(Thread) I know I'm a bit late to the party, but I've decided to weigh in on the ICS subdivisions, and the start of the “Meghalayan” at 4.2 ka. For the past year, my colleagues @hannahkolus , @DKaufman1 and I have been looking into the Holocene significance of the 4.2 ka event.
Specifically, we’ve been wondering whether the event stands out as an unusual period of abrupt change (broadly defined) at that time, regionally or globally, or whether the combination of societal impact and confirmation bias has over-inflated its importance.
So @hannahkolus has been leading a study to take all of the paleoclimate data that we can get our hands on that are sufficiently resolved to characterize abrupt change in the Holocene and develop some objective tests to identify multiple types of abrupt change.
We compared the results with a robust null hypothesis, and then took a look at 4.2, and how it compares with the rest of the Holocene (especially the known event at 8.2)
First, although a lot of relevant Holocene data are now available, there are a lot of datasets, especially in the 4.2 literature that aren’t publicly available. We’ve made a special effort to track down data used in this literature (which is a slight bias) and incorporate them.
This lack of access to the data used to identify the event is frustrating, and needs to improve. Access to the data is fundamental if this is the major geologic change in human history. This the main point of a comment that @DKaufman1 and I just posted clim-past-discuss.net/cp-2018-82/
Second, @hannahkolus is just about to submit an abstract to present at AGU in December, and will be submitting a manuscript on this in the near future, so you’ll have to wait to get all the gory details of her analysis, but here’s the upshot:
Globally, the evidence of abrupt climatic changes, as an excursion (severe drought) change in long term trend (tipping point), or change in mean (regime change), at 4.2 ka is very consistent with those types of changes that occur throughout the Holocene (except for 8.2 ka event) 
Re: 8.2 ka, using Hannah’s event detector (adapted from Morrill et al., 2013) the event stands out as a strongly significant cold excursion in the Atlantic, enough so that it has global significance:
In contrast, looking for any kind of excursion at 4.2 ka (wet/dry/cold/warm) shows some significance in East Asia, but even in the broader Mediterranean, there are enough high resolution records that don’t show it as a significant excursion, that its not significant.
The same story emerges looking at other types of abrupt change (regime change, tipping points) - yes these events occur at 4.2 ka, just not broadly or frequently enough that they stand out against the Holocene background.
So - back to the “Meghalayan”. I doubt I will ever use this term, although there is value in having the community agree on what they mean when they say “Late Holocene”. And 4.2 is pretty much as good as other candidates for that boundary.
But is there objective evidence for a major climatic disruption at that time? Not as far as we can tell with the all the data we have available (>500 datasets). If we’re missing data, make them public, and we’ll gladly add them in.
p.s., this is twitter, and this is unpublished work led by an ECR, so we’re not showing everything of course. And whether to chime in or not with unpublished work was a difficult decision. But in the end we just wanted to share the data’s perspective.
And look forward to AGU and @hannahkolus paper for the full, measured, nuanced approach. (end)
