"Cool temperatures and heavy rains resulted in failed harvests in Britain and Ireland" -- but not elsewhere. "Famine was prevalent in north and southwest Ireland" -- but not elsewhere. This is more an indication of the marginal conditions for agriculture in the British Isles; there are many places in Europe which won't see massive crop failures just because summers are 1 degree less warm.

What specifically is the mechanism for dust in the atmosphere? Dust put there from global fires will have different outcomes than dust put there from a volcanic eruption.

When is the story set? Today, we could cope with greenhouses to a degree. Animals (farm and wild) would be at greatest risk.

@AlexP I recall reading that the year after the Tambora eruption there were crop shortages all over the world. I specifically recall the southern U.S. being mentioned in materials I have read. It was not just "summer was 1 degree less warm." There was snow in places that generally don't see any (I recall Florida being mentioned specifically), blizzards into late spring/early summer, weather patterns disrupted, etc. I agree that 1 degree temperature difference would be negligible, all other things being equal; but all other things were not equal and it was not limited to Britain/Ireland.

@L.Dutch thanks for the quote edit, I was hoping someone would do it for me. I was on my phone and couldn't remember the text shortcut.

@AlexP I don't have citations right now, but I've read about this event... I'm pretty sure crop failures (of varying degrees, obviously) were global. I am guessing the info on Britain just reflects who edits WikiPedia most.

@AlexP - Britain/Ireland are hit by this change a bit worse. Britain by location should be frozen over, however it keeps warm through a process called thermohaline circulation. Waters warm in the Caribbean and make their way across the Atlantic to Britain, keeping it significantly warmer and wetter. The system depends on a large chunk of the atlantic warming in the equatorial sun..less warming, less warm water to Britain, less rain and colder temperatures = bigger impact on Britain than most locations (Pacific northwest america also sees the same effect).

@EveryBitHelps - Are you opposed to multiple eruptions? The Earth is decently resilient and it's hard to get an elongated nuclear winter without something actively replenishing particulate in the air (new eruptions). Do you want this a single event, or can you have multiple converging events (Toba and Krakatoa erupt 15 years apart)?

In a modern society it will actually be the political problems that will cause the collapse of society, unless there exists a World government and if that government is capable of preventing a civil war. The main problem is that a disaster that doesn't predominantly strike poor countries will have global effects because of global trade. So, if China runs out of food, it will have enough financial reserves to buy all the food it needs, but this will trigger export bans to China because no country is going to allow all its food from being exported away to China.

This then triggers the collapse of the World economy.

What do you mean when say society survive? It's about king's death or civilan war? Changing some habits in the society? Or about post-apocalyptical scenario?

I mean if you have dozen people you have some society. So society could only change and the question is how significant changes do you expect?

@Twelfth, I originally thought everything would happen at once but i was having trouble thinking of a reason for dust to stay in the atmosphere so long without a bigger initial impact. spreading out some of the carnage would make a lot of sense. The first set of incidents set off a chain of reactions and famine etc ensues. Just when everyone is getting used to things another event happens and essentially kills any chance of the old world order surviving unscathed.

@ADS, I am with you on the idea that any grouping of humans is a society. Asher's comment made me find out that apparently when we talk about 'societal collapse' we are talking about a group of humans living in a somewhat structured and ordered community. In my question I'm not concerned when the king die's as I can make that happen any time I like but rather with the break down of society as a whole. This isn't post apocalyptic as I am trying to figure out how long this apocalypse (that isnt an outright apocalypse) has to be before everything falls apart. Does that make sense?

@JoeKissling, a couple of small asteroids hit the planet near and around the equator. City killer size only, they are not the dinasour killers. I know one hits or triggers a massive lava flow and another a huge eruption in 2 seperate areas. There is also a huge tsunami along a desert coastline and flooding of a Mediterranean sized area (not to sure how much dust those last two contribute). I'm sure a couple of earthquakes can cause at least one candle to fall in a bakers shop somewhere. It won't cause a global fire but cities and crop fields will burn.

@JoeKissling. I realised that but tunguska hit in the middle of nowhere just destroying a forest. That's why I have some of them hitting natural features setting off volcanoes and earthquakes. It's the volcanoes themselves that cause the dust in the atmosphere. I can couple it with a general cooling period like the little ice age which also contributed to the 1815 Tambora event.

@EveryBitHelps Touching off a volcano is going to take more than a city killer

@JoeKissling If the volcanoes are already primed and just need something to set them off? Maybe a direct hit or a hit on a fault line that triggers an earthquake that sets off the volcanoe

@EveryBitHelps hitting a fault might cause an earthquake, and if you hit a volcanic dome like Mt. St. Helens you might trigger a volcanic eruption. But a volcano like Tambora would be hard to trigger with such a small impact.

@JoeKissling good points. So I either have to have a few big asteroids or think of some additional factors. But I still need to figure out how long society can survive in a dusty scenario!