A Year Without Summer
The agricultural hardships of 1816 are, for the most part, long-forgotten to general history but at the time were a severe crisis that tested the post-Aix order.
[1] Historians now trace the climatic anomaly to the eruption of Mount Tambora in the British East Indies the previous April (along with a number of smaller previous eruptions), thrusting a cloud of ash and dust into the atmosphere that dimmed the sun and dramatically cooled the Northern Hemisphere for much of the following year, not only preventing a proper summer growing season but also ashy snow and strange fogs. Rainfall was abnormally higher, and massive floods - particularly on the Rhine - wiped out entire farms and towns, and the clammy, cold conditions made conditions ripe for particularly severe typhus epidemics, which struck the damp British Isles disproportionately hard. Coming at the tail end of several years of lean crops, food shortages erupted across much of Europe, triggering an important demographic episode as the worst famine to strike the continent in the 19th-century occurred.
While contemporary art captured the dismal time eloquently, the impact of the Year Without Summer was mostly on helping supercharge two important population shifts that would accelerate and compound in the Napoleonic world. The first was a general move to the cities. With farms struggling, young men and women - particularly those not born first - migrated to cities that early on could not absorb them that quickly. Work was scarce, as was food, and violent crime skyrocketed, as did prostitution and burglaries. In Cologne, food riots forced city leaders to flee under cover of night; in Warsaw, curfews were imposed to prevent unrest. But though 1816 was an hour of misery, these new arrivals to the cities triggered a wave of urbanization across west-central Europe, particularly in France, that would help jumpstart the continental Industrial Revolution. The other major change triggered was a major rise in emigration, either to the Americas, or in the case of the Dutch fleeing some of Europe's most impoverished hinterland
[2] to the Cape. The postwar emigration boom, as birth rates jumped and Europe's population grew enormously, can be traced back to the strange summer of 1816. (Of course, many emigres found their way to a North America experiencing the same bizarre climatic episode, but those who arrived in Brazil saw no such events and Rio de Janeiro thus for many years earned a reputation for sunshine and plentiful land that the United States did not).
In addition to the demographic and economic changes triggered by 1816, it also helped introduce another one - state interventionism. In France, Napoleon ordered tangible aid be made to those without food or work, inaugurating a
Bureau des Pauvres which worked to alleviate cases of dire poverty, whether it be coordinating food shipments throughout the country or dispatching the unemployed to build housing on the periphery of the cities for new residents. Compared to modern social welfare schemes, the
Bureau des Pauvres was fairly threadbare in structure and in assistance, but by the standards of the day, it was a revolutionary innovation and another step on the path of revolutionizing the relationship between government and citizen already underway in France and, soon enough, the rest of Europe...
[1] I'll preface this, though, that Europe economically and demographically is much better off ITTL without all the fighting between 1808 and 1814 that bled the continent IOTL, especially France. So the post-Napoleonic calamity that struck an exhausted continent is nowhere near as severe.
[2] The post-Napoleonic Netherlands outside of Amsterdam/Rotterdam were some of the poorest parts of Europe