Throughout history, volcanoes have been the source of many cataclysmic disasters.

Their eruptions can cause  all the things you expect: explosive destruction, catastrophic  ashfall, and devastating wildfires, not to mention whole swathes of land  getting swallowed by lava and debris. But one thing volcanoes aren’t  usually responsible for… is pandemics.

Except for the time they might  have caused the Black Death. Because scientists now think volcanic  eruptions could have touched off a cascade of dominoes that  lead to a deadly bacterium killing up to half of the  population of Medieval Europe. [♪ INTRO] Beginning in 1347, the Black  Death swept across Europe, killing an estimated 50 million people. Now granted, this was Medieval  times, but that’s about seven times the worldwide death toll from COVID-19, and comprised up to half of the total  population of Europe at the time.

The deadly pandemic was an outbreak  of plague, an infectious disease which causes fever, painfully swollen  lymph glands, and black necrotic tissue – hence the ‘Black’ part of the name. The infection also affects the blood, lungs, and can move through the body so fast that  it causes death in just hours or days. But it was another 550 years before anyone figured out what was actually doing the killing.

A 19th Century physician  studying a new plague outbreak in Hong Kong found the tiny bacterium  Yersinia pestis to be the culprit. And in 2011, DNA analysis of  skeletons found in plague pits confirmed that Yersinia pestis was the  killer in the Black Death pandemic, too. The bacteria is normally carried by animals, so it’s thought that they were transferred  to humans via fleas that infected wild rats.

More than 650 years later,  I still feel like this is what fuels our collective hatred and fear of rats. Most recently, in 2022, the particular  strain of Yersinia pestis that caused the Black Death has been traced to  Kyrgyzstan, and was known to have made it’s way across Asian trade  routes to the Black Sea by the 1340s. But it’s still not clear how the  bacterium made the jump to western Europe, why it spread so quickly,  and why it was so deadly.

But now, a 2025 study claims  to have found an answer, using a combination of  climate data from tree rings, atmospheric compositions preserved in ice cores, and historical written records from  chronicles, treaties and even poetry. Together, these clues point to a chain  of events like a carefully constructed line of toppling dominoes,  which starts with a seemingly unrelated natural disaster,  halfway across the globe. Naturally, the first domino we’re going  to talk about is a volcanic eruption.

Now, written records from this  time aren’t all that complete, and there’s no definitive historical account matching the eruption the researchers describe. However, scattered writings from all  over the world, including Japan, China, Germany, France, and Italy  report unusual cloudiness in the years before the Black Death. Some records also mention dark lunar eclipses.

During a lunar eclipse, when the  Earth casts its shadow on the moon, light filtering through the  atmosphere makes the moon appear red. A darker eclipse implies that  less light than usual is making it through to illuminate the moon,  suggesting there’s more stuff in the atmosphere getting in the way. Like particles from a volcanic eruption.

Now all of this would be fairly circumstantial, if it wasn’t for a smoking gun discovered in ice core records from Greenland and Antarctica. In these cores, which are drilled  vertically down through the ice caps, there are layers that correspond  to each year’s snowfall. Layers of snow are buried, year on  year, turning to ice and sealing in a chemical record of what was  going on in the atmosphere that year.

And, for a number of years  preceding the Black Death, researchers found spikes in the concentration  of sulfur dioxide in the atmosphere, as recorded in the icy layers. In pre-industrial times, the main source  of sulfur in the atmosphere was volcanoes, and since its concentration in the  ice corresponds to the concentration in the atmosphere, these pulses  can be used to pinpoint the time, and to some extent the size,  of past eruption events. Researchers found small sulfur spikes  in layers from 1329, 1336, and 1341, followed by a really big one in 1345, two years before the Black  Death took off in Europe.

That last one represents around 14 million  tons of extra sulfur in the atmosphere, which has the potential to cause many of the phenomena reported in written records. For comparison, when Mount  Pinatubo in the Philippines erupted in 1991 it ejected  about 6 million tons of sulfur. So at least by ice core-reckoning, the 1345 event was more than twice the size of Pinatubo.

The researchers don’t actually know  exactly where the eruption happened, but the global distribution of sulfur  implies it was somewhere near the equator, which would have allowed the volcanic gases to move north and south in equal quantities. One possible candidate is  Mount Rinjani, in Indonesia, which is known to have erupted and produced a big sulfur spike a century earlier, in 1257. If that volcanic region  had continued to be active, then one big eruption or several  smaller ones could have produced the sulfur we see in the ice, and the  global reports of changeable skies.

Wherever it was, though, that  volcanic outburst set the stage for the second domino to fall, in the  shape of a volcanic winter and famine. The sulfur dioxide from volcanic  eruptions is an aerosol in the atmosphere, which scatters and reflects  sunlight, causing temporary cooling. Mount Pinatubo’s eruption, with  its 6 million tonnes of sulfur, cooled the planet by half a degree  Celsius in the year after the eruption, temporarily bucking the global warming trend.

So 14 million tonnes from a 1345 eruption could be expected to have an even greater effect. And researchers found evidence  for such a cooling event in the wood of trees all across Europe. In the study of tree rings,  known as dendrochronology, each ring in a slice through the trunk  represents a year of that tree’s growth, much like the layers of ice in cores.

This time, though, it’s the size and structure of the rings that gives  information about past conditions. In northern Spain, tree slices  from this period consistently have what’s known as blue rings. Now these rings aren’t  actually blue to the naked eye, but rather show up when  microscope slides are prepared.

Paper-thin wood samples are  stained with a special combination of chemicals to show their  structure and chemistry. In this, normal wood appears bright  pink, while abnormal layers show up blue. Blue rings indicate plant cells with  little to no lignin in their cell walls.

And since lignin is the tough  fiber that makes wood stronger than normal plant stems, it’s as if  these layers were just never finished. Scientists aren’t sure exactly what  makes trees leave behind blue rings, but in more recent records they have seen a link between these structures and cold summers. And while it’s fairly common to  have one anomalously cold summer, several together is much more  rare, and it hints at something bigger going on in the atmosphere.

And that is consistent with  other data from tree rings. In thousands of trees measured across Europe, rings between 1345 and 1347 consistently have lower density wood than usual  from the summer growing season, suggesting cold temperatures and  non-ideal growing conditions. And it wasn’t just the trees that suffered  from these cold, volcanic winters.

The researchers believe  that the lower temperatures would have disrupted normal patterns  of rainfall all across Europe. And written accounts from this time confirm this, with reports of exceptionally cold,  wet summers between 1345 and 1347, leading to poor harvests and crop  failures across southern Europe. Before long, people in the  Mediterranean were facing famine, which was compounded by the fact that many of these populations  had been drawn to big cities.

Growing urbanization meant that the  Italian city-states like Venice, Genoa, and Pisa, were so densely populated  that they couldn’t be self-sufficient. There wasn’t enough space to expand nearby  agriculture to support their citizens, and so they had to rely on grain that  had been shipped in from further afield. Usually, they’d use maritime trade routes  to import their grain from southern Italy, Sicily, Sardinia and northern Africa.

But when all of these crops failed, it left a lot of people with  not a lot of food to go around. But before our third domino falls,  we need to pause for a quick break. Thanks to Storyblocks for  supporting this SciShow video!

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That discount is only available  through the link in the description. Now, we arrive at our third domino. In the past, a famine like this  would have meant that many, many people would have died by starvation.

But these late-medieval Europeans were  far too sophisticated to starve to death. The key was those Italian city states  and their overseas trade links. The cities had been cultivating a  relationship with the Mongols in central Asia for the last 100 years, and now their governments called on those contacts to save their citizens.

Further north, the Asian grain-producing  regions are thought to have not been so severely affected by the climatic downturn. And there were already established trade routes between Italy and the Sea of Azov  in the north of the Black Sea. Ok, sure, so the two peoples were in  the middle of a war while all this was going on, but the famine forced them  to negotiate a ceasefire.

Funny that. Trade routes were reopened,  and grain began to flow again. The Italians were saved from starvation, but unbeknownst to everyone, something  else was lurking on those grain ships.

Our final domino: the arrival  of the plague in Europe. Those ships that carried grain from the Black Sea also carried fleas infested with Yersinia pestis. It’s likely that these fleas  didn’t even need rats as a host, which may have been ousted from  the ships if found in the grain.

Instead, the tiny insects survived the  journey feeding directly on the grain dust. The Mongols had been dealing with  their own outbreak of plague, after that new strain from Kyrgyzstan we  talked about earlier had spread over land. But at the time, nobody knew what caused  the disease, or how it was transmitted, so the Italians had no way of understanding  the risk of trading with Asia.

The ships arrived, the grain was unloaded, and the infected fleas spread to rodents, domestic animals and, before long, humans. When you look at the timing and locations  of the first Black Death outbreaks, the link to the Mongol grain seems clear – whenever grain shipments arrived, the  first cases of plague soon followed. For instance, in Venice, the first cases occurred less than 2 months after the grain shipment.

And when there was enough surplus for  grain exports to resume from Venice to another Italian city, Padua, we  see a plague outbreak there, too. Meanwhile, larger Italian  cities like Milan and Rome were more agriculturally self-sufficient,  and dedicated grain-producing regions like Verona and Ravenna managed  to get by without importing grain. And all of these places were protected  from the first wave of plague.

And so, the Black Death in Europe was as much a product of global circumstance as  it was an infectious bacterium. Chance had set up these  medieval dominoes just right: natural events, global climate,  demographic distributions, and socio-economic responses were aligned in a perfect storm to create the  worst of all possible outcomes. In fact, this is an early  example of how globalization can be both a blessing and a curse.

Those far-reaching trade links helped save everyone in northern Italy from starvation. But they were ultimately responsible  for killing just as many, if not more, with the plague. [♪ OUTRO]