05-08-2024, 01:56 AM
(This post was last modified: 05-08-2024, 01:18 PM by Cejo.
Edit Reason: words, sources
)
I wanted to post some climate data for the area and time-frame highlighted in the Lazaridis paper, basically based on this map:
First up are some measurements taken from peat samples, near the southern extent of area 1. These charts show a very significant increase in temperature and a decrease in precipitation between 5500-5000 ybp. The coldest months notably go above freezing, while the warmest months spike to 2x the long-term average, since then. Precipitation drops by 40-50%. Unfortunately, measurements only cover back to 5500 ybp, or 3550 BCE, while the Lazaridis paper indicates migration from this region ca. 4400-4000 BCE (6350-5950 ybp).
However, I also made some plots from climate models/simulations, which fortuitously focus on 6000 ybp, the period when migration from 1 is indicated. These show 100-year average temperatures for summer and winter months. These show winter temperatures in area 1 generally at or above freezing south, decreasing north. Summer temperatures are warm, in the 23-25 range. Combined with the peat samples above, this indicates that there was a sharp dip in temperatures between 6000 ybp and 5500 ybp, before temperatures spiked again.
Next up is a look at sediment samples from Lake Van, near area 4 from the study. These temperatures and precipitation plots are a little more stable. While there are oscillations, the magnitude is generally smaller. Potentially worth noting is that precipitation increased roughly 33% prior to indicated migration into the region, and while it oscillated from there, the long term average remained generally elevated for the next 1000 years, dropping again around the time of migration from area 3.
Finally, I have included precipitation and temperature plots from peat samples near area 3. To me, the only really notable change is prior to migration from 1 to 3 (shown in black dotted line), specifically regarding coldest-month temperatures. The indicated date of migration immediately follows recovery from a cold period, during which winters dropped below freezing, returning to above-freezing temperatures year-round. There is a similar reduction and recovery in GDD5 (growing degree-days). I don't notice any massive changes on the later date (grey dotted line) which corresponds to migration from area 3 and expansion across the Steppe.
I'm still working on conclusions. It is hard to identify a climate-driven motivation for migrations from area 1, given discontinuity in data, aside from a hint of some temperature changes. However, one general pattern that emerges is expansion into areas recovering from changes. In other words, it doesn't seem to have been necessarily local climate changes driving people from an area. Rather, it may have been that nearby areas were put under stress and became relatively depopulated, opening up room for expansion and drawing people to new areas. I think particularly if these areas were inhabited by farmers, and precipitation or temperatures drastically changed, this would have put stress on that kind of economy. This would have been particularly true if plague also accompanied it. None of this is groundbreaking work, but I think that's what I'm seeing here, and seeing this made me think about it a little differently.
Edit: Adding sources...
Oghaito et al 2021 - https://gmd.copernicus.org/articles/14/1195/2021/ (ultimate source of the gridded model climate data I used)
https://esgf-node.ipsl.upmc.fr/search/cmip6-ipsl/ (where I accessed the gridded model climate data)
https://lipdverse.org/Temp12k/current_version/ (where I accessed peat and lake sediment samples)
First up are some measurements taken from peat samples, near the southern extent of area 1. These charts show a very significant increase in temperature and a decrease in precipitation between 5500-5000 ybp. The coldest months notably go above freezing, while the warmest months spike to 2x the long-term average, since then. Precipitation drops by 40-50%. Unfortunately, measurements only cover back to 5500 ybp, or 3550 BCE, while the Lazaridis paper indicates migration from this region ca. 4400-4000 BCE (6350-5950 ybp).
However, I also made some plots from climate models/simulations, which fortuitously focus on 6000 ybp, the period when migration from 1 is indicated. These show 100-year average temperatures for summer and winter months. These show winter temperatures in area 1 generally at or above freezing south, decreasing north. Summer temperatures are warm, in the 23-25 range. Combined with the peat samples above, this indicates that there was a sharp dip in temperatures between 6000 ybp and 5500 ybp, before temperatures spiked again.
Next up is a look at sediment samples from Lake Van, near area 4 from the study. These temperatures and precipitation plots are a little more stable. While there are oscillations, the magnitude is generally smaller. Potentially worth noting is that precipitation increased roughly 33% prior to indicated migration into the region, and while it oscillated from there, the long term average remained generally elevated for the next 1000 years, dropping again around the time of migration from area 3.
Finally, I have included precipitation and temperature plots from peat samples near area 3. To me, the only really notable change is prior to migration from 1 to 3 (shown in black dotted line), specifically regarding coldest-month temperatures. The indicated date of migration immediately follows recovery from a cold period, during which winters dropped below freezing, returning to above-freezing temperatures year-round. There is a similar reduction and recovery in GDD5 (growing degree-days). I don't notice any massive changes on the later date (grey dotted line) which corresponds to migration from area 3 and expansion across the Steppe.
I'm still working on conclusions. It is hard to identify a climate-driven motivation for migrations from area 1, given discontinuity in data, aside from a hint of some temperature changes. However, one general pattern that emerges is expansion into areas recovering from changes. In other words, it doesn't seem to have been necessarily local climate changes driving people from an area. Rather, it may have been that nearby areas were put under stress and became relatively depopulated, opening up room for expansion and drawing people to new areas. I think particularly if these areas were inhabited by farmers, and precipitation or temperatures drastically changed, this would have put stress on that kind of economy. This would have been particularly true if plague also accompanied it. None of this is groundbreaking work, but I think that's what I'm seeing here, and seeing this made me think about it a little differently.
Edit: Adding sources...
Oghaito et al 2021 - https://gmd.copernicus.org/articles/14/1195/2021/ (ultimate source of the gridded model climate data I used)
https://esgf-node.ipsl.upmc.fr/search/cmip6-ipsl/ (where I accessed the gridded model climate data)
https://lipdverse.org/Temp12k/current_version/ (where I accessed peat and lake sediment samples)