Monday, September 12, 2016

22,000 Years Of Climate Change And History In One Comic

xkcd has an excellent comic summing up 22,000 years of climate change and the associated pre-historic and historical events taking place along the way in one oversized comic.

Two bits of commentary are in order.

First, xkcd has repeatedly demonstrated that often there is just no good substitute for drawing something to scale, even if that means it won't fit on the confines of an ordinary sized comic, or an ordinary sized piece of paper or book.  This is just such a case.

Second, while the comic is largely spot on when it comes to historical dates, the smoothing in the temperature diagram (which the author, in fairness, acknowledges is present in the comic) comes at the cost of a pretty important feature of the data from the period of about 40,000 to 10,000 years ago, which is that the temperature was wildly unstable and oscillated dramatically over much of that time frame, which is one important reason that farming had trouble establishing itself. It is hard to establish agriculture when one generation one set of crops work, and two generations later, an entirely different climate prevails.

To recap:
The key paleoclimate data are presented in the chart below.

Ignoring for the moment their simulated data in light gray bar graph form: 
Estimated dates of some well-studied cases of the initial emergence of cultivation are on the horizontal axis (85455). Climate variability (Left) is an indicator of the 100-y maximum difference in surface temperature measured by levels of δ18O from Greenland ice cores (SI Appendix). A value of 4 on the vertical axis indicates a difference in average temperature over a 100-y period equal to about 5 °C.  
As the chart indicates, intermittent periods of wild temperature variation over the span of just a few generations was the norm for the entire Upper Paleolithic era (about 40,000-50,000 years ago), after which temperatures became much more stable starting at the beginning of the Holocene era about 10,000 years ago when farming first emerged in the Fertile Crescent and China and the middle latitudes of the Americas (farming arose independently at later times in the New Guinea Highlands, Sub-Saharan Africa and the Eastern United States).
This feature is material to the presentation because the big point of the comic is to show how extreme and unprecedented rapid climate change in the present is relative to the past.  And, compared to the last 10,000 years that is correct.  Moreover, there is extremely solid evidence showing that climate change now is man-made, which was not true of climate instability from 40,000 to 10,000 years ago. But, this pretty important detail does make for a more complicated story.

This rapid fluctuation during the last glacial maximum around 20,000 to 22,000 years ago also helps explain while the world experienced an intense population bottleneck at the time even though much of it was not covered with ice.

And, this rapid fluctuation may have been a driver of mass migrations across the globe by Upper Paleolithic populations whose existing habitats became uninhabitable again and again.  This placed Upper Paleolithic modern humans under severe selection pressure for adaptability, which is a trait that it is fair to say distinguished us more from other archaic human populations, like Neanderthals, that did not survive, than straight "IQ".

1 comment:

andrew said...