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Why a 15 Percent Slow-Down in North Atlantic Ocean Circulation is Seriously Bad News

“We know somewhere out there is a tipping point where this current system is likely to break down. We still don’t know how far away or close to this tipping point we might be. … This is uncharted territory.” — Stefan Rahmstorf

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The North Atlantic ocean circulation (often called AMOC or the Great Ocean Conveyor) is now the weakest its been in sixteen centuries.

Increasing melt from Greenland due to human-forced warming of the atmosphere through the deep ocean is freshening the ocean surface of the far North Atlantic. To the south, higher ocean temperatures are increasing surface salt content through greater rates of evaporation. Fresh water prevents ocean water from sinking in the north and rising salt content generates increased sinking in the south. As a result, the rate at which waters move from the Equator toward the Pole is slowing down. Since the mid 20th Century, this critical ocean circulation has reduced in strength by 15 percent on decadal time-scales.

(Deep water formation in the North Atlantic is driven by the sinking of cold, salty water. Over recent years, this formation, which drives larger ocean circulation and atmospheric weather patterns, has been weakening due to increasing fresh water flows coming from a melting Greenland. Image source: Commons and the NASA Earth Observatory.)

Movement of warm Equatorial waters northward and their subsequent overturning and sinking in the North Atlantic drives a number of key weather and climate features. The first is that it tends to keep Europe warm during winter and to moderate European temperatures during summer. The second impact is that a fast moving current off the U.S. East Coast pulls water away from the shore keeping sea levels lower. The third is that warm water in the North Atlantic during winter time tends to keep the regional jet stream relatively flat. And the fourth is that a more rapid circulation keeps the ocean more highly oxygenated — allowing it to support more life.

A slowing down of ocean circulation in the North Atlantic therefore means that Europe will tend to cool during winter even as it heats up during summer. Sea level rise will accelerate faster for the U.S. East Coast relative to the rest of the world due to a slowing Gulf Stream combined with the effects of melting land glaciers and thermal ocean expansion. The North Atlantic jet stream will tend to become wavier — with deep troughs tending to form over Eastern North America and through parts of Europe. These trough zones will tend to generate far more intense fall and winter weather. Finally, a slowing ocean circulation will tend to increase the number of low-oxygen dead zones.

(Cool pool formation near Greenland juxtaposed by a warming and slowing of the Gulf Stream as it is forced southward is an early indication of ocean circulation slow-down. During recent years, this phenomena — which is related to larger human-forced climate change — has become a prevalent feature of North Atlantic Ocean climate and weather patterns. An indicator that climate change and ocean system changes for this region are already under way. Image source: Earth Nullschool.)

A 15 percent slow down in ocean circulation is not yet a catastrophic event. It is, however, enough to produce odd weather and climate signals. We have tended to see higher rates of sea level rise off the U.S. East Coast, we have tended to see more extreme winter weather across the North Atlantic basin. The long term trend for increasing ocean dead zones is well established. And European weather has become more and more extreme — with hot summers and severe winters.

With rates of Greenland melt increasing, there is a risk that the historic observed North Atlantic circulation weakening will increase further and more radically — producing still more profound results than we see today. In the event of large melt outflows coming from Greenland during abnormally warm summers or due to warming deep water melting glaciers from below — a possibility that rises with each 0.1 C of global temperature increase — we could see a very rapid weakening of ocean circulation above and beyond that which has already been recorded.

(Like Antarctica, Greenland features a number of below sea level locations directly beneath its largest ice masses. This feature makes Greenland more vulnerable to rapid ice loss and large melt outflows. Image source: NASA JPL.)

If such a tipping point event is breached — and there is increased risk for it as global temperatures enter a range of 1.5 to 2.5 C above 1880s averages during the 2020s through the 2040s — then we can expect far more profound weather and climate disruptions than those we have already experienced.

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World Ocean Heartbeat Fading? ‘Nasty’ Signs North Atlantic Thermohaline Circulation is Weakening

Scientists call it Atlantic Meridional Overturning Circulation (AMOC). But we may as well think of it as the heartbeat of the world ocean system. And when that heartbeat begins to slow down, we’d best sit up and start paying attention:

(New video produced by climate hawk Peter Sinclair and featuring top scientists Stefan Rahmstorf, Michael Mann, and Jason Box, issues warnings about an observed disruption to ocean circulation due to water freshening in the North Atlantic. This is the kind of work I mentioned last week in my KPFA interview. The kind that should be showing on major network news every single night. Since that probably won’t happen, I urgently ask you to spread this video, together with its critical information, as far and as wide as possible.)

Global Warming Poses Risk to Ocean Circulation, Life Support

For nearly three decades now, prominent climate scientists have been warning policymakers that salt and heat driven circulation of the world ocean system (called thermohaline — thermo for heat and haline for salt) could be disrupted by cold water outflows from Greenland. There, in the North Atlantic, salty, dense, ocean water issuing from the tropics along the Gulf Stream begins to cool. The heavier water, burdened with salt, sinks to the bottom in the North Atlantic. This sinking, in turn, drives a massive ocean conveyer belt. It delivers colder, oxygenated water to the deep ocean. It dredges less oxygen rich bottom waters to the surface where they can be reinvigorated. And it drives this ocean revitalizing train of currents through every major corner of the world ocean.

A disruption of this ocean water mixing machine would ripple through the world oceans like a gunshot to a vital circulatory organ, reducing oxygen levels throughout the whole ocean system, and greatly reducing the oceans’ ability to support life. It would be a major shift toward a stratified, less life supporting ocean, and one step closer to the nightmare ocean state called a Canfield Ocean (named after its discoverer — Dr. Donald Canfield).

Warmer, salty water cooling and sinking in the North Atlantic is an essential cog in the wheel of this massive ocean water overturning machine. It has also been described (as Dr Box notes in the video above) as the Achilles Heel of global ocean circulation.

But I like to think of it more as the world ocean’s beating heart. The reason is that any disruption of the overturning process in the North Atlantic basically kills off a life-giving circulation to the entire world ocean system.

Cooling in Exactly the Wrong Place

AMOC Temperature Trend

(Linear temperature trend from 1900 through 2013 produced by Stefan Rahmstorf in his new study. Note the anomalous cool pool just south of Greenland. That’s exactly the kind of temperature signature you don’t want to see. One that is indicative of cold, fresh water outflows from Greenland interfering with North Atlantic and World Ocean Circulation. Also see: RealClimate.)

Now, a new 2015 report headed by Dr. Stefan Rahmstorf finds that the world ocean system is cooling in exactly the wrong place — the North Atlantic just south and east of Greenland. This cooling is an indicator that a high volume outflow of cold, fresh water is entering this region of ocean. A cold, fresh outflow that comes directly from the melting glaciers of Greenland itself. A cooling and freshening that creates a physical block to salt water down welling in the North Atlantic. The kind of block that can directly disrupt the Gulf Stream and the rest of ocean circulation on down the line.

Dr Rahmstorf explains the findings of his study in his notes at RealClimate:

The North Atlantic between Newfoundland and Ireland is… the only region of the world that has defied global warming and… cooled. Last winter [this region] was the coldest on record – while globally it was the hottest [such period] on record. Our recent study (Rahmstorf et al. 2015) attributes [ anomalous North Atlantic cold water] to a weakening of the Gulf Stream …, which is apparently unique in the last thousand years.

It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway. (emphasis and a little clarity added)

To Dr Rahmstorf’s point that the North Atlantic was experiencing a Gulf-Stream threatening record cold while the world was under a pall of record warmth, we need only look at NOAA’s Land-Ocean temperature anomalies map for the winter of 2014-2015 below:

NOAA land ocean temperatuer anomalies

(NOAA Land Ocean temperature anomalies map for 2014-2015 shows extraordinary record cold pool of water south of Greenland in a record warm world. The smoking gun for large glacial outflow and thermohaline disruption in the North Atlantic. Image source: NOAA via ClimateCrocks and MeltFactor.)

Other Concerns Regarding North Atlantic Cooling

Unfortunately, an expanding pool of cold, fresh water in the North Atlantic is not just a threat to ocean health. It also represents a zone of anomalous cold in a region surrounded by atmospheric and ocean warming. As such, it represents a zone of likely expanding atmospheric instability — one involved in the shift of the cold center of circulation from the polar zones and more toward Greenland and Canada. Parcel to the kinds of weather disruptions that have been described in the theories of Dr. Jennifer Francis and during some of the later works of Dr. James Hansen (alluded to in The Storms of My Grandchildren).

As such, cold water bleeding from the great glaciers of Greenland not only poses a threat to ocean circulation, it also poses a risk for generating significant disruptions to atmospheric winds and related weather as well. Ones that could set off increasingly intense storm events in the Northern Hemisphere similar to what was seen for the US Northeast this winter (but likely worsening with time) and the extraordinarily powerful barrage of storms hitting England during the winter of 2013-2014.

Dr. Hansen in his Greenland Ice Sheet Loss: Exponential? paper warned of the potential for continent-sized frontal storms packing the strength of hurricanes under some rapid Greenland melt scenarios by mid-century.

Hollywood dramatizations aside, this is more than enough real world weather and climate trouble to pose serious cause for concern. And as Dr. Rahmstorf, Peter Sinclair, Dr. Jason Box and Dr. Mann allude to the header video — the policy makers were warned well in advance.

Links:

A Nasty Surprise With the Greenhouse

What’s Going on With the North Atlantic?

Exceptional 20th Century Slowdown in North Atlantic Overturning Circulation

Unpleasant Surprises in the Greenhouse

NOAA

MeltFactor

Greenland Ice Sheet Loss: Exponential?

Linking Weird Weather to Rapid Warming in the Arctic

Canfield Ocean

Hat tip to Today’s Guest Is…

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