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2016 is the 200th anniversary of the so-called “year without a summer.” This post was originally published in 2015.

Two hundred years ago, a massive volcanic eruption in Indonesia rocked the globe. Ash shot miles into the sky when Mount Tambora exploded. A wall of hot gas and rocks sped down the mountainside and destroyed the city of Tambora. The ash that was lofted into the atmosphere spread far across the Northern Hemisphere and left the Earth in the midst of a year without a summer.

[Photo gallery: Colbuco volcano erupts in Chile]

Volcanic eruptions are climate wild cards. No eruption illustrates that more than Mount Tambora, which blew its top in April 1815. Tambora’s blasts and the tsunamis they caused killed an estimated 92,000 people, including those who starved to death because the explosion of debris killed livestock and crops.

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Though no one realized it at the time, we now know that Tambora’s eruption was the major cause of the following year, 1816, being cooler than average across the Northern Hemisphere.

Tambora’s ash-cloud of sulfur-dioxide shot more than 20 miles high into the atmosphere — higher than where our day-to-day weather takes place — to a layer called the stratosphere. Weather and winds are very stable in the stratosphere, and once something gets to that level, it takes a long time to “wash” it out.

The newly-formed, sunlight-absorbing cloud spread far and wide around the globe, preventing the sun’s rays from warming the surface of the Earth. The average temperature of the globe dropped a mere 3 degrees, but that small change proved dramatic for not only Earth’s climate but for life in the Northern Hemisphere.

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[Chilean village a ghost town after Calbuco’s blasts]

Some of the worst effects, including summer-time frost, impacted the eastern U.S. as far south as Virginia. In Europe and Great Britain the volcano’s effects included not only the coolest summer on record, but day after day of cloud-covered skies and above normal rainfall, which caused widespread crop failures, outbreaks of disease and famine.

Thus the year was marked in the climate annals as the Year Without a Summer.

In addition to its effects on weather, the Tambora eruption triggered literary side effects that are with us today.

During the summer of 1816, the poets George Gordon Lord Byron, Percy Bysshe Shelley, his wife-to-be, Mary Wollstonecraft Godwin, Mary’s stepsister Claire Clairmont, and Bryon’s physician, John William Polidori, spent that summer on Lake Geneva in Switzerland.

“At first,” the future Mary Shelley wrote, “we spent our pleasant hours on the lake or wandering on its shores. But it proved a wet, ungenial summer and incessant rain often confined us for days to the house.” The group passed the gloomy days reading and talking about German ghost stories. Byron suggested a ghost story writing contest, which encouraged Mary to write about a “Dr. Victor Frankenstein,” who creates a monster that he brings to life. Shelley encouraged Mary to enlarge the story into a novel, which was published in 1818 as Frankenstein or The Modern Prometheus.

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Polidori wrote a short story, The Vampyre, a prototype of the long line of vampire stories that are still very much with us.

While no one in 1816 realized that Tambora had global effects, another huge Indonesian eruption — Mount Krakatoa on Aug. 23, 1883, which killed an estimated 36,000 people — was evidence that what happens around a volcano doesn’t stay around the volcano.

By 1883, the world was connected by undersea telegraph cables and overland telegraph lines. Within hours of the Krakatoa eruption, news spread around the globe, including to the community of professional and amateur sky watchers, who in the weeks and months after the eruption shared their observations.

Krakatoa’s ash — tiny pieces of rock the blast had pulverized — first spread around the tropics and then slowly north and south. By November it was in the sky as far north as England, where the poet Gerard Manley Hopkins wrote of a green, blue, copper, and magenta sky: “… more like inflamed flesh than the lucid reds of ordinary sunsets… the glow is intense; that is what strikes everyone; it has prolonged the daylight, and optically changed the season; it bathes the whole sky, it is mistaken for the reflection of a great fire.”

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Krakatoa’s effects on global weather weren’t as pronounced as those following the much larger 1815 Tambora eruption. But in 1884, the average Northern Hemisphere summer temperatures fell by as much as 2 degrees. Some regions recorded unusual weather, such as the record amounts of rain that fell in California during the July 1, 1883 through June 30, 1884 water year. The 38.18 inches of rain that fell in Los Angeles during this period remains the city’s wettest on record.

The global spread of Krakatoa’s ash gave atmospheric scientists their first clues that high-altitude, high-speed winds (what we now know as jet streams) circle the globe.

By the early 1980s, some scientists were thinking that sulfur dioxide in volcanic eruptions, not the ash, causes the widespread climatic effects of some volcanoes. Their hypothesis was that the sulfur dioxide a volcano shoots high into the air combines with water vapor to form sulfuric acid, which quickly condenses into tiny sulfate aerosols. As winds carry these aerosols around the earth, they absorb some solar radiation, which warms the stratosphere where absorption is occurring while cooling earth’s surface below.

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Changing the locations of areas of warm and cold air affects patterns of high and low atmospheric pressure from the stratosphere down to the surface. These changes, in turn, affect the speeds and directions of winds aloft and at the surface with related changes in temperature and precipitation patterns.

The 1982 eruption of the El Chichon volcano in Mexico, in conjunction with the 1980 eruption of Mount St. Helens helped confirm the hypothesis. While the Mount St. Helens eruption had lowered global average temperatures by roughly 0.1 degrees Celsius, the much smaller amount of ash from El Chichon cooled the globe three to five times as much.

Data from NASA’s Total Ozone Mapping Spectrometer (TOMS), which had been launched in 1978, showed that sulfur aerosols probably were the culprit. TOMS, which measures other atmospheric gases as well as ozone, showed that El Chichon produced 40 times as much sulfur dioxide as Mount St. Helens.

In addition, while a volcano’s ash falls out of the stratosphere within days or at most weeks, the veil of sulfuric acid takes months to precipitate out. It has more time to affect climate.

Ash from each volcano has a unique signature based on the different amounts of various kinds of rocks the blast shattered to create the ash. Scientists who measured amounts of sulfur dioxide found in ice cores in Greenland, Antarctica, and various glaciers found have found that “signatures” of Tambora, Krakatoa and some other major volcanoes correlate well with subsequent, short-lived climate changes.

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The June 1991 eruption of the Mount Pinatubo volcano in the Philippines left no doubt that sulfur dioxide from volcanoes is the primary way they affect climate. Pinatubo was the 20th century’s second-largest eruption, behind only the 1912 eruption of Novarupta on the Alaska Peninsula in 1912.

Its sulfur dioxide cloud was larger than the one from El Chichon or any other volcano since the TOMS satellite was launched. Pinatubo’s cloud cooled the Earth’s surface by almost one degree Fahrenheit. In its 2007 report the Intergovernmental Panel on Climate Change called Pinatubo the “best-documented explosive volcanic event to date… The growth and decay of aerosols resulting from this eruption have provided a basis for modeling (the effects) due to explosive volcanoes.” In the United States the summer of 1992, was the third coldest and third wettest in 77 years.

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