Here are more thoughts: if North America's dirty industries, like steel mills, were the culprit, then we should be able to trace back to those times a significant regional cooling differential in the downwind regions of places like Pittsburgh, PA.. (realclimate.org)
A shutdown today would not lead to another ice age, but it could cause some serious regional cooling over Europe, among other impacts that we don't fully understand. (climatesight.org)
(Right) LGM regional cooling compared to LGM global cooling as simulated in PMIP - 2, with AOGCM results shown as red circles and EMIC (ECBilt - CLIO) results shown as blue circles. (ipcc.ch)