The Arctic Ocean is contained in a polar climate characterized by persistent cold and relatively narrow annual temperature ranges. Winters are characterized by the polar night, extreme cold, frequent low-level temperature inversions, and stable weather conditions. Cyclones are only common on the Atlantic side. Summers are characterized by continuous daylight (midnight sun), and air temperatures can rise slightly above 0 °C (32 °F). Cyclones are more frequent in summer and may bring rain or snow. It is cloudy year-round, with mean cloud cover ranging from 60% in winter to over 80% in summer.
The temperature of the surface water of the Arctic Ocean is fairly constant at approximately −1.8 °C (28.8 °F), near the freezing point of seawater.
The density of sea water, in contrast to fresh water, increases as it nears the freezing point and thus it tends to sink. It is generally necessary that the upper 100–150 m (330–490 ft) of ocean water cools to the freezing point for sea ice to form. In the winter the relatively warm ocean water exerts a moderating influence, even when covered by ice. This is one reason why the Arctic does not experience the extreme temperatures seen on the Antarctic continent.
There is considerable seasonal variation in how much pack ice of the Arctic ice pack covers the Arctic Ocean. Much of the Arctic ice pack is also covered in snow for about 10 months of the year. The maximum snow cover is in March or April — about 20 to 50 cm (7.9 to 19.7 in) over the frozen ocean.
The climate of the Arctic region has varied significantly during the Earth's history. 55 million years ago during the Paleocene–Eocene Thermal Maximum, when global climate underwent a warming of approximately 5–8 °C (9–14 °F), the region reached an average annual temperature of 10–20 °C (50–68 °F). The surface waters of the northernmost Arctic Ocean warmed, seasonally at least, enough to support tropical lifeforms (the dinoflagellates Apectodinium augustum) requiring surface temperatures of over 22 °C (72 °F).
Currently, the Arctic region is warming twice as fast as the rest of the planet.
Due to the pronounced seasonality of 2–6 months of midnight sun and polar night in the Arctic Ocean, the primary production of photosynthesizing organisms such as ice algae and phytoplankton is limited to the spring and summer months (March/April to September). Important consumers of primary producers in the central Arctic Ocean and the adjacent shelf seas include zooplankton, especially copepods (Calanus finmarchicus, Calanus glacialis, and Calanus hyperboreus) and euphausiids, as well as ice-associated fauna (e.g., amphipods). These primary consumers form an important link between the primary producers and higher trophic levels. The composition of higher trophic levels in the Arctic Ocean varies with region (Atlantic side vs. Pacific side), and with the sea-ice cover. Secondary consumers in the Barents Sea, an Atlantic-influenced Arctic shelf sea, are mainly sub-Arctic species including herring, young cod, and capelin. In ice-covered regions of the central Arctic Ocean, polar cod is a central predator of primary consumers. The apex predators in the Arctic Ocean - Marine mammals such as seals, whales, and polar bears, prey upon fish.
Endangered marine species in the Arctic Ocean include walruses and whales. The area has a fragile ecosystem, and it is especially exposed to climate change, because it warms faster than the rest of the world. Lion's mane jellyfish are abundant in the waters of the Arctic, and the banded gunnel is the only species of gunnel that lives in the ocean.
Petroleum and natural gas fields, placer deposits, polymetallic nodules, sand and gravel aggregates, fish, seals and whales can all be found in abundance in the region.
The political dead zone near the center of the sea is also the focus of a mounting dispute between the United States, Russia, Canada, Norway, and Denmark. It is significant for the global energy market because it may hold 25% or more of the world's undiscovered oil and gas resources.
The Arctic ice pack is thinning, and a seasonal hole in the ozone layer frequently occurs. Reduction of the area of Arctic sea ice reduces the planet's average albedo, possibly resulting in global warming in a positive feedback mechanism. Research shows that the Arctic may become ice-free in the summer for the first time in human history by 2040. Estimates vary for when the last time the Arctic was ice-free: 65 million years ago when fossils indicate that plants existed there to as recently as 5,500 years ago; ice and ocean cores going back 8,000 years to the last warm period or 125,000 during the last intraglacial period.
Warming temperatures in the Arctic may cause large amounts of fresh meltwater to enter the north Atlantic, possibly disrupting global ocean current patterns. Potentially severe changes in the Earth's climate might then ensue.
As the extent of sea ice diminishes and sea level rises, the effect of storms such as the Great Arctic Cyclone of 2012 on open water increases, as does possible salt-water damage to vegetation on shore at locations such as the Mackenzie's river delta as stronger storm surges become more likely.
Global warming has increased encounters between polar bears and humans. Reduced sea ice due to melting is causing polar bears to search for new sources of food. Beginning in December 2018 and coming to an apex in February 2019, a mass invasion of polar bears into the archipelago of Novaya Zemlya caused local authorities to declare a state of emergency. Dozens of polar bears were seen entering homes and public buildings and inhabited areas.
Sea ice, and the cold conditions it sustains, serves to stabilize methane deposits on and near the shoreline, preventing the clathrate breaking down and outgassing methane into the atmosphere, causing further warming. Melting of this ice may release large quantities of methane, a powerful greenhouse gas into the atmosphere, causing further warming in a strong positive feedback cycle and marine genera and species to become extinct.
Other environmental concerns relate to the radioactive contamination of the Arctic Ocean from, for example, Russian radioactive waste dump sites in the Kara Sea Cold War nuclear test sites such as Novaya Zemlya, Camp Century's contaminants in Greenland, or radioactive contamination from Fukushima.
On 16 July 2015, five nations (United States, Russia, Canada, Norway, Denmark/Greenland) signed a declaration committing to keep their fishing vessels out of a 1.1 million square mile zone in the central Arctic Ocean near the North Pole. The agreement calls for those nations to refrain from fishing there until there is better scientific knowledge about the marine resources and until a regulatory system is in place to protect those resources.