I’m a little suspicious of this chart for the reasons you describe. And as a certified dumb guy, even I find it hard to believe that there is some kind of nexus between global cooling and volcanic activity. Any thoughts?
Four major eruptions would cause lots of ash and debris to cloud the atmosphere, causing less heat from the sun to reach earth? I’m assuming, much like the concept of nuclear winter?
Solar output dips, Climate cools, Earths' crust cools, shrinks and moves causing earthquakes and volcanoes.
Well, off the top of my head - volcanic activity could eject a lot of crap into the atmosphere that could reflect back the suns radiation and cause cooling. But then it could also absorb more radiation and cause heating. So it could do both. The science is settled - the debate is over.
The "nexus" is between reduced solar activity and volcanic activity. The reduced solar activity alone would account for the temperature reductions. In addition, the increased volcanic activity on ocean floors increases evaporation. More snow increases surface albedo. More heat/light is reflected than absorbed.
That one seems pretty well established with “years without summer” in the history books following big eruptions.