You may remember from childhood that tree rings are used to measure the age of trees, but they can also help researchers reconstruct climate conditions dating back thousands of years.
Tree rings are formed yearly, and each ring is a record of the growing conditions the year it was formed.
Researchers from the University of Arizona and the Swiss Federal Research Institute WSL are using tree ring data to create a historical record of the North Atlantic jet stream. The research has revealed that weather extremes in Europe, such as flooding, heat waves, and drought, are the result of changes in the positioning of the jet stream since the 1960s.
The new study, published in the journal Nature Communications, is the first to create a record of changes in the jet stream dating back as far as 1725.
Having such a rich collection of data allowed the research team to see how changes in the jet stream between southern and northern positions correlated with major weather events, and that there has been an increase in extreme jet stream changes since 1960.
For the study, the researchers focused on latewood density, as latewood is the part of the tree ring that forms later in the summer growing season and shows temperatures during that time.
In an extreme northern position, the jet stream is linked to summer heat waves, drought, and wildfires in the British Isles and western Europe and heavy precipitation in southeastern Europe.
When the jet stream is in the extreme southern position, the weather events reverse, and western Europe experiences heavy flooding while southeastern Europe has drought and higher temperatures.
North America is also susceptible to the jet stream’s positions, and current weather events can be traced to changes in the stream.
These weather extremes are further aggravated by climate change, which led the researchers to question if the increase in the North Atlantic jet stream’s positioning is because of rising global temperatures.
“There’s a debate about whether the increased variability of the jet stream is linked to man-made global warming and the faster warming of the Arctic compared to the tropics,” said Valerie Trouet, the lead author of the study. “Part of the reason for the debate is that the data sets used to study this are quite short — 1979 to present. If you want to see if this variability is unprecedented, you need to go further back in time — and that’s where our study comes in.”
Trouet and the research team hope to create a record of the North Atlantic jet stream’s positions dating back 1,000 years, which will help show how climate change influences the stream and how the stream affects weather in North America.