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New warnings from the medieval warm period about climate change


Published : 05 Aug 2023 07:58 PM | Updated : 05 Aug 2023 07:58 PM
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Before our current, carbon-fueled global warming trend took off during the 20th century, the most consequential temperature bump in recorded history was the Medieval Warm Period. This week, scientists announced in the journal Nature that they’ve found a new way to decipher the climate of this period — and their findings match with current climate models in a way that gives researchers more confidence in their ability to predict the future. 

Scientists don’t know exactly what caused the Medieval Warm Period, which lasted from about 800 to 1400, though they agree it was the result of natural fluctuations in Earth’s climate. Whatever the exact cause, the warming — probably less than 1C at the peak — changed human affairs for better and for worse. Wine grapes grew prolifically in England and citrus fruits thrived in China. Throughout northern Europe, grazing lands and farms spread northward and up mountains, into the once-forbidding islands of Iceland and Greenland. But in the American Southwest and in Central America, repeated megadroughts contributed to the collapse of civilizations. 

Until recently, scientists have disagreed over exactly how warm the Medieval Warm Period was, said Fredrik Charpentier Ljungqvist, professor of historic geography at Stockholm University who was not an author of this recent paper. Studies over recent decades have drawn different conclusions — some showing it was even warmer than it is today, and others saying it didn’t come close.

“This new reconstruction shows that it’s probably still very warm, but less warm than the last 10, 20 years.” Today’s climate has already warmed 1.1C or 1.2C above the preindustrial baseline, scientists estimate.

Studies tracing out the past climate are extremely valuable for revealing the natural background against which human-generated global warming is occurring. They should also remind us that even small swings in the climate can alter the course of human history.

The new paper’s lead author, Jesper Bjorklund, a tree-ring expert with the Swiss Federal Institute for Forest Snow and Landscape Research, said what’s particularly relevant to today’s situation is the fact that their tree-ring measurements line up with climate models for the first time. 

Until now, critics of climate modeling could argue that the models, when projected backwards, didn’t predict the Medieval Warm Period as measured by previous techniques. 

“I think just by sharpening our vision on the observation part, we were able to bring together these two communities,” said Bjorklund, referring to scientists who use climate models and those who read natural records. “And by doing that, I think the predictions will be more credible.” 

Studies tracing out the past climate are extremely

 valuable for revealing the natural background 

against which human-generated global warming

 is occurring. They should also remind us that even

 small swings in the climate can alter the course of human history

Tree rings can reach back about 8,000 years. The oldest trees in the world live about 5,000 years, but scientists can delve further into the past by using dead wood, whether on the ground, in manmade structures, or preserved in oxygen-poor sediments at the bottom of lakes and bogs.

Bjorklund’s team was able to essentially read the tree rings right down to the fine print by looking at their microscopic cell walls. It’s what they call a “wood anatomical technique,” and it’s giving them a more precise way to measure past climates.

He and his colleagues looked at trees from a particular region, Fennoscandia, in Northern Scandinavia, but they plan to expand the new technique to other regions. The thickness of tree rings can give a rough estimate of the climate during the year the ring was made, and over the years scientists have refined that technique by using X-rays to measure the wood density. Now, he said, they are mapping the thickness of the individual cell walls in the tree rings. 

For this study, they were able to incorporate 50 million cell walls, he said. They refined the data further by looking at the most climate-sensitive parts of the cell walls, which are parallel to the rings and the bark surface. Previous work on tree rings revolutionized society’s understanding of our current era of climate change. Scientists had known since 1896 that carbon dioxide was increasing, and that even though it’s a small component of our atmosphere, it plays a big role in warming the Earth. 

But the relative rate of change became much clearer in 1998, when scientists used tree rings as well as other natural records to create what became known as the hockey stick graph. (Despite repeated attacks and misinformation campaigns, the hockey stick has been refined and replicated by many climate scientists.) The most recent graphs of past global temperatures show a gentle rise followed by a cold period starting in the 1400s, known as the Little Ice Age, followed by a rise starting in the mid 1800s and becoming steep by the mid 20th century.

Stockholm historian Ljungqvist said there are winners and losers in past climate changes. Warm spells tend to bring drought to the Americas, which in medieval times drove the Mayans from their cities and into wetter country, and devastated farming people in what’s now New Mexico. 

Yet in Europe, the population more than tripled, and the Norse expanded their dairy industry and farming into Iceland, Greenland and Newfoundland, where for the first time they encountered Inuit — descendants of people who came East across the Bering Strait thousands of years earlier. 

But by the 1300s, the climate was growing less stable, with unpredictable rainfall and temperature swings. The plague known as the Black Death devastated Europe — and the unstable weather probably contributed. Unpredictable crop yields led to poor nutrition, which left people weaker, and in the case of black plague, changes in climate led to booms and busts in the populations of rodents, which often carried infected fleas.

The Little Ice Age also drove the Norse in Greenland to try to supplement their diets with fish and sea mammals, but eventually through cold and lack of rain their crops and livestock died, and they disappeared some time during the 15th century.

Of course, many factors drive history — technological advances can help humanity as much as good weather. Warfare can hit a civilization as hard as a drought. But there’s no escaping the climate. Bolstered by observations like tree-ring data, those climate models are looking more and more robust, an alarming sign for our still-unfolding future. 


F.D. Flam is a Bloomberg Opinion columnist covering science. She is host of the “Follow the Science” podcast. 

Source: Bloomberg