parties. It guarantees freedom of scientific investigation and the free exchange of results. Seven signatories hold territorial claims, some overlapping, that other countries do not recognize; Article IV freezes the status quo. On land, plant life is mostly hardy mosses, lichens and liverworts, but the Southern Ocean supports whales, seals, seabirds and penguins, all depending on krill, whose swarms BAS says can be seen from space.

White snow and ice bounce sunlight back into space. This helps keep the whole Earth cool.

Scientists study the poles to learn how our planet is changing.

Bright white snow and ice act like a mirror. NSIDC says snow-covered sea ice can bounce back as much as 90 percent of the sunlight that hits it, while the dark ocean soaks up most of the sunlight that reaches it. That is one way the poles help keep the whole planet cool.

When sea ice melts, more dark ocean is uncovered, it absorbs more heat, and the warming grows. This makes the poles very sensitive to climate change. Scientists have measured that Arctic summer sea ice has become much thinner and smaller since the 1980s. Polar science has helped the world before: British scientists in Antarctica discovered the ozone hole, and their discovery led to an agreement between countries in 1987, the Montreal Protocol.

Ice and snow are bright. NSIDC notes that bare sea ice reflects 50 to 70 percent of incoming solar energy and snow-covered sea ice as much as 90 percent, while the darker ocean absorbs most of the sunlight that reaches it. When sea ice melts and exposes the ocean, more energy is absorbed, warming the surroundings further: a feedback that makes the poles the regions most sensitive to climate change. Measurements show a dramatic decline in both the thickness and extent of Arctic summer sea ice since the 1980s, consistent with a warming Arctic, and NSIDC stresses that what happens in the Arctic does not stay in the Arctic.

The poles are also scientific observatories. Air bubbles trapped in the Antarctic ice sheet let scientists measure atmospheric conditions from more than 100,000 years ago. BAS scientists discovered the Antarctic ozone hole, caused by CFCs and halons; the discovery led to the 1987 Montreal Protocol, and BAS says that, with recovery under way, the hole is not expected to fully repair until around 2070.