April 11th 2018 saw the publication of two studies (here and here) on the supposed ‘slowdown’ of the Atlantic Meridional Overturning Circulation (AMOC) and what it might mean for the climate of Northern Latitudes in the coming decades (cooling, basically). The Day After Tomorrow being a perennial favourite of climate alarmists, inevitably headlines like Gulf Stream current at ‘record low’ with potentially devastating consequences for weather, warn scientists started to appear all over the popular press the day after.
The Indie breathlessly ‘informed’ its readers:
Catastrophic changes in global weather patterns could be on the horizon as scientists confirm the warming Atlantic current has reached a “new record low”.
The Gulf Stream current, which has not been running at peak strength for centuries, is now at its weakest point in the past 1,600 years.
Climate change resulting from rising levels of carbon dioxide in the atmosphere is a likely cause of this phenomenon.
Carbon Brief summarises the media response to the catastrophic revelation that North Atlantic currents are at their weakest in 1600 years:
The Atlantic current that brings warmth to northern Europe is at its weakest in at least 1,600 years, widely covered new research says. For the Guardian, a collapse of the Gulf Stream – part of the weakening circulation known as AMOC – “Would see western Europe suffer far more extreme winters, sea levels rise fast on the eastern seaboard of the US and would disrupt vital tropical rains.” New studies suggest global warming is weakening AMOC, report the Financial Times, BBC and Associated Press. The slowing of this Atlantic circulation suggests “one of the most feared consequences [of climate change] is already coming to pass”, says the Washington Post. Scientific American and Mail Online both focus on the potential for extreme weather if the weakening of AMOC deepens. Climate models predicted a weakened Gulf Stream would be one consequence of climate change, explains one of the study’s authors, Stefan Rahmstorf, at the RealClimate blog. He says a key question is whether a slowdown is already underway, noting the question is easier asked than answered.
I can summarise CB’s summary even more succinctly: ‘We’re DOOMED!’.
OK. Reality check. One study (Rahmstorf’s) says that AMOC has weakened by about 15% since the 1950’s and that this is ‘unprecedented’ in the last 1600 years and therefore is mainly attributable to the sharp increase of atmospheric GHGs (which have melted the Arctic) since then. The other study suggests that the 15% decline in the strength of AMOC (not the Gulf Stream note, which is only a small part of the North Atlantic ocean circulation) “occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years”. They were unable to distinguish one possible scenario from the other. The important point is that it started as a result of the termination of the Little Ice Age which, despite the attempts by climate alarmists to attribute to the beginning of the Industrial Revolution and changing land use, was almost certainly an example of past natural global warming.
It is entirely possible that Rahmstorf et al’s identification of an ‘unprecedented’ decline in AMOC since 1950 is also due to a combination of multidecadal natural variability (which RAPID measurements confirm can be an order of magnitude greater than the decline predicted by climate models) and a millennial decline in the strength of AMOC instigated by not only the ‘recovery’ from the LIA, but by the most intense solar activity in 8000 years.
However, AMOC variability appears to be much more complicated than what the simple ‘freshwater hosing’ climate model experiments suggest. Climate scientists keep telling us that a melting Arctic inhibits the formation of deep salty water in the north Atlantic by adding trillions of tons of freshwater at the surface, thus slowing down the subsurface transport of deep cold water to the south, which in turn affects the ‘return’ currents of warmer surface waters from the south. In truth, nobody really understands the intricate mechanisms which drive the Atlantic thermohaline circulation and predictions of abrupt slowdowns of AMOC initiated by melting glaciers at high latitude are probably a gross over-simplification. As such, the theory of an anthropogenically enhanced or caused rapid slowdown in AMOC is very suspect, to say the least.
North Atlantic Cold Events
Throughout the Holocene, there have been a series of nine abrupt climate changes of varying severity and duration which have affected northern latitudes, resulting in cooler Northern Hemisphere temperatures and altered global circulation patterns. Several of these cold events are associated with catastrophic collapse of the AMOC due to massive fresh meltwater input into the north Atlantic. Notably, most occurred during the early Holocene when there were still extensive glaciers covering parts of the northern hemisphere and orbital forcing of northern latitudes was more significant than it is today. Other north Atlantic cold events are not associated with freshwater pulses and appear to correlate with significant downturns in solar activity, the last of which occurred during the LIA.
This paper is a review of studies of abrupt climate changes (ACCs) during the Holocene published during the past ten years. North Atlantic cold events are indicators of ACCs. As indicated by North Atlantic ice-rafted debris (IRD), there were nine confirmed cold events during the Holocene, occurring at 11.1 kyr, 10.3 kyr, 9.4 kyr, 8.1 kyr, 5.9 kyr, 4.2 kyr, 2.8 kyr, 1.4 kyr, and 0.4 kyr respectively according to most representative results from Bond et al. (1997) . . . . . . . .
Studies have indicated that the four ACCs occurring in the early Holocene may be related to freshwater pulses from ice melting in the northern part of the North Atlantic, and the other five ACCs that occurred during the middle and late Holocene may be related to the decreased solar activity.
Sharp readers will notice that one of the April 2018 studies above details a decline in AMOC beginning when the LIA was ending. According to what is written above though, we might expect AMOC to be already weak during a cold event. I suggest it was very weak throughout the 17th and 18th centuries and started picking up in strength as the sun became more active following the Maunder Minimum (1645-1715). This suggests that the Rahmstorf paper which details a more dramatic decline occurring after 1950 may be nearer the mark. Warming was well under way and solar activity picked up strongly post 1910, peaked during the 1950s and remained high until the end of the 20th century. Thus, whether due to freshwater input from melting glaciers (particularly during the very strong Arctic warming of the 1930s/40s) or due to increasing solar activity (or a combination of both), we might reasonably expect AMOC to start to decline significantly around that time. But of course it didn’t cool after 1950 and it’s highly likely that cooling is abrupt once AMOC reaches a certain minimum strength – I hate to use the term ‘tipping point’ but that’s probably what we’re looking at, where the climate responds abruptly to a gradual change.
Climate alarmists are obsessed with the idea of amplified Arctic GHG warming creating this AMOC ‘tipping point’; yet they can’t even convincingly attribute the majority of modern Greenland ice melt to GHG warming. Just for example, a recent paper found that most of the Greenland ice melt of recent times was due not to warmer air temperatures but to increased solar insolation at the surface caused by changing weather patterns and circulation resulting in less cloud cover. Undoubtedly, freshwater input to the North Atlantic does have an impact, but is it enough of an impact with the modest melting we have witnessed thus far? The shutdown of AMOC in the early Holocene was initiated by absolutely vast amounts of freshwater being deposited into the north Atlantic (as evidenced by geological deposits of ice rafted debris (IRD), not the modest amounts we have seen to date. RAPID data conclusively shows that AMOC can decrease naturally by 10% or more over just a year or two (as it did in 2009/10). Patently, this was not due to a meltwater pulse in high latitudes.
The fact is, AMOC variability can be driven by factors other than so called ‘freshwater hosing’. In demonstration, I give you this study: Hosed vs. unhosed: interruptions of the Atlantic Meridional Overturning Circulation in a global coupled model, with and without freshwater forcing.
However, other model simulations have shown that spontaneous changes in the AMOC can occur in the absence of freshwater inputs (Winton, 1993; Sakai and Peltier, 1997; Hall and Stouffer, 2001; Ganopolski and Rahmstorf, 2002; Schulz, 2002; Loving and Vallis, 2005; Wang and Mysak, 2006; Colin de Verdière, 2007; Friedrich et al., 2010; Arzel et al., 2011; Kim et al., 2012; Drijfhout et al., 2013; Peltier and Vettoretti, 2014; Vettoretti and Peltier, 2015). Although uncommon, these “unhosed” oscillations show that the AMOC can vary as a result of processes internal to the ocean–atmosphere system, which have been linked to oscillations in the strength of the vertical density gradient in the North Atlantic (Winton, 1993; Arzel et al., 2011; Peltier and Vettoretti, 2014) as well as to the existence of unstable states of sea-ice extent in the North Atlantic (Li, 2005; Li et al., 2010; Siddall et al., 2010; Petersen et al., 2013).
So, on the evidence we have so far, The Day After Tomorrow will never come however hard climate alarmists rub their magic lanterns in the hope of conjuring up wished for catastrophes. AMOC may however decline further and we may pass a ‘tipping point’ where the global climate (especially that of northern latitudes) shifts to a rather abrupt cold state. The wheels were probably put in motion early in the 20th century and thoroughly oiled from 1950 onwards, so such a shift may be inevitable, or it may not. I have an idea we might know for sure within the next decade or two. Meanwhile, install a multi-fuel burner before they’re banned by Western governments and stock up on plenty of wood!