thumb|Golfstrømmen og den Nordatlantiske strøm
Golfstrømmen er en varm, kraftig og hurtig havstrøm, som fører vand fra Den Mexicanske Golf gennem Floridastrædet og ud i Atlanterhavet i nord-nordøstlig retning. Den følger den amerikanske østkyst til mellem Cape Hatteras og Newfoundland, hvor den drejer mod øst og tager retning mod Nordeuropa. Golfstrømmen bliver drevet af Grønlandspumpen og er en del af det globale thermohaline kredsløb, også kaldet det store transportbånd, som er et sammenhængende strømsystem, der involverer alle klodens oceaner.
På grund af sin betragtelige varmetransport virker Golfstrømmen som en slags ovn, der opvarmer de berørte områder, så deres klima er betydeligt varmere og mildere, end det kunne ventes ud fra deres geografiske bredde. Ifølge denne kilde er det overdrevet. americanscientist.org: The Source of Europe's Mild Climate Citat: "... The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth...The conservation of angular momentum, it turns out, causes the mountains of North America to contribute substantially to the dramatic difference in temperatures across the Atlantic..." 2. feb 2007, Ing.dk: Golfstrømmens betydning er overvurderet Citat: "...Når bjerge som Rocky Mountains har så stor betydning for klimaet her på den anden side af Jorden, skyldes det, at bjergkæden afbøjer vinden mod sydøst ved et samspil imellem bjergene og Jordens rotation...Eigil Kaas er enig med David Battisti og Richard Seager, at Golfstrømmen ikke har nogen særlig betydning for hverken temperaturen i Europa eller for temperaturforskellen hen over Nordatlanten..." Ifølge en anden kilde er virkningen af en større svækkelse af den atlantiske termohaline cirkulation (inkl. Golfstrømmen) massiv og verdensomspændende.Oregon State University (2010, August 5). Ancient Hawaiian glaciers reveal clues to global climate impacts. ScienceDaily. Retrieved August 10, 2010 Citat: "...That coincides almost exactly with a major slowdown of what scientists call the Atlantic meridional overturning circulation, or AMOC, in the North Atlantic Ocean...But scientists have found that the AMOC does more than just keep northern Europe habitable. Its effects can extend far beyond that..."The new data from Mauna Kea, along with other findings from geological archives preserved in oceans and lakes in many other areas, show that the decline of the AMOC basically caused climate changes all over the world," Clark said. "These connections are pretty remarkable, a current pattern in the North Atlantic affecting glacier development thousands of miles away in the Hawaiian Islands. "The global impact of the AMOC changes," Clark added, "was just massive."..."
I praksis har Golfstrømmen i snit ikke ændret sig mærkbart i tidsperioden 2002-2009. 29 March 2010, BBC News: Gulf Stream 'is not slowing down' Citat: "...Between 2002 and 2009, the team says, there was no trend discernible – just a lot of variability on short timescales...The NOC team now has a chain of instruments in place across the Atlantic, making measurements continuously. "In four-and-a-half years of measurement, we have found there is a lot of variability, and we're working to explain it," said NOC's Harry Bryden...Professor Bryden's team calculates that their system is good enough to detect a long-term change in flow of about 20% – but it has not happened yet..." Hvis Golfstrømmen havde ændret sig mindst 20 % i snit, ville man have kunnet måle det. En anden omfattende undersøgelse udført af University of Rhode Island og Stony Brook University fandt ingen mærkbare gennemsnitlige ændringer i Golfstrømmen i perioden 1993-2013.University of Rhode Island. (2014, March 3). Oceanographer refutes claims that climate change is slowing pace of Gulf Stream. ScienceDaily Citat: "...But this is exactly what has been underway at the University of Rhode Island and Stony Brook University for the last 20 years: measurement of the strength of the Gulf Stream....These new results reinforce earlier findings about the stability of Gulf Stream transport based on observations from as far back as the 1930s..."There are variations of the current over time that are natural -- and yes, we need to understand these better -- but we find absolutely no evidence that suggests that the Gulf Stream is slowing down."..."
Forskning fra 2015 viste at store mængder koldt ferskvand (smeltevand) laver "rod" i den globale varmefordeler syd for Island, hvilket kan give mere ekstremt vejr i Europa og USA.23. marts 2015, videnskab.dk: Mystisk havfænomen kan give ekstremt klima i Danmark. Citat: "...Det kolde område syd for Island laver rod i denne globale varmefordeler. »Problemet med dette kolde område er, at det består af koldt ferskvand. Da ferskvand er lettere end saltvand, vil det ikke synke til bunds, men lægger sig som et låg over de globale havstrømme syd for Island. Det betyder at hele bevægelsen i havstrømmene aftager. Derfor har vi forskere været meget interesserede i at finde ud af, hvorfor dette kolde område overhovedet findes, og hvorfor det bliver ved med at blive større,« forklarer Jason Box...Undersøgelser viste, at havstrømmene i Atlanterhavet aldrig har været svagere, end de er i dag, og at det siden 1970’erne er gået hastigt ned ad bakke. »Klimaforandringerne har været med til at svække de atlantiske havstrømme dramatisk, og det ser ud til, at det kommer til at blive endnu mere markant i fremtiden,« siger Jason Box..."University of South Florida (USF Innovation). (2016, January 22). Melting Greenland ice sheet may affect global ocean circulation, future climate: University of South Florida and international scientists find influx of freshwater could disrupt the Atlantic Meridional Overturning Circulation, an important component of global ocean circulation. ScienceDaily Citat: "...Researchers say the it could impact the future climate in places such as portions of Europe and North America..."We derived a new estimate of recent freshwater flux from Greenland using updated GRACE satellite data," said USF professor Tim Dixon. "The data suggest that the influx of freshwater from Greenland is accelerating, and has changed the Labrador Sea in ways that could lead to a weakening of the AMOC...Increased freshwater flux could weaken the AMOC, resulting in a number of consequences, both local and global, said the researchers...According to Dixon, the global impacts are less certain, but potentially more consequential..."
Forskning fra 2016 af Helmholtz Centre for Ocean Research Kiel (GEOMAR) viste i en simuleringsmodel, at 80% af Grønlandsk smeltevand omkring år 2016 holdes væk fra et følsomt Golfstrømsområde. Dog vil der ske betydende ændringer fra 2036 til 2046 i Labradorhavet, men inden da vil der kunne ses ændringer i havcirkulationen. Der står dog ikke direkte hvilken effekt, det vil have på Golfstrømmen. Helmholtz Centre for Ocean Research Kiel (GEOMAR). (2016, June 20). Breathing space for the Gulf Stream: Scientists calculate the fate of the Greenland meltwater. ScienceDaily Citat: "...The model calculations showed that more than half of the meltwater is transported southwards along the Canadian coast by the Labrador Current. Less than 20 percent remain in the area between Greenland and Labrador which is critical for the Gulf Stream system. The model suggests that the melting-induced impact on ocean salinity is, to date, only half as large as natural variations measured in the past decades...However, the simulation also shows a progressive trend in the freshening of the Labrador Sea. "If we project the rise in Greenland melting rates into the future, we expect first noticeable changes in the Labrador Sea in two or three decades," emphasizes Professor Böning, "in this sense the Gulf Stream may just get some breathing space." "Meltwater fluxes from Greenland have been accelerating in recent years and if, as seems likely, this trend continues we could see changes in ocean circulation even sooner" explains co-author Jonathan Bamber, Professor at the University of Bristol..."
Ifølge dansk forskning fra 2021 og 2023, kan de nordatlantiske havstrømme gå i stå, meget hurtigere end forventet.25. feb 2021, ing.dk: Nordatlantiske havstrømme kan svækkes hurtigere end hidtil antaget Citat: "...Kompleks havmodel forudser, at de nordatlantiske havstrømme risikerer at gå i stå hurtigere, end forskning tidligere har påpeget. Det kan føre til en helt ny form for uforudsigelighed i vores klimasystemer, advarer danske forskere...Klimamodeller har gang på gang forudset, at den globale opvarmning vil sænke farten på havstrømmene, men nu tyder det altså på, at udviklingen vil ske endnu hurtigere end tidligere antaget...For når havstrømmen når det vendepunkt, hvor den går i dvale, er der ingen vej tilbage, fortæller forskerne...", backupPNAS March 2, 2021 118 (9): Risk of tipping the overturning circulation due to increasing rates of ice melt, backup25. jul. 2023, videnskab.dk: Danske forskere med vild udmelding: Afgørende havstrømme kollapser omkring 2057, backup Citat: "...Mere specifikt angiver forskerne i deres studie, at det ud fra omfattende statistiske beregninger er sandsynligt, at AMOC kollapser på et tidspunkt efter 2024 og senest i 2095. Årstallet med størst statistisk vægt i regnestykket er 2057..."Open Access: 25 July 2023, nature.com: Warning of a forthcoming collapse of the Atlantic meridional overturning circulation, backup Citat: "...In this work, we show that a transition of the AMOC is most likely to occur around 2025-2095 (95% confidence interval)..." Ifølge forskning fra 2021 ved det tyske Potsdam Institute for Climate Impact Research (PIK) har Golfstrømmen ikke været svagere i de sidste 1000 år end i det 20. århundrede.Potsdam Institute for Climate Impact Research (PIK). (2021, February 25). Gulf Stream System at its weakest in over a millennium. ScienceDaily Citat: "...Never before in over 1000 years the Atlantic Meridional Overturning Circulation (AMOC), also known as Gulf Stream System, has been as weak as in the last decades. Researchers compiled proxy data, reaching back hundreds of years to reconstruct the AMOC flow history. They found consistent evidence that its slowdown in the 20th century is unprecedented in the past millennium...", backup
Ifølge forskning fra 2021 af Francois Lapointe og Raymond S. Bradley kollapsede AMOC (incl. Golfstrømmen) i slutningen af det 13. århundrede, hvilket forårsagede den lille istid.December 18, 2021, scitechdaily.com: Winter Is Coming Paradox: Researchers Uncover the Surprising Cause of the Little Ice Age Citat: "...But in the late 1300s, AMOC strengthened significantly, which meant that far more warm water than usual was moving north, which in turn cause rapid Arctic ice loss. Over the course of a few decades in the late 1300s and 1400s, vast amounts of ice were flushed out into the North Atlantic, which not only cooled the North Atlantic waters, but also diluted their saltiness, ultimately causing AMOC to collapse. It is this collapse that then triggered a substantial cooling..."15 Dec 2021, science.org: Little Ice Age abruptly triggered by intrusion of Atlantic waters into the Nordic Seas Citat: "...Here, we show that the LIA was preceded by an exceptional intrusion of warm Atlantic water into the Nordic Seas in the late 1300s...", backup
__TOC__ thumb|Golfstrømmen er her vist i orange og gule farver, repræsenterende en højere vandtemperatur i Atlanterhavet. Golfstrømmens første møde med den kolde Labradorstrøm ved Cape Hatteras ses tydeligt. Kilde: NASA.
Navngivning Den engelske faglitteratur bruger begrebet Golfstrøm-systemet med en opdeling i Floridastrømmen – strømmen gennem Floridastrædet Egentlige Golfstrøm – strækningen fra Floridastrædet til Cape Hatteras Golfstrøm-forlængelsen – strækningen fra Cape Hatteras til området syd for Newfoundland.