I am also a novice candidate to GRE. But I would try and describe my suggestions to the Essay.
flaws that i saw :
The author mentions that the Scientists' discovery of Earth being significantly cooler, and the accounts of Asia and Europe supported it, this doesnt consolidate that the 'whole' earth (and not only Asia and Europe)actually became cooler we would need facts regarding the other continents having been registered with a change in whether pattern.
The authors suggestion that a big meteorite might have caused a dust cloud and that would have eventually made the earth cooler, suggests that the meteorite crash with earth would have given a significant blow to the earth and that might have been registered by the Scientists/Archieologists of that time.
The dimming of the sun and the cold temperatures can very well be 2 different occurances, neither dimming of Sun assures of cold temperatures nor the viceversa case can be assured.
There can be many other factors that might cause the Earth to be suddenly cooler like path deviations of earth or even it could be a large Black Hole in Sun, which might very well refute the justification of a eruption or a Meteorite.
Please do let me know if these flaws that i outlined are itself having flaws
Thanks a lot
As a Large Meteorite striking/creating a large flash, remains a possibility it is very well possible that there was no brigt flash even if the meteorite did strike earth, moreoever the records cannot be depended upon as its long ago, that we are talking about and the records might have been very well lost.