Strictly for amusement value… I have a few more odd associations with ol’ Louis Agassiz than you’d commonly expect to find.

The long period of warming after the last ice age (roughly from 18k years before present) saw the ice sheet that was covering much of northern America melt and retreat. Some of the melt formed what has been called Lake Agassiz, after the geologist Louis Agassiz. There are shoreline deposits over wide areas of the US interior (Dakota for instance). It is thought that perhaps this broke its banks and flooded into the Atlantic, perhaps through the St Laurence river area.

Simple model experiments show the thermohaline conveyor belt, that originates with warm, saline water moving north and sinking, will be stopped by a large freshwater input. Your b option was thought for some time to be the mechanism and is perhaps plausible (the switch seems to be pretty sensitive when it’s close to it’s switching threshold), though the evidence is stacking up for a). c is still on the table – the latest is a possible meteor strike over the US, which may have accounted for the megafauna and the Clovis people also, the hypothesis goes.

There is an ash layer and some evidence of ET material just under the Younger Dryas sediments. Did the meteor hit Lake Agassiz? You can get to some interesting hypotheses andevidence by searching clovis, meteor and dryas.

The detectives are still sifting through the evidence to find the culprit, or even work out whether the Lake, the Meteor and perhaps even another mystery party were in cahoots.

All of these natural phenomena are used by some to suggest that all global change events are natural. This might be plausible if the Earth was a simple mechanical system with no feedbacks. It isn’t, and responds to all kinds of phenomena, including us. It’s this fascination with the natural world and its complexity that got me into science as a career in the first place.

The logical follow-up question (at least to me, for the purposes of what I’m trying to get a grasp of) would be, what caused the precipitous glacial melt in North America (and also, was it even all that precipitous?)? Could it be a) a sudden-onset warming spike which caused the glacial melt and subsequent spill into the Atlantic current over a short time period, b) a slow-and-steady onset of glacier-melting-friendly temperature increases, which over time released new water into the Atlantic current but only reached a critical volume at a certain point after a long process, thus triggering an (only seemingly) sudden onset even though the cause itself was not sudden, or c) something else entirely which I haven’t thought of?

Naturally occurring rapid climate changes are called Dansgaard-Oeschger events. As I understand it each has it’s own cause, so I reckon the notion of priodicity is a bit of a stretch.

With the Younger Dryas they think it was caused by an interruption of the thermohaline circulation which brings warm tropical water to the Northern Atlantic via the Gulf Stream (the warm surface current). During the process of warming Europe about 5C more than it otherwise would be water evaporates making it more salty and denser. The dense water sinks, which is an important function in driving the thermohaline circulation system.

During the deglaciation of North America a huge lake formed in the middle called
Lake Agassiz. The favoured theory is that the lake burst suddenly creating a huge freshwater flux which interrupted the thermohaline circulation.

If so it’s unlikely to happen again quite like that, although I’ve seen stuff on the slowing of the thermohaline by as much as 30% in recent decades.

I did notice in the UN Climate Science Compendium that there is a story about it raining more in higher latitudes meaning more outflow into the Arctic sea. Not sure whether this is an issue.

One potential cause, which you can worry about if you’ve got nothing better to do, would be if a giant slab slipped off a continental shelf into the deep, disturbing methane clathrates which would then greenhouse the climate into a hotter state. Not sure that would be fast enough to qualify as sudden though.

I’ve just checked the Copenhagen diagnosis and they reckon if the world peaks in 2020 we’ll need to reduce by 9% pa from there (Figure 22), compared with Meinshausen’s emissions reduction path (Figure 3) where they say 6%. So they must have upped the ante as it were.

I notice that Meinshausen’s boss Schellnhuber was involved in the Copenhagen exercise.

the Younger Dryas is thought to have been caused by a large flood of freshwater from continental ice sheet melt in N Am hitting the north Atlantic and shutting off the warm currents heading north – these are the ones that keep Europe warm. They had started up as part of the post glacial warming phase. This isn’t agreed to by all but it’s a plausible mechanism. The lake project shows how fast it might have happened – this has surprised most.

One of the reasons it would have had such a quick effect is there was so much ice around, that it took no time for the ice sheets to re-advance (Lots to work with). This kept things cold for another 1300 years until the currents switched back on.

Warming can be pretty fast if large amounts of GHGs hit the atmosphere but not that fast.