The major problem with the history of climate variation in the Northern Hemisphere has never been explaining why it warms up to temperatures better than today’s. It has been explaining why it cools down. The Northern Hemisphere has had several protracted warm periods that include an exceptionally long period during the European Bronze Age that ended abruptly around 1159 BCE. We also had the medieval optimum, which lasted about two hundred years.
The evidence points to a natural climate regime as warm as we are now experiencing that left alone will eliminate perennial ice in the Arctic and enhance growing conditions in the north. That little extra heat slowly pushes the trend line to the optimum over as many years as it takes. In this case, since the little ice age, it has taken a good two hundred years and more.
I had a chance to see a program on ancient Ireland and got an eyeful of the tree ring evidence. It was startling, in that occasional cold years were common and that conditions for the Bronze Age did not stand out that much. What did stand out was the eighteen year growing break between 1159 to 1176 BCE. Afterwards, the climate recovered as it normally did after a bad year or two.
One advantage with the Irish data is that the Irish climate is profoundly moderated by the Gulf Stream. This means that any modest increase or decrease in the available heat will be generally buffered by the incoming winds. It will always want to be cold, wet and miserable. As we go into the Baltic, we both get and expect a much broader variation in conditions.
More plainly, a cold summer in Ireland translates into a disaster in most of Eastern Europe.
Because the moderating effect is dominant, normal variation is suppressed strongly. When you get a sharply defined event, it is certain to be a global event. We get this with the 1159 event which is marked in the tree ring record as an eighteen years stretch of zero summer growth.
This means a major temperature drop in Northern Europe and that has been substantiated with other records. That is deep enough to end any cropping for a generation over vast areas. Fortunately the bulk of the inland folk relied on cattle husbandry, which perhaps contracted but they were likely able to sustain themselves.
This period is associated with the onslaught of the sea peoples throughout the Mediterranean and the movement of peoples from the north in general. It is nice to police up the actual decades of this historic movement.
What is becoming much more apparent is that both the protracted eighteen year cold spell of 1159 and the many shorter cold year events are caused the same way. We need to look for major volcanic activity.
Remember that our own knowledge of volcanism is still in its infancy. Surely we learned that after Mt St Helens. We simply do not have the full range of geologically inferred activity available to us for study.
We know that a single major blast can drop global temperatures for a year or so. We also know that an active spewing volcano that operates for months will do the same thing.
A very likely disaster is the activation of a large magma mass at near surface that erupts for years. Iceland has the needed features and I believe it is quite capable of flooding the atmosphere with fine dust for eighteen years. It would also be limited to the high latitudes, unlike any of the monsters in Indonesia.
My conclusion is that the one clear mechanism able to chill the northern hemisphere is volcanic activity in all its various expressions. A number of known cold intervals have in fact been associated with known volcanic events. The proposition that all chilling events in the Northern Hemisphere are directly linked to volcanic events is strongly supportable.
I have never seen any popular media actually attempt to describe the full range of volcanic possibilities. The onshore mega volcanoes appear very dormant, but that includes Yellowstone Park, Iceland and Hawaii. You get the picture. Imagine any of those becoming truly active and throwing many cubic mile of dust into the atmosphere for decades. This is no risk with Hawaii but that give as us a clear measure of the real size of these structures. The global temperature would nosedive and crops would be largely impossible outside the tropics.
It is quite clear that a year or two is quite survivable because of our current global infrastructure. It would just be inconvenient as hell and we all would be on wartime rations. Sustaining populations past that would require a crash program aimed at massive food production in the tropics. We would all learn to enjoy our recently recognized cattail starch ration.