Phd Student Tzu Tung ”Sassa” Chen  next to a tree bole
Photo: Annika Söderpalm

Studying our health using the annual rings of trees


Weather data is only available for about the past 100 years. To go further back in time, the annual rings of trees can be used to see both temperature and precipitation. This is what Sassa Chen, a doctoral student at the Department of Earth Sciences, works with. She wants to know how our health is impacted by climate.

“We know that quick changes in weather make us more susceptible to illness but that more long-term climate changes can also impact us. We might think mostly of a warmer climate but a colder climate can result in smaller harvests, forcing people to migrate and leading infections to spread to new areas.” 

Sassa Chen studied geology at National Taiwan University, where she also worked with tree rings of huge tropical trees. In Sweden, the trees grow slower and don’t get as large but can be several hundred years old.
“Trees grow more quickly with warmth and rain. This causes the annual rings to be wider and from these we can reconstruct the climate in that location.”

Climate data brought answers about old malaria deaths in Sweden

She is just writing an article about deaths from malaria in Sweden
“Many people are surprised to hear that there is malaria in Sweden, but there is.”

Using reconstructed climate data and church records, she and her colleagues have shown that during the past 100 years, many people have died of malaria the year after an extremely warm summer. This is because the parasites need two months with at least 16 degrees to develop.
“During the 18th century, malaria was even called the Gothenburg Sickness, since a tenth of the city’s population died. We no longer live as densely as before and malaria does not spread in the same way.

The research gave Sassa the opportunity to experience unique natural places 

Sassa Chen became a geologist because she loved fieldwork and being outdoors.
“Sweden is absolutely amazing! As a geologist, you go to place where tourists don’t set their foot. Deep in the forest, in fjords, and far in the north.”

For her research, it’s also good that it isn’t so warm. Trees that end up in lakes become conserved and are valuable material for studying tree rings. In Taiwan, that type of material decomposes because it is too warm. But it was neither nature or the temperature that got her to come to Sweden. It was a Swedish engineer that came to repair a device at the university lab where she worked. They now live together in Gothenburg.

Tzu Tung Chen shows a core samples from tree
When the core sample from a tree is polished, the width of the annual rings can be seen. These can then be used to see how the climate has varied.
Photo: Annika Söderpalm

A new view on being a doctoral student

When she first came to Sweden, she studied Swedish for three months and then had a three-month placement, in part at the Department of Earth Sciences. She had already considered becoming a doctoral student in Taiwan, so when an appropriate doctoral position opened up, she applied. In Taiwan her view of what it meant to be a doctoral student differed from here in Sweden.
“There, you are a student and the supervisor decides everything. In one way, it can feel safe knowing exactly what you have to do but you don’t have the same freedom as here. Here, you have to do more multitasking. You alternate between different roles. You teach, you are an employee and are part of a department with meetings about the work environment and gender equality, and then you have your own research. You are colleagues even if you are at different levels, not just boss and student like in Taiwan. I really like it.”

The educational course was a bit of a shock but useful, according to Sassa Chen. It is required if you want to be able to teach and since she didn’t get a spot on the English course, she signed up for the Swedish one instead.
“It was very instructive to be forced to formulate myself in Swedish, and it helped me get into Swedish culture in a completely different way than if I had taken the course with the other international students.”

In the future, she would consider working as a consultant within geological data collection.
“People are really tough,” she says with admiration in her voice. “Women in overalls and helmets who don’t care what the weather is like and head out in rubber boats. And then they have fika and drink hot coffee. I can see myself being one of them in the future,” she says with a laugh.

Tzu Tung ”Sassa” Chen

Is: Doctoral student in geology
Born: Grew up in Taitung in eastern Taiwan
Age: 33
Interesting facts: Could speak quite a lot of Swedish when she moved to Sweden. She has the children’s shows on SVT Play to thank for that.