…and why did scientists spend 3 days in Pisa discussing it?

From January 10-12th, 2018, about 60 paleoclimate researchers from all over the world gathered in Pisa, Italy, to discuss their research on the 4.2 ka BP climate event. In short, there is evidence of droughts and cooling in some parts of the world approximately 4200 years ago. Researchers have noted the general coincidence of this climatic anomaly to rapid transitions (also termed as declines or collapses) in the some of the world’s greatest ancient civilizations, including Old Kingdom Egypt, the Akkadian Empire in Mesopotamia, the Liangzhu culture in China, and the Indus civilization in modern-day India and Pakistan.

Leaning tower of Pisa

Throughout the Holocene (the last 12,000 years), one other similar-duration climatic event occurred at around 8200 years ago, when a large meltwater pulse was released from the Laurentide ice sheet in North America and triggered a slowing of the North Atlantic thermohaline circulation, subsequently causing cooling and droughts for over a century in several regions of the world. Evidence for the 8.2 ka BP event is quite well-documented, along with its likely trigger, but the 4.2 ka BP event continues to puzzle scientists in regards to its spatial extent, trigger, and large-scale characteristics. The 4.2 ka BP event does not appear to have the same driving mechanism as the 8.2 ka event, and thereby presents a conundrum for the paleoclimate community. This has prompted several interesting questions, such as:

Why do we have large climatic shifts or events occurring in certain parts of the world, while in other areas records show no change at all?

Why does the timing of the onset of the event appear to vary from anywhere between 4.5-3.8 ka BP in different locations and records? [In fact, even apparently reliable records close to each other often do not come to the same conclusions].

Finally, what could be a possible cause for the 4.2 ka BP event, if we accept that a global driver is responsible?

At the 4.2 ka BP workshop, most of the 39 talks discussed reconstructions based on speleothems, tree rings, marine foraminifera, coral reefs, ice cores, peat bogs, and lake cores from various parts of the world. A few talks presented statistical syntheses from global databases, as well as model outputs and regional syntheses from the Middle East, China, India, and the Mediterranean. Several talks generated considerable controversy by discussing potential mechanisms for the 4.2 ka BP event, such as volcanic eruptions from Hekla 4 in Iceland, increased dustiness from Megalake Chad and the Bodélé depression, and shifts in the North Atlantic Oscillation (NAO) or El Nino Southern Oscillation (ENSO) regimes.

Though no overall take-home message was formally agreed upon by the participants, the three-day workshop did illuminate a few important concepts, highlighted a few new reconstructions, and generated several new questions. Several papers were of direct interest to the scope of the TwoRains project.

One of the most-cited speleothem records showing the 4.2 ka BP event from northeastern India, Mawmluh cave, is about to be challenged by an upcoming publication on a new speleothem from the same cave (G. Kathayat et al., in preparation). While the old Mawmluh cave record (Berkelhammer et al., 2012) found a drought beginning around 4.3 ka and a sharp spike in aridity between 4.1-3.9 ka, the new record shows drought starting later and lasting 4.0-3.92 ka BP.

I presented a co-authored paper with new data from the well-established Staubwasser foraminifera record from the mouth of the Indus, which highlighted what appears to be a general trend towards aridity from 4.5-3.6 ka, with a possible increase in this signal around 4.3-3.8 ka.

In a summary paper, A. Sinha noted the paucity of records from subcontinental India, joking that it made preparing his synthesis presentation much easier than expected. He stressed the importance of selecting suitable locations for reconstructions that could be expected to show strong correlations to Indian Summer Monsoon rainfall. Only the records from Rara lake, Kotla Dahar, Staubwasser’s marine foraminifera, Lonar Lake, Sahiya cave, and Mawmluh cave qualified as possibly reliable reconstructions of past rainfall in India over the 4.2 ka BP period.

What can we all expect as a future outcome from this workshop? A special issue dedicated to the 4.2 ka BP event is being planned for the journal Climate of the Past, and we may see a proposal to PAGES to create a working group focusing on arid extreme events and societal impacts in the Holocene. Overall, the participants agreed that the mysteries surrounding the timing, global impact, and cause of the 4.2 ka BP event were far from clear and deserve a more detailed examination in our future research, and I hope that my work on the TwoRains project will contribute to that goal.