No one can say for certain, but odds are good that much of the country won’t get a white Christmas this year. Temperatures have remained unseasonably mild for weeks. Boston recorded its ninth-warmest November since 1872. Minnesota farmers had their best growing season in 50 years due to the long autumn. Coatless New Yorkers practiced yoga in Central Park.

Experts say the blame for this unwinter weather belongs not to climate change but to a shift in air pressure at the North Pole. The “polar vortex” that buried the East Coast in snow last winter has so far been kept at bay by a phenomenon known as Arctic Oscillation. El Niño, the cyclical warming of the surface temperature of the Pacific Ocean, is also doing its part to make mid-December feel like mid-April.

Warm weather is forecast to last through Christmas and well into the New Year, raising an intriguing possibility: could this be the year without a winter? It would be a welcome prospect in cities where municipal snow-removal budgets were busted early and often in 2015; less so among owners of ski resorts and snowmobiling businesses. Students of history, however, will note a climatic coincidence: 2016 marks the 200th anniversary of what has come to be known as the year without a summer.

On April 5, 1815, Indonesia’s volcanic Mount Tambora awoke from a centuries-long slumber. Over the course of several days, repeated thunderous explosions from the 14,000-foot mountain hurled massive amounts of molten rock, ash, and dust into the sky above the island of Sumbawa. The blasts were so intense that British naval officers hundreds of miles away presumed a mighty battle was underway. Ten thousand people in the village of Tambora were killed instantly. Tens of thousands more on Sumbawa and surrounding islands ultimately died from starvation. On April 10, a mighty final explosion tore the top off Tambora and triggered a tsunami. An ash cloud of unprecedented size rose from the site of the eruption, blotting out the sun and causing temperatures in the region to plummet. Along with the ash cloud, Tambora had belched an estimated 55 million tons of fine sulfur particles and sulfur-dioxide gas into the stratosphere.

As William and Nicholas Klingaman describe in their 2013 book The Year Without Summer, when Tambora’s sulfur-dioxide gas reached the stratosphere, it mixed with plentiful hydroxide gas in a chemical reaction that created as much as 100 million tons of sulfuric acid. Condensing into minute droplets, the acid became aerosolized and formed a massive cloud. Hitching a ride on the mighty jet stream, the cloud began spreading. “By the winter of 1815,” write the Klingamans, “the nearly invisible veil of ash covered the globe, reflecting sunlight, cooling temperatures, and wreaking havoc on weather patterns.”

News of the colossal eruption traveled painfully slow in the pre-telegraphic era. The world was left to wonder about the strange weather over the ensuing months. Londoners gaped in awe at the unmatched beauty of that summer’s sunsets, unaware that the glorious hues and glowing evenings were likely caused by the Tambora cloud’s stratospheric dispersion of the sun’s rays. That winter, blizzards dumped red- and brown-colored snow on the mountains of Europe.

In the United States, the summer of 1816 announced itself as truly unusual on June 7. In the heavily populated northeast, temperatures hovered around freezing all day. In Ohio and Pennsylvania, budding fruit trees succumbed to the frost. At higher elevations in New York State, snow covered the ground. Migratory birds fell dead. Recently shorn sheep died when they could find no forage on the frozen ground.

Weird weather persisted all summer and into the early fall. High winds kept the air dry. Drought caused wells to fail, streams to dry up, and wildfires to rage. August felt more like October, with freezing temps and heavy frost killing crops from Maine to South Carolina. Farms failed across New England, leading many who relied on the land for a living to pick up and move west.

Old timers declared there had never been anything like it in living memory. “We have had the most extraordinary year of drought and cold ever known in the history of America,” wrote 73-year-old Thomas Jefferson to a former colleague. Theories abounded. Some blamed sun spots. Others chalked it up to a change in the flow of “electrical fluid” between the atmosphere and the earth’s surface. Some said it was all in God’s hands.

Then, as now, some claimed that the climate was changing permanently and irreversibly. The real culprit was Tambora’s aerosolized sulfur cloud, which would stay draped over the earth like a curtain over a bird cage for the better part of 1817. “[T]he thin veil of sulfuric acid droplets continued to affect weather patterns for at least another two years,” the Klingamans write.

By the summer of 1818, things had leveled out. The earth’s climate had proved its resiliency, though the truth of what happened would not be confirmed until the late twentieth century. In the unlikely event that 2016 turns out to be a year without winter, it will surely add fuel to the debate over humanity’s contribution to global warming. Is it our fault that the climate is changing? Will we be able to adapt to a warmer climate? While those who claim to know the answers to these questions are legion, the lesson of the year without summer is that, when it comes to the earth’s complex climate, only hindsight is 20/20.

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