There is a sweeping arc to human history, but it is punctuated by irregular blips marking events that sharply changed things for peoples and nations. These include natural disasters like volcanoes, earthquakes, hurricanes, drought and floods. They may be pandemics or technological discoveries like the cultivation of rice, invention of the steam engine, vaccines, integrated circuits or genetic engineering. These events may have demographic and social or cultural effects and usually economic ones. Some are immediate, some take longer. Some are permanent, some passing. Some alter current income or production; others alter wealth or net assets. Take low-rainfall years like 1988 or excess rainfall ones like 2019. For both, farm output fell sharply. Farms produced less, and gross domestic product was lower than otherwise. Ag machinery sales fell. Farmers lost net worth. But the effect largely was on income and not on assets. Several dry years and bad crop practices caused the Dust Bowl, coinciding unfortunately with the Great Depression. It cut farm output, GDP and incomes. It changed farming practices, population densities and rural economies permanently. Large migrations from affect...