Which of the following is a more effective tool for learning about past changes in Earth’s climate: measurements from paleoclimate records or outputs from climate model simulations? Depending on who you ask, you will probably get a different answer to this question. Through my research on climate in Antarctica, I’ve been convinced that both tools are equally important. In fact, both tools are necessary in order to maximize understanding of the Earth’s climate system. The challenge for climate scientists is to effectively combine information from both data and models, which are often used by separate scientific communities.

In the final phase of my PhD work at the University of Washington, I’ve combined climate model results with ice-core data to improve understanding of glacial-interglacial changes in Antarctica. Through an Interdisciplinary Graduate Fellowship funded by the Program on Climate Change, I brought together my research analyzing measurements of the South Pole ice core with a climate model analysis of fundamental atmospheric processes. Not only did this project bridge the gap between data and models, it created a collaboration across departments, connecting my work with Eric Steig in Earth and Space Sciences to my work with Dargan Frierson in Atmospheric Sciences. This is exactly the type of interdisciplinary collaboration needed in the field of paleoclimate research to create innovative progress on important questions.

My focus in analyzing the record of the South Pole ice core has been a temperature reconstruction of the most recent deglacial warming, which occurred about 20 thousand years ago. There’s been a longstanding discrepancy over East Antarctic temperature estimates between ice-core data reconstructions and climate model results. Compared to previous ice-core-based estimates, climate models do not produce a cold enough glacial period, leading scientists to conclude that something is missing from the models (Crowley and North 1991, Masson-Delmotte et al., 2006). My new results show that it’s actually the ice-core interpretations that have neglected an important process; in fact, the climate models agree well with an improved interpretation of the ice-core data. There’s just one wrinkle. The agreed-upon climate change is a bit small compared to what might be expected from polar amplification. This newfound model-data agreement motivates the overarching question of this project: what fundamental physics in climate models can explain this smaller-than-expected glacial-interglacial temperature change in East Antarctica?

To answer this question, I simulated highly-idealized glacial-interglacial transitions in Antarctica. I used an aquaplanet GCM (Kang et al., 2008) to examine the temperature response to changes in atmospheric CO₂ levels across different configurations of Antarctic topography. I tested a flat topography, a cone topography, and an asymmetrical cone with slightly lower elevation in West Antarctica (Figure 1). Under CO₂ levels of 180 and 280ppm, I ran each topography to equilibrium and compared the resulting temperature response.

The main result I found was that higher topography damps the Antarctic surface temperature response to an increase in CO₂. Furthermore, when comparing the mean temperature change in Antarctica compared to that of the tropics, the results show that polar amplification of warming is also damped by higher topography. As seen in Figure 2, the flat topography has the largest factor of polar amplification of warming, while the cone topography has the smallest factor. The East and West regions of the step topography represent intermediate scenarios. This relationship between height and temperature change has been seen before in the presence of high surface albedos over the ice sheet (Salzmann et al., 2017, Hanh et al., 2019). However, albedo is low over the aquaplanet ocean surface, so these results show that there must be a more fundamental process in the atmosphere that contributes to this effect of topography.

The jury is still out as to what exactly these fundamental processes are that can explain the relationship between height and polar amplification of warming. But even at this point, the result has an important implication for my ice-core data reconstruction. The relatively small glacial-interglacial temperature change that I reconstruct should be expected simply because of the high elevation of East Antarctica!

With my ice-core data analysis alone, my thesis shows that previous ice-core temperature estimates were missing an important piece of their interpretation. However, only with a simple climate model experiment can I start to understand the physical processes underlying this result. On their own, climate models and data are useful tools, but when used together they can illuminate new perspectives and deeper understanding. In this way, this interdisciplinary project greatly expanded the scope of my work. Including an entirely new analysis approach has added an additional dimension to my research.

Going forward, this project has introduced me to new collaborators in the climate modeling world that I will continue to work with as we finish analyzing these results. In general, more such connections need to be developed between the paleoclimate data and the climate modeling communities. Instead of stopping at a simple verification between model and data results, comprehensive projects that dive deeply into both the model and data side of a question can take advantage of the powerful combination of these tools.

My main takeaway from this project is to always talk to those folks who use different approaches to answer similar questions. You never know what new dimensions and perspectives can be added to the status quo of a given technique in a scientific field! Huge thanks to the Program on Climate Change for making this work possible.

Emma Kahle earned her PhD from the University of Washington where she studied how Earth’s climate has changed in the past using data from Antarctic ice cores. She is now engaged in a variety of outdoor projects around the world related to studying, teaching, and communicating climate change science. You can reach her at emmackahle@gmail.com.

Crowley, T. J., and G. R. North (1991). Paleoclimatology. New York, NY: Oxford University Press.

Masson-Delmotte, V., Kageyama, M., Braconnot, P., Charbit, S., Krinner, G., Ritz, C., and Gladstone, R. M. et al. (2006). Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints.  Climate Dynamics, 26(5), 513-529.

Kang, S. M., Held, I. M., Frierson, D. M., and Zhao, M. (2008). The response of the ITCZ to extratropical thermal forcing: Idealized slab-ocean experiments with a GCM. Journal of Climate, 21(14), 3521-3532.

Salzmann, M. (2017). The polar amplification asymmetry: role of Antarctic surface height. Earth System Dynamics, 8(2), 323-336.

Hahn, L., Armour, K., Battisti, D., Pauling, A., Donohoe, A., and Bitz, C. M. (2019, December). Understanding Asymmetries in Arctic and Antarctic Lapse-Rate Feedbacks and Polar Amplification. In AGU Fall Meeting 2019. AGU.