30.08.2023

Past abrupt changes in North Atlantic Overturning have impacted the climate system across the globe

Tübingen researchers part of international team comparing data on cave deposits worldwide with climate modeling - findings help to better understand natural climate fluctuations

A new study shows that abrupt climate changes have affected precipitation patterns worldwide in the past, especially in the tropical monsoon region. Professor Kira Rehfeld and postdoctoral researcher Nils Weitzel from the University of Tübingen's Environmental and Geoscience Center are part of an international research team that used data from cave deposits and climate model simulations to investigate the global effects of rapid temperature increases in the northern hemisphere. The comparison of stalagmite and model data shows in unprecedented detail how these abrupt changes, and the associated changes in the Atlantic overturning circulation, or AMOC, affected global atmospheric circulation. The study was published in the journal Proceedings of the National Academy of Sciences PNAS.

Dansgaard-Oeschger events are abrupt temperature jumps in the northern hemisphere during which temperatures in Greenland rose by up to 15 °C in a few decades during the last cold periods. "These events are the archetype for abrupt climate changes, and a better understanding of these climate oscillations is crucial for a more reliable assessment of the risk and potential impact of future tipping events in the climate system," says study co-author Niklas Boers of the Potsdam Institute for Climate Impact Research and the Technical University of Munich.

In the study, the research team examined the SISAL database, a global collection of cave minerals curated by more than 100 researchers worldwide. In a sense, stalactites serve as climate archives from whose calcite matrix geochemical information on precipitation changes may be obtained. "Our results show that atmospheric circulation and associated precipitation patterns changed drastically and abruptly worldwide as a result of the Dansgaard-Oeschger events," says first author Jens Fohlmeister, who was working at the Potsdam institute at the time of the study. "The effects of these abrupt past climate changes were felt worldwide, but most strongly in tropical monsoon areas."

The study compared the spatial patterns and magnitude of changes across abrupt climate transitions with information from complex climate models and found clear similarities. Here, the study builds on recent developments in climate model development, note co-authors Kira Rehfeld and Nils Weitzel: "For a long time, climate models were only examined to see if they reproduced anthropogenic warming, with little focus on climate variability. However, both the national and international scientific communities have moved forward in this regard."

The study took off in 2019 at an international workshop where speleologists and climate modelers noted that the data available for rapid abrupt transitions in the last cold period has never been better. Rehfeld, a participant at the time, points out, “Global data collections - like the stalagmite database used here - have made it possible for the first time to study even distant-past climate changes on land." Stalagmites can be radiometrically dated in absolute terms and thus have few temporal uncertainties. Moreover, they are found on all continents outside Antarctica. The compilation of published data is thus an important complement to polar ice core studies. 

"Added to the improved real-data situation is the realization that climate models should also reflect natural variations for an assessment of future climate risks, which motivates international climate model comparisons and these joint studies." Data-model comparisons of such experiments could be used in the study - finding that real data and global climate models showed extremely similar patterns.

To date, there are still large margins of uncertainty in estimates of changes in, for example, the Atlantic Meridional Overturning Circulation (AMOC), despite years of extensive research. "The study shows that the joint systematic and careful comparison of climate model data and proxy indicators, with consideration of their respective strengths and weaknesses, makes it possible to decipher past rapid climate changes," Rehfeld says.

She said the work could help better understand what mechanisms led to abrupt climate transitions and how, for example, sea ice dynamics, changes in AMOC, or changes in the height of polar ice sheets contributed to them, "This is an important contribution to better assess potential risks from nonlinear climate transitions in the future."

Written according to a report by the Potsdam Institute for Climate Impact Research (PIK)

Current publication:

Jens Fohlmeister, Natasha Sekhon, Andrea Columbu, Guido Vettoretti, Nils Weitzel, Kira Rehfeld, Cristina Veiga-Pires, Maya Ben-Yami, Norbert Marwan, Niklas Boers: Global reorganization of atmospheric circulation during Dansgaard-Oeschger cycles. Proceedings of the National Academy of Sciences PNAS, https://doi.org/10.1073/pnas.2302283120

Further information:

• SISAL Database - global collection of speleothems, grouped by cave system: 
  Comas-Bru, L., Rehfeld, K., Roesch, C., Amirnezhad-Mozhdehi, S., Harrison, S. P., Atsawawaranunt, K., Ahmad, S. M., Brahim, Y. A., Baker, A., Bosomworth, M., Breitenbach, S. F. M., Burstyn, Y., Columbu, A., Deininger, M., Demény, A., Dixon, B., Fohlmeister, J., Hatvani, I. G., Hu, J., Kaushal, N., Kern, Z., Labuhn, I., Lechleitner, F. A., Lorrey, A., Martrat, B., Novello, V. F., Oster, J., Pérez-Mejías, C., Scholz, D., Scroxton, N., Sinha, N., Ward, B. M., Warken, S., Zhang, H., and SISAL Working Group members: SISALv2: a comprehensive speleothem isotope database with multiple age–depth models, Earth Syst. Sci. Data, 12, 2579–2606, https://doi.org/10.5194/essd-12-2579-2020, 2020.
• Malmierca-Vallet, I., Sime, L. C., and the D–O community members (inc. K. Rehfeld and Nils Weitzel): Dansgaard–Oeschger events in climate models: review and baseline Marine Isotope Stage 3 (MIS3) protocol, Clim. Past, 19, 915–942, https://doi.org/10.5194/cp-19-915-2023, 2023. 

Contact:

Professor Dr. Kira Rehfeld
University of Tübingen
kira.rehfeldspam prevention@uni-tuebingen.de

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