TUCSON, Ariz. – From the early 1700s until the 1960s, the fast moving river of wind known as the North Atlantic Jet Stream, which drives weather extremes over Europe, was pretty steady on its course.
Then it became less predictable. But instrument data alone can’t tell the jet stream’s movements for comparison over the centuries, given that scientists began keeping records of weather events via instruments only in the late 19th century.
The rings of trees, however, offer a far more complete historical picture of climate variations. As they age, trees grow outward from the center, and each year a new, distinct circle of dead wood is created around the trunk. In that ring lives information about precipitation, temperature and other data about that year.
A team led by Valerie Trouet, a dendrochronologist, sampled 400 trees from the Balkans and 200 in Scotland — including what might be the oldest known tree in Europe, a Bosnian pine in Greece named Adonis, which is 1,075 years old. The jet stream flows between these regions, and trees revealed the range of temperatures in their rings and the frequency of fires, an expansive chronicling of jet stream behavior.
“More extreme positions create more extreme climate events, especially heat waves and storms,” in Europe, Trouet said. And the tree rings show “big fires happen in the Balkans when the jet is in its southerly position.”
The fact that the stream has become more variable only in recent decades suggests that the shift is the result of humans’ effects on climate, Trouet said. “The recent rise in variance is unprecedented in 300 years,” she said.
More analysis is underway to look back to even earlier centuries.
The Laboratory of Tree-Ring Research at the University of Arizona in Tucson was founded in the 1930s by A.E. Douglass, an astronomer who turned to trees to better understand the connection between sunspots and climate.
The lab has helped establish other labs around the world. There are now roughly a dozen large labs globally and data from 4,000 sites on all continents except Antarctica. The information is stored in the International Tree Ring Data Bank, open to all researchers. As more data becomes available, a richer picture forms of the nexus of past climate, ecosystems and human civilization.
At this time, the most essential role for tree rings is probably their use in reconstructing past climate and providing much greater context. “The instrumental period provides a snapshot” of past climate, said researcher David Meko, “but the tree rings are a panorama.”
This window into the deep climate past has become vital in a rapidly warming world, to show how the climate of the last half-century is far outside the historical norms going back thousands of years.
Living bristlecone pine trees, for example, are several thousand years old and their information is added to by those that died thousands of years ago, but remained intact in their cold, dry high-altitude environment.
It’s hard to argue with tree rings that huge environmental changes are not occurring. Climate change seen in the past six or seven decades has few, if any, comparisons in the far past, researchers say.
The current two-decade-long drought on the Colorado River, for instance, is the longest since medieval times when a drought lasted for 62 years — with no very wet years in between dry years.
Moreover, conditions in some recent years are the hottest and driest in many centuries. Other sources — lake sediments; ice core samples; coral; the otolith, or ear bone, of fish; and even the shells from living and long dead geoducks, a large bivalve with a snakelike appendage — add to the broader picture.
“We have divers sucking up ancient geoducks off the ocean floor,” said Bryan Black, a dendrochronologist. Combined with long dead geoduck shells, data could go back many thousands of years. Shells off Iceland go back 1,000 years. “They show that the last century is unprecedentedly warm,” he said.