Jonny McAneney’s Tusk comment on his collaborator Mike Baillie’s paper:
This is an important paper, but it is equally important to point out that scientific understanding has moved on in the decade or so since this was published. I have posted before on this site about the corrections of some of the contents of this paper, but given that this paper now has its own blog topic, I thought it prudent that these details be posted along with it. This way, the people who come across it will not be mislead by the errors that we now know to have existed when the paper published.
The main errors in this paper all arise with the mis-dating of the Greenland ice cores. First and foremost I should point out that at the time of publishing, the ice core dates that Mike was drawing upon was believed to be correct, but they have since been proven to be in error.
So let us imagine that we are back in 2007, and we have questions about the cause of environmental catastrophes as evidenced in a plethora of precisely dated tree-ring chronologies. The big question of this time was what caused the so called AD 536/540 event. So we can look to ice core information which preserves a chemical record of the atmosphere in each calendar year. In 2007 we effectively had four good Greenland ice core chronologies to draw from – the European workers’ DYE3, GRIP and NGRIP cores, which were used to derive the Greenland Ice Core Chronology (GICC05), and the independent American-workers GISP2 ice core. A point to note is that GISP2 was drilled 30 km from GRIP as a form of independent replication. Of the two camps, it was considered that the European cores were better due to the claim of internal replication with the DYE3, GRIP and NGRIP cores, and also because GISP2 was analysed with much lower resolution than the European cores, and also suffered from periods of missing data due to damaged ice coming up from the American bore holes. Indeed, replication between the American and European cores began to break down below AD 1000, and GISP2 had a 14 metre gap of “trashed ice” (around 70 years worth of ice layers) beginning around the AD 540s. So GISP2 may not be useful in determining the cause of the 536/540 event, so emphasis was placed upon the European GICC05 cores.
So when one looked at the European cores it was found that there were two large volcanic acid signals around the 530s, one in 529, and another around 533. The uncertainty of these dates at this depth was quoted as +/-2, so the later date could well have caused the AD 536 event, but there was no volcanic evidence for the cause of the 540 event. This lead to two camps, one was that the 533+/-2 eruption caused the 536 event and the 540 event, and the other was that the 533+/-2 eruption caused only the AD 536 event, but something else caused the AD 540 event, leading Mike to postulate that it could have been a cosmic impact. Just to be clear, Mike never postulated a cosmic impact in AD 536, only that there may have been an impact in AD 540. This is an important correction since many people afterwards worked on the idea that there was an impact in AD 536!
However, Mike showed in 2008 and 2010, and again in 2015 with myself, that there was likely an ice core dating error during the 1st millennium AD, with the ice cores being approximately 7 years too old with respect to volcanically induced climatic effects observed in tree-ring chronologies. What this meant was that the 529 and 533 dates in teh ice cores were actually AD 536 and AD 540, and so the AD 536/540 event was caused by two large volcanic eruptions. Shortly after our 2015 paper, a paper was published on a newly obtained Greenland Ice core (NEEM) that showed that the GICC05 timescale was indeed too old by 7 years in the 6th century AD. This was achieved by synchronising the isotopic signals in deposited in trees and ice cores from the AD 775 cosmic radiation event (now known as the AD 775 Miyake event).
So to bring it back to the discussion of Mike’s “significant impacts” paper above, we note that Mike was using the chemistry of the GICC05 ice cores. Mike discovered a large Ammonium spike dated to 539 on GICC05, but we now know that this is 7 years too old and so actually dates to around AD 546 and so cannot have caused the AD 540 event (which is now known to be due to a tropical/equatorial eruption). Note we do not observe ammonium in the GISP2 core around AD 540 (when we correct for the offsets in its core), as we find that the 14 meter gap begins around 535/536! We now also know that the 1014 Ammonium spike is around AD 1020, and so it no longer coincides with the English sea floods in late September 1014 which could have been caused by a storm surge in the North Sea coinciding with a spring tide (the flood occurred the day after the new moon). Ammonium does occur at around 1020 in GISP2 (and we know that GISP2 is accurately dated at this depth). Another point to make then is that the ammonium in these ice cores now dated to AD 1020 do not coincide with the close comet passage of AD 1014.
In 2007 the logical chain of thought for evidence of an impact was generally sound. To recap, GISP2 has ammonium in AD 1908.48, which coincides with the Tunguska impact, and when we look at the ammonium record in the GICC05 cores we find an ammonium signal prior to the AD 540 event, and we also find an Ammonium signal around a sea flood event which some historians have coinciding with clouds and stuff falling from the sky in the same year (note not necessarily in the same time of year). It makes a great story, until we realise that those linkages are not actually correct. Indeed, we can even question the 1908 GISP2 ammonium signal. Science is all about replication, and when one looks at the NEEM ice core, we find no large ammonium signal in 1908, although there is a small spike in 1907, but nothing comparable to the 546 and 1020 Ammonium spikes which NEEM replicates. So the question is did the Tunguska event leave ammonium on the ice sheets at all? If it didn’t, then again the logic chain breaks down further.
This leads us then Mike’s suggestion of a possible cosmic event around 1490/1491. The GICC05 timescale should be correct.