At a Glance
Ice core studies from the Allan Hills Blue Ice Area in Antarctica have yielded ice dating back to 2,000,000 years ago, the oldest ever retrieved. Analysis of ice dating to 1,000,000 years ago suggests that links between climate and carbon dioxide (CO2) are similar to those of more recent glacial cycles.
Many scientists have looked to characteristics of past climates as one guide to our planet’s future climate. A wide range of “paleoclimate” archives have been studied for this purpose. Of these archives, ice cores drilled through the Greenland and Antarctic ice sheets are unique in that they preserve fossil air that was trapped more or less when the snow fell. Ice core samples have been used to characterize variations in the CO2 concentration of air, among other properties. Through ice core studies, we have a detailed record of atmospheric CO2 variations back to 800,000 years ago. The results show that global climate varies closely with CO2 concentration changes during the ice ages. Heretofore, no older ice had been retrieved. This is a serious limitation because glacial cycles were different before 800,000 years ago: earlier glacial cycles were less intense and lasted for a shorter time. We want to understand the difference in conditions between earlier ice ages (1,000,000-2,000,000 years ago) and the past 800,000 years.
My colleague John Higgins and I, working with a group from the University of Maine, along with graduate student Yuzhen Yan, have searched for old ice in a previously unexplored Antarctic environment: the Allan Hills Blue Ice Area. Preceding ice core studies have drilled cores where the ice age at the surface is zero, and age increases progressively with depth. In the Allan Hills, the Transantarctic Mountains block the flow of ice to the oceans. They guide old ice from the bottom of the ice sheet to the surface (Figure 1.4). In three expeditions to the Allan Hills over the past six of years, we have recovered ice dating back to 2,000,000.
We have analyzed ice dating to 1,000,000 years ago for CO2, methane (CH4), and other properties. The data suggest that links between climate and CO2 are similar to those of more recent glacial cycles. During the last expedition (austral summer of 2015-2016), we recovered ice as old as 2,000,000 years. To date we have focused on the time-consuming process of dating the ice. Our Maine collaborators have analyzed the isotopic composition of the ice, which turns out to reflect past temperatures of the study region. We will begin making analyses of greenhouse gases (CO2 and CH4) shortly.
Studies of million-year-old ice showed that, at that time, Earth’s climate properties co-varied as during more recent times. For example, the relation between CO2 and Antarctic temperature was the same 1,000,000 years ago as during the larger climate fluctuations of more recent times. With our new samples, we will determine if this pattern holds true further back in time. We will also be able to test the hypothesis that the long-term climate cooling of the last 3,000,000 years ago was a consequence of declining levels of CO2 in air. Overall, this research will contribute to the objective of studying Earth’s past climates to advance our understanding of the response of climate to changing levels of CO2 and other greenhouse gases.