I read that.
I like the explanation by professor Mike Baillie in his excellent book, Exodus To Arthur
Although the trees worldwide record a catastrophic event at that time, Baillie explains that there isn't an acid layer (at this time) in the Ice Core samples that is indicitive of a volcano. He argues for a celestial event.
"The five harshest environmental events showing in the dendrochronology records are events at 2354-2345 BC, 1628-1623 BC, 1159-1141 BC, 208-204 BC, and 536-545 AD. In terms of climate, these time periods appear similar in that the growth ring evidence implies colder than usual temperatures and unusual rainfall patterns."
I believe the 1168BC event was the Exodus period and when Santorini/Thera/Akatori in the Mediterranean blew it's top.
Keys makes a good case for a truly dramatic event around 535 A.D. affecting climate worldwide.
Exodus to Arthur:
Catastrophic Encounters with Comets
by Mike Baillie
paperback
reviewed by Steven Zoraster