The Northern Hemisphere winter of 2014-15 was the warmest on record globally, according to the National Oceanic and Atmospheric Administration. But if you look closely at global temperature maps, it becomes clear that one area of the North Atlantic conspicuously bucked the trend, as it has during many years since 1970.

That region was, in fact, the coldest it has been since the dawn of instrument records, at up to 3.6 degrees Fahrenheit colder than average. According to a new study, this cold pool may be an indicator of a dramatic slowdown in the Gulf Stream, which transports vast amounts of heat north from the equator to the pole, passing off the East Coast of the U.S. and into the North Atlantic.

If true, this is vindication for those who think global warming is likely to trigger so-called "tipping points" in the climate system, which, once set into motion, cannot be stopped. The U.N. Intergovernmental Panel on Climate Change had judged that there is up to a 10% likelihood of a Gulf Stream shutdown before year 2100, though many climate scientists estimate this likelihood is even higher.

"Evidence is mounting that the long-feared circulation decline is already well underway," says co-author Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany, in a blog post for RealClimate.

The slowdown in this current, the study finds, is unprecedented in hundreds to perhaps as long as 1,000 years, and is most likely related to another tipping point, which is the melting of the Greenland ice sheet. The influx of freshwater from the ice sheet is one of the main sources of freshwater inflow into the North Atlantic Ocean.

Land and ocean temperature departures from average during December through February 2015. This shows the cold spot south of Greenland. Image: NOAA/NCDC

As it pours into the Atlantic, the freshwater is lighter and colder than heavier, salty water that typically occupies that area. It therefore tends to sit on top of the water column, accumulating over the years and interfering with the formation and sinking of dense, cold and salt-enriched waters. This chokes off the northward flowing Gulf Stream, slowing it down, and affecting ocean circulation downstream as well.

No new ice age, at least not yet...

While it's not anywhere close to the apocalypse that a rapid Gulf Stream shutdown was shown to be in the 2004 blockbuster disaster film The Day After Tomorrow, a rapid slowdown in this current would boost sea level rise rates along the highly populated Mid-Atlantic and Northeast coasts of the U.S. It could also bring much cooler conditions than is currently the norm to parts of northern Europe.

The study also calls into question many of the assumptions made by climate modelers in designing state of the art computer models, since the study shows that these models may be underestimating the speed and magnitude of ocean current trends in the North Atlantic. Most models show a progressive weakening of the Gulf Stream as global warming continues, but few have suggested it would be so significant, so soon.

Michael Mann, a coauthor of the study and director of the Earth Systems Science Center at Penn State University, told Mashable in an email:

Once again, we are learning that the climate model projections may be too conservative. In this case, the fact that the Greenland Ice Sheet is loosing mass and contributing to freshwater runoff into the North Atlantic decades ahead of schedule may be the reason that the Atlantic Meridional Overturning Circulation (AMOC) is weakening decades ahead of schedule as well.

"[This is] Another reminder that uncertainty is not our friend when it comes to human-caused climate change. It appears to be cutting against us, rather than in our favor, once again," Mann said.

The Gulf Stream is one part of a vast global undersea infrastructure known as the thermohaline circulation, also referred to as the "Global Conveyor Belt." This circulation, which extends from pole to pole and throughout every ocean, is powered by density differences in ocean waters in different areas of the world. If you disturb any part of this circulation, the entire thing is liable to have a major hiccup, like an escalator with something stuck in its gears.

The study makes a strong case that we have already stuck that wrench in the ocean circulation's gears by melting more polar land ice, although it provides for the possibility that natural variability or other factors are also to blame for the post-1970 slowdown in the AMOC of which the Gulf Stream is a crucial part.

Changing mass balance of the Greenland ice sheet, showing a sharp increase in runoff into the ocean (blue line). Image: Box and Colgan via meltfactor.org

It is the sinking motion of North Atlantic bottom water that powers the Gulf Stream current by pulling warmer, less dense water northward from the tropics, like a group of weight lifters involved in a tug-of-war against a weaker team. Freshwater from the melting ice sheet may be leading to less bottom water formation, the study finds.

"Greenland ice takes on a new role in the climate change story, not just indicating change and contributing to sea level rise, but possibly playing an important role in destabilizing regional if not global ocean circulation that naturally exchanges heat north-south," said Jason Box of the Geological Survey of Denmark and Greenland, and a study co-author, in an email to Mashable.

A new index and proxy data

There is no reliable observational evidence of the strength of the Gulf Stream over time, since even modern measurements are relatively scarce. To get around this problem, the study's authors created an index based on sea surface temperatures to infer the strength of the current over time. Specifically, they took into account the temperature difference between the area most influenced by changes in the strength of the circulation, which is that telltale cold patch in the North Atlantic, and the rest of the Northern Hemisphere.

The study also uses so-called "proxy" measurements, including data gleaned from coral reefs and tree rings, to extend large-scale surface climate conditions back to 900 AD. The study found that it's likely that the weakening trend observed since the mid-1970s is unique in that entire period.

Furthermore, the study found that the index they devised to track the current's strength over time closely matched modeled trends, which lends some confidence to the findings.

"Of course there is uncertainty as to how well our temperature-based index captures real AMOC variations," study co-author Rahmstorf told Mashable in an email. "The problem here is that nobody knows what the true AMOC variations were. In the paper we present quite a bit of corroborating evidence that we are indeed looking at AMOC variations with that index. Other temperature-based methods... have come to similar results."

Many uncertainties remain about what is going on with the Gulf Stream, and any changes in the broader Global Conveyer Belt as a whole. Direct measurements will help reduce these mysteries, and efforts are underway to fill some data gaps in the North Atlantic and the Southern Ocean. These are the two most important regions where salty, dense bottom water forms, powering ocean currents thousands of miles up and downstream.