Lab 6: Glacial-Interglacial Cycles
The previous labs have allowed you to examine the the concept of seasons, the general composition and characteristics of the Earth’s atmosphere, and the impacts of solar radiation, greenhouse gases, and albedo on Earth’s surface temperature. This lab ties it all those factors together as you explore glacial periods and interglacial periods over the past 400,000 years. You already have had some exposure to the last glacial period: you explored Earth’s surface temperature during the Last Glacial Period, which occurred approximately 20,000 years ago.
By the end of this lab, you should be able to answer the following research questions:
Why do glacial periods occur?
What happens when Earth transitions from a glacial period to an interglacial period?
How does an interglacial period differ from a glacial period?
Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:
Fact based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor, (2) found in background sections, or (3) determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.
Synthesis based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.
Hypothesis based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.
So let’s begin with a little recap that will take us all the way back to Lab 1. In the first lab, we learned about how the intensity and duration of solar radiation affect the Earth’s temperature and therefore produce the seasons. In the last lab, we examined how other factors like albedo and greenhouse gas concentrations can impact temperature. In this lab, we are going to look at one more set of factors that can influence the Earth’s temperature. What we are really interested in is what controls temperature at a global scale – and the only way to do that is to consider the whole set of factors together.
We are currently in an ice age that began 2.6 million years ago. Yes, we are in an ice age. Once the shock of reading that last statement wears off, realize that the only requirement for labeling the Earth as being in an ice age is that there has to be permanent (year-round) and somewhat extensive ice cover somewhere on the planet. Right now, that ice cover is on the Arctic, Greenland, and the Antarctic.
While ice ages generally involve a lowering of the average global temperature, there are regular fluctuations during any ice age between slightly cooler periods and slightly warmer periods. These fluctuations occur over long time scales – thousands of years. During glacial periods, the temperatures are below the overall average which, as you might expect, causes ice sheets to extend their range. During interglacial periods, the temperatures are above the overall average which causes ice sheets to retreat. Interglacial periods separate glacial periods and vice-versa; this is the glacial-interglacial cycle. Scientific measurements indicate that about 80% of a glacial-interglacial cycle is spent in the glacial period – i.e. the cooler periods are longer than the warmer ones.
But how do we know any of this? During the Prezi, you were introduced to the process of obtaining and making measurements on ice core samples. One of the places where this work is done is Vostok; click Vostok to open a Google™ Earth file and see where this research station is located. As the image below shows, this site has been chosen because it provides information about conditions on Earth as far back as 422,000 years.
Now, we are going to take a look at some of the data that has been generated at the Vostok research station. Click Vostok_Temperature to open the Excel file of interest. Do the following to create a graph from the data in that file:
• Select cells in rows 2-424 of columns A and B.
• Under the Insert tab, select Line and then choose the first 2-D line.
• The resulting chart shows temperature changes — as differences from the average temperature — over the past 422,000 years. Interglacial periods typically have temperatures at least 3° C above the average.
Based on the graph and the other information above, answer the following questions:
Q1: Where is the Vostok station located?
Q2: Are we currently in a glacial period or an interglacial period? How do you know?
Q3: When did the most recent ‘maximum’ in a glacial period (which is really a temperature minimum or valley in the graph) occur?
Q4: What is the approximate difference in temperature between a maximum in a glacial period (i.e. a valley) and a maximum in a preceding or succeeding interglacial period (i.e. a peak)? (Before moving onto Q5, consider this temperature difference – it should be surprising to you. Why?)
Q5: What is the approximate time interval between maximums in glacial periods (i.e. the time between ‘valleys’ in temperature)?
Q6: What is the approximate time interval between maximums in interglacial periods (i.e. the time between ‘peaks’ in temperature)?
You have just observed an example of climate change. The Intergovernmental Panel on Climate Change (IPPC) defines climate change as a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). In other words, climate change is a long-term shift in the statistics of the weather (including its averages). The climate change you observed was totally natural and occurred over long time scale (i.e. thousands of years). You will examine in later labs the possibility of human activities causing climate change over short time scales (e.g., several decades).
In Part 2 of this lab, we were able to use temperature data collected at the Vostok research station to see that there have been regular time intervals between glacial and interglacial periods during the course of Earth’s history – well, at least the last 422,000 years of that history. The next logical item to consider is what factors cause those regular shifts between glacial and interglacial periods. It turns out that three of the most critical factors are related to the position and orientation of the Earth with respect to the Sun: eccentricity, precession, and obliquity (or tilt). To get a sense of what these terms mean, you are going to examine a simulation that when clicked takes you to the simulation.
if you haven’t done so already, click on the image above to go to the simulation. Now, do the following:
• After reading the important disclaimer in the grey box, click on <OK>.
• In the lower left hand corner, click on <Show Top View>.
• For the time being, remove the check from <Labels>.
• Next to <Labels>, check the <Eccentricity> box.
• Move over to the right site of the animation window, where there is a graph that will have temperature data from the Vostok ice core in green; this data should look familiar from Part 1. The graph also possesses a time slider running from ‘Now’ down to ‘400,000 years ago’. Move the slider through that time scale and notice what happens in terms of the position / orientation of the Earth relative to the sun when you do this …
Q7: Based on what you see happening as you move the time slider, what does ‘eccentricity’ mean?
Q8: What is the general relationship between the minimums and maximums in eccentricity (in purple) and the temperature (in green)?
Q9: What is the approximate length of the eccentricity cycle (i.e. how many years occur between periods of minimum eccentricity or how many years occurs between periods of maximum eccentricity)?
• Move the slider back to the present day (‘Now’), uncheck the ‘Eccentricity’ box and check the ‘Labels’ box.
• You will notice the following terms in yellow appear and disappear during one complete revolution around the Sun: Spring Equinox, Summer Solstice, Aphelion, Autumnal Equinox, Winter Solstice, and Perihelion. You learned what equinoxes and solstices were in the Solar Radiation & Seasons lab. Aphelion and perihelion should be new terms to you …
Q10: Based on the meaning for eccentricity you determined above, what does perihelion represent and what does aphelion represent?
Q11: In the present day, at what time of year do perihelion and aphelion occur?
• The three terms you just explored all have to do with the distance between the Earth and Sun. It is important to know that, despite the way the animation may portray things, the Earth’s orbit is nearly circular and there is only a 3.4% difference between the Earth’s distance from the Sun in the aphelion and in the perihelion.
• Uncheck the <Labels> box and click on the <Tilt> box. [Note that obliquity is another term for tilt.]
• You should already have an understanding of what tilt means, so lets focus on some of the changes in the Earth’s tilt over time. Slowly move the time slider from ‘Now’ down to ‘400,000 years ago’. Stop the slider at some of the peaks (purple line is to the right) in the tilt plot, as well as some of the valleys (purple line is to the left).
Q12: What is the relationship between the minimums and maximums in tilt and the Earth’s temperature?
Q13: What is the approximate length of the tilt cycle (i.e. how many years elapse between periods of minimum tilt or how many years elapse between periods of maximum tilt)?
• The Earth’s tilt has a minimum value of about 22.05o and a maximum value of 24.5o. Although this range is rather small, it has a significant effect on the Earth’s temperature. As you learned in the Solar Radiation lab, the tilt of the Earth is the main reason for the seasons.
• Uncheck the <Tilt> box and click on the <Precession> box.
• Making sure the time slider is back at ‘Now’, follow the Earth through a couple of orbits …
Q14: Precession relates to a feature of the Earth’s tilt. What do you notice is happening to the Earth’s tilt when <Precession> is highlighted that could explain what this term means? [This NASA video may help also.]
• Slowly move the time slider from ‘Now’ down to ‘400,000 years ago’, stopping at some peaks and valleys to note what is happening to the precession when you do this …
Q15: What is the approximate length of the precession cycle (i.e. how many years occur between periods of minimum precession or how many years elapse between periods of maximum precession?)
What you just observed were the Milankovitch cycles. You estimated the cycles for the eccentricity, obliquity, and precession. The change in the shape of Earth’s orbit around the Sun (i.e. the change in eccentricity) is very small, and eccentricity has a cycle of approximately 100,000 years. The change in the obliquity (tilt) of Earth’s axis is also very small (about 2.5o), and obliquity has a cycle of approximately 41,000 years. Precession – also known as precession of equinoxes – has a cycle of approximately 26,000 years. As we will explore in the next part, it is during periods of overlap of those three cycles when dramatic temperature changes occur on our planet.
The Milankovitch theory proposes that glaciation is triggered by minima in summer insolation near 65° N, enabling winter snowfall to persist all year and therefore accumulate to build Northern Hemisphere ice sheets. The onset of the last glacial period occurred approximately 116,000 years ago.
Open Insolation_Eccentricity_Obliquity to open the file in Microsoft Excel and view changes in insolation at 65 N, eccentricity, and obliquity over the past 500,000 years. Insolation ranges from 388 W m-2 to 497 W m-2. Eccentricity ranges from 0.004595 to 0.049305. Obliquity ranges from 22.08° to 24.48°. Insolation is the quantity of incoming solar radiation. You learned about this in Solar Radiation & Season lab.
Q16: How did insolation change from 128,000 years ago to 116,000 years ago?
Q17: What were the insolation, eccentricity, and tilt values 116,000 years ago?
Open Orbit to determine on what seasons aphelion and perihelion occurred 116,000 years ago. Precession controls the timing of aphelion and perihelion.
Q18: When did aphelion occur?
Q19: When did perihelion occur?
Q20: Why did the high latitudes of the Northern Hemisphere receive such a small amount of insolation during the summers approximately 116,000 years ago?
Consider this set of relationships: You put money in a savings account. That money gains interest, which causes the amount of money in your savings account to increase … which causes the amount of interest you gain on your savings account to increase … which … you get the idea. The product of one process feeds back on that process causing it to increase or be enhanced. This is the idea of a positive feedback loop.
Without two positive feedback mechanisms the ice would not have kept advancing from 116,000 years ago to the Last Glacial Maximum. The two positive feedback mechanisms are ice-albedo feedback and greenhouse-gas concentrations. Ice-albedo feedback in the context of glaciation works as follows: cooling tends to increase snow and ice cover and thus the albedo, thereby reducing the amount of solar energy absorbed and leading to more cooling. This video can help explain the topic in the opposite direction (i.e. warming and decrease of snow and ice cover). You learned in the Global Surface Temperature lab that a small increase or decrease in greenhouse-gas concentrations can cause relatively large changes in the global surface temperature. CO2 concentrations can change dramatically between peaks in interglacial periods and peaks in glacial periods due to changes in oceanic processes. For example, CO2 is more soluble in colder waters than in warmer waters. Examine the picture below, which shows changes in temperature and CO2 concentrations at Antarctica over approximately the past 400,000 years.
Q21: What is the relationship between temperature and CO2 concentrations during glacial-interglacial cycles?
Q22: What is the difference in CO2 concentrations between maximums in interglacial periods (i.e. peaks in the graph) and maximums in glacial periods (i.e. valleys in the graph)?
Q23: Why do atmospheric CO2 concentrations increase and decrease so much during glacial-interglacial cycles?
Watch the video below to visualize the retreat of ice sheets from the Last Glacial Maximum (21,000 years ago or 19,000 B.C.) to the present. This visualization was developed at the Zurich University of Applied Sciences. Snow and ice cover at the end of the summer is shown. Please note that retreat of ice sheets occurs at a glacial pace: you will notice in the animation that the ice sheet moved just 100 meters — which is about the length of a football field — per year as it was retreating from present-day central Indiana to the present-day border of the United States and Canada. The yellow line shows the actual shoreline. Also shown in the animation are CO2 concentrations, average global temperature, sea level, and the global population. Make sure your cursor is not over the video when it is playing; you want to be able to see the change in date as you see the change in land cover.
Q24: Compared to the present-day global average temperature, how much lower was the global average temperature 21,000 years ago?
Q25: How much did CO2 concentrations increase from 21,000 years ago to 1500 A.D?
Q26: Why have CO2 concentrations increased by at least another 100 ppm from 1500 A.D. to the present?
Click LGM to view in Google™ Earth the extent of ice and other types of land cover during the Last Glacial Maximum. The product is part of a larger project by the Zurich University of Applied Sciences. Focus on the Northern Hemisphere and notice just how far south the ice sheets extended. The ice sheet that covered Canada and parts of the United States is the Laurentide Ice Sheet. It may have been up to 3 km thick over northeastern Canada, but it was much thinner at its edges. As noted earlier, the climate was much different 21,000 years ago than it is today. For example, much of the eastern United States that wasn’t under the Laurentide Ice Sheet was boreal forest; the boreal forest is now restricted mostly to high-latitude areas.
Q27: Why was the sea level so low? If the water wasn’t being stored in the oceans, where was it being stored?
Q28: How did the low sea level affect coastlines and how did that in turn affect the surface albedo of the planet? Hint: Look at Florida and turn off and on the Last Glacial Maximum layer.
Write responses of one to two sentences for each of the following big questions of the lab.Q29: Why do glacial periods occur? Q30: What happens when Earth transitions from a glacial period to an interglacial period? Q31: How does an interglacial period differ from a glacial period?