A decade ago, Bradley Markle, an assistant professor at the Institute of Arctic and Alpine Research at the University of Colorado Boulder, noticed an odd pattern while sifting through temperature records from the end of the last ice age in Antarctica. The records seemed to defy prevailing theories of how temperatures vary across the Antarctic continent.
The anomaly piqued Markle's interest, but he had to file it away in favor of finishing his Ph.D. at the University of Washington. Now, ten years later, Markle has resolved the mystery.
Along with his former advisor, Eric Steig , Markle published a paper in The Proceedings of the National Academy of Sciences (PNAS) outlining a new principle of Antarctic temperature variation. The authors found that the greenhouse effect — the process by which atmospheric gases trap heat — explains why some places in Antarctica tend to warm or cool more dramatically than others.
"Because the greenhouse effect is nonlinear, it amplifies changes in warmer places more than colder places," Markle said. "Water vapor is the strongest greenhouse gas, and its concentration increases as the temperature goes up."
Antarctica plays an important role in our planet's climate system. It is one of two regions on Earth that radiate more energy than they absorb (the other is the Arctic). As such, it acts as an exhaust valve for excess heat. Markle and Steig's new finding, if adopted by other researchers, could fundamentally reshape scientists' understanding of climate dynamics, not just in Antarctica, but across the globe.
The role of the atmosphere
While all of Antarctica is cold by nonpolar standards, there is a wide range of local climates. Antarctica contains roughly half of Earth's total surface temperature range.
"The temperature change you'd experience, on average, going from Abu Dhabi to the coast of Antarctica is about the same as going from the coast of Antarctica to the coldest place in Antarctica," Markle said. "There's this huge range of surface temperatures."
As the global climate warms, so too does Antarctica. But that warming doesn't affect all regions equally.
In the past, scientists looked to a physical principle called Planck response to design models of how Antarctic regions respond to overall temperature change. The Planck Response reasons that, as an area gets warmer, it emits more heat back into the atmosphere. By this logic, warmer areas should respond less dramatically to climate shifts than colder ones.
However, a growing body of evidence ran counter to this principle. Temperature records derived from chemical analyses of ancient ice, however, suggested the opposite.
Initially, colleagues suggested that the anomaly could be explained by variations in ice sheet thicknesses, marine temperature dynamics, or atmospheric heat transport. But these explanations did not satisfy Markle. Instead, he hypothesized that the greenhouse effect played a more important mediating role.
"There was something more fundamental at play, because we saw these relationships for a variety of forcing mechanisms and over a variety of timescales," Markle said. "The best predictor of how much temperature change happened was the temperature an area started at."
It took many more years of investigation to gather enough evidence to publish the theory. First, Markle and Steig refined ice core analysis methods to produce a highly detailed reconstruction of Antarctic surface temperatures going back 160 thousand years. Then, Markle compared this record to a mathematical model of surface temperature changes based on his greenhouse effect theory. Finally, he compared that relationship to a simulation from an atmospheric model produced by scientists at the National Center for Atmospheric Research (NCAR).
The results were striking. All data sources pointed toward Markle's hypothesized relationship, where warmer regions respond more dramatically than cold ones.
"Over the specific surface temperatures of Antarctica, the greenhouse effect is nonlinear, it's curved," Markle explained. "That means, for a given change in surface temperature, you get an increasingly bigger change in the greenhouse effect over that range of average surface temperatures from minus 60 to minus 20."
A new framework
The discovery comes with a knock-on bonus. Markle found that one particular local variation from his greenhouse effect model could be explained by changes to the local thickness of the Antarctic ice sheet. Essentially, Markle had created a new way to reconstruct past ice sheet thickness across the full span of the ice core record.
"Before this, people more or less assumed that differences between warming and cooling across Antarctica on these time scales was caused by the ice sheet itself," Markle said. "Knowing this, we can now better understand how much the ice sheet actually did change."
Now that the results are out there, Markle hopes the scientific community can stress-test the theory. If the relationship is integrated into modeling efforts, it could help researchers investigate how Antarctica will react to current and future climate warming.
"This pattern really shows up when the average temperature change is large," Markle said. "Today, there are all of these weird atmospheric changes occurring with warming, and I'm really curious to see if this underlying pattern holds up or is overwhelmed."
Markle stresses that this kind of research would not be possible without analyzing ice core records that stretch back millennia beyond instrumental climate records. While this kind of research is sometimes viewed as less immediately relevant, the results reveal something about our present climate system that in situ records could never produce.
"One reason I like this study is it represents the value of looking at the climate in the past," Markle said. "The majority of paleoclimate research is more about history, but this is more about a process."
