North Atlantic Deep Water Flow Unreliable
One of the scary scenarios frequently trotted out by climate change alarmists is the possible shutdown of the ocean currents in the Atlantic Ocean. This would disrupt northern hemisphere climate, particularly in Europe. Indeed, one Hollywood disaster movie had frozen military helicopters falling from the skies in the UK and Manhattan buried under a tsunami of ice. We are told this could happen at any time, if the world gets too hot from all that CO2 our species is churning out. Now a shocking new paper in the journal Science implies that the standard view of a relatively stable interglacial circulation may not hold for conditions warmer/fresher than at present. Why? Because it happened before, over 100,000 years ago, without the help of man made global warming. Another catastrophic climate threat is shown to be totally natural and to have happened before our species began burning coal and driving SUVs.
Everyone knows about the Gulf Stream, the warm current that carries heat from the tropics up the coast of North America, across the North Atlantic to warm the British Isles and the rest of Europe. There are other currents involved with the giant heat circulation system that redistributes heat energy around the world, gigantic rivers flowing in the sea that dwarf any river on land. One of these is the North Atlantic Deep Water (NADW) sometimes described as part of the “great ocean conveyor belt.” It is the return flow that balances the warm water moving northward, flowing unseen deep beneath the surface of the ocean.
Cold and dense, the NADW forms in two areas of the northern North Atlantic: in the Labrador Sea (between Greenland and Canada) and in the GIN Sea (between Greenland, Iceland and Norway). Oceanographers identify the NADW by its salinity and temperature, which is about 2.5 – 5°C for the water from the Labrador Sea and about 0 – 2.5°C for the water from the GIN Sea. Due to its higher temperature deep water from the Labrador Sea is a little less dense, and flows southwards above the colder water from the GIN Sea. This creates two distinct layers of North Atlantic Deep Water, an upper layer (UNADW) that forms in the Labrador Sea, and a lower layer (LNADW) that forms in the GIN Sea.
Cold, dense, deep water forms in two areas of the northern North Atlantic: an upper layer (UNADW) that forms in the Labrador Sea, and a lower layer (LNADW) that forms in the GIN Sea.
“Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales,” the authors of a new study state, giving the motivation for their work. The article, “Rapid Reductions in North Atlantic Deep Water During the Peak of the Last Interglacial Period,” by E. V. Galaasen et al., which just appeared on Science online, sheds new light on the behavior of this important part of Earth's climate system. It seems that during the last interglacial period (LIG), more than a thousand centuries ago, the NADW underwent a number of sudden fluctuations. Here is how the paper begins:
Future climate could be affected on a global scale if the circulation of North Atlantic Deep Water (NADW), the main water mass ventilating the deep Atlantic (Fig 1), is altered. Such changes could have widespread and long-lasting impacts—including for example on regional sea level, the intensity and pacing of Sahel droughts, and the pattern and rate of ocean acidification and CO2 sequestration. However, the response of NADW to high-latitude warming and ocean freshening, both of which would decrease source region density and potentially inhibit NADW formation, remains a key uncertainty in future climate projections. Model estimates range from nearly no change to ~50% reduction in Atlantic Meridional Overturning Circulation by 2100 AD. Compounding the uncertainty, models may inherently underestimate the possibility for abrupt and large changes and there may even be critical stability thresholds in surface ocean buoyancy that, if crossed, could switch circulation into an equilibrium state without strong NADW formation. The current consensus is that we are far from any such stability thresholds, and that the modern style of vigorous NADW ventilation is a robust feature of warm interglacial climates. However, large but shorter-lived transient anomalies might be possible even in the midst of a generally vigorous interglacial circulation.
In other words, a smoothly working ocean circulation system helps keep our climate calm, but interruptions in the return flow could cause all sorts of trouble. This is the classic example of what climate change alarmists tout as a tipping point, a sudden transition to a different climate state triggered by crossing an unknown threshold.
Fig 1, from the paper showing the NADW.
A core of ocean sediment, taken in 2003 from the sea floor just beyond the southern tip of Greenland, has revealed new detail during the warm period between the two previous glacial periods 115,000 to 130,000 years ago. Previously, scientists had thought that time, known as the Eemian interglacial, to be a tranquil enough time for North Atlantic circulation. Instead, Galaasen and colleagues found three episodes during which the flow of the NADW had slowed, stopped, or risen toward the surface. Each of the two more recent reductions in NADW flow lasted a few hundred years, but the earliest one consisted of several centuries-long drop-offs spanning a period of 2500 years.
The critical question is what caused the shutdowns? Was it the temperature? The prevailing opinion is that the Eemian climate was warmer than that of the Holocene by a few degrees. But the worst outbreak of shutdowns happened at the beginning of the Eemian, when the temperatures were lower than today. The authors conclude that sudden infusions of fresh water into the North Atlantic or Arctic triggered the disruptions. “The largest and longest NADW anomalies occur early in the LIG when, ice melting was strongest and residual ice masses from the prior glaciation persisted,” they report.
Similar disruptions of ocean circulation are thought to have occurred during the transition from the last glacial period to the current Holocene warm period. Named after Louis Agazziz, the father of the theory of Ice Ages, Lake Agassiz was an immense glacial lake located in the middle of the northern part of North America. Though the lake drained several times before the ice sheets covering Canada fully abated, geologists have found evidence that a major outbreak around 13,000 years ago drained north through the Mackenzie River into the Arctic Ocean.
The extent of Lake Agassiz during the end of the last glacial period.
Many think that this caused a shutdown in the Meridional Overturning Current (MOC), which in turn caused a sudden dip in temperature that is known as the Younger Dryas. From this and other events we know that a disruption of the NADW could cause a sudden shift in northern hemisphere temperatures, possibly for centuries. Here is how Galaasen et al., summed up their findings:
Our results call for a reevaluation of the notion that the deep Atlantic ventilation is relatively stable and vigorous during interglacial periods. Our records resolving centennial-scale variability provide clear evidence for large changes in deep Atlantic water mass geometry—similar in magnitude to glacial millennial-scale events—punctuating the LIG. The most prominent NADW reductions occurred during periods of ice sheet melting and known freshwater outbursts—attesting to the importance of surface buoyancy forcing in triggering such events. Concerns about the future evolution of the thermohaline circulation have revolved around the potential existence of inherent tipping points that, if crossed, could lead to long-term perturbations in the mode of ventilation, although the existence of such bistability is debated.
Is our world nearing a tipping point, about to be cast in a real world version of a Hollywood disaster film? Probably not. The really big, persistent disruptions seem to require a massive infusion of fresh water as a trigger. There are no gigantic glacial lakes to be found on present day Earth. Elevated global temperatures could possibly release significant volumes of fresh water by melting the ice sheets on Greenland, but even with a temperature rise of several degrees melting Greenland would take more than a thousand years. Never say never.
More interesting is the fact that these shutdowns occurred during the last interglacial warm period, a time during which our ancestors were stone age hunter-gatherers. There were no automobiles, no industry, no profligate burning of fossil fuels, yet the vaunted tipping point was tripped. So such things can happen under purely natural conditions.
If such a thing happens again it will undoubtedly be blamed on anthropogenic global warming, but no one will be able to separate humanity's role in triggering the change from nature's. The lesson here is that sudden, violent swings in climatic conditions can happen without any help from mankind, and blaming every bout of bad weather or change in nature on us is simply misanthropic hubris.
Be safe, enjoy the interglacial and stay skeptical.