Seven hundred years ago this month, people across northern Europe saw a comet in the sky and feared the worst. They were already running out of food. It had rained too much in 1315—sometimes every day for weeks at a stretch. Wheat, barley, and oats rotted in the fields, and it was too wet to make hay. Then, after an unusually cold winter, the rains started again, and the 1316 harvest failed, too. Grapes in vineyards were covered with a fuzzy mildew, and, one observer wrote, “there was no wine in the whole kingdom of France.” There wasn’t much bread, either. The historian William Chester Jordan, in his book “The Great Famine,” recounts how Parisians first put to the wheel and then exiled a group of bakers whom they accused of bolstering their loaves with waste. Across the Continent, there was also a severe shortage of salt—used to make cheese and to preserve food—since there was not enough sun to dry the salt pans on the Baltic and North Sea coasts. In 1317, the rains came again. Storms washed away not only newly planted grain—which was already scarce, because farmers had begun eating their seed corn to survive—but also topsoil and dikes. Sheep and cattle, standing in cold, muddy pastures, began dying of infection. People died, too, from malnutrition and illness.

In some regions of Europe, the Great Famine of 1315-17 killed a tenth of the population, shattering social norms and local economies. Villages were abandoned, religious houses were dispersed, and minor feudal lords pawned their land to whoever could pay. Peasants and the urban poor were left to fend for themselves. And yet the Great Famine is not as well known as it might be; William Rosen, the author of “The Third Horseman,” calls it “the famine history forgot.” In part, this is because of what followed it: the Black Death, which reached Europe in 1347 and killed a third of the population; and the Hundred Years’ War, which was fought between 1337 and 1453, and was as brutal a slog as it sounds. Those catastrophes, though, were visited on a population that had been left physically weak and divided by the famine, which, in turn, increased the damage they did. If nothing else, the hard times of seven centuries ago demonstrate that hunger has both moral and political costs.

Another reason the Great Famine has been passed over is that its origins seemed so prosaic, compared with the dynastic struggles of the time. It’s a story about the weather. The famine, as Rosen notes, occurred almost precisely at the dividing point between what is known as the Medieval Warm Period (M.W.P.), which was three centuries of fairly mild weather in Europe, and the Little Ice Age, which lasted until about 1850. But even just half a century ago some historians treated the rain as incidental to the famine. For them, the problem was that Europe’s population had grown too quickly, and the peasants irresponsibly farmed land so marginal that the slightest disruption could be disastrous. The famine was a Malthusian issue, as those historians saw it, or one of agricultural practices, not primarily a climate-related disaster.

More recently, politicians who think that worrying about climate change is a waste of time have pointed to this period as proof that there is nothing strange about things getting a little warmer every now and then. The M.W.P. is a particular obsession of Senator James Inhofe, of Oklahoma, the chairman of the Environment and Public Works Committee, who has said that scientists have ignored it in their attempt to perpetrate the “hoax” of climate change. One problem with that position is that, while parts of Europe were warmer during the M.W.P., the world as a whole was not. For this reason, some scientists prefer to call it the Medieval Climatic Anomaly. And today, it appears, the world as a whole is warmer.

One of the most important insights of recent studies is that, when the climate changes, it can do so swiftly and relentlessly. It is possible, in a human lifetime, to see sea levels rise and ice shelves break away, and, when they do, nothing about what happens next can be taken for granted. The climate record is full of sudden disasters. Studies have also clarified some of the mechanisms of the relationship between climate and shorter-term weather; a 2010 report on the M.W.P., published in the journal Climate Dynamics, looked at the connections between the rain in Europe, the temperature of the Indo-Pacific warm pool, and the flood levels of the Nile. The Great Famine looks like a fourteenth-century example of what we now call extreme weather. We are also learning how, in our own time, changing ocean temperatures can cause shifts in El Niño, the name given to a collection of weather patterns that originate in the Pacific and stretch across the globe; a “Godzilla El Niño” is credited for the oddly warm weather in the Northeast this winter. We have built cities and economies on assumptions about the seasons that may prove unstable. The best models we have now project that, as a consequence of climate change, the frequency of extreme-weather events, from superstorms to droughts, will increase sharply.

A particularly alarming prospect is the sustained failure of the South Asian monsoon. The food supply for more than a billion people relies on the rains of the monsoon season. Models suggest that, in the next century, monsoons will become more and more erratic and extreme. A failed monsoon can mean that the rain hasn’t come, or that it has come in the wrong place for the wrong amount of time. In recent years, India has experienced droughts but also floods, like the one that wreaked havoc in Chennai in December. Last year, in a report on possible monsoon failures, The Economist noted that “immense cloudbursts in Uttarakhand killed over 6,000 people in 2013.” And India’s polluted, particle-heavy air can make the rain fall harder.

At the outset of the Great Famine, when peasants first took the measure of their ruined fields, many of them thought that they were alone—that the disaster was confined to their area. Soon, though, as travellers returned or hungry armies passed through, and when the peasants themselves went to market, and saw how steeply prices were rising, they learned the extent of it. “The whole world was troubled,” a chronicler in Salzburg wrote. Many peasants took to the road, joining a migration to cities, only to die there and be buried in mass graves. In Paris, priests led barefoot worshippers in processions meant to show contrition. Seven hundred years later, scientists armed with climate models met with politicians in the same city. They didn’t need to watch for signs and wonder, as in the days of the comet of 1315-16. They knew what was coming. ♦