The hidden history in the Notre Dame attic

The attic's beams were cut from trees dating back over 1,500 years. Scientists are now decoding the climate history preserved in their remains.

The shocking fire that struck Notre Dame in April 2019 offers an unusual opportunity. By sifting through the iconic cathedral’s remains, researchers plan to reconstruct the medieval climate and possibly shed light on how the climate works today. 

Valérie Daux always looked forward to walking past Notre Dame on her way to work. The beloved Paris cathedral – with its giant, colorful rose windows and two tall towers –  is considered one of the greatest examples of French Gothic architecture. But the fire that rushed through Notre Dame’s attic left much of the building in ashes.

Now, Daux, a climate scientist with Laboratoire des Science du Climate ed de L’Enviornment, sees in the remains an opportunity, because it opens access to an archive of climate history preserved in the beams and logs.

The Notre Dame attic was known as “The Forest” because of the lattice of ancient beams that were added to the structure in 1220. About 13,000 beams in total went into the framework, each made of a single 800-year-old oak tree. The old, dry wood fueled the fire, but some beams survived with only superficial burns. Those are the beams Daux is most interested in. Under the burns, the tree rings are still intact.

Trees trap secrets in their wood each year, in a layer just under the tree bark called “cambium.” They form a new layer each year, giving the cross-section of the tree its typical bullseye appearance. The rings contain patterns – annual patterns, decadal patterns. Once a tree ring is formed, they don’t change at all. The information is locked in, like a fossil.

Daux wants to extract that history and use it to reconstruct the climate record of the Medieval Warm Period – a period known for atypically high temperatures – because it is a critical time for understanding how climate worked before it was modified by human activities. Few other trees exist from that time period and are available for research.

Historically, direct climate measurements were not widespread and, with very few exceptions, don’t go earlier than about 200 years ago. If scientists want to know more and extend their data further back in time, they have to use “proxy” data – data collected from indirect measurements. To do that, they use historical records, like ship’s logbooks, or they use natural recorders of climate, like sediments, ice cores, pollen fossils, or tree rings.

Researchers aren’t arguing about whether the Medieval Warm Period, also called the Medieval Climatic Anomaly, was unusually warm. But how much warmer is still in question. Daux says that maybe by studying Notre Dame’s logs, they could answer the question of whether there was a time in the relatively recent past that was as warm as today.

Paul Sheppard, a dendrochronologist at the University of Arizona, says studying Notre Dame’s trees is “taking advantage of a unique opportunity.”

Trees can live up to 1,000 years or more, but most logs are cut before they reach that age. The trees used in Notre Dame’s attic all needed to reach a large size before they were cut, so they are all approximately 800 years old. There are few monuments existing today with beams that old.

“It is like a first look at ancient trees from a long time ago. There are simply no other trees in Europe right now that might date back to that time. So those are going to be very interesting trees to sample,” Sheppard, who does not work with Daux, says.

Scientists agree that modern climate change is fueled, at least in part, by human activity, namely burning fossil fuels. But Sheppard says the next question is to determine how resilient our planet is. How well can Earth recover from these dramatic changes?

To know that, we have to understand climate history on a long-term scale, as Sheppard says, “not just human memory of 50 years, or recorded history of the last 100 years, but in the last 1,000 years?” Understanding the warming period, and the changes after it will help answer that question.

Under the burns, the tree rings are still intact, which contain annual patterns that are locked in like a fossil.

To decode climate data, Daux says that she will cut into the trees from the Notre Dame attic, ring by ring, extracting cellulose to determine the oxygen and carbon composition of each ring. Then by comparing that to oxygen and temperature data found in the beams of a Southern France castle, she can begin to construct the climate history.

“You can understand climate only if you have understood all the mechanisms now and before it was impacted by human activities,” Daux says, adding that the relatively recent boom of cars, fossil fuels, and population growth marks a dramatic change in human activity.

Right now, workers are still cleaning up the toxic mess left by the fire. Deep in the Notre Dame attic, they are sorting and tagging logs, then sending them to a team to determine when and where they were grown.

When Daux gains access to the trees, the real work begins. The damage to Notre Dame hit at the heart of citizens of Paris and echoed around the globe. But, maybe, just maybe, all is not lost, and the chance to peek into the past will give insight into today’s climate.

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