Palaeoclimate: Ice Cores, Isotopes and the Record of Past Change

How do we know what Earth's climate was like thousands of years before thermometers existed? Scientists read natural archives instead. Two of the best are deep sea sediment cores and ice cores. Layer by layer, an ice core preserves old snowfall, and the tiny air bubbles sealed inside it hold actual samples of the ancient atmosphere. This module introduces oxygen isotopes and shows how their ratio acts as a thermometer from the past: the mix of heavy and light oxygen in ice or in shells shifts with temperature and with how much water was locked up in ice sheets. You then practice the real skill, working with actual climate data to investigate the Laurentide Ice Sheet, the giant ice mass that once covered much of North America. The exercises ask you to analyze records yourself rather than just read conclusions, which is exactly what palaeoclimate scientists do: measure a proxy, state the assumption behind it, and build the story of past change from the evidence.

This unit shows how palaeoclimatologists reconstruct climates from long before anyone could measure them. One key method uses isotopic ratios as stand-ins for past temperature, because the balance of heavy and light isotopes preserved in rocks, shells, and ice changes with the conditions in which they formed. You then apply that thinking to two dramatic ancient episodes. The first is Snowball Earth, a time in the Neoproterozoic when ice may have reached nearly to the equator. The second is the Paleocene-Eocene Thermal Maximum, a burst of extreme warmth linked to a huge release of greenhouse gases. Comparing a deep freeze with a hothouse shows that climate can swing far, and that greenhouse gases are tied to those swings. The bigger point is that modern climate change is not a new kind of event. It is the latest chapter in a very old relationship between carbon and temperature that the geologic record lets us read.

This lab makes the isotope thermometer feel real. Water molecules come in slightly different weights because oxygen has isotopes, and the balance between heavy and light water in ice shifts with the temperature at which the snow fell. To build intuition for this, you use pennies of different masses as stand-ins for isotopes, measure them, and watch how the mix oscillates over time, just as isotope ratios in a real ice core oscillate between cold glacial periods and warmer interglacials. Then comes the payoff: you compare your penny results with actual isotope measurements from real ice cores. The activity teaches the central habit of proxy science. You never measure past temperature directly. You measure something that moved with temperature, check the assumption connecting the two, and only then make an inference about how warm the Earth was.

The ocean keeps its own climate diary. Tiny shelled creatures build their shells from seawater, and the oxygen isotope ratio in those shells records how much of the world's water was frozen into ice sheets when the shell formed. In this activity you work with real data from Ocean Drilling Program core 677, one of the classic records of Quaternary climate. As you plot the isotope values down the core, a rhythm appears: ice sheets grew and shrank in regular cycles over the last few million years. Those cycles match the slow wobbles in Earth's orbit, which pace the ice ages, while feedbacks like ice reflectivity and greenhouse gases amplify the response.

Glaciers are slow, but they are honest recorders of climate. This lab begins with land ice itself: how snow compacts into glacial ice, how glaciers flow, and over what timescales they grow and shrink. Those timescales matter because they tell you what a glacier can and cannot record. Next, an online interactive lets you explore how glaciers give scientists evidence of climate change, from the layers in their ice to the marks they leave on the landscape as they advance and retreat. Finally, you measure change yourself. Using image processing software, you track how much area a real glacier has lost as temperatures have risen. The lab connects the deep past to the present: the same ice that archives ancient atmospheres in its bubbles and isotopes is now shrinking in front of our cameras, giving a modern, visible signal of warming.