Vostok Ice Core
Objectives
• Identify glacial and interglacial time periods and describe how long they last • Compare the current interglacial to previous interglacials
• Describe relationship between temperature and greenhouse gas concentration changes
• Compare CO2 and CH4 concentrations between glacial and interglacial periods
• Use linear regression to find correlations and make predictions about the relationship between two
variables
• Compare the concentrations of greenhouse gases for different climate periods (glacial, pre-industrial,
current)
• Compare current changes in greenhouse gases and temperature to changes in the past
• Identify possible causes of climate change
Introduction
Ice cores provide the data to reconstruct changes in the atmospheric concentrations of greenhouse gases such as carbon dioxide before direct instrumental measurements began at Mauna Loa (Hawaii) in 1957. Snow falling in the polar and alpine regions of the earth (e.g. Greenland, Antarctica, and high mountain ranges) is preserved as annual layers within glaciers and ice sheets, provided that these layers are not destroyed by flow of the ice or melting during summer. These annual layers provide a record of the earth’s climate that reaches back as much as 800,000 years. Ice cores are drilled through glaciers and ice sheets and analyzed to recover these climate records.
In this assignment you will work with one of the key global change datasets – the Vostok ice core. The Vostok ice core was drilled in the 1980s by French, Russian, and American scientists at an old Soviet station in East Antarctica. We will be looking at the changes in temperature, carbon dioxide, and methane as a function of depth and age in the core. The overall questions to be addressed are:
• How did climate conditions at the height of the last Ice Age (around 20,000 years ago) differ from today’s conditions?
• What is the relationship between greenhouse gas concentration and temperature changes?
• How do changes in temperature and greenhouse gas concentrations since the Industrial Revolution
compare to the changes that took place near the end of the Ice Age?
This is a computer-based assignment.
You will need to use a computer that has Microsoft Excel on it.
A note on Stable Isotopes as an Indicator of Temperature Changes …
Stable isotopes of hydrogen and oxygen in water, as recorded in glacier ice, are indicators of temperature change. We talked about Oxygen isotopes in class. But we also look at Hydrogen isotopes. The two stable isotopes of hydrogen are “normal” hydrogen with one proton and no neutrons, and “heavy” hydrogen (called deuterium) with one proton and one neutron. The deuterium isotope is twice as heavy as the normal hydrogen isotope, and so water molecules containing a deuterium atom are more difficult to evaporate, especially at colder temperatures. This means that water vapor, and the snow that precipitates from it, contains fewer heavy water molecules (those that contain deuterium) than the original source
water (the ocean). This difference is measured by D, the ratio of heavy to light hydrogen isotopes (D/H) in a water sample. Water vapor and precipitation, such as snow that forms glacier ice, will always have a negative D value because the light water molecules are easier to evaporate while the heavy water molecules tend to remain in the ocean.
The preference for the light versus the heavy water molecules increases at colder temperatures. In other words, during an Ice Age when temperatures are colder, the heavy isotope is even harder to evaporate. This makes the D of snowfall, as recorded in ice cores, more negative at colder temperatures because it contains fewer heavy water molecules. Therefore, we can use measurements of D in ice cores as an indicator of temperature change. Past temperatures at Vostok are estimated from the isotopic composition (D) of the ice based on the observed relationship between temperature and D in modern snowfall:
Temperature (oC) = -55.5 + [(D + 440) / 6]
All of this is to say: we can estimate the PAST TEMPERATURE of the earth based on the composition of the ICE locked up in ICE SHEETS and GLACIERS.
Procedure
The Vostok data excel file contains both the actual data as well as graphs of the data to help you visualize what the data is showing. See the tabs at the bottom of the window to move between temperature, CO2, and methane data and graphs.
The data is graphed versus age, or years before present. So today is year 0 and the age increases as we go back in time. For example, the current interglacial runs from today to its start 11,000 years ago, or 0- 11,000 years on these graphs. When a graph window is active(click on it), moving the mouse over the graph will give you the values at the point – first number is age (x), second number is Y (temperature, CO2, or methane at that age, depending on the graph you are looking at.
Refer to the graph of estimated temperature at Vostok to answer the following questions.
1. How many interglacial periods, when the temperature at Vostok was at least as warm as or warmer than today’s average temperature of -55oC, are recorded in the Vostok ice core? Hint: count the number of different time periods that temperature at Vostok reaches or exceeds -55oC. Do not count every wiggle. Each period of time temperature gets above -55oC and stays close to or above -55oC counts as one interglacial. (The answer is less than 10.)
2. How long do interglacials last, approximately? Hint: move the mouse over the start and end of each interglacial to find the year that the interglacial started and ended. The difference is how long the interglacial lasted.
3. How many glacial periods, when the temperature atVostok was colder than today’s average temperature of -55oC, are recorded in the Vostok ice core? Hint: count the number of different time periods that temperature at Vostok remains continuously colder than -55oC. Again, the answer is less than 10.
4. How long do glacial periods last, approximately?
5. Which is the normal mode of climate during the last 420,000 years, an interglacial climate similar to today or a glacial climate? In other words, have the last 420,000 years been spent mostly in an interglacial climate or in a glacial climate? Explain your answer.
Now, compare the current interglacial (0 to 10,000 years ago) to previous interglacials with respect to temperature and duration (length of time). Note that there is more detail to the current interglacial – this represents a difference in sample resolution rather than an actual difference. There are more samples per unit of time closer to the surface because the ice compresses with depth. In other words, a given thickness of ice corresponds to less time near the surface of the ice sheet, and more time near the base of the ice sheet. That’s why there is more detail to the graph closer to time = 0 (the present). The small details are lost with depth as each ice sample represents a longer period of time.
6. Were previous interglacials warmer or colder than the current interglacial, which has a mean annual temperature at Vostok of -55 oC? Explain your answer. Hint: Move the mouse over the interglacials to find the temperature at their highest point.
7. How does the duration (length) of the current interglacial compare to previous interglacials? Explain your answer.
Relationship of Greenhouse Gas Concentrations to Temperature Changes
Refer to the plots of carbon dioxide (CO2) and methane (CH4) versus age to answer the following questions. Carbon dioxide concentrations are in parts-per-million by volume (ppm), methane concentrations are in parts-per-billion by volume (ppb). A part per million of carbon dioxide literally means that there is one molecule of carbon dioxide per every million molecules of air. One part per billion is 1/1000 of one part per million – other words 1000 ppb = 1 ppm.
8. Compare the plots of carbon dioxide and methane to your graph of temperature changes at Vostok. Describe how carbon dioxide and methane concentrations change in relation to temperature.
9. Discuss, in general terms, how CO2 and CH4 concentrations during glacial periods compare to those during interglacial periods.
10. Note the timing of the four major warming events representing deglaciations (the end of an ice age and the beginning of an interglacial climate). Then look at how CO2 and CH4 change during the same time. Can you tell which changes first, temperature or greenhouse gas (CO2, CH4) concentrations? Does it matter? Explain your answer. Hint: Can you find any examples of time periods when temperature changed without greenhouse gas concentrations also changing? Or any time periods when greenhouse gas concentrations changed without temperature also changing?
Use the graphs of carbon dioxide (CO2) vs. temperature and methane (CH4) vs. temperature to answer the following questions. Unlike the previous graphs, which plotted the variables (temperature, CO2, CH4) versus time (age), these graphs are scatterplots that graph the variables versus each other – CO2 versus temperature and methane versus temperature. This allows us to directly examine the relationships between variables. These graphs illustrate what you should have already noticed: there is, in the past, a strong relationship between temperature and the concentrations of the greenhouse gases carbon dioxide and methane. Each of these plots has a linear trendline with an equation and R2value, or correlation that describes this relationship between greenhouse gases and temperature mathematically. An r2value of 1 indicates a perfect 1:1 relationship. An r2value of 0.5 is generally considered a strong relationship. An r2value less than 0.3 is often considered weak.
11. Which is better correlated (has a higher r2value) with temperature, CO2 or CH4? Explain why. Hint: how do atmospheric CO2 concentrations compare to methane concentrations?
12. Use the past relationship between CO2 and temperature to predict the expected average temperature at Vostok now given the recent increase in the amount of CO2 in the atmosphere. To do this, plug in today’s CO2 concentration (you’ll need to look this up) in for x in the equation given by the scatterplot of CO2 vs. temperature and solve for y (temperature). Report your answer and show your work.
13. Do you think this a good estimate for today’s temperature at Vostok? Explain why or why not. Hint: We have considered only the effects of CO2 on temperature. Do you think this makes for an accurate prediction? Are there other factors that might influence temperature? What other factors in the climate system might have an important affect on temperature?
14. The current mean annual temperature at Vostok is around -55°C. However, we predict a much warmer temperature should exist at Vostok today, given the observed relationship between CO2 and temperature that existed in the past. Brainstorm and list possible reasons for this apparent discrepancy, such as other factors that affect temperature besides CO2, or reasons why warming may lag behind the CO2 increase.
How Do Recent Changes Compare to the Changes at the End of the Last Ice Age?
Let’s take a closer look at the changes in temperature and greenhouse gas concentrations that occurred at the end of the last Ice Age. Return to the graph of temperature versus age. Right click on the X axis and choose Format Axis. A dialog box appears. Set maximum value to 25,000, major unit to 5000, and minor unit to 1000. Click close. The graph is now zoomed in on the period 0-25,000 years ago- which encompasses the time from the last glacial maximum (the coldest part of the most recent Ice Age) to the present day. Do the same for the graphs of CO2 and CH4 versus age and answer the following questions.
15. What was the average CO2 and CH4 concentration during the last glacial maximum (20,000-25,000) years ago? Hint: use Excel to calculate the average over the given time period by entering this formula in any blank cell to calculate the average over an appropriate range of cells: =average(cell1:cell2) – for example, the average CO2 concentration from 20,000-25,000 years ago is given by entering the formula =average(b20:b24) into any blank cell on the GHG vs Temp tab.
CO2:
CH4:
16. The current interglacial is called the Holocene and began ~11,000 years ago. What are the Holocene averages for these gases in the Vostok ice core (0 to 11,000 years ago)? Note that the Vostok data ends 2300 years ago because it takes time for the very slow snow accumulation rate at Vostok to bury snowfall to a sufficient depth for densification to seal the bubbles and trap a sample of the atmosphere, so your average for the Holocene will only cover the time period for which we have data from the Vostok ice core and exclude any recent changes due to industry and agriculture.
CO2:
CH4:
17. What is the difference in CO2 and CH4 between the last glacial maximum and the Holocene average?
CO2:
CH4:
18. Current concentrations of CO2 and CH4 are 390 ppmv and 1865 ppbv, respectively. What is the difference between the average Holocene values and today’s concentrations?
CO2:
CH4:
19. How much warming (in degrees) occurred at Vostok between the last glacial maximum and the Holocene? Use the graph of temperature changes over time to answer this question.
20. How do the human-caused increases in CO2 and CH4 since the industrial revolution compare to the increases at the end of the last Ice Age? What does this suggest about the amount of warming we might expect over the coming decades and centuries, given the increase in CO2 and CH4 that has already occurred?
