Howdy. A little something: a sort of fancified version of Beedok's "Yellowstone Blows 1948" map, with the assumption that the Big Boom brings on an actual ice age rather than just a few years of horrid cold. I may do one with the other assumption at some other time.
The US did not collapse immediately: although death tolls in the area of heaviest ashfall (the Great Ash Desert) were over 90%, the east and gulf coasts receieved a "merely" airplane-stopping and road-messing-up fall of ash (things were a bit worse on the west coast), and the ashfall being in mid-January, there were no crops to ruin. Of course, things descended into Extreme Badness as the year went on and summer failed to arrive, but the US remained a functional society long enough to make some important historical moves, such as the evacuation of some of the population to less hard-hit areas as the Phillippines and other parts of the west Pacific, Cuba, etc., the transfer of most of the small US nuclear arsenal and the airforce with which to deliver it to western European allies (deterrence, you know) and making heroic efforts to move the agriculture of the far south from cotton, peanuts, etc. to such things as winter wheat and potatoes and other plants which will grow in cool climates.
Still, as everywhere in the first couple years, the famines were horrendous, and the mass move of northern refugees from their ice-immobilized cities brought about a violent reaction by southerners already starving: in the end, the US disintegrated, with at one point eight states claiming to be the "true" heirs to the US. A century later, there were still five, although three had dropped the US title.
Stalin had the advantage of a brutally regimented police state to prevent disorders, and the move of vital materials, party officials, secret policemen, and lots and lots of soldiers and their families south begin within a few months, although much of northern Russia was so paralyzed by unnaturally severe cold that it's inhabitants had to be written off. In the end, more than half the population of the old USSR was essentially "triaged" - and of those who survived the move south, many in turn starved, for the agricultural production had plummeted even along the shores of the mediterranean, although things weren't as bad as in the USSR proper. The Soviet Union, however, survived.
The southern hemisphere had a little time to prepare, since the ash spread only slowly across the equator: still, South America would still suffer demographic catastrophe over the next couple years, and for those who fled south to Australia and South Africa there were bitter dissapointments as cold and darkened skies followed them.
The French took over the US occupation of Italy, and later moved into Spain after Franco's state collapsed in mass famine and social breakdown. A government survived in the south and in North Africa through extreme measures which would warp French politics for the century to come. (Nobody will know exactly how many Arab and Berber North Africans were eaten during those years, and how many of them had died of natural causes first: after the first year, the French government certainly made no bones about recycling European dead).
The very worst of the cold was over after a few years, but the planet failed to return to former levels of warmth: so much snow had fallen that the albedo of the planet had changed beyond a certain tipping point...
A century later, civilization has survived and finally surpasses pre-Yellowstone levels of development.
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The most powerful international block is the Democratic alliance, a close association of mostly Asian and Pacific nations which are at least nominally democratic and mostly capitalistic in their economics.
The Pacific Union is a union of states to some extent "Americanized" by the settlement of US refugees and their control by US military in the decades after the eruption. Although northern Japan was largely depopulated, the great cold was sufficiently weakened further south for centers of civilization to survive in Kyushu and Shikoku, as well as in the Phillippines and Taiwan. Nowadays the largest population in the Union is Philippino, and the capital is in Manila, although the Union is currently a constitutional monarchy with the Emperor of Japan as it's nominal head. English is the Lingua Franca and the speech of the elite, but Chinese and Japanese and Tagalog are still spoken, and the American refugees have largely assimilated (although this Japan _does_ have quite a few blondes and redheads).
The Chinese civil war was interrupted by everyone starving, and in the consequent chaos no real government survived: it was not till some decades later that new centers of government would again take up the task of unifying China. The nation is currently divided between a Communistic south and a Republican (and fairly nominally democratic) north which also holds an enclave around Hong Kong (controlled during the chaos years by the Pacific Union and Australia, handed over to the North a couple decades back): much of the land of the north is no longer suitable for agriculture and it is less populous than the south, but richer.
Australia is another influential state, but the south has become largely useless for agriculture due to desertification over the last century: New Guinea in this world is a major center of agribusiness. The British Royal Family in exile live here, as do the heirs to the Dutch and Swedish thrones (may they one day thaw out!)
The US (Raleigh capital) is now a bigger player than Australia, and has ambitions to reunite the US: the pugnacious and largely fascist Montgomery/US is the major obstacle to this happening, and energetic propaganda campaigns and efforts to detatch the south from its European alliance continue. Texas/west Louisiana is perhaps a bit too lefty for US/Raleigh tastes, and given to fiery religiosity, but they're a democracy, unlike their unpleasant neighbors.
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The League of Paris is an alliance of hard-right, authoritarian states. The French realized they couldn't hold an empire without bringing the Italians and the Spaniards in, and nowadays have an essentially fascistic, racist, anti-Left one-party state with a "Roman" ethic. They are very defensive over the whole Mass Ethnic Cleansing thing and quick to point out the Badness of other regimes during the early years. The few surviving central Europeans -Germans, etc. - who made it into French and Italian territory (with central Europe already essentially a mass of ruins in 1948, there weren't that many who made it out alive after that first terrible winter without a spring) have in large part been resettled in areas of Africa and South America.
The (Montgomery) US has a two-party system: alas, far-right-wing and further right wing. Blacks (those who haven't made it out to the north or west) are still segregated, and after some allocation choices made during the famine years, somewhat fewer in numbers proportionally compared to OTL. The government is paranoid, and northern spies and provocateurs are seen in every bush. The absorbtion of Texas is feverently hoped for, and there is quite a bit of "welcomed with flowers" rhetoric circulating in the heavily censored press.
India entered its own era of chaos, although Gandhi, who died of malnutrition a few years later (refusal to take more than his "fair share") was a tower of moral strength in the effort to maintain some sort of functional government. The current northern government is, alas for his hopes, a right-wing Hindu nationalist regime with a major hard-on against the Soviets and aspirations to reunite the peninsula.
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The League of Paris used to include more of south America, but in the fading of Soviet revolutionary fever and continued hard times, right-wing juntas have been widely displaced by left-wing ones inspired by Red Mexico, whose ruthless rulers have created that most unusual thing, a highly efficient and well-run Latin American dictatorship: lefty, but with some room for market economics, universalist, heavily armed, interventionist, and inspirers of Latin American pride: their takeover of much of the western US (mostly wasteland at present, but still, it's the principle of the thing) has put a spring in many a radical South American's step. The Mexican People's Republic is quite hostile to the Paris League and the Democratic Alliance for ideological reasons, and are quite peeved over the existence of DA states to the south and to the NE, which are seen as major obstacles to a long-term plan for unifying all Latin America.
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The Soviet Union has perhaps more blood on its hands than anyone else: the population of its Middle Eastern territories is nowadays more Slav than Turkish/Arab/Persian: food in those first bad years went to the loyal or to those Muslims with productive farms. Very little of the Middle East's urban population survived. The Balkans weren't quite as bad, in that the Russians, Ukrainians, etc. were usually willing to go a little way to share with fellow Slavs, and there were local communists whose loyalties could be maintained by _not_ taking the food from local mouths, but the ethnic balance was still rather different afterwards (as in the middle ages, Greece is full of Slavs, and there are no gypsies to be found, although the family names of some party members yield clues). Russian is the universal language, and all but the most basic schooling is conducted in it.
Still, after a century, things have become more civilized, Islam is undergoing a bit of a revival, and resettlement in the mountainous lands of the Kurds was thin enough that they now have their own state. The USSR is still a ruthless dictatorship, but it has moderated its economic policies, and between increased small capitalism and state allocation of resources to public consumption, a feeble prosperity has come to the Red Empire. World revolution is largely off the plate, and the biggest concern is the increased need for food imports: populations bounced back after the initial great die-back, and increased drying over the last century has made irrigation more difficult.
Soviet allies generally follow a more "relaxed" and less militant version of Communism than the Mexican brand, with the notable exception of the Chinese, who are making noises about joining the Veracruz Pact due to the USSR's disinterst in supporting them in military ventures against the ROC.
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French and British holdout forces in the Levant and Egypt delayed Soviet movement into that area, although the need for more agricultural land eventually led to their occupation of Iraq in the face of feeble protests on the part of Australia. The Arabs of the Peninsula and Egypt, the Jews of Palestine, and the various peoples of the Levant managed in the face of catastrophe to find common cause of the "hang together or be hung separately" variety, also sparked by British efforts to resettle some of their population in the region. In famine and revolution, a political alliance of sorts was established, and Arab and Jew formed a common front against colonizers either British or Soviet (not to mention the French).
A fair number of Jews from freezing Europe or North America made it to Jerusalem Alliance territory: as per terms of the alliance, they were resettled in Egypt and Syria and Lebanon, so as to maintain the fragile accord between Palestinian Muslims and Jews. A century later the now-renamed Union of the Children of Abraham is actually only 5% Jewish, but then imagine the US if it had 15 rather than 6 million Jews. By intensive and low-moisture agricultural methods pioneered by the Jews the Union manages to feed itself in spite of extreme desertification, although much of Arabia has become depopulated, its inhabitants moving to the crowded cities along the Med (where massive-scale desalination projects are growing) or the Nile. It loathes both the Latins and the Soviets, but has improved relations (a bit) with the Reds in recent years.
Given the risky nature of travel nowadays around icy Cape Horn, the Suez canal is more important than ever as a travel link between east and west...
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The SE Asian Economic Cooperative, led by Burma (which managed to adapt to climate changes fairly well, and is by the year 2049 actually rather more populous than OTL) is lefty, but also has a fairly vigorous internal market. It is scrupulously neutral in international affairs and trades with everyone, and has a fairly formidable joint military command directed to keeping the Indians and the Chinese out. Governments include oligarchy, one-party state, the near-theocracy of Aceh and a couple actual democracies.
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The Kongo Empire was the construction of a rather successful local warlord backed by Paris, which wanted to restore enough order to the chaotic parts of central Africa to get some serious resource extraction going. To the eternal annoyance of the Latins, his son and heir managed to extract his state from puppet status with the aid of the south Africans: in spite of several efforts on the part of the Latins to break up the state or assasinate its ruler, the grandson of the initial warlord is still in charge.
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South Africa is currently in negotiation to join the Democratic Alliance: this is a Big Thing, since South Africa is one of the three largest economies on earth which will make the lead of the DA over the other power blocks and nations secure. It was alienated from the democratic nations due to the opressive nature of its government, local whites and European refugees on top, blacks on the bottom, and was for a long time friendly to the US/Birmingham and the Latin Union, although the more democratic nature of its government (for whites, that is) prevented it from becoming a full member of the Paris League. Over the last quarter century, however, south Africa has made a slow and often painful transition to majority rule, and it is now more democratic than some members of the Alliance (I'm looking at you, North Haiti). Although the west and southwest are heavily desertified, the east - especially Mozambique - still provides some healthy agricultural production, and South Africa is a net food exporter.
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World population declined sharply after the event, and recovered only slowly: world population is only about 3.5 billion in 2049. It's a multipolar and diverse world, with relatively weak "great powers" rather than superpowers. No single nation really has global reach. Society is more conservative than OTL, with children, family, and social order being more central to people's thinking. It's a leftier world: even the *Fascistic nations see the importance of Big Government to make sure the people are fed, dry lands are irrigated, resources obtained, and infrastructure maintained. European derived nations are more religious: God seems more necessary in a world where modern civilization can be smashed like a egg-crate by a random eructation of the planet's innards.
Late 40s' technology was mostly retained, but much of its full application could not be carried out due to limited resources: it was a while before either US could restart their nuclear program, or the USSR invest in rockets rather than irrigation canals and artificial fertilizer. However, as the world has slowly adapted to new conditions, populations have increased again and the economy has expanded: real investment in scientific research and technological development became practicable again by the 2000s, and by 2049 some fields of technology (medicine, chemistry, biotech...) are on a 1970s level.
Rocket technology is retarded, the development from the V-weapon having been interrupted. Currently there are a few satellites in orbit, and there are international conventions against putting atomic weapons on long-range missiles: alas, the atom bomb was re-invented before too long, and there are eight nuclear powers, although as yet arsenals remain fairly small. Due to the Soviet occupation of the middle east and the inaccessability of various ice-bound oil fields, the world actually uses more atomic power as a percentage of power production than ours does.
Of course, lots of coal is also burned: indeed, rewarming the planet through increased CO2 has been suggested since the 1980s, and the coal industries make a lot of advertising hay re the patriotism of burning more coal. Some people worry loudly re the effects on the planetary environment of acid rain, ocean acidification, etc: they are generally loudly shouted down, for although it's no longer getting colder, it is still getting drier as more and more moisture is locked up in the growing glaciers.
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Currently, international tensions are spiking as Latin ally Hindustan invades the Republic (hah!) of South India on some rather feeble excuse: everyone knows that Hindustan intends to reunite India by hook or by crook, and it looks like they've decided on a big, heavy, spiky crook. The Soviets, against which Hindustan has some major territorial claims, are rather outraged.... will there be a third world war?
Bruce