In recent decades, the jet stream - the eastward winds at ~10km above the earth surface that orchestrate much of the climate at the earth surface - has slowed down and has shown a wavier course. To put these recent changes in a long-term context, and to help answer the question whether they are linked to anthropogenic climate change, we have used tree rings to reconstruct climate, and particularly jet stream, variability over the past few centuries.

In particular, we have reconstructed snowpack in the Californian Sierra Nevada over the past 500 years and found that the 2015 snowpack was a record low over this period of time (Belmecheri et al. 2016). We further reconstructed winter jet stream variability over the western Pacific Ocean over the past 400 years (Wahl et al. 2019) and have shown that California wildfire activity over this period was modulated by jet stream variability. We have also found that North American jet stream variability orchestrates ecosystem functioning across the continent, including plant phenology (Hudson et al., in prep., a) and monarch butterfly migration ((Hudson et al., in prep., b).

We have further developed (Klippel et al. 2019) and used tree-ring data sets to reconstruct summer jet stream variability over the North Atlantic Ocean and Europe over the past 1000 years (Trouet et al. 2018, Xu et al., in prep.). In the process, we discovered the oldest living tree in Europe (1075 years old; Konter et al. 2017). Our summer North Atlantic-European jet stream reconstructions show a recent, increasing trend in the number of extreme positions of the jet stream: more and more summers show a jet stream in an extremely northern or southern position, which influences the summer weather conditions over Europe. The number of jet stream extremes has started increasing since the 1960s and this increase is unprecedented over the past 300 to 1000 years. We have further found that such northern or southern extremes of the summer North Atlantic-European jet stream creates a dipole in summer weather over Europe: in summers when it is (relatively) warm and dry in northwestern Europe, it is cool and wet in the Balkans and vice versa (Belmecheri et al. 2017). This jet stream-driven climate dipole has resulted in past climate extremes in both regions (e.g., wildfires in Greece, heatwaves in the British Isles; Trouet et al. 2018), that can also be linked to historical events (e.g., the fire in Constantinople in August 1782, the failed harvest and famine due to a cold summer in Scotland in the same year; Xu et al., in prep.). Like in North America, we find that North-Atlantic-European jet stream variability is an important driver of ecosystem functioning over Europe, for instance of forest productivity (Dorado et al., in prep.).

In addition to the research component, this grant has provided me the opportunity to (1) organize a broad-audience lecture series in Fall 2018, highlighting the cutting-edge research done by women scientists at the University of Arizona; (2) develop and teach a graduate-level scientific writing course, in which we discuss writing practice and productivity, the journal paper publication process, and self-editing. Up to 16 graduate students per semester in the class also form weekly writing groups and learn about writing accountability. In addition to this, our project research has been covered by many national and international media outlets (e.g., New York Times, Washington Post, The Guardian, Reuters, National Geographic, etc.) and in public lectures and podcast interviews. Finally, I have written a broad audience book about dendrochronology that includes most CAREER research results. Tree story: The History of the World Written in Rings was published by Johns Hopkins University Press in April 2020 (https://jhupbooks.press.jhu.edu/title/tree-story). The Dutch translation ('Wat bomen ons vertellen') was published in May 2020, and translations in six additional languages (including Spanish, Italian, and Chinese) are forthcoming.

Last Modified: 01/15/2021
Modified by: Valerie Trouet