The Global Conveyor Belt

A massive, global circulation of seawater buffers the effects of greenhouse gas emissions - and gives us time to deal with climate change

The oceans of the world are giving us breathing space in our race toward a warmer planet.  About 40% of the carbon dioxide we have emitted so far has been dissolved in seawater, and as much as 90% of the excess heat retained by the Earth system due to increasing greenhouse gas concentrations now resides in the oceans as well.  Were this not the case, global temperatures would be rising even faster than they are!

That capacity exists not just in the surface ocean but at depth as well because of a phenomenon discovered just a few decades ago: The global ocean conveyor belt.

Some History

Understanding ocean winds and currents has been crucial to the survival of sailors and the success of their voyages since humans first took to the open ocean.  Those voyages began more than a thousand years ago, and stories of early ships and their adventures, from Chinese junks to Polynesian canoes to Viking warships (and perhaps to St. Brendan's currach), make rich and rewarding reading.

While those early sailors knew regional currents well, a global view of ocean circulation awaited circumnavigation of the planet beginning in 1522.  By 1675 some rather fanciful maps of continents and ocean currents were starting to appear.

Benjamin Franklin played a role in the early scientific description of one of the most important currents, being credited with the first detailed map of the Gulf Stream printed in 1786.  Note the circular current in the upper left inset.  It was known even then that faster crossings of the Atlantic could be made by following this circular currents and winds to the north when going to Europe and to the south when your destination was the Americas.

By 1876, a more complete map captured the major features of the global ocean currents, and by 1943 that included a more accurate mapping of the "roaring 40s" or the nearly-constant high winds and rapid currents surrounding the Antarctic continent.

Global Currents and Gyres

A new term and a more comprehensive and consistent view of large-scale ocean circulation was added in the 1950s when individual currents named for locations or famous explorers began to be subsumed into the concept known as gyres.  The Gulf Stream and three other named currents make up the North Atlantic Gyre that circles the Sargasso Sea.  With gyres, that early circular pattern in Franklin's map came to be recognized as a general feature.

Perhaps some of the romance and history of ocean exploration has been lost in the process, but gyres have been identified and named for all the major oceans.  We have lost the Humboldt Current as an isolated flow, but gained the South Pacific gyre.

Imagine how much energy it takes to drive these currents.  The Gulf Stream moves water at the rate of 30-150 Sverdrups.  Sverdrups?  Named for an early Norwegian oceanographer, the use of Sverdrups was invented by another oceanographer who got tired of repeatedly writing "millions of cubic meters per second" when describing ocean currents.  One Sverdrup is equivalent to a flow rate of 1 million cubic meters of water (or 264 million gallons) per second!

The energy that drives these gyres is embedded in related surface winds and comes ultimately from the sun.  Our weather/climate system is one large heat engine trying to redress the unequal distribution of solar energy received between the tropics and the poles. 

And the Gulf Stream is a large part of that engine, transporting huge amounts of tropical warmth up the coast of North America and delivering it to northern Europe, making that region far warmer than it ought to be at that latitude.  Consider that Oslo and Stockholm are well north of Moscow and at the same latitude as much of Siberia, or that London and Dublin, with their mild winters and occasional palm trees, are at the same latitude as Calgary, Winnipeg, and the southern tip of Hudson Bay.  The Gulf Stream is woven deeply into the history, culture, and economics of the region.

Carbon Dioxide and Heat are Transferred to the Oceans

Wherever ocean currents flow, winds and waves and tides together create a rough surface that facilitates the transfer of both gases and heat from the atmosphere to seawater, and here is where we get to the buffering effect of the oceans on our impact on global atmospheric chemistry and temperature.

Carbon dioxide dissolves in water, creating carbonic acid.  As we continue to inject carbon dioxide into the atmosphere, much of that (about 40% so far) becomes carbonic acid in the oceans; enough to cause the level of acidity in surface seawater to increase (causing the pH to drop), with important negative effects on coral reefs and other sea life.  The heat capacity of the oceans is immense as well, so that small changes in temperature mean that a tremendous amount of energy has been absorbed.  That, in turn, leads to thermal expansion contributing to rising sea levels.

So there is capacity in the oceans to resist the changes in both chemistry and temperature that we are forcing on the atmosphere, but with consequences for marine systems. 

And the surface oceans are only part of the story.

The Global Ocean Conveyor Belt

As early as the 1930s, oceanographers determined that at the northern end of the Gulf Stream, not all delivered water returned south as part of surface currents, but that much of it actually sank - and the concept of North Atlantic Deep Water was born.  In a classic 1982 paper, Wallace Broecker and a colleague stated that a more realistic quantitative representation of the ventilation (as they termed it) of the deep sea "remains one of the major unsolved problems in oceanography."  

Where does all the deep water go, and why does it sink?

By 1987, Broecker had sketched out the Thermohaline Circulation, or ocean conveyor belt, a connected, integrated cycle of sea water flow among the oceans, both at the surface and at depth.  The name derives from the initial idea that this circulation was caused by gradients in temperature (thermo) and salt concentration (haline) that increased the density of seawater in the far north Atlantic, causing it to sink, driving the formation of North Atlantic Deep Water, and perhaps driving the entire global circulation pattern as well.

A more recent if less descriptive name for this global pattern, one that allows for a wider range of drivers, is the Global Meridional Overturning Circulation (GMOC) with the Atlantic portion of this given the acronym AMOC.  Analogies and metaphors for the climate system running amok are unavoidable.

It always amazes me how recently some of the most fundamental pieces of our global weather/climate system have been discovered.  Atmospheric rivers were unnamed until the 1990s when improvements in remote sensing technologies allowed us to see them.  Jet streams could not be fully understood and mapped prior to high elevation aviation.  The global system of mid-ocean ridges that demonstrates plate tectonics was not fully visualized until the 1970s.  The ocean conveyor belt remained unknown and unnamed until the 1980s, and details are still being worked out.

And a new report has added additional complexity to the study of ocean currents.  A first global map of seamounts, remnants of seafloor volcanoes that do not rise high enough to reach the surface and become islands, reveals more than 45,000 of these features, some 14,000 feet tall!  Turbulence caused by these mountains can complicate the neatly described arrows in the maps above.

If the creation of deep water starts the cycle, where is that cycle completed?  Where does upwelling, or the return to the surface, happen?

On the map above, wherever the deep water blue arrows change to surface orange suggests upwelling.  Major regions include the north Pacific and Indian Oceans, and the circumpolar currents surrounding Antarctica.  The total flow of water through the conveyor is thought to be about 100 times that of the Amazon River, or 16 times all of the world's rivers combined.  Despite that high flow rate, it is estimated that it takes about 1,000 years for seawater to complete one cycle through the system.

But there are other local and important coastal areas where winds, tides and currents also drive both upwelling and downwelling.  Essentially, whenever surface winds drive seawater away from the coastline, deeper water will rise to replace it.  Onshore winds have the opposite effect.  Upwelling waters tend to be colder and richer in nutrients, and can support higher productivity of algae and fish.

This map from NOAA emphasizes that these local upwelling and downwelling events can occur along most coastlines, with important local impacts. 

At least one of these upwelling sites has both regional and global impacts apart from GMOC: The El Niño/La Niña or ENSO circulation that drives surface sea water from Peru to Australia and returns that flow at depth to the coast of South America.  Cyclic changes in this system drive major changes in the fisheries off the Peruvian coast as well as the energy balance of the atmosphere and the distribution of major weather patterns.  This cycle is not represented on most GMOC maps, and it is hard to find discussion of the interactions between GMOC and ENSO (my apologies for using these acronyms!).

If both resistance to climate change and a viable climate in some northern regions depend on the vigor of the Gulf Stream and the larger global circulation system, how might a changing climate affect or be affected by changes in the system?  And has this global flow of seawater changed in the past?

Monitoring Change in Ocean Currents

That remains something of a mystery.  Presentations can be found suggesting that the flow rate is basically unchanged over the last century, or that it has slowed significantly over the last decade or two, or that it is much slower now than at any time in the last 1,600 years.  Records of climate change preserved in ice cores going back 120,000 years or more suggest that there have been periods of rapid climate change in Greenland, at the northern end of the Gulf Stream, and these might reflect periods when the Gulf Stream, and AMOC, and GMOC, all collapsed.

Importance and uncertainty make careful monitoring of ocean currents a top priority for assessing our changing climate system, and there are several programs in place to do this.   

The intensity of coverage can be seen in these recent maps of deployments by the Global Drifter and Argo Programs that use passive or programmable buoys to measure conditions from the surface to a depth of more than a mile.

Another program named RAPID uses tethered buoys arrayed across the Atlantic at 26^o north latitude to monitor the doings of the AMOC.  While this network has been in place for 20 years, that is too short a time to detect trends in a process that may vary on a multi-decade to century time frame.

Still, models, theories and the ice core record all suggest that dramatic changes in global ocean circulation might be in our future.  It is clear that the state of the Gulf Stream is one key indicator of where our changing climate system is taking us, and, along with trends in other ocean currents, needs to be closely watched. 

The role the oceans have played so far in delaying the impacts of greenhouse gas emissions might change.   

Sources

Here is an interesting site that summarizes the history of human seafaring cultures over the last 2,000 years.

http://oceanmotion.org/html/background/timeline16-1700.htm

The map from 1675 is from:

https://longstreet.typepad.com/thesciencebookstore/2015/10/a-gl.html

Franklin's map of the Gulf Stream can be seen here:

https://www.smithsonianmag.com/smart-news/benjamin-franklin-was-first-chart-gulf-stream-180963066/

The ocean maps from 1876 and 1943 are here:

https://commons.wikimedia.org/wiki/File:Ocean_currents_1943_%28borderless%29.png

https://commons.wikimedia.org/wiki/File:Ocean_currents_1943.jpg

The complete map of surface ocean currents is from:

https://oceanservice.noaa.gov/facts/gyre.html

Early articles by Wallace Broeker on Thermohaline Circulation include:

Broeker, W. S., and T. H. Peng. "Tracers in the sea: Palisades." NY (Lamont-Doherty Geol. Observ. Publ.). New York. 1982

Broecker, W. S., 1987: The biggest chill. Natural History, No. 97, 74–82.

The map of the global ocean conveyor belt is from:

https://www.aoml.noaa.gov/noaa-scientists-detect-reshaping-of-the-meridional-overturning-circulation-in-southern-ocean/

Background on measuring ocean depth and mapping seamounts can be found here:

https://earthobservatory.nasa.gov/images/147437/taking-a-measure-of-sea-level-rise-gravimetry

And the map of upwelling sites is from:

https://oceanservice.noaa.gov/education/tutorial_currents/media/supp_cur03b.html

Maps of ocean sampling by the Global Drifter and Argo programs, and the schematic of a programmable drifter are from:

https://www.aoml.noaa.gov/global-drifter-program/

https://www.aoml.noaa.gov/phod/gdp/faq.php

https://www.aoml.noaa.gov/two-decades-argo-program/

https://oceanservice.noaa.gov/education/tutorial_currents/06measure4.html

A description of the RAPID program and the graph shown are from:

https://rapid.ac.uk/