Shifts in the global climate happen naturally over millennia and they can leave a number of clues behind, including in tree rings. Trees store information about climatic conditions in the annual growth rings they lay down each year. Some trees can live to be thousands of years old, giving scientists valuable information about past climate conditions that can help us understand the changing climate of today’s world.
At the University of Arizona, paleoclimatologist Valerie Trouet uses dendrochronology, the science of dating objects through the study and comparison of tree rings, to learn about the ancient climate on Earth. Her new book, “Tree Story: The History of the World Written in Rings,” details some of the discoveries scientists have made.
“The way [this] works is that if it's a good year, and the tree is happy, it will grow a lot and grow a wide ring,” Trouet explains. “On the other hand, if it's a very dry year…the trees will be unhappy and form a very narrow ring in that year. So, if you have a 500-year-old tree from Arizona, by measuring how wide or narrow each of those 500 rings in the tree is, that'll give you an idea of which ones were the wet years, represented by the wide rings, and which ones were the dry years, represented by the narrow rings.”
It's a different story in places like the European Alps, or in Canada and Alaska, where it’s rarely too dry for trees to grow a lot but it can get too cold, Trouet notes. The trees in these regions record temperature, rather than moisture.
“We can really study the rings in pretty much anything that's made out of wood, as long as it has enough rings in it. ... We can date a lot of wood material very precisely, to the exact year, by using dendrochronology.”
Dendrochronology is not limited to living trees alone, Trouet adds. “We can really study the rings in pretty much anything that's made out of wood, as long as it has enough rings in it. Through the process of matching…the pattern in tree rings, we can also date wood that we don't necessarily know when it was used. But by matching it to the patterns of living trees, we can date when, for instance, archeological sites were built or when historical buildings were built, or when paintings were painted, or when musical instruments were made. We can date a lot of wood material very precisely, to the exact year, by using dendrochronology.”
To sample living trees, dendrochronologists take a small sample of the wood about the length and diameter of a pencil, a process that does not harm the tree. “It's a bit like a biopsy or drawing blood from a tree. ... So you extract a ‘pencil’ out of the tree that shows all the tree rings on it.”
Dendrochronologists have sampled old wood from Roman times in Europe and compared it to wood from living trees and historical buildings to develop a climate reconstruction going back 2,500 years, Trouet says. They discovered that the reign of the Roman Empire was a period of severe climate change.
“It would be wet for a couple of decades and then dry again for a couple of decades; wet again for a couple of decades,” Trouet explains. “So, we then hypothesized that…these extremes in the climate might have contributed to the fall of Rome, or the disintegration of the Western Roman Empire.”
On the other hand, dendrochronologists studying trees in Mongolia discovered that the period when Genghis Khan established the Mongol Empire was one of the wettest periods in Mongolian history. “It's almost [the] opposite of the story of the Roman Empire, where climate helps to explain, or is one of the factors contributing to the fall of Rome,” Trouet says. “In Genghis Khan's case, it's the other way around, where the really good climate conditions help him to build and expand his empire.”
Reconstructing climate patterns of the past reveals the “extraordinary character” of climate change in our own era, specifically how global warming today compares with changes in previous eras, Trouet says.
Scientists have been measuring temperature and precipitation on a global scale since the early 1900s, she notes, well after the start of the industrial revolution, when human activity began “really messing with the climate.” Instrumental data alone, therefore, doesn’t provide “a good record of what natural climate variability looks like,” Trouet explains. “We only have a record of what the climate is like when we are influencing it as humans.”
Tree rings allow scientists to look further back in time at natural climate variability, which puts current climate change into a much longer-term context. The findings of dendrochronologists largely track with the 1998 “hockey stick” climate report, for example. This study was controversial at the time because it tried to reconstruct global temperatures over the past 1,000 years. The researchers graphed a line showing temperatures slowly cooling between the year 1000 to about 1850 (the stick) and another line showing a steep increase from 1850 to the present (the blade).
“Wherever I go, where the trees are sensitive to temperature, you always see an unprecedented warming in the 20th century. It’s everywhere.”
“This steep increase — you keep seeing it in the tree rings,” Trouet explains. “Wherever I go, where the trees are sensitive to temperature, you always see an unprecedented warming in the 20th century. It’s everywhere.”
Dendrochronology “sits at the nexus between ecology, climatology, and archaeology,” Trouet says. “So, as we move forward in a warming world where climate is going to have a very big impact on ecology and ecosystems — it’s also going to have a big impact on humans — being able to provide a longer-term and historical context to…what is happening here and now, I think is exciting.”