I'm surprised no one's addressed the elephant in the room: WAR.
The world is about to face severe economic depression. Food availability is going to suddenly drop. Millions will be left without power, or adequate shelter as the temperatures drop day, after day, after day.
In a perfect world we would simply reorganize our resources, and struggle to help everyone out.
However reality is a harsh mistress.
Faced with starvation millions of people would pack their belongings and simply move south. This would constitute a migration similar to what we've seen in Europe over this past year.
As resources are already strained, and the further south you go, the less stable the governments become, this will invariably lead to political, and maybe even military problems.
Hundreds of thousands of Canadians and Americans flee to Mexico as the temperatures drop. The Mexican government, already rocked by allegations of corruptions, collusion with cartels, and dealing with massive waves of cartel violence can't properly accommodate these newcomers.
The new comers are not welcomed by the Mexicans, who feel invaded by pretentious northerners. The cartels start kidnapping influential, or rich migrants, running protection rackets, etc. The newcomers demand better conditions, refugee status, etc. .
In parallel, as the conditions deteriorate, the Canadian military and government relocate to more temperate locations in the US (with their government's approval, and under certain conditions, etc.). As the Mexican government begins cracking down on "illegal" American and Canadian refugees, as well as seeing their own resources (especially real estate and food production) dwindling, the Canadian and American governments have to make a choice.
This choice comes in the form of a massive invasion of Mexican territory, under the pretense of defending their immigrated citizens from the corrupt Mexican authorities, and rampant cartel violence. Realistically, this invasion will succeed in taking down organized Mexican government and military in a few short weeks, if not days.
The Canadian and US authorities now take control of the Mexican territory and begin flushing out the cartels, establishing food production, etc.
This is a realistic scenario. Anyone suggesting that northern countries would somehow sit quiet while being buried under mountains of snow, or that countries such as the US or Russia would stand by and allow their populations to starve are being .. let's say naive.
Speaking of Russia, they would probably invade Ukraine, and other states, such as Afghanistan. Only this time, they won't be screwing around. They will put entire villages to the torch, because now it's a matter of survival. They will have a limited amount of time in which to move their major manufacturing centers south - before they are buried under so much snow that the machines and technology is lost to them forever.
China will likely experience massive unrest. The northern population will seek to escape to the already crowded south. The government will have to step in and stem the tide of desperate people who will be fleeing wherever they can. Already a massive importer of food, China cannot hope to feed their 3 billion strong population. They will have to face either allowing a very large number of people to starve/freeze, or invade a neighboring nation to provide them with the needed agricultural and living space.
Seeing how India and Pakistan are both nuclear powers that will be quite an interesting conflict.
And so, is there anything that North American and Europe can do?
Frankly, the best, and realistically only thing they can do is invade their southern neighbors. I expect that a new age of colonialism will be sparked, with European countries invading African nations, and Canada/USA invading Mexico, to start with, most likely Cuba, and maybe some other nations as well.
In a time of global panic, crashing economic markets, dwindling food supplies, and facing the end of their civilizations, these nations will have little choice other than breaking out of their "We're all friends" mentality, and resorting to more time-worn traditions such as "the guy with the bigger gun survives".