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Medieval Monks Could Have Unknowingly Recorded The Ferocity of Volcanic
Activity

NATURE
06 April 2023
ByHEATHER HANDLEY & JONTI HORNER, THE CONVERSATION
Lava exploding from volcano top and pouring into a glowing river
(Nicole Schriber/iStock/Getty Images Plus)

Before humans started heating the planet by burning fossil fuels in the
19th century, Earth had experienced centuries-long widespread cool
period known as the Little Ice Age.

Scientists believe this cold spell may have been triggered, in part, by
volcanic eruptions which made the atmosphere hazier, blocking some
incoming sunlight.

Records of these eruptions are sparse, and much of our knowledge of them
comes from the traces left behind in polar ice and tree rings, which are
fragmentary and sometimes contradictory.

In a new study published in Nature, an international team of researchers
led by Sébastien Guillet at the University of Geneva has found another
way to learn about these historical eruptions: by studying descriptions
of lunar eclipses in medieval manuscripts.

Dark eclipses
The researchers compiled hundreds of records of lunar eclipses from
across Europe, the Middle East, and Asia, documenting 187 eclipses
between 1100 and 1300.

In particular, they searched for descriptions that provided information
on the brightness and colour of the Moon during the eclipse. Most of
these turned out to be from European monks or clerics, writing in Latin.

Based on these descriptions, the researchers ranked the colour and
brightness of the Moon reported in each total eclipse. The brighter the
eclipse, the clearer the atmosphere at the time: darker eclipses
indicated a higher level of aerosol particles in the upper atmosphere –
a marker of recent volcanic activity.

The next step was to put the eclipse data together with simulations of
how aerosol particles behave in the atmosphere, modern satellite
observations, and climatic evidence from historical tree ring records.

This allowed the researchers to estimate the timing of the culprit
eruptions more precisely than from previous ice core records – and
determine which eruptions reached the stratosphere and would be more
likely to generate climatic cooling effects.

What lunar eclipses tell us about the state of the atmosphere
A total lunar eclipse is a beautiful sight. When the Sun, Earth, and
Moon align perfectly, our planet blocks direct sunlight from reaching
the Moon's surface.

However, Earth's atmosphere bends sunlight around our planet. As a
result, some sunlight reaches the Moon even during a total eclipse.

Earth's atmosphere also scatters sunlight, acting as a giant colour
filter. The bluer the light, the more it is scattered – which is why the
sky is blue in the daytime, and why the Sun appears ruddy at dawn and dusk.

During a total lunar eclipse, the sunlight reaching the Moon has been
filtered by Earth's atmosphere, removing much of the blue and yellow
light. The light that reaches the Moon is effectively the sum of all the
dawns and all the dusks occurring at that time.

And the state of Earth's atmosphere at that time controls just how much
light is filtered.

How volcanoes affect lunar eclipses
If you've ever seen a sunset during a dust storm, or on a very smoky
day, you know the extra particles clogging up the sky can produce deep,
vibrant reds and oranges.

Imagine a total lunar eclipse occurring while wildfires rage overseas.
The fires would pump smoke and dust into Earth's atmosphere, making the
Moon redder and darker during the eclipse.

Which brings us to the effect of volcanoes. The largest volcanic
eruptions pump vast amounts of material into Earth's stratosphere, where
it can remain for many months.

The spectacular volcanic sunsets seen throughout Australia in the months
following the Tongan volcanic eruption of January 2022 are a great
example. And that material, once in the stratosphere, will spread around
Earth.

What effect does this have on lunar eclipses? It turns out the
brightness of the Moon during a lunar eclipse depends the amount of
material in our stratosphere. In the months after a large eruption, any
lunar eclipse would be markedly darker than normal.

How volcanoes affect the climate
Volcanic eruptions can eject huge amounts of ash, sulphur dioxide, and
other gases high into the atmosphere. Eruptions can cause either cooling
or warming (both temporary). The effect depends on exactly what the
volcano spews out, how high the plume reaches, and the volcano's location.

Sulphur dioxide is particularly important. If it reaches the
stratosphere, it reacts with water vapour to form a lingering veil of
sulphate aerosols. These aerosols, along with the volcanic ash, block
and scatter solar radiation, often leading to cooling at the Earth's
surface.

Large volcanic eruptions, such as the 1991 Mount Pinatubo eruption in
the Philippines and the infamous 1815 eruption of Tambora in Indonesia,
slightly lowered global temperature in the years after the eruption.
After Tambora, Europe and North America experienced a "year without a
summer" in 1816.

On the other hand, water vapour and carbon dioxide from volcanic
eruptions have a warming effect. It's only small, as all present-day
volcanic emissions produce less than 1percent of the carbon dioxide
released by human activities.

The past and future of volcanoes, eclipses, and the climate
Eyewitness accounts through historical reports and oral traditional
knowledge are often overlooked in the study of volcanoes. However, the
inclusion of broader sources of knowledge is incredibly valuable to help
us understand past impacts of volcanic eruptions on people and the
environment.

In this study, the combination of historical observations with ice
records and climate reconstructions from tree rings has enabled more
precise timing of those ancient eruptions. In turn, this has allowed us
to better understand their potential impact on the climate during the
European Middle Ages. Such information can help us to understand the
role these eruptions may have played in the transition to the Little Ice
Age.

In the future, volcanoes may have to work a little harder to create a
"dark" eclipse. As the atmosphere warms, the altitude of the
stratosphere will increase. As a result, it may take a bigger eruption
to put significant amounts of aerosols into the upper layer where they
will hang around to darken the Moon for future generations!

Heather Handley, Associate Professor of Volcanology and Geoscience
Communication, University of Twente and Adjunct Associate Professor,
Monash University and Jonti Horner, Professor (Astrophysics), University
of Southern Queensland

This article is republished from The Conversation under a Creative
Commons license. Read the original article.