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‘Like blood, then turned to darkness’: how medieval manuscripts link lunar eclipses, volcanoes and climate change


Before humans began to warm the planet by burning fossil fuels in the 19th century, Earth had gone through a centuries-long widespread cool period known as the Little Ice Age.

Scientists think this cold snap may have been possible partly caused by volcanic eruptions making the atmosphere more hazy and blocking some incident sunlight.

Records of these eruptions are scarce and much of our knowledge of them comes from the traces left in them polar ice And tree ringswhich are fragmentary and sometimes contradictory.

In a new study published in NatureAn international team of researchers led by Sébastien Guillet of the University of Geneva has found another way to learn about these historic outbursts: by studying descriptions of lunar eclipses in medieval manuscripts.

Dark eclipses

The researchers collected hundreds of data on lunar eclipses from across Europe, the Middle East and Asia, documenting 187 eclipses between 1100 and 1300.

In particular, they looked for descriptions that provided information about the moon’s brightness and color during the eclipse. Most of these turned out to be from European monks or clergy, who wrote in Latin.

Based on these descriptions, the researchers ranked the moon’s color and brightness reported with each total eclipse. The brighter the eclipse, the brighter the atmosphere at the time: Darker eclipses indicated a higher level of aerosol particles in the upper atmosphere – a sign of recent volcanic activity.

During a total lunar eclipse, the moon turns red due to sunlight refracted by the Earth’s atmosphere. A particularly dark eclipse indicates more aerosols in the atmosphere, which is a sign of recent volcanic activity.
Chris Harwood/Shutterstock

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

This allowed the researchers to estimate the timing of the culprit’s eruptions more accurately than from previous ice core records — and determine which eruptions reached the stratosphere and were likely to generate climate-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 are perfectly aligned, our planet blocks direct sunlight from reaching the lunar surface.

However, Earth’s atmosphere bends sunlight around our planet. As a result, some of the sunlight reaches the moon, even during a total solar eclipse.

Earth’s atmosphere also scatters sunlight, acting as a giant color filter. The bluer the light, the more it is scattered. That’s why the sky is blue during the day and the sun looks ruddy at sunrise and sunset.

During a total lunar eclipse, the sunlight that reaches the moon is filtered through the Earth’s atmosphere, removing much of the blue and yellow light. The light reaching the moon is actually the sum of all sunrises and sunsets that occur at that time.

And the state of the Earth’s atmosphere at that time determines how much light is filtered.

NASA video released to explain total lunar eclipse seen from America in December 2011.

How volcanoes affect lunar eclipses

If you’ve ever seen a sunset during a dust storm or on a very smoky day, you know that the extra particles clogging the air can produce deep, vibrant reds and oranges.

Imagine a total lunar eclipse as wildfires rage abroad. 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 massive amounts of material into the Earth’s stratosphere, where it can remain for many months.

The spectacular volcanic sunsets seen across Australia in the months following the January 2022 Tongan volcanic eruption are a case in point. And that material, once in the stratosphere, will spread over the Earth.

What effect does this have on lunar eclipses? It turns out that the brightness of the Moon during a lunar eclipse depends on the amount of material in our stratosphere. In the months following a major outburst, any lunar eclipse would be significantly darker than usual.

How volcanoes affect the climate

Volcanic eruptions can eject massive amounts of ash, sulfur dioxide and other gases high into the atmosphere. Eruptions can cause cooling or warming (both temporary). The effect depends on what exactly the volcano spews out, how high the plume reaches and the location of the volcano.

Read more: Climate explained: how volcanoes affect climate and how their emissions compare to what we produce

Sulfur dioxide is particularly important. When it reaches the stratosphere, it reacts with water vapor to form a persistent veil of sulfate 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 lower global temperatures in the years following the eruption. After Tambora, Europe and North America experienced a “year without summer”of 1816.

A photo from the International Space Station showing white puffy clouds over the ocean and a dark gray plume from a volcanic eruption.
The ash and smoke plume from the 2022 Hunga Tonga-Hunga Ha’apai eruption was visible from the International Space Station.
EPA/NASA/Kayla Barron

On the other hand, water vapor and carbon dioxide from volcanic eruptions have a warming effect. It’s only minor, as all current produce volcanic emissions less than 1% of the carbon dioxide released by human activities.

The past and future of volcanoes, eclipses and the climate

Eyewitness accounts through historical accounts and oral lore are often overlooked in the study of volcanoes. However, incorporating broader sources of knowledge is incredibly valuable in helping us understand the effects of past volcanic eruptions on humans and the environment.

Read more: When the Bullin guild: Aboriginal memories of volcanic eruptions thousands of years ago

In this study, the combination of historical observations with ice records and climate reconstructions of tree rings allowed for more accurate timing of those ancient eruptions. In turn, this has enabled us to better understand their potential impact on climate during the European Middle Ages. Such information can help us 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 height of the stratosphere will increase. As a result, a larger outburst may be needed to get significant amounts of aerosols into the upper layer, where they hang out to obscure the moon for future generations!

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