AMOC: A Non-Tipping point
Gabriel Oxenstierna, Ph.D
The Atlantic Meridional Overturning Circulation (AMOC) is one of Earth’s major ocean circulation systems—it redistributes heat on our planet and is a key driver of climate variability. There is a northward transport of heat throughout the Atlantic, comprising one quarter of the global heat flux (reaching a maximum of 1.3PW at 25°N).
Figur 1. AMOC is the Atlantic section of the global Meridional Overturning Circulation (MOC). One part of it is the Gulf stream, that transports heat northwards from the tropics.
The heat transport is a balance of the northward flux of the warm Gulf Stream, Ekman pumping, and southward fluxes of cooler thermocline, and cold North Atlantic deep water. The circulation is completed after a very long time as the deep waters rise to the surface again in the Southern Ocean. The turnover period of AMOC is many hundreds of years.
Forecasts of cascading tipping points
Several studies have found evidence that both the Gulf stream and the AMOC have weakened during the last 40 years or so.[1][2][3] One highly publicized report warns that a continued weakening would have “severe impacts” and increase the risk of “cascading problems” for other major Earth systems, “such as the Antarctic ice sheet, tropical monsoon systems and Amazon rainforest” – see the figure below illustrating the major climate crisis teleconnections related to AMOC.[4] Climate crise effects would occur in many other areas as well. Stormier weather, more floods, collapsed plankton production, and widespread oxygen death in the oceans (anoxia) are forecast. The issue of AMOC’s whereabouts is therefore of great interest.
Figur 2. AMOC is a centerpiece in the cascades of tipping points thought up by alarmist researchers.[4]
What the IPCC says
The IPCC AR6 report highlights AMOC as a main building block in the climate and that it potentially is one of the most important ‘tipping points’.[5] Over the years, the IPCC has had dramatic projections for the AMOC. In the latest climate report (AR6) they claim that the AMOC currently is at its weakest for the last 1600 years and forecast a dramatic future decline.
Figur 3. AMOC-flow anomalies according to IPCC model simulations. The thick grey and black lines are the history as simulated by the two latest model generations. The colored lines are forecasts from the models according to selected emissions scenarios. Flows are in Sverdrups (Sv, million cubic meters per second). Source: AR6 WG1 fig. 9.10, which is taken from [3]
The IPCC claims that AMOC is “very likely” to weaken over the 21st century under all emission scenarios.[ch 4.3.2.3] An almost monotonous reduction by 25 to 50 percent in 2100 is predicted, depending on which scenario is chosen.
IPCC only has “medium confidence” that there will not be an “abrupt collapse” before 2100. If it collapsed, the world’s weather patterns would be dramatically impacted.
Model fudging begets a history revision…
For the historical part, figure 3 is model based.[3] Comparing the modelled history in the older CMIP5 and the newer CMIP6 computer model ensembles, we see that AMOC is bumped up in CMIP6, especially during the second half of the 20th century up until 1990. As a result of this history revision in the models, the CMIP6 models now show a much more pronounced weakening from around 1980.
The revised history neatly fits the narrative that a decline in the AMOC is mainly caused by increased levels of greenhouse gases, i.e., that it is caused by anthropogenic global warming (AGW).
…but no decline seen in recent data from the Atlantic
Recently, we got an update from the measuring stations called RAPID, which directly measure the various flows that together constitute AMOC. RAPID measures AMOC at a series of stations, located at 26.5 N. The flows are measured in the Gulf Stream, another part is the Ekman transport, and finally the southbound return flow of different layers of cold water. These three components together form AMOC, see the red time series in the figure below (copied from the RAPID website).
Figur 4. Readings of the AMOC flows in Sverdrups (Sv). (A)MOC is the red curve with a flow of 16.8 ±4.6Sv. It comprises the Gulf stream (green, 31.3 ±3.1 Sv), the Ekman transport (black, 3.7 ±3.5 Sv), and the cold, deep southbound flow (lila, -18.2 ±3.4 Sv). The latest data are up until March 2022. Source: RAPID.
As can be seen from the figure, the AMOC did have a certain decrease at the very beginning of the RAPID measurement period, from 2004 until 2010, but after that AMOC is trend-stable. The Gulf stream shows a slight decrease, in line with the above-mentioned research, whereas the other components are stable. Volatility is also stable over time within a fairly large short-term variability.
Data collected directly in the Atlantic Ocean thus do not provide any support for the IPCC’s forecasts of an ongoing collapse of the AMOC.
IPCC’s forecasts disputed by field research
In recent years, a number of research reports have been published that put the IPCC forecasts into question.[6][7] Some of the researchers behind RAPID write that they can’t find any signs of a weakening AMOC during the last 30 years.[6] This is the very period where the revised history from IPCC claims there is a steep decline, due to their climate models. Reality vs. fiction.
The researchers write that AMOC rather seems to be “a decadal oscillation, which is superimposed on a multidecadal cycle”. Paleoclimatic records also show distinct multidecadal variability of the AMOC.
Which are then the dominant feedbacks and their associated timescales in AMOC’s natural variability? Is AMOC variability periodic, or quasi-periodic? The timescales, as well as the mechanisms behind these natural variations remain unexplained. This is no wonder, given the current’s extended turnaround time (100’s of years). The IPCC itself is not too sure about the state of the AMOC: “Given the large discrepancy between modelled and reconstructed AMOC in the twentieth century and the uncertainty over the realism of the 20th century modelled AMOC response (Section 3.5.4.1), we have low confidence in both.” (p. 9-32)
Summing up
The model makers have managed to create an impression of a steeper decline in the AMOC from 1990 and onwards by manipulating the models from the CMIP5 to the CMIP6 model generations. This manipulation of the history fits the climate crisis narrative that a decline in the AMOC is caused by greenhouse gas emissions and AGW.
The IPCC is not too convinced and gives a “low confidence” to the models. This doesn’t stop the IPCC from forecasting a sharp and monotonous decrease of the AMOC as “very likely“. They promote a climate crisis narrative entirely built on models they themselves give a low confidence rating.
The issue of natural variability is pertinent to all discussions on the AMOC, but remains unresolved. This doesn’t stop the IPCC from giving the primary role in AMOC developments to the effects of greenhouse gas emissions and AGW.
The empirical data on the water flows in the various strands of the AMOC in the Atlantic show no decline in the last 30 years. The AMOC is stable and doesn’t show any sign of decline.
References
[1] Robust Weakening of the Gulf Stream During the Past Four Decades Observed in the Florida Straits, Piecuch and Beal, 2023, https://doi.org/10.1029/2023GL105170
[2] Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation, Niklas Boers, Nature 2021, https://doi.org/10.1038/s41558-021-01097-4
[3] Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations, Menary and 13 co-authors, 2020, https://doi.org/10.1029/2020GL088166
[5] IPCC SROCC “Extremes, Abrupt Changes and Managing Risks”, Chapter 6.7, https://www.ipcc.ch/site/assets/uploads/sites/3/2022/03/08_SROCC_Ch06_FINAL.pdf
[4] Exceeding 1.5°C global warming could trigger multiple climate tipping points, Armstrong McKay and 5 co-authors, Science 2022, https://doi.org/10.1126/science.abn7950
[6] A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline, Worthington and 5 co-authors, 2021, https://doi.org/10.5194/os-17-285-2021
[7] A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s, Fu and 4 co-authors, Science 2022, https://doi.org/10.1126/sciadv.abc7836
Gabriel Oxenstierna is a PhD at Stockholm University and one of the Clintel signatories.
Classic cultist fear porn, full of contradictions for the discerning mind.