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How Volcanoes Caused The Black Death
How Volcanoes Caused The Black Death
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0:00
Throughout history, volcanoes have been
0:02
the source of many cataclysmic disasters.
0:04
Their eruptions can cause all the things you expect:
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explosive destruction, catastrophic ashfall, and devastating wildfires,
0:11
not to mention whole swathes of land getting swallowed by lava and debris.
0:15
But one thing volcanoes aren’t usually responsible for… is pandemics.
0:20
Except for the time they might have caused the Black Death.
0:24
Because scientists now think volcanic eruptions could have touched off
0:27
a cascade of dominoes that lead to a deadly bacterium
0:31
killing up to half of the population of Medieval Europe.
0:35
[♪ INTRO]
0:38
Beginning in 1347, the Black Death swept across Europe,
0:42
killing an estimated 50 million people.
0:44
Now granted, this was Medieval times, but that’s about seven times
0:48
the worldwide death toll from COVID-19,
0:51
and comprised up to half of the total population of Europe at the time.
0:55
The deadly pandemic was an outbreak of plague, an infectious disease
0:59
which causes fever, painfully swollen lymph glands, and black necrotic tissue –
1:04
hence the ‘Black’ part of the name.
1:06
The infection also affects the blood, lungs,
1:08
and can move through the body so fast that it causes death in just hours or days.
1:13
But it was another 550 years before anyone
1:16
figured out what was actually doing the killing.
1:18
A 19th Century physician studying a new plague outbreak
1:21
in Hong Kong found the tiny bacterium Yersinia pestis to be the culprit.
1:26
And in 2011, DNA analysis of skeletons found in plague pits
1:30
confirmed that Yersinia pestis was the killer in the Black Death pandemic, too.
1:35
The bacteria is normally carried by animals,
1:37
so it’s thought that they were transferred to humans via fleas that infected wild rats.
1:42
More than 650 years later, I still feel like this is
1:46
what fuels our collective hatred and fear of rats.
1:49
Most recently, in 2022, the particular strain of Yersinia pestis that caused
1:52
the Black Death has been traced to Kyrgyzstan, and was known to have
1:56
made it’s way across Asian trade routes to the Black Sea by the 1340s.
2:00
But it’s still not clear how the bacterium made the jump to western Europe,
2:05
why it spread so quickly, and why it was so deadly.
2:08
But now, a 2025 study claims to have found an answer,
2:12
using a combination of climate data from tree rings,
2:15
atmospheric compositions preserved in ice cores,
2:19
and historical written records from chronicles, treaties and even poetry.
2:24
Together, these clues point to a chain of events like a carefully constructed
2:27
line of toppling dominoes, which starts with a seemingly
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unrelated natural disaster, halfway across the globe.
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Naturally, the first domino we’re going to talk about is a volcanic eruption.
2:39
Now, written records from this time aren’t all that complete,
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and there’s no definitive historical account
2:44
matching the eruption the researchers describe.
2:47
However, scattered writings from all over the world, including Japan, China,
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Germany, France, and Italy report unusual cloudiness
2:54
in the years before the Black Death.
2:56
Some records also mention dark lunar eclipses.
3:00
During a lunar eclipse, when the Earth casts its shadow on the moon,
3:03
light filtering through the atmosphere makes the moon appear red.
3:07
A darker eclipse implies that less light than usual is making it
3:11
through to illuminate the moon, suggesting there’s more stuff
3:15
in the atmosphere getting in the way.
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Like particles from a volcanic eruption.
3:20
Now all of this would be fairly circumstantial,
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if it wasn’t for a smoking gun discovered in
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ice core records from Greenland and Antarctica.
3:28
In these cores, which are drilled vertically down through the ice caps,
3:31
there are layers that correspond to each year’s snowfall.
3:35
Layers of snow are buried, year on year, turning to ice and sealing
3:39
in a chemical record of what was going on in the atmosphere that year.
3:43
And, for a number of years preceding the Black Death,
3:45
researchers found spikes in the concentration of sulfur dioxide in the atmosphere,
3:50
as recorded in the icy layers.
3:52
In pre-industrial times, the main source of sulfur in the atmosphere was volcanoes,
3:57
and since its concentration in the ice corresponds to the concentration
4:00
in the atmosphere, these pulses can be used to pinpoint the time,
4:05
and to some extent the size, of past eruption events.
4:08
Researchers found small sulfur spikes in layers from 1329, 1336, and 1341,
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followed by a really big one in 1345,
4:18
two years before the Black Death took off in Europe.
4:21
That last one represents around 14 million tons of extra sulfur in the atmosphere,
4:26
which has the potential to cause many
4:28
of the phenomena reported in written records.
4:31
For comparison, when Mount Pinatubo in the Philippines
4:34
erupted in 1991 it ejected about 6 million tons of sulfur.
4:38
So at least by ice core-reckoning, the 1345 event
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was more than twice the size of Pinatubo.
4:43
The researchers don’t actually know exactly where the eruption happened,
4:47
but the global distribution of sulfur implies it was somewhere near the equator,
4:51
which would have allowed the volcanic gases
4:53
to move north and south in equal quantities.
4:56
One possible candidate is Mount Rinjani, in Indonesia,
5:00
which is known to have erupted and produced
5:02
a big sulfur spike a century earlier, in 1257.
5:05
If that volcanic region had continued to be active,
5:08
then one big eruption or several smaller ones could have produced
5:12
the sulfur we see in the ice, and the global reports of changeable skies.
5:16
Wherever it was, though, that volcanic outburst set the stage for
5:20
the second domino to fall, in the shape of a volcanic winter and famine.
5:24
The sulfur dioxide from volcanic eruptions is an aerosol in the atmosphere,
5:28
which scatters and reflects sunlight, causing temporary cooling.
5:32
Mount Pinatubo’s eruption, with its 6 million tonnes of sulfur,
5:35
cooled the planet by half a degree Celsius in the year after the eruption,
5:39
temporarily bucking the global warming trend.
5:42
So 14 million tonnes from a 1345 eruption
5:45
could be expected to have an even greater effect.
5:48
And researchers found evidence for such a cooling event
5:51
in the wood of trees all across Europe.
5:54
In the study of tree rings, known as dendrochronology,
5:56
each ring in a slice through the trunk represents a year of that tree’s growth,
6:01
much like the layers of ice in cores.
6:03
This time, though, it’s the size and structure
6:06
of the rings that gives information about past conditions.
6:09
In northern Spain, tree slices from this period consistently
6:13
have what’s known as blue rings.
6:15
Now these rings aren’t actually blue to the naked eye,
6:17
but rather show up when microscope slides are prepared.
6:20
Paper-thin wood samples are stained with a special combination
6:23
of chemicals to show their structure and chemistry.
6:26
In this, normal wood appears bright pink, while abnormal layers show up blue.
6:31
Blue rings indicate plant cells with little to no lignin in their cell walls.
6:35
And since lignin is the tough fiber that makes wood stronger
6:38
than normal plant stems, it’s as if these layers were just never finished.
6:42
Scientists aren’t sure exactly what makes trees leave behind blue rings,
6:46
but in more recent records they have seen
6:48
a link between these structures and cold summers.
6:51
And while it’s fairly common to have one anomalously cold summer,
6:55
several together is much more rare, and it hints at something
6:59
bigger going on in the atmosphere.
7:01
And that is consistent with other data from tree rings.
7:03
In thousands of trees measured across Europe,
7:06
rings between 1345 and 1347 consistently have
7:10
lower density wood than usual from the summer growing season,
7:14
suggesting cold temperatures and non-ideal growing conditions.
7:18
And it wasn’t just the trees that suffered from these cold, volcanic winters.
7:22
The researchers believe that the lower temperatures
7:24
would have disrupted normal patterns of rainfall all across Europe.
7:28
And written accounts from this time confirm this,
7:31
with reports of exceptionally cold, wet summers between 1345 and 1347,
7:37
leading to poor harvests and crop failures across southern Europe.
7:41
Before long, people in the Mediterranean were facing famine,
7:44
which was compounded by the fact that
7:46
many of these populations had been drawn to big cities.
7:49
Growing urbanization meant that the Italian city-states like Venice, Genoa,
7:53
and Pisa, were so densely populated that they couldn’t be self-sufficient.
7:58
There wasn’t enough space to expand nearby agriculture to support their citizens,
8:02
and so they had to rely on grain that had been shipped in from further afield.
8:07
Usually, they’d use maritime trade routes to import their grain from southern Italy,
8:11
Sicily, Sardinia and northern Africa.
8:13
But when all of these crops failed,
8:15
it left a lot of people with not a lot of food to go around.
8:18
But before our third domino falls, we need to pause for a quick break.
8:23
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8:26
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9:48
Now, we arrive at our third domino.
9:50
In the past, a famine like this would have meant that many,
9:53
many people would have died by starvation.
9:56
But these late-medieval Europeans were far too sophisticated to starve to death.
10:00
The key was those Italian city states and their overseas trade links.
10:05
The cities had been cultivating a relationship with the Mongols in central Asia
10:09
for the last 100 years, and now their governments
10:12
called on those contacts to save their citizens.
10:15
Further north, the Asian grain-producing regions are thought to have not been
10:18
so severely affected by the climatic downturn.
10:20
And there were already established trade routes
10:23
between Italy and the Sea of Azov in the north of the Black Sea.
10:26
Ok, sure, so the two peoples were in the middle of a war while all this
10:30
was going on, but the famine forced them to negotiate a ceasefire. Funny that.
10:34
Trade routes were reopened, and grain began to flow again.
10:37
The Italians were saved from starvation,
10:40
but unbeknownst to everyone, something else was lurking on those grain ships.
10:45
Our final domino: the arrival of the plague in Europe.
10:49
Those ships that carried grain from the Black Sea
10:52
also carried fleas infested with Yersinia pestis.
10:55
It’s likely that these fleas didn’t even need rats as a host,
10:59
which may have been ousted from the ships if found in the grain.
11:02
Instead, the tiny insects survived the journey feeding directly on the grain dust.
11:07
The Mongols had been dealing with their own outbreak of plague,
11:10
after that new strain from Kyrgyzstan we talked about earlier had spread over land.
11:15
But at the time, nobody knew what caused the disease, or how it was transmitted,
11:19
so the Italians had no way of understanding the risk of trading with Asia.
11:23
The ships arrived, the grain was unloaded,
11:26
and the infected fleas spread to rodents,
11:29
domestic animals and, before long, humans.
11:31
When you look at the timing and locations of the first Black Death outbreaks,
11:36
the link to the Mongol grain seems clear –
11:39
whenever grain shipments arrived, the first cases of plague soon followed.
11:43
For instance, in Venice, the first cases occurred
11:46
less than 2 months after the grain shipment.
11:48
And when there was enough surplus for grain exports to resume from Venice
11:52
to another Italian city, Padua, we see a plague outbreak there, too.
11:56
Meanwhile, larger Italian cities like Milan and Rome
11:59
were more agriculturally self-sufficient, and dedicated grain-producing regions
12:04
like Verona and Ravenna managed to get by without importing grain.
12:08
And all of these places were protected from the first wave of plague.
12:12
And so, the Black Death in Europe was as much a
12:16
product of global circumstance as it was an infectious bacterium.
12:20
Chance had set up these medieval dominoes just right:
12:23
natural events, global climate, demographic distributions,
12:27
and socio-economic responses were aligned in
12:29
a perfect storm to create the worst of all possible outcomes.
12:34
In fact, this is an early example of how globalization
12:37
can be both a blessing and a curse.
12:39
Those far-reaching trade links helped save
12:42
everyone in northern Italy from starvation.
12:44
But they were ultimately responsible for killing just as many,
12:47
if not more, with the plague.
12:50
[♪ OUTRO]
