Milakovitch Cycles and Climate Change in the Recent Past
(Reading #2 for my course in Climate Change, Alan Holyoak, PhD)
By the end of this topic (reading and class discussion) you should be able to understand and explain:
- Why the Earth has seasons.
- What the three Milankovitch Cycles are, as well as their time periods and combined effect on insolation.
- What the sunspot cycle is, and its effect on climate.
Understanding factors that affected past climates helps us gain perspective on present and possible future climate change. This reading introduces you to some factors that are highly predictable and that almost certainly had an effect on Earth’s climate history. Detailed climate data do not exist for every period of Earth’s history, so in this reading we will focus mainly on climate throughout the last 500,000 years. This timeframe is extremely useful because data for it are amazingly complete, and because many of the same factors that affected Earth’s climate then also affect our climate today.
Why The Earth Has Seasons
You would get a surprising set of answers if you were to ask 100 people on the street to explain in as much detail as possible what causes the seasons. Scientists pondered and studied this question for many years before they finally worked it all out. The basic answer is that the Earth’s axis is tilted in relation to the plane of the Earth’s orbit around to the sun, and this produces seasons. If this is new information to you, you may be thinking, “What does THAT mean?”
As you know, the Earth spins on its axis. This rotation gives us our cycle of night and day. Earth’s axis is, however, currently tilted 23.4o from perpendicular in relation to the plane of its orbit around the sun (Fig. 1). This means that part of the year the northern hemisphere is angled away from the sun and part of the year it is angled toward it. During northern hemisphere summer months the northern hemisphere is tilted toward the sun, and therefore has longer days and receives a greater intensity of solar energy, technically called insolation, than the southern hemisphere (Fig. 2). During northern hemisphere winter months the northern hemisphere tilts away from the sun and has shorter days and receives a lower amount of insolation than the southern hemisphere. This is what causes our summers and winters. Of course, during the spring (vernal) and fall (autumnal) equinoxes the northern and southern hemispheres receive equal amounts of energy.
So why isn’t the hottest time of year when we have the longest days, i.e., right around the summer solstice? The oceans absorb a huge amount of heat before the temperature of surface waters change very much. This characteristic of water causes the actual timing of the hottest and coldest days, as well as the onset of spring warming and fall cooling to be delayed slightly when compared to the dates of equinoxes and solstices.
Figure 1. Seasonal differences in the angle of the axis of rotation of Earth in relation to the sun. (Image courtesy of DOE.gov)
Figure 2. The relative amounts of solar energy that strike different latitudes of the Earth’s surface in June and December. (Image modified from Dr. Gerken, NMSU.)
All right, but why is it always warm in the tropics? Take another look at Fig. 2 and you will see that the surface of the earth near the equator receives a fairly constant amount of solar energy during all seasons of the year. This is why it’s warm there year-round.
Back to the question about the reason we have seasons. Many people think that we have seasons because the Earth’s orbit is elliptical, and that when Earth is closest to that Sun that’s when we have northern hemisphere summer. While it’s true that the Earth’s orbit is slightly elliptical, Earth is actually closest to the sun in January! This means that the angle of the Earth’s axis is more important to determining the seasons than the shape of Earth’s orbit. What would happen if the angle of Earth’s axis changed? Read on!
The current orientation of the Earth to the sun, and the shape of Earth’s orbit around the sun do not tell the whole story of seasons and climate. It turns out that the orientation of the Earth in relation to the sun and the shape of Earth’s orbit are not constant; they undergo predictable cycles. As these cycles occur they affect Earth’s climate. These factors are:
- Precession – cyclic shifts in the direction the Earth’s axis points.
- Obliquity – cyclic shifts in the angle of Earth’s axis.
- Eccentricity - cyclic shifts in the shape of Earth’s orbit around the sun.
These three constantly shifting cycles and the way they interact with each other to affect Earth’s climate are called Milankovitch Cycles. These cycles are named after Milutin Milankovic. He did not discover these cycles, but he studied the cycles and calculated how their interplay would impact the relative position and orientation of the Earth to the sun, and how they might affect climate.
FYI, Milankovic was born in Serbia in the late 1800s, and he studied engineering and science. His early work on this topic was carried out while he was a POW during WWI, and the culmination of his calculations and the theory they supported was published in a book in 1941 on the eve of Nazi Germany’s invasion of Yugoslavia. His book was reportedly being printed when the printing company was bombed, and only one copy of the original first edition survived – the one he had taken for himself after the first day of the publication run.
An introduction to the Milankovitch Cycles is presented below.
A shift in the direction the Earth’s axis points is called precession. Over time the direction pointed toward by the Earth’s axis moves in the shape of a circle (see Fig. 3). This type of motion is much like the wobble you see when a toy top starts to slow down. It takes about 23,000 years for the Earth to undergo one complete cycle of precession. The Earth’s axis currently points toward Polaris, also known as the North Star. That is the brightest star in the constellation Ursa minor (the little dipper). As earth’s precession continues the direction of the axis will shift away from Polaris. Be advised that precession alone does not change the tilt of Earth’s axis, only the direction it points.
Figure 3. One cycle of Earth’s precession is completed every 23,000 years. (Image: NASA.gov.)
The amount of tilt in the Earth’s axis also shifts in a predictable cycle. The obliquity of the Earth’s tilt ranges from 22.1o to 24.5o (see Fig. 4), and is currently at an angle of 23.45o in relation to the plane of its orbit around the sun. It takes about 41,000 years for Earth to complete one cycle of obliquity. While the cycles of precession and obliquity are taking place the shape of the Earth’s orbit around the sun also shifts.
Figure 4. The range of obliquity of the Earth’s axis – current obliquity is 23.4o. (Image: NASA.gov)
The Earth’s orbit around the sun is weakly elliptical, and the shape of this elliptical orbit also undergoes a predictable cyclic change (see Fig. 5). Because the Earth has a slightly elliptical orbit, the distance from the Earth to the sun is not the same all year long. In fact, Earth is currently closest to the sun on 3 January and is farthest away on 3 July. Think about that and the timing of seasons in the northern hemisphere.
The eccentricity of an orbit is perfectly circular when the orbiting object is always the same distance from the object it is orbiting. A perfectly circular orbit has an eccentricity value of 0.0. When an orbit is elliptical, however, its eccentricity value is greater than 0.0. The range of Earth’s orbital eccentricity is small, from a minimum of only 0.005 to a maximum of 0.058. Earth’s current eccentricity is about 0.02. In its current shape, Earth is 3% farther away than when it is closest. It takes about 100,000 years for Earth’s orbit to go through a complete cycle of its eccentricity.
Figure 5 (right). This figure shows the two extremes in the eccentricity of the orbit of one object around another object; at one extreme its orbit is closer to circular (yellow orbit), and at the other extreme it is more elliptical (red orbit). (Image: NOAA.gov)
Effects of Milankovitch Cycles
The Milankovitch cycles are all happening at the same time, but they do not affect the total amount of solar energy received by the Earth in a year. Together they do, however, combine to have an effect on the intensity of solar radiation received at different latitudes. For example, altering axis tilt (obliquity) changes the intensity of seasons.
Look back at Figure 2 and take some time and ponder on whether you think Earth would have more intense seasons when its axis is 22.1o or 24.5o.
A change precession affects the seasons in which Earth is closest to and farthest from the sun. Currently we are farthest from the sun during the northern hemisphere summer. How might that affect temperatures during high latitude winters? Summers? And, of course, shifts in the eccentricity of Earth’s orbit will also have an effect on climate.
Climate and Milankovitch Cycles
The importance of these cycles to climate seems to be in variations of intensity of solar radiation received at higher latitudes at different times of the year, since total solar radiation in the lower latitudes, i.e., near the equator, remains constant. In addition, there is greater total landmass in the northern hemisphere than in the southern, so the climate effects will be largest in the north. This is because it takes more energy to raise the temperature of water than it takes to change the temperature of land. A continent will therefore heat up and cool down more quickly than an ocean.
Milankovic’ calculated the changes in intensity of radiation received in high latitudes over long periods of time. He saw predicted short-term variability and long-term changes in climate that corresponded to the three overlapping cycles discussed earlier in this reading. He hypothesized that shifts in the three Milankovitch Cycles might explain the timing and extent of glacial and interglacial periods of the past few million years.
Data from polar ice cores allows scientists to collect observations needed to test Milankovic’s hypothesis. In Figure 6 the yellow line shows the calculated variations in solar insolation at 65o N over the past 500,000 years based on Milankovic’s calculations. The lower graph in Fig. 6 indicates changes in global climate obtained from ice cores. Glacial periods are in blue and interglacial or warm periods are in red. Notice that there is a correlation between glacial and interglacial periods and changes in solar insolation due to Milankovitch cycles. The connection is statistically too strong to be ignored. These data support the explanation that there is a relationship between Milankovitch Cycles and the cycles of ice ages and interglacial warm periods. If, however, climate were driven solely by Milankovitch cycles we should currently be experiencing a trend of global cooling.
Figure 6. Types of Milankovitch cycles (top), calculated amount of solar insolation at 65oN during the month of July (middle), and observed climate record from ice cores based on oxygen isotope concentrations (bottom) with glacial periods indicated in blue and interglacial periods in red.
Before we assume that the primary cause and effect of recent ice ages and warm interglacial periods is now sorted out, there are still some nagging questions. First, the changes in insolation intensity due to Milankovitch Cycles are quite small, and alone are probably not large enough to cause such dramatic climate effects on their own. In addition, while eccentricity appears to have the greatest influence on climate, it actually has the smallest impact on insolation of the three cycles. So while there is clearly a statistical correlation between the cycles and climate, there are issues that have yet to be resolved. Do not, however, make that mistake of assuming that because science has not yet answered all questions related to this complex question that we know nothing about paleoclimate and driving forces of ancient and modern climate. The current interpretation is that small changes in solar insolation may trigger planetary feedbacks that drive the Earth’s climate system past tipping points that drive the planet into or out of ice ages.
We will return to ice core data, how these data are collected, what they represent, and how they are interpreted later in the course. The reason they will not the handled in this reading is that there are other foundational topics that you need to be introduced to before we discuss ice core data. These include the formation and effects of global surface winds, surface and deep oceanic currents, and a more complete introduction to paleoclimatology and proxy temperature data. But, before we leave the topic of astronomical cycles that can affect climate there is one more cycle that you may have heard of and that you will want to be aware of: the sunspot cycle.
Solar Cycle: Sunspots
Scientists discovered that the Sun’s output of solar energy fluctuates. This output varies naturally in conjunction with the number of sunspots present at any given time (see Fig. 7). Sunspots were reportedly first observed by the naked eye in China as early as 28 BCE (Before Common Era = BC), and we have a reliable scientific record of sunspots that goes back to 1610. Scientists have been using satellites to record the number of sunspots and solar output since 1978. These satellite data reveal that solar output varies by about 0.08%, with the lowest output occurring when no sunspots are present and the highest when many are present.
Sunspots are regions on the surface of the sun that are cooler than the surrounding surface. This is counterintuitive because total solar irradiance increases when sunspots are present. Total intensity increases because whenever sunspots form so do solar faculae. Solar faculae are bright, extra hot regions of the sun’s surface where solar irradiance is higher than normal.
The 400-year record reveals that there is an 11-year sunspot cycle. It also shows that the maximum number of sunspots produced varies from cycle to cycle (see Fig. 8). There were periods of time in the 400-year record when few to no sunspots were visible for extended periods of time. It is possible that these occurrences may represent a longer-term solar cycle, but data confirming or refuting that possibility are still lacking. Extended periods of time when very few sunspots were present correlate with periods of cooler temperatures on Earth. The longest recorded period of time when the sun produced no sunspots is called the Maunder Minimum. It began in the mid-1600s and continued until about 1700. During that time the Earth experienced a cool period sometimes referred to as “The Little Ice Age.”
Figure 7. The sun with sunspots in 2000 (left) and no sunspots in 2009 (right). (Image courtesy of NASA.)
Figure 8. 400-year record of sunspots. The Maunder Minimum correlates with a cooling event called the “Little Ice Age”. (Image courtesy of NASA.)
Most of the time sunspots appear and disappear on the predictable 11-year cycle, but climatologists discovered that total insolation differences between low and high points in a given cycle account for only a small fraction of observed changes in global climate. The current sunspot cycle is shown in Fig. 9. These data show that we ended one sunspot cycle in 2009-2010 and that we are now entering a new sunspot cycle. If our climate were driven solely by changes in solar irradiance, 2000-2010 should have been a decade of global cooling, but we observed the opposite.
Figure 9. The number of sunspots present per month from January 2000 through September 2011. (Image courtesy of NOAA.org.)
Natural long-cycle events are now well known, and climatologists continue to investigate how these factors together with shorter-term events and other forcing factors affect Earth’s climate.
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Hipps, LE. 2010. Personal communication and readings produced by Dr. Hipps. Professor of Atmospheric Science, Department of Plants, Soils, and Climate. Utah State University.
Mathez, EA. 2009. Climate Change: The Science of Global Warming and Our Energy Future. Columbia University Press. NY. 318 pp.
NASA.gov. 2004. URL: http://www-istp.gsfc.nasa.gov/stargaze/Lseason.htm
NASA.gov. 2005. URL: http://www-istp.gsfc.nasa.gov/stargaze/Sseason.htm