The Gulf Stream plays an immensely important role in moderating the climate of eastern North America and Europe. Moreover, Greenland melt impacts ocean current processes in the North Atlantic. For years, contradictory research has alternately said the Gulf Stream was slowing and that it was not slowing. The latest research looks at the Gulf Stream in a different way and has found that Greenland melt has been influencing the Gulf Stream for more than a century and that the evidence has been staring us in the face for a generation or more.
The Day After Tomorrow was a science-fiction climate change movie (mostly fiction) about a collapse of the Gulf Stream and how it triggered an instant ice age combined with a sea level jump of 30 feet or more. While a real collapse of the Gulf Stream would not be anywhere near as dramatic, this is a very big climate science discovery, or at least it is the most recent confirmation that the Gulf Stream is actually slowing.
There is a massive hole in the global warming signature in the North Atlantic - the only place on earth where the temperature has fallen since 1901. It's this hole that has provided clues.
The great ocean currents are intricately tied to our climate system and the Gulf Stream is a critical part of the largest ocean current on earth. Implications of what these researchers are calling an "exceptional 20th century slowdown" of the Gulf Stream are critically important to climate as we know it.
Ocean circulation changes related to the Gulf Stream have likely been involved in dramatic climate changes in our recent prehistoric past.
Abrupt climate changes of 9 to 15 degrees Fahrenheit globally, and 25 to 35 degrees in Greenland, have been identified from ice core evidence, and the primary driver in these changes is very likely increased fresh water from melt and iceberg discharge in Greenland. Over 20 of these events can be seen in layers of highly accurate ice cores and in very distinctive changes in ocean floor sediments in the North Atlantic. Most of these abrupt change events, both warming and cooling, occurred in a few to half a dozen decades to as little as a few years, and they typically persisted for several thousand years. (2)
Ongoing changes in the flow of the Gulf Stream are no guarantee of an abrupt climate change, but the evidence from the past is clear. Ocean circulation changes related to the Gulf Stream, because of increased fresh water in the North Atlantic, have very likely been involved in extremely dramatic climate changes in our recent prehistoric past.
This new research (Rhamstorf 2015) looks at the Gulf Stream in a different way than previous work that has simultaneously shown slowing and no change. Evidence of the stream's slowing - the hole in the warming signature - has been apparent in global temperature change evaluation since the first Intergovernmental Panel on Climate Change (IPCC) report in 1990. (3)
The location of this "hole" in the North Atlantic is in a crucial area where Gulf Stream water sinks. This sinking of warm water buries heat and carbon dioxide in the deep ocean in a massive global current that stretches all the way around the tip of Africa to the Eastern and North Pacific. This current has an influence on our climate that is just as massive as its size would indicate. (4)
(5) It flows at an average jogger's speed of about 4.5 miles per hour; throughout its upper several hundred feet, it's about 50 to 60 miles wide and is about 2,000 to 3,000 feet deep. The total flow in the Gulf Stream is greater than all of the world's rivers combined. All of this warm water from the tropics ends up in the North Atlantic between Newfoundland and Norway.The scale of the Gulf Stream is immense. It is the fastest ocean current on earth. It flows from the tropics to the Arctic and represents a segment of the largest ocean current system on the planet (the Thermohaline Circulation, see below). At its peak off of Cape Hatteras, the Gulf Stream transports 150 sverdrups, or 21 cubic miles of water per day.
The Thermohaline Circulation
The Thermohaline Circulation (thermo-temperature, haline-salt) extends from Greenland, around the tip of Africa, all the way to the Pacific Ocean. The circulation itself is caused by sinking water in the North Atlantic and to some extent around Antarctica. The circulation moves along the surface from the Indian Ocean toward Greenland and as it flows, evaporation makes it saltier and heavier. As it cools in the North Atlantic, it becomes even heavier, as cold water is heavier than warm. At some point, it gets heavy enough to sink. This happens between Canada and Scandinavia, at the exact location of the warming hole. In general, the water that has sunk is called bottom water, but water sinking to the south of Greenland, east of Labrador, is called Labrador Slope Water.
Much of the sinking water goes all the way to the ocean floor where it flows south all the way down the Atlantic, back around the tip of Africa, then around Australia, with some of the water surfacing in the Indian Ocean. The rest of the circulation rises up off the coast of South America and begins its surface journey back toward Greenland. The entire trip takes about 1,000 years.
Greenland's Ice Loss Creates a Plug in the Gulf Stream
The 10-year average for Greenland ice loss has increased more than 500 percent from 2001 to 2011, or from about 34 to 215 gigatons. (6) Also deserving mention, Greenland shattered its ice loss record in 2012, with a 600-gigaton loss reported. (7) This 600-gigaton record is 157 cubic miles of melt or 1.6 millimeters of sea level rise globally.
It is noteworthy that the work Rhamstorf and team have been pursuing is a relatively long-term evaluation and does not really feel the impact of recent melt, especially as related to the 2012 record.
This tremendous discharge of fresh water moves south along either side of Greenland into the North Atlantic where it is blocked from further southward movement by the northward flow of the Gulf Stream. Because fresh water is less dense than salt water, it floats on the surface and basically acts like a plug in the stream. The density variation between the two water masses keeps them from mixing very well, so the plug persists.
The Latest Research - Rhamstorf
This new research (Rhamstorf 2015, see reference 4) is from the German National Environmental Agency (Potsdam Institute), Penn State, Spain's Environmental Institute and the University of Bristol in Rhode Island. These researchers have combined an analysis of temperature with modeling to evaluate where in the North Atlantic the Thermohaline Circulation is most vulnerable to disruption, and confirmed that weakening of the Gulf Stream is unprecedented in the last 1,000 years.
"The subpolar gyre reaches nearly its lowest temperatures of the past millennium in the late 20th century, despite global warming."
They found that what they call the "subpolar gyre," or that area between Greenland, Iceland, Newfoundland and Norway, is not only the coolest place on earth relative to 20th century warming, but that: "Remarkably, the subpolar gyre reaches nearly its lowest temperatures of the past millennium in the late 20th century, despite global warming."
Their work showed that melt in Greenland increased from about the turn of the 20th century up until about 1970. Then until about 2000, Greenland was stable; ice loss did not grow. By 2000, ice loss began to grow again and as the IPCC says, it has increased over 500 percent since 2001.
Rhamstorf also reports that a new Atlantic current measurement buoy system (RAPID ARRAY) has shown a measured decrease in flow of 7 percent per year from 2008 to 2012. (8) They note that although the measurement period is brief, the decline when taken into the context of this new research could likely be important.
One of the most interesting pieces of previous research highlighted in this new evaluation is from deepwater corals off Nova Scotia. Corals grow in annual layers, so chemical fingerprints in these layers can tell us a lot about ocean conditions where the corals grew, and some of the corals grew many centuries ago.
The way they know is that nutrients in the form of nitrogen isotopes vary depending on where the corals grew. The chemical fingerprint of Labrador Slope Water is very different from the chemical fingerprint of tropical water, so a decrease in flow of the Gulf Stream will increase the amount of Labrador Slope Water located off Newfoundland. This will change the nitrogen isotope content of the corals' growth layers. This research concludes:
The persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate. (9)
Rhamstorf's work also looked at the differential between South Atlantic and North Atlantic Ocean temperatures and found that the South Atlantic has warmed more than the North Atlantic. This is largely consistent with a slowing of the Gulf Stream that would cause warm water to accumulate in the South Atlantic, as the Gulf Stream is after all a part of the global Thermohaline Circulation. If one part of the circulation slows, something must happen elsewhere to account for the slowing.
The Slowdown and the "Extreme" East Coast Sea Level Rise Event of 2009-2010
More work on this issue out of the University of Arizona and the National Oceanic and Atmospheric Administration (NOAA) has documented an extreme sea level rise event in 2009-2010 along the East and Northeast Coast of the United States. The average global sea level rise is three millimeters per year and the average along the Northeast Coast is 2.5 millimeters per year, but with a very high variability. In 2009-2010, the average along the Northeast Coast jumped by almost 100 millimeters representing a one in 850-year event. (10) The authors' found that a 30 percent slowdown in the Gulf Stream was significantly responsible for this sea level rise event. Their abstract concludes:
The extreme nature of the 2009-10 sea level rise event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the 21st century, climate models project an increase in magnitude and frequency of extreme interannual sea level rise events along this densely populated coast.
There's a hole in the warming. It's been there for decades; we just haven't really understood what it meant until now.
1. Greenland ice discharge has increased over 600 percent since 2000 … IPCC 2013, Summary for Policy Makers (SPM), E.3 Cryosphere, page 5, second bullet. http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf
2. Abrupt Climate Changes and Collapse of the Gulf Stream ... Bond 1993 and Broeker 1994 did the original work on Hienrich events. These events consisted of large iceberg flows that deposited what is called ice-rafted debris on the ocean floor consisting of layers of gravel and sediment from Canada and Greenland. The resulting freshwater incursions into the North Atlantic are thought to be the main trigger for Dansgaard-Oeschger events that resulted in rapid climate fluctuations from phase changes in the Atlantic Meridonal overturning (AMO). Evidence from the Greenland ice sheet shows that over 20 of these events have occurred in the last 100,000 years. Menviel et al., in the most recent work on this subject, has confirmed Dansgaard-Oeschger events through modeling.
Menviel et al., Simulating the Dansgaard-Oeschger Continuum-mechanisms, patterns, timing, Climate of the past, January 14, 2014. http://www.clim-past.net/10/63/2014/cp-10-63-2014.pdf
Evidence from Greenland as fast as a few years ... Abrupt Climate Change - Anticipating Surprises, National Research Council of the National Academies of Science, December 2013, Preface, page vii, second paragraph. http://www.nap.edu/download.php?record_id=18373 9 to 15 degrees across the globe ... Alley, The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future, Princeton University Press, 2000, page 119, Figure 12.2. http://press.princeton.edu/titles/6916.html
Data for figure 12.2 is from Cuffey and Clow, Temperature, accumulation, and ice sheet elevation in central Greenland through the last deglacial Transition, Journal of Geophysical Research, volume 102(C12), pp 26,383 to 26,396.
Greenland temperature change is twice that of the global average ... Chylek and Lohmann, Ratio of Greenland to global temperature change - comparison of observations and climate models, Geophysical Research Letters, July 2005, Chylek and Lohmann say the Greenland temperature change is 2.2 times greater than the global average. From Alley's Figure 12.2 (Cuffey and Clow), the 25 to 35-degree Fahrenheit abrupt changes in Greenland would equal 9 to 15 degrees average across the globe. http://www.iac.ethz.ch/doc/publications/Chylek-Lohmann-GRL2005.pdf
Also see: 25 to 35 degrees in Greenland ... National Research Council, Abrupt Climate Change: Inevitable Surprises, Committee on Abrupt Climate Change, 2002. Figure 2.5, page 37.
3. Evidence has been known since the first IPCC report ...
Intergovernmental Panel on Climate Change, Scientific Assessment of Climate Change, Chapter 7, Figure 7.13, page 216. http://www.ipcc.ch/ipccreports/far/wg_I/ipcc_far_wg_I_chapter_07.pdf
4. Global warming "hole" the key to this new research ... Rhamstorf et al., Exceptional 20th century slowdown in Atlantic Ocean overturning circulation, Nature Climate Change, March 23, 2015.
Complete paper: http://www.meteo.psu.edu/holocene/public_html/Mann/articles/articles/RahmstorfEtAl_NatureCC15.pdf
5. 150 sverdrups Gulf Stream flow ... Coastal Carolina University, 03/29/2015. http://www.coastal.edu/marine/ One sverdrup is 1 million cubic meters of water per second (0.00024 cubic miles per second, 21 cubic miles per day or 7,600 cubic miles per year, and at 4.6 gigatons per cubic mile is 35,000 gigatons per year. 150 sverdrups is 5.25 million gigatons per year. For comparison, earth's freshwater lakes and rivers contain 22,339 cubic miles of water (http://water.usgs.gov/edu/gallery/global-water-volume.html) or at 62.4 pounds per cubic foot (ice is 8 percent less), and 74 gigatons per cubic mile is 1.7 million gigatons.
6. Greenland melt, iceberg production and ablation in 2012 ... 600 gigatons, 86,434,573,829,531.81 gallons or 78 cubic miles or 8 percent of the annual Gulf Stream flow. At 3.82 gigatons per cubic mile, Greenland melt in 2012 peaked at 157 cubic miles of fresh water. At 94.8 cubic miles per millimeter of sea level rise, this is 1.6 millimeters of sea level rise.
Tedesco et al., Evidence and analysis of 2012 Greenland records from space-borne observations, a regional climate model and reanalysis data, The Cryosphere Discussions, November 30, 2012.
7. IPCC: Ice loss increased 532 percent since 2001 ... "The rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [-6 to 74] gigatons yr-1 over the period 1992 to 2001 to 215 [157 to 274] gigatons yr-1 over the period 2002 to 2011." This is a 10-year average. 2013 IPCC, summary for Policy Makers, B3 Cryshpere, Bullet 2, page 9.
8. Slowed 7 percent per year, 2008 to 2012 ... Smeed et al., Observed decline of the Atlantic meridional overturning circulation 2004-2012, Ocean Science, February 6, 2014, abstract.
9. Deep water corals unique in 1,800 years ... Sherwood at al., Nutrient regime shift in the western North Atlantic indicated by compound-specific δ15N of deep-sea gorgonian corals, PNAS, January 18, 2011, abstract.
10. An extreme event of sea level rise along the Northeast Coast of North America ... Goddard et al., An extreme event of sea level rise along the Northeast Coast of North America in 209-2010, Nature Communications, February 24, 2015, abstract.
Press release: http://www.gfdl.noaa.gov/news-app/story.106