Climate History & the Cryosphere
Part B: Ice Cores
Ice Core Climate Records
In order to fully understand the implications of how climate is changing today, it is important to look at historical records to see how climate has changed in the past. Current climate data collection methods, including satellite observations, only cover a very small window of Earth's long history with respect to climate change time scales. Luckily, clues to past climatic conditions, dating hundreds of thousands of years back in time, are recorded in glacial ice all over the world. Paleoclimatologists (scientists who study past climate) make inferences based on indirect measures of proxy databiological, geological, or chemical indicators that reflect climate conditions. For example, glacial ice is made up of layer upon layer of compacted snowfall that contains dust, pollen, gas bubbles, and other materials that give us clues about what climate was like at different times in the past.
- Watch the video below to learn more about how scientists use ice cores to study past climate. Then answer the Checking In questions.
- As you watch, take notes and write down any questions you have about ice cores.
Ice Core Data
This activity was adapted from Vostok Ice Core: Excel (Mac or PC).
Several different climate indicators can be measured from samples of the ice:
- dust: The amount of dust in each annual layer provides information about airborne continental dust and biological material, volcanic ash, sea salts, cosmic particles, and isotopes produced by cosmic radiation that were in the atmosphere at the time the dust was deposited in the ice. The color contrast between dust and snow also provides a visual indicator of boundaries between different ice layers.
- air bubbles: Bubbles trapped in ice cores give scientists actual samples of air from hundreds of thousands of years ago. By analyzing the composition of the air in these bubbles, we can find out what the atmosphere was like long ago.
The maximum amount of moisture that air can hold drops with decreasing temperatures. When humid air cools, the water molecules will condensate to form precipitation. Heavier isotopes have a slightly higher tendency to condensate, so humid air gradually loses relatively more and more of the heavier water molecules (H218O or HD16O). Every time precipitation forms, the air mass becomes more depleted in heavy isotopes. During cold conditions (e.g., during winter or in a cold climatic period), the air masses arriving in over ice sheets have cooled more and have formed more precipitation, which means that the remaining vapor is more depleted in heavy isotopes. Deuterium depletion (δD) therefore, can be used as a proxy for temperature.
The Vostok core was drilled in East Antarctica, at the Soviet station Vostok from an altitude of 3488 m, and has a total length of 2083 m. Ice samples have been analyzed with respect to isotopic content in 2H (δD), dust, and methane and carbon dioxide trapped in air bubbles. The profiles of 2H, methane, and carbon dioxide concentrations behave in a similar way with respect to depth in the core, showing a short interglacial stage, the Holocene, at the top, a long glacial stage below, and the last interglacial stage near the bottom of the core. The record goes back in time about 160,000 years.
Part 1: Gas Age vs. Ice Age
Age is calculated in two different ways within an ice core. The ice age is calculated from an analysis of annual layers in the top part of the core, and using an ice flow model for the bottom part (the details of which are beyond the scope of this unit). The gas age data accounts for the fact that gas is only trapped in the ice at a depth well below the surface where the pores close up.
- Download the Vostok ice core data (Excel 44kB Dec18 12) and open the spreadsheet in Excel. If you are unfamiliar with Excel, go here for tips and tutorials or use the program's Help function to assist you.
- Plot both the ice age and the gas age as a function of depth on the same graph.
Stop and Think
1: Does age increase or decrease down the core? Why?
2: Is the thickness of an annual layer of ice smallest at the top or bottom of the core? Why?
Part 2: δD as a proxy for temperature
Next you will calculate the temperature based on the isotopic composition of the water in the ice core.
- Insert a blank column into the table to the right of the delta-deuterium column (δD). Label the new column Temperature (degrees C).
- Calculate the temperature at the Vostok station based on the following formula describing the empirical relationship between temperature and deuterium concentration. *Be sure to save your work!*
Temperature (°C) = -55.5 + (δD + 440) / 6
- Plot your calculated temperature vs. ice age.
Part 3: CO2 and Dust
- Plot CO2 as a function of gas age. Compare it to the graph you made of temperature vs. ice age. How closely does the plot of CO2 resemble that of temperature?
- Now make a plot of dust as a function of ice age. Compare this to the temperature vs. ice age plot; how well do the changes in dust concentration correlate with the temperature changes?
Stop and Think
3: The present atmospheric CO2 concentration is 379 ppmv (IPCC, 2007). Calculate the change in CO2 concentration between the last glacial maximum (~20,000 years ago) and the 18th century, and between the 18th century and today. You can assume that the shallowest ice core measurements represent the environmental conditions in the 18th century. Why were CO2 and dust concentrations different during the glacial time as compared to the 18th century?
4: Why are these ice core paleoclimate records so important to our understanding and prediction of climate change?
5: Note that there were two major warming events representing two deglaciations in the Vostok calculated temperature data. Look at how CO2 changes during those deglaciation periods. From the data provided in this lab, can you tell which changes first, temperature or CO2 concentration? Why is this important?
Optional Extension: Tropical Ice Cores
Although most of the planet's glacial ice is found near the poles, there are also many mountain glaciers near the equator that can provide scientists with different clues about Earth's climate history, particularly about climate events like El Nino and monsoons, which don't occur near the poles.
- Read Background Essay: Tropical Ice Cores Measure Climate (Acrobat (PDF) 46kB Jul4 11).
- Watch this NOVA scienceNow video segment Tropical Ice Cores Measure Climate about glaciologist Lonnie Thompson and his research of tropical mountain glaciers. Then answer the Checking In questions.