Dr. Grieman gave four goals of understanding for her seminar: (1) what paleoclimate proxies are and why they are used, (2) the basics of ice core science and how they are collected, (3) how chemicals trapped in ice cores are measured, and (4) how these chemicals can be used to investigate ice sheet change.
Dr. Grieman explained paleoclimate proxies as models that show changes in global temperature over long periods of time, while simultaneously allowing scientists to make predictions for how future climate scenarios could manifest by finding analogous instances of climate changes in Earth’s distant past. Because humans have reliable global temperature data going only as far back as 1850, paleoclimate proxies, such as those that can be modeled from ice core analysis, allow scientists to find data from much further back in time. Dr. Grieman explained that there are in fact many different ways paleoclimate proxies can be obtained through analyzing stalagmites in deep caves, river and lake sediments; non-quantitative historical records; coral samples; and so on. There are also three primary variables scientists need to consider when deciding which method to use. One, how far back in time do you want to go? Two, what is the temporal resolution of the record? Does it get seasonally specific, or does it only show changes by several millennia at a time? Three, how far of a range does the proxy give you? A tree ring can give you information about a forest, but not much about global climate.
For Dr. Grieman’s study, ice cores were the proxy used. Ice cores are drilled mainly in Greenland and Antarctica, but sometimes on glacial sheets in subarctic areas too. The location of the drill site can also affect how old the ice core will be. Cores are drilled from sheets, which form over millions of years in reliable arctic and Antarctic regions of the planet, due to snowfall becoming continually compressed, trapping air from the environment during the period of the sheet’s formation. Cores must also be drilled at the peak of an ice sheet’s flow, giving the researchers easily delineated, horizontal lines of ice deposits to analyze. Cores can tell scientists a multitude of different things about the era being analyzed, such as temperature, greenhouse gasses, wind speed, sea ice extent, volcanism, solar activity, atmospheric chemistry, biological productivity, and much more.
Dr. Grieman and her team’s goal was to determine at what point in recent paleoclimatological history Earth’s temperatures were as high or higher than they are today, in order to analyze what the climatological features of that kind of environment are. Dr. Grieman explained that the period of time they found to match this criterion was called the Last Interglacial, and took place approximately 129,000 to 116,000 years ago. At that time, Antarctic temperatures were what scientists predict them to be in the year 2100, and sea levels were about six to nine meters higher than the present. The causes of the Last Interglacial period were changes in Earth’s orbit and ocean circulation, not greenhouse gas-related. Dr. Grieman’s team was also attempting to determine how much Antarctic ice contributed to sea level rise during that time.
The specific motivation of the team was to analyze an ice core from the West Antarctic Ice Sheet to see if it collapsed during the Last Interglacial period, as well as if the Ronne ice shelf retreated. The team’s approach was to drill a 651 meter ice core at a location where ice was likely still present during the Last Interglacial period, and then to analyze the gasses trapped inside to predict the environment of the time.
The team was at the Antarctica site for three months drilling dozens of cores for analysis. Determining age scales for the cores can be done with analysis of chemical impurities with annual cycles and matching up methane levels with other collected cores. The key indicators of ice sheet/shelf change—what the team was analyzing—are water isotopes, which can show elevation and climate pattern changes; sea salt content, which shows ocean proximity at the time; and overall air content, which can also show elevation change.
To measure these indicators, cores are melted, then that water is pumped through various instruments, such as an ion chromatograph. The age of the cores is manually counted until 2,000 years back, or the first 300 meters of the core. After this, the cores are compared with methane and isotope records to map further into the past. In measuring the amount of sodium, magnesium, and calcium present in the cores, Dr. Grieman saw that levels peaked after the Last Interglacial, suggesting that the ice sheet did not collapse during the period of time predicted, as chemical traces are much higher when an ice sheet is closer to the ocean. If the ice sheet collapsed, it would then have been after the end of the Last Interglacial, not during.
Dr. Grieman and her team will be continuing their research on this topic, primarily to determine the reasons for the surprising outcomes of their work, namely the fact that the ice cores showed a sea salt content peak just slightly after the end of the Last Interglacial, and another unexpected sea salt increase around 7,000 years ago, which does not occur in other ice core records.
Dr. Grieman’s work on this subject is essential to our understanding of Earth’s climate history, and how that might help us contend with the climate change we are now experiencing.