The Denmark Strait is the arm of the North Atlantic that separates Greenland from Iceland. It is about 480 km (300 mi) long and 290 km (180 mi) wide at its narrowest point, between Straumnes on Iceland's Hornstrandir peninsula and Cape Tupinier on Greenland's Blosseville Coast. The International Hydrographic Organization runs the boundary between the Arctic and Atlantic oceans across it, from Straumnes to Cape Nansen, 132 km south-west of Cape Tupinier. The strait connects the Greenland Sea, an extension of the Arctic Ocean, to the Irminger Sea of the Atlantic. Its importance lies mostly under the surface. Cold Arctic water pours over a ridge on the seabed here, forming the largest known underwater waterfall, which helps drive the Atlantic overturning circulation that carries warm water north. The East Greenland Current carries sea ice and icebergs south through the strait, and its waters hold important fisheries. Today it faces a warming climate, a weakening Atlantic circulation and arguments over fish and oil. Its protections are spread across nature reserves, fishing limits, a 1997 boundary deal, a 2021 oil ban and a war grave on the seabed.
The Greenland pump: an overflow larger than any river
In Danish it is called Grønlandspumpen, the Greenland pump. South of the strait, the Irminger Sea is warmer and lighter. North of it, water from the Nordic Seas is colder and denser. Where the two meet on the Greenland–Iceland Rise, the dense northern water spills over the ridge and slides down the seafloor to a depth of around 3,000 m. This is the Denmark Strait overflow, also known as the Denmark Strait cataract. It carries around 3.2 million cubic metres of water a second, in a stream about 200 m thick that runs down the slope for roughly 1,000 km. The water drops more than 3.5 km in all, over three times the height of Angel Falls.
The overflow supplies one of the main inputs to North Atlantic Deep Water and accounts for about half of the dense water flowing from the Nordic Seas into the North Atlantic. That makes it a working part of the Atlantic meridional overturning circulation (AMOC), the system that brings warm surface water north. Because of this, anything that changes the temperature or saltiness of the water arriving at the strait matters well beyond Greenland and Iceland.
Melting ice and a slowing Atlantic circulation
The broadest threat to the strait is climate change. The East Greenland Current runs south through it from Fram Strait. It is the main route by which sea ice leaves the Arctic, and it carries cold, low-salinity surface water, between 0 °C and −1.7 °C in its top 150 m. As Greenland's ice melts and Arctic sea ice thins, more fresh water reaches this current. Fresher water is lighter, and the overflow depends on the northern water being dense. The risk is that it sinks less readily.
Whether the AMOC is already slowing has been argued over for two decades. Direct measurements began only in 2004 with the RAPID moorings at 26°N. In 2014 the data appeared to show a decline about ten times faster than models predicted, but later readings suggested part of that was natural variation. A March 2022 review concluded that global warming may cause a long-term weakening, but that it is hard to detect in the changes since 1980, which include periods of both weakening and strengthening, with magnitudes estimated at anywhere from 5% to 25%. In November 2024 a study in Nature Geoscience reconciled the models with the observations and found a slowdown of 0.46 sverdrups per decade since 1950. One sverdrup is a million cubic metres a second.
Sea ice also links the strait to Iceland in a more direct way. Polar bears do not live in Iceland, but they reach it now and then by drifting across on sea ice from Greenland.
Fish, oil and the lines drawn at sea, 1975 to 2021
Fishing is the main economic use of the strait and the main source of conflict over it. Icelandic capelin spawn inshore in winter and then migrate north in spring and summer to feed in the plankton-rich waters between Iceland, Greenland and Jan Mayen. They return between September and November, and the spawning run begins north of Iceland in December and January. Their routes are tied to ocean currents and water masses, which is why changes in the currents through the strait matter to the fishery. Fishing accounts for more than 90% of Greenland's exports, mostly shrimp and fish, and the fjord towns of Iceland's Westfjords depend on the same grounds.
Iceland's protection of these waters came through the Cod Wars with Britain. It extended its fishing limit from 3 to 4 nautical miles in 1952, to 12 in 1958, and to 200 in 1975. Three confrontations followed (1958–61, 1972–73 and 1975–76), with Royal Navy frigates escorting British trawlers and Icelandic coast guard vessels cutting their nets. A NATO-brokered agreement in 1976 ended them, and Britain accepted Iceland's 200-mile zone. It ended more than 500 years of unrestricted British fishing in these waters, and since 1982 the 200-mile exclusive economic zone has been the international standard under the UN Convention on the Law of the Sea. That zone created a new problem just north of the strait. Iceland measured part of its limit from Kolbeinsey, a tiny islet 105 km off its north coast. Denmark, acting for Greenland, objected, because this gave Iceland about 9,400 km² of sea that would otherwise have been Greenland's. An agreement in 1997 between Iceland, Denmark and Greenland fixed the boundary for good, giving Iceland 30% of the disputed area and Greenland 70%.
Oil has been held back by policy. Greenland set up a state company, Nunaoil, to develop hydrocarbons. In July 2021, however, Greenland's government banned all new oil and gas exploration, with officials saying the environmental "price of oil extraction is too high". The ban applies across Greenland's territory, including its side of the strait.
24 May 1941: HMS Hood and a protected war grave
The strait's most deliberately protected site lies 2,800 m down. On 24 May 1941 the German battleship Bismarck and heavy cruiser Prinz Eugen tried to break out through the Denmark Strait into the Atlantic to raid Allied convoys, an operation the Germans called Rheinübung. The British battlecruiser HMS Hood and battleship HMS Prince of Wales intercepted them. Less than ten minutes after the British opened fire, a shell from Bismarck struck near Hood's aft magazines. The ship blew up and sank within three minutes, and only three of her 1,418 crew survived. Prince of Wales, completed only in March 1941, had trouble with her new quadruple gun turrets and broke off the fight. Bismarck had been hit in her forward fuel tanks, gave up the breakout and was sunk three days later while heading for occupied France. Prinz Eugen escaped to Brest.
The wreck of Hood was found in 2001 by a team led by David Mearns, lying in two debris fields at about 2,800 m. In 2002 the British government designated it a war grave, protected under the Protection of Military Remains Act 1986. Expeditions follow a strict look-but-do-not-touch rule. The one exception was a permitted 2015 expedition that raised one of the ship's bells on 7 August 2015. Princess Anne unveiled the bell at the National Museum of the Royal Navy in Portsmouth on 24 May 2016, the battle's 75th anniversary. The strait's place in the GIUK gap, the naval choke point between Greenland, Iceland and the United Kingdom, kept it militarily important through the Cold War.
Hornstrandir, Látrabjarg and the protected Icelandic shore
On land, the strictest protection is at the strait's narrowest point. Hornstrandir, Iceland's northernmost peninsula, covers 580 km² at the northern end of the Westfjords, north-west of the Drangajökull glacier. Its last farming families left in the 1950s, and in 1975 it became the Hornstrandir Nature Reserve, under some of the strictest conservation rules in Iceland. Hunting is banned, which has made it a refuge for the Arctic fox, Iceland's only native land mammal. The bird cliffs around Hornvík rise to more than 500 m. Much of the land is still privately owned, and some descendants of the former residents have rebuilt their family houses.
Farther south, Látrabjarg marks the westernmost point of Iceland. Its cliffs run for 14 km and reach 440 m, making it Europe's largest bird cliff. Millions of puffins, northern gannets, guillemots and razorbills nest there, along with one of Iceland's largest kittiwake colonies. The whole Westfjords region, which faces Greenland across the strait, had only 7,168 residents in 2024. On the Greenland side the coast is even emptier. Tasiilaq, with 1,985 people in 2020, is the largest community on the entire east coast. The Blosseville Coast opposite Hornstrandir is steep and hard to reach by ship, with ice floes, frequent fog, dangerous currents and few good anchorages. It is named after the French lieutenant Jules de Blosseville, who sighted it in 1833 and then vanished with his ship and 83 other men; it was not properly mapped until Georg Carl Amdrup's expedition in 1900. That emptiness is itself a kind of protection, and much of what threatens the strait comes from far away: carbon emissions, melting ice and fishing demand.
To follow the water upstream, read our guide to the Greenland Sea, and for the global system the overflow feeds, see our explainer on thermohaline circulation. The Icelandic shore of the strait is part of the landscape covered in our article on Iceland's geography.