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One way to explore the way that climate is changing now is to learn ways that it has changed in the past. So consider this cataclysmic event from almost two centuries ago. That's when the world experienced the biggest volcanic eruption ever recorded in human history. It killed approximately 100,000 people in Indonesia. And that was just the beginning. It affected climate around the world. That eruption is the latest subject of Climate Connections, our series with National Geographic.

NPR's Michael Sullivan reports.

MICHAEL SULLIVAN: Alain Vauthier owns Chateau Ausone in St. Emilion, France. It's one of the oldest vineyards in Bordeaux and a place where they like to keep a fair bit of wine from each vintage in the cellar. It's an impressive collection. But there are only a few bottles from 1816. And that, Vauthier says, is as it should be.

Mr. ALAIN VAUTHIER (Owner, Chateau Ausone): It's not a good vintage. We keep only for the history of the vineyard. It's a bad time, bad weather, bad summer, 1831, 1947, '49, not very good, 1816, no.

SULLIVAN: Daniel Lawton is the owner of Bordeaux's oldest wine brokerage house. His assessment of the 1816 vintage is even less charitable.

Mr. DANIEL LAWTON (Wine Brokerage House Owner): Detestable, you understand? Horrible year. A quarter of a normal crop, it was very difficult to make good wine.

SULLIVAN: A bad year for wine and even worse year for a continent trying to cope with the aftermath of the Napoleonic Wars.

Dr. PATRICK WEBB (Dean, Academic Affairs, Tufts University's Friedman School of Nutrition Science): You had mass demobilization of large numbers of soldiers flooding on the labor market and you had economies disrupted, infrastructure damaged, governments in limbo, and so the conditions were already ripe for something to go wrong.

SULLIVAN: And, says Patrick Webb, something did go wrong - very wrong. Webb is a dean at Tufts University's Friedman School of Nutrition Science. He describes what happened during what's now known as the year without summer. When temperatures drop, crops failed and people starved.

Dr. WEBB: Hundreds of thousands of people died. People were reduced to eating rats and fighting over roots. And, you know, so most of these people were killed by epidemic disease related to the starvation. They simply couldn't find enough food.

SULLIVAN: In the U.S. it snowed in New England well into the summer where the average temperatures in July and August of 1816, Webb says, were five to ten degrees below normal. And all of this was triggered by something that happened more than a year earlier on the other side of the world.

(Soundbite of people shouting)

SULLIVAN: That something occurred here, on the island of Sumbawa in the middle of the Indonesian archipelago.

Professor HARALDUR SIGURDSSON (Volcanologist, University of Rhode Island): That's Tambora. That's the volcano that has the greatest historical eruption on Earth in 1815.

SULLIVAN: That's Haraldur Sigurdsson, a volcanologist at the University of Rhode Island. He knows more about Tambora than anyone and has been coming here for the past 20 years collecting data on what he says is the biggest and most overlooked eruption in recorded history - 10 times bigger than Krakatao, more than 100 times bigger than Vesuvius or Mount St. Helens.

Prof. SIGURDSSON: The eruption went up about 43 kilometers up into the stratosphere. That's was about 30 miles and emitting a volume that is about 50 to 100 cubic kilometers of molten rock in the form of ash and pumice. So that volume is by far the largest volume of any volcanic event on Earth.

SULLIVAN: But it was the enormous cloud of gas released by the eruption, some 400 million tons of it, Sigurdsson says, that produced the year without summer.

Prof. SIGURDSSON: That gas, it reacts with the water vapor in the atmosphere and it forms tiny little droplets of almost pure sulfuric acid. And those tiny droplets are suspended in the stratosphere. It is a veil around the Earth, back scattering some of those on radiation, acting like little mirrors bouncing back radiation back into space, decreasing the amount that reaches the Earth's surface.

SULLIVAN: The amount of heat that reaches the Earth.

Prof. SIGURDSSON: The amount of heat, right, causing global cooling.

SULLIVAN: Of course, no one really knew that at the time and few people know about it now. It wasn't until the early 1980s, Sigurdsson says, that he caught the Tambora bug, when researches taking core samples in the ice in Greenland made an amazing discovery.

Prof. SIGURDSSON: You drill down into - through the ice and you can count the rings just like in a tree. And people started to do chemistry on those layers. And they'd found out that there was a (unintelligible) great sulfur concentration of one particular layer: 1816. And that was the first evidence really that Tambora had a global reach but it was understudied and we needed to get much more information on what really happened here.

(Soundbite of digging)

SULLIVAN: He's been gathering that information ever since. His task is made easier, he says, by the scrupulous recording keeping done by the earth itself. The layers of the soil here not unlike the layers of ice in faraway Greenland.

Prof. SIGURDSSON: Each layer is like a page in a book. These layers that we see here, they're really a graphic representation of the eruptions. So they're writing down for us the history of the volcano and they don't lie.

SULLIVAN: While he was digging, Sigurdsson discovered something else, what he calls the Pompeii of the East - the Lost Kingdom of Tambora.

(Soundbite of shovel)

Mr. SIGURDSSON: I studied the deposits in Pompeii and Herculaneum - the same mode of destruction, the same mode of the death of the people. The difference here is that the human remains are much more carbonized. They are also entirely carbonized. The bones are piece of charcoal, not much higher temperature, much hotter.

SULLIVAN: And yet, all the big eruptions - Tambora, Krakatao, Pinatubo - have ended up cooling the earth, causing temperatures to drop. And that, Sigurdsson says, has got some people thinking about replicating the Tambora effect in an effort to slow global warming.

Prof. SIGURDSSON: People have proposed that we should induce artificial volcanoes by bringing sulfur up into the stratosphere to produce this effect. But hey, do you want to counteract one pollution with another one, I don't think so.

SULLIVAN: But there are still much to be learned from eruptions like Tambora, Sigurdsson says - information that can be used in models being generated for studying global climate change. Global warming is viewed by many as the most pressing, most dangerous threat. But Sigurdsson warns that catastrophic climate change might come from an unexpected yet familiar direction.

Prof. SIGURDSSON: Somewhere on the Earth, within the next thousand years, there will be a comparable eruption. And we'd better be aware of the consequences of such an event.

SULLIVAN: Aware, he says, and prepared.

Prof. SIGURDSSON: Eruption like this occurring today would cut out telecommunications globally. Interferes with the particles in the atmosphere, and the aerosol would also cut out much of the aviation, and societally(ph), a huge political impact and economic impact. No one really has attempted to evaluate what the impact would be of this type of eruption on modern society. That's big issue to be studied.

SULLIVAN: Sigurdsson hopes to study and learn more about Tambora when he returns next year.

Michael Sullivan, NPR News.

INSKEEP: You could take a virtual tour of the Tambora excavation site at npr.org/climateconnections.

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