From time to time cold spots emerge in the North Atlantic (NA). New research paints a ‘consistent picture’ of change to the Atlantic’s ‘conveyer belt’, which plays a major role in world’s weather. Its finding says that a region of Newfoundland defies global warming. Is a cold blob in the North Atlantic a matter to be concerned? That may depend on whether the authors (L. Caesar et al; Fn.1) took the following three aspects into consideration:
a. The average depth is about 3’300 meters, and colder than + 4°C. A cold spot showing up at the sea surface is presumably a very small fraction from entire water volume of the North Atlantic, presumably less than 0,1%.
b. Are considerations mainly based on air temperatures from 1900 to 2013 (see Fig. at top), of any help in the climate change debate.
c. Do the most pronounced climatic shifts since 1850, the strong warming from 1919 to 1939 and the lasting cooling from 1940 to mid-1970, played any role with regard to the topic? (Details HERE)
On none of these elementary points the research paper in NATURE titled: “Current Atlantic Meridional Overturning Circulation weakest in last millennium”, pays attention that puts the paper in the category of speculation. Here after at first the paper’s Abstract, followed by a few comments.
The Atlantic Meridional Overturning Circulation (AMOC) — one of Earth’s major ocean circulation systems — redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about ad 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades.
The AMOC is popularly known as the Gulf Stream System. The Gulf Stream is typically 100 kilometres wide and 800 metres to 1,200 metres deep. That is certainly a lot of water, but how much compared with the entire NA water volume. Is it 0,5%, or more, or less, and is this flow of water and its surrounding observed, and data available in reasonable numbers?
One would have expected that the paper analysis sub-sea-surface observations. Nothing! Instead the research’s combines several different types of climate “proxy data”. “Proxy data” is a term given to natural records that can be used to study past changes to the world’s climate, respectively are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer fraction of the Earth’s history. Examples of proxy datasets include ice cores, tree rings and ocean sediments (Fn. 2). The researcher’s claim that they have not only looked at few, but up to a dozen, finding them sufficient to “tell a consistent story of how the AMOC evolved over the last 1600 years”. Such boosted assessments seem to become frequently, since, for example Stefan Rahmstorf in 2015 at “realclimate.org”, covered already 1100 years.
How can one take seriously a study that ignores the subject of investigation, namely the North Atlantic? Using proxy data instead is misguided and irresponsible. At most proxy-data may tell that air sea temperatures have been warmer or colder for some time, but nothing at all about the status and movement within the water-body at any time in the past. To claim that one can make statements about the future behavior of the water masses is speculative, arrogant and naive.
None of the above-mentioned points (a-c) is discussed by the authors. At least they should have mentioned the huge dimensions involved, and the huge supremacy of ocean temperature over air temperature, which is a too big story to be raised here.
However, it is shocking to reckon the author’s inability to include the two most pronounced climatic shifts since the end of the Little Ice Age at about 1850. Both events (see above; item C) have had their origin in the North Atlantic. The warming from 1919–1939 was particular pronounced in the Atlantic Sector of the Arctic, and felt all over the Northern Hemisphere (in America until 1933, and elsewhere until 1939). The causing of the global cooling from winter 1939/40 lasted until the mid-1970, came from the oceans in the Northern Hemisphere, the North Atlantic and the North Pacific. Both events stand in a very close correlation with the naval warfare during the two World Wars. By human activities at sea huge water masses were churned, altering ocean temperature and salinity structure.
Even if science is unwilling to consider whether the two most prominent changing trends in the climate during the last Century had been anthropogenic, their findings about the processes of the AMOC in recent decades, or during the last 1000 years”, are incomprehensible. None of the internal ocean processes are explained, the dimensions and parameters involved ignored, no detailed observation data discussed, but instead assessed by computer-modeling.
Meanwhile Wikipedia summaries the state of research as it follows:
A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation. A 2015 study suggested that the Atlantic meridional overturning circulation (AMOC) has weakened by 15–20% in 200 years.
The only fact known is, there is sometimes a cold blob. No surprise if the waterbody is in permanent flow and has only a mean temperature of plus 4°C., and the Gulf Current only a small part of the total. Discussing in abstracts about the AMOC is an easy task. On the other hand clarifying what has caused the AMOC to support the major climatic shifts in the last Century (see above, item. c.), seems to be a too difficult task for science.
Fn. 1: L. Caesar, G. D. McCarthy, D. J. R. Thornalley, N. Cahill & S. Rahmstorf
Fn. 2: More examples of proxy data: ice cores, fossil pollen, ocean sediments, ratios of oxygen isotopes in air bubbles trapped in ice masses, , lake levels; pollen sediments in lakes/ rivers/oceans/ and coastal areas; pack-rat middens; glacial termini, borehole temperature; coral bleaching; ; archeological information . https://meteor.geol.iastate.edu/gccourse/history/paleoclimate/examples2.html