A Critical Control Point in Our Climate System?
Ocean Circulation, the Gulf Stream and a Possible Tipping Point
The big climate news last week, beyond the largest fires ever in California and Southern Europe, was the release of Climate Change 2021: The Physical Science Basis. This volume describes in excruciating detail our understanding of how the climate system works, and how humans are driving changes in that system. This is the scientific basis for the 6th Assessment currently being produced by the Intergovernmental Panel on Climate Change, or IPCC.
The first IPCC report appeared in 1990, and while projections of the direction we are heading have not been altered all that much in the ensuing 30 years, the certainty of the science has increased dramatically, as has our understanding of the implications for sea level rise, disappearance of ice, increased temperatures and the general likelihood of weather extremes. Island nations may disappear, coastal cities will be threatened. Mitigation will be unthinkably expensive. The basic conclusions are unequivocal. The climate system is changing directionally, not naturally or randomly. The world is getting hotter, and we are the cause. The IPCC report offers several alternative futures depending on what we do about greenhouse gases. The biggest variable in our climate future is us.
The pathways to our new climate reality described in the IPCC report are presented mostly in terms of continuous changes. Multiple graphics show direct measurements that document warming and its consequences. Most accelerate seamlessly into a model-predicted future, without major leaps, or as some like to call them, tipping points.
The dire predictions about our climate future, and how it can be deflected by reducing greenhouse gas emissions, would seem to be warning enough to consider actions to stem the tide (literally). But as this site is not about politics or policies, no more to be said on that score. We should be clear, though: presenting what is known about the climate system and how it is changing is not in itself a political act.
But what about tipping points? Are there super-sensitive parts of the complex climate system that might be pushed to cause a sudden change in state or accelerate the rate of change?
The case is made below that a familiar part of the ocean circulation system, the Gulf Stream, may be part of just such a critical system controller, but to address this concept, we first need a quick tour of the climate system so fully described in Climate Change 2021.
The global climate system is one massive heat/energy machine that attempts to redress the unequal distribution of received solar energy from equator to poles through the movement of air and water in response to gradients in temperature, pressure, and humidity in the atmosphere or salinity in the oceans.
In general, heat moves from the tropics toward the poles. It is not a direct or simple path. Warm tropical air rises and moves towards the poles, but not in a smooth, stable or predictable way. Long-term cycles of rising humid air and falling dry air give us rainforests at the equator, temperate forests in mid-latitudes, and a preponderance of deserts around 30 degrees north and south, all subject to local variation due to geography.
The rising and falling of heated and cooling air, along with the spinning of the Earth under these moving currents, gives us the trade winds familiar to sailors, and the westerlies that drive storms west to east in the north temperate zone (across North America and Europe).
Wind patterns drive ocean currents as well, and air and water in concert, modified by geography, give us El Niño and the Gulf Stream. They also combine to produce the “Roaring Forties,” that continuous band of wind and ocean currents moving at high speed west to east north of Antarctica and south of Africa, South America and Australia that make the passage from the Atlantic to Pacific around Cape Horn so treacherous. I enjoy a good sailing adventure story, and those like the Aubry/Maturin tales by Patrick O’Brien always seem to feature a deadly passage around the Horn.
This striving to redress global energy inequalities in the climate system is what creates weather and climate. The transfer is rarely smooth. Turbulence at the interfaces between air masses from warmer and colder regions brings rain to water croplands, but also storms and floods that can wash away both fields and towns. Where these interfaces become becalmed, droughts can lead to water shortage, crop failures and fires. Blocking patterns in the atmosphere can keep rainfall or drought in place for long periods of time, accentuating the damage.
The IPCC 6th Assessment science report tells us that the weather/climate system is getting more energetic, warmer, and probably more violent, with less ice and rising, more acidic oceans. Maps that provide detailed descriptions for all regions of the world are on offer in this most recent report.
One reason that it has taken 30 years and 6 rounds of IPCC assessments to arrive at the dead certain understanding of where we are now headed is that there are within the global climate system a number of “oscillations” or places in this complex weather/climate machine that cycle among different states. These cyclic switch points add “noise” or confusing variation to what otherwise might be steady changes in temperature and other indicators; confusing only because we don’t understand them completely.
El Niño (or ENSO – El Niño Southern Oscillation) is perhaps the best known as we have experienced some important weather disasters at its hands. Major events in 1972/3, 1982/3 and 1997/8 led to severe storm damage along the west coast of California. The strongest El Niño ever in 2015/6 did not bring rain to relieve the ongoing drought in that state, but that is a story for another time. Strong variations in El Niño/La Niña can alter global temperatures significantly.
The Polar Vortex (related to the Arctic Oscillation) was a major topic in 2013-5 as outbreaks of record-breaking cold air and record-breaking snow fall affected the northeastern U.S. even in a year that continued the upward march of global average temperatures. Where extreme events occur tends to determine how much attention they get!
While the direction of climate change is clear, we are still learning how to predict when and how these oscillations will change, and how they might be altered in a warmer world.
Are there oscillations that are so strong or have such high impact that they might alter what the IPCC report presents as a steady march to our warmer and more violent weather world?
The biggest actor here, and a continuing source of concern, is the Atlantic Meridional Overturning Circulation, or AMOC. A more familiar term relating to AMOC is the Gulf Stream (Figure below), a huge current of ocean water warmed by passage across the tropical Atlantic and then deflected north along the east coast of North America. The Gulf Stream is one of the strongest manifestations of our climate system’s efforts to redress unequal energy from the sun. The Gulf Stream moves tropical warmth north at a rate measured in billions of gallons per second.
The Gulf Stream is just one link in a continuous, global cycle of sea water that features regions where deep water rises or “upwells” to the surface, and others where water sinks toward the depths (see figure on thermohaline circulation above). This cycle is estimated to take between 1,000 and 4,000 years to complete, and has provided a buffer against more dramatic climate changes than we have seen so far. Only About 50% (depending on your time frame) of the carbon dioxide we have emitted into the air by burning fossil fuels remains in the atmosphere. Much of the rest has dissolved into seawater. An estimated 90% of all the excess heat accumulated in the Earth’s climate system in response to greenhouse gas emissions is in the oceans as well. This circulation system carries much of the carbon and heat to lower depths, again providing a short-term buffer against human impacts.
No, this is not the solution to climate change, it just says that things would be getting hotter faster if this circulation did not occur. Differences in temperature and precipitation would also shift if this massive transfer of tropical warmth did not occur.
There is growing consensus that the Gulf Stream, and the fate of the water it delivers to the seas around Iceland and Southern Greenland, plays a key role in driving the global ocean circulation system. Tropical water becomes cooler and saltier heading north, and at this northern end is then dense enough to sink toward the sea floor. The momentum and pressure provided by this sinking is seen as important in driving the entire global ocean circulation system.
If there is a key point in the global climate system, one that could generate a “tipping point” response, AMOC and the Gulf Stream would be a good place to look. At its northern terminus, the warmth delivered by the Gulf Stream causes Northern Europe to be much warmer than it “should” be for being so far north. The strength of AMOC and the Gulf Stream current, then, is crucial to weather and climate, particularly in Northern Europe.
So a few questions: what would happen if AMOC shut down? Has it happened before? What do we know about the rate of change of the northern flow the Gulf Stream and sinking of delivered water at the northern end? Are there predictions for the response of the Gulf Stream and AMOC over the next few decades, and do they suggest a continuous change or a tipping point? The key question really is – will our climate future, already a huge global challenge, arrive gradually, or in a rush?
This format and forum allow only a quick summary on these crucial questions.
Has it happened before?
In an older but insightful and accessible treatment of climate reconstruction from ice cores, Richard Alley (in The Two Mile Time Machine) provides evidence for several abrupt changes in climate over the last 110,000 years as recorded in the Greenland ice cap. He describes three types of breakdown in AMOC related to the two different locations where deep water formation takes place (shown in the figure above). A brief summary would say that sinking could be reduced or shutdown in one of these locations separately, or in both simultaneously. If one shuts down (two of the three shutdown scenarios) then relatively minor climate changes would follow. If both shut down at once (third scenario) serious disruption to AMOC and climate would ensue. Detailed analyses of the ice record suggests changes of tens of degrees in mean temperature over the course of years to decades in and around Greenland. A probable cause cited for these rapid changes is increased outflow of fresh water into the salty seas at the northern end of the Gulf Stream due to ice cap and glacial melting, or breaking of ice dams. Water with lower salt content will be less likely to sink.
Bottom line here is that the ice core record suggests rapid changes are not uncommon and could be expected even without human intervention in the climate system.
What are the possible impacts?
In The Little Ice Age, Brian Fagan records historical disruptions to human cultures around the world as a result of changes in global average temperatures of just 1 degree Fahrenheit, much less than predicted for our climate future. The medieval warm period (from about 900-1250) saw first settlement and then abandonment of Southern Greenland by the Norse, as recorded by Jared Diamond in Collapse. Fagan captures the cooling of the little ice age (from about 1400 to 1850) in iconic paintings of ice skaters on the canals of Holland. Ice on the canals is now a rare event.
So it appears that AMOC and climate might have been related over the last thousand years or more, and that the ice core record shows much larger sudden shifts in climate over the 10,000 and 100,000 year time frames.
Is the rate of flow in the Gulf Stream Changing?
By definition, we can’t see a tipping point clearly before it arrives. Which leaves us with trying to understand what controls the rate of flow in the Gulf Stream and the rate of sinking at its northern end. We might also measure current rates of flow with enough precision to detect minor changes in rates or other characteristics that might foretell a larger shutdown. Looking at that satellite image above of the flow of Gulf Stream warmth, with all the variable eddies and changes in course, might give an idea as to how difficult it might be to measure an average rate of northern flow.
The longest running data set related to AMOC is a reconstruction of North Atlantic sea surface temperatures reaching back to 1850. This figure show the difference between changes in water temperatures at the north end of the gulf stream (50-60 degrees north) and it’s mid-point (35-50 degrees north). Blue areas indicate colder temperatures at the northern end which would be consistent with a slowing of AMOC. The depth of the blue bars since 2010 suggests slowing is occurring. While reconstructions are more difficult the farther back in time you look, what data are available show a similar drop in temperature from 1855 through the 1860s, near the end of the little ice age.
Previous IPCC science reports have projected long-term and continuous declines in AMOC, with reductions in flow of maybe 20% by 2100, but the variation around these model predictions is very large. The 6th Assessment science report says there is “medium confidence” that there will not be an abrupt change in AMOC, but that if such a change occurred, climate impacts would be substantial.
Given the importance of the Gulf Stream and AMOC, it is surprising that only in the last 17 years has there been a long-term, intensive effort to measure the actual rate of flow. On the other hand, given the technological challenges facing such an effort, that “late” start is not so surprising after all. That program carrying out these measurements has the acronym RAPID, and the technologies involved and data acquired to date are detailed on the RAPID website.
A recurring theme of these essays is the amazing technological and organizational expertise required to make the kind of complex measurements needed to monitor the state of the global climate machine. RAPID is another great example.
Data from the first 15 years of RAPID measurement shows wide variation in the rates of flow, and no discernible trend, but then that record is too short to find a trend in a process that changes at decadal time scales. An early result however, reported on the RAPID site, is that minor slowdowns in the rate of flow from 2009-2011 coincided with cold winters in Europe, suggesting a role in inter-annual weather variability.
So IPCC models and what measurements we have suggest that a tipping point for AMOC is unlikely. A recent paper projects a different view.
When that paper by Niklas Boers (citation below) was released, it received considerable media attention due to the nature of the prediction made. Boers presents studies saying that AMOC exhibits two different states or conditions, one with strong northern flow and one with weak to none, and that the system switches quickly from one to the other – a classic tipping point.
Boers presents some sophisticated statistical analyses of increasing variability in AMOC characteristics to suggest that the measured decreases in sea surface temperatures at the northern terminus of AMOC (as in the figure above) portend not a continuous change, but a nearly complete loss of stability, and a pending switch to the slow flow state. There is no indication as to when this might occur, or what changes in temperature or other characteristics would trigger it.
It should be noted that other sources and references cast doubt on the two-state nature of AMOC, opting for continuous change rather than a rapid switch in state.
In the end, AMOC remains one of the bigger uncertainties in models of our climate future. We just don’t know. Scientists (like myself) prefer to state what we do know and prefer not to dwell on the “don’t knows.” One of the many things I like best about the writings of Neil deGrasse Tyson on astrophysics is that he uses that phrase (we don’t know) on a regular basis. When we don’t know we should say so – that is where the science gets interesting.
The rate of flow of the Gulf Stream, and the state of AMOC, bear very close watching as we move into our climate future. The ice core record suggests rapid switches have occurred, and the Boers paper would suggest destabilization towards a tipping point. The measurement record, still incomplete, and the models of the IPCC suggest a continuous change, the likely outcome of which could still be colder winters in Northern Europe, and major changes in other parts of the global climate/energy machine.
Which will it be, continuous change or tipping point? Either one suggests we need to prepare for a future with a diminished AMOC and Gulf Stream. I have ended some of these essays with a call for preparedness and humility in the face of the changing climate machine. What we know and don’t know about AMOC makes that call a little more urgent.
Sources
The IPCC report, Climate Change 2021: The Physical Science Basis is here:
https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/
This contains estimates of the fate of emitted carbon dioxide
Satellite images of the Gulf Stream and thermohaline circulation are from NASA and can also be found here:
https://commons.wikimedia.org/wiki/File:Golfstrom.jpg
https://en.wikipedia.org/wiki/Thermohaline_circulation#/media/File:Thermohaline_Circulation_2.png
NASA currently has a great visualization of the full ocean circulation system here:
https://gpm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt
the 90% of warming being in the oceans is from:
https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content
The rate of flow in the Gulf Stream is measured in Sverdrups – named for the inventor of the term. One Sverdrup is equal to 260 million gallons per second. Rates of flow in the Gulf Stream can range from 30-150 Sverdrups.
https://en.wikipedia.org/wiki/Sverdrup
The long-term NOAA data set is here:
The RAPID website is here:
https://rapid.ac.uk/
The Boers paper is:
Boers, N. 2021. Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change 11:680-688
Wikipedia has good pages on almost every concept and term used in this essay.
The three books cited are:
Richard Alley. 2008. The Two Mile Time Machine. Princeton University Press
Jared Diamond. 2005. Collapse. Viking Press
Brian Fagan. 2000. The Little Ice Age. Basic Books
Acknowledgements
Thanks to Allison Leach and John Pastor for reviewing this essay and suggesting important improvements!