At a recent meeting of the U.S. House of Representatives Committee on Science, Space and Technology, Rep. Steve Stockman (R-TX) asked White House Science Advisor John Holdren why “global wobbling” was not included in climate models of modern climate change (YouTube). After all, if global wobbling was so important in ending the last ice age, why does it not matter now.
The answer has to do with timescale. Dr. Holdren did an admirable job answering this question during the hearing, but we’ll go into some more detail here. Representative Stockman’s “global wobbling”, refers to cyclical changes in Earth’s orbit around the sun. There are three primary ways in which the Earth’s orbit changes (check out the National Geographic video with Richard Alley linked below that illustrates the cycles quite well):
1) Eccentricity: Earth’s orbit around the sun is not a perfect circle; it is actually slightly elliptical. How elliptical or eccentric the orbit is changes through time. The degree of eccentricity of our orbit changes how close we are to the sun and thus the amount of solar radiation we receive at different points during the year. Our orbit goes from nearly circular to to elliptical and back over the course of about 100,000 years.
2) Obliquity (Tilt): Earth’s rotational axis is also tilted relative to the orbital plane. It is this tilt that gives us our seasons; summer occurs when the Northern Hemisphere (or Southern Hemisphere) are pointed toward the sun. Likewise, it is winter when we are pointed away from the sun. So the greater the angle of tilt, the more extreme our seasons would be, i.e. warmer summers and colder winters. Earth’s axis is currently tilted at about 23.5 degrees. The tilt varies from 22.2 degrees to 24.5 degrees and back over a period of 41,000 years.
3) Precession of the Equinoxes: Our third orbital changes is the real “wobble.” As the Earth rotates and revolves around the Sun, it also slowly wobbles like the spinning of a top. This wobbling changes the direction the Earth’s axis is pointing at different locations in Earth’s orbit. For example, in our current configuration, the Northern Hemisphere points towards the sun (summer) at the furthest point in the orbit (aphelion) and away from the sun (winter) at our closest point (perihelion). This configuration acts to make the Northern Hemisphere seasons a little milder. The orientation is reversed in the Southern Hemisphere, making their seasons a little more extreme than they would be otherwise. This wobble of the Earth’s axis has a period of about 23,000 years. So in about 11,500 years, we will have a bit warmer summers and colder winters because the Northern Hemisphere will point towards the sun during perihelion (all else being equal of course).
These changes in Earth’s orbit do not affect the total amount solar radiation absorbed by the Earth very much, but they do change the latitudinal and seasonal distribution of that radiation. It turns out this distribution is very important if you want to grow or melt an ice sheet. The cooler the summers are in the Northern high latitudes the more likely it is that snow is going to survive unmelted through the summer. Over time, that snow that survives the summers will accumulate into an ice sheet. If you have warmer summers, there is more melting than the amount of snow that accumulates during the winter and the ice sheet shrinks. It is now recognized that these changes in Earth’s orbit played an important role in setting the timing of the glacial-interglacial transitions. However, it is important to note that the orbital changes are not enough to drive the entire glacial-interglacial transitions. They got the ball rolling, but feedbacks such as albedo and greenhouse gases are necessary to explain the entire change.
This brings us back to timescales. As we can see, Earth’s orbit changes very slowly. The 20th century represents only about 0.4% of the cycle of the parameter (our “wobble”) with the shortest period. The 20th century represents only 0.1% of an eccentricity cycle. These orbital parameters are predictable can also be calculated for times in the past and into the future. NASA has an orbital calculator based on the work of Andre Berger that you can use to find out what the orbital parameters were for any time during the last million years or a million years into the future. Other researchers have made similar calculations for farther back in the geologic past.
The big point here is that the mechanisms of climate change vary depending on the timescale that we are considering. This idea is fundamental to the field of geology: that small changes over long periods of time can result in major changes to the Earth System. Tectonic plates only move on the order of centimeters per year, but over millions of years that motion can build mountain ranges. Over the ten to hundred thousand-year timescale of the glacial cycles, these orbital changes matter, but over the decade to century timescale of anthropogenic climate change, they are a drop in the ocean.
I hope to use this platform to talk about this connection between climate change and timescales and to delve into some of the lessons that the geologic record of climate can teach us about what’s happening now. In an upcoming post, I will continue this discussion about climate change and timescales by looking at the mechanisms for climate change on even longer timescales. One of the most remarkable aspects of anthropogenic climate change from a geologist’s perspective is how humans have short-circuited the geologic carbon cycle causing a slow process, the weathering of fossilized organic carbon in sedimentary rocks, that usually only influences climate on million-year timescales to become important on decadal timescales.
Thanks for reading and I look forward to your comments.
Resources and Further Reading:
1) National Geographic Video with Richard Alley, http://channel.nationalgeographic.com/channel/videos/ice-age-cycles/
1) NASA Earth Observatory feature on Milankovitch: http://earthobservatory.nasa.gov/Features/Milankovitch/
2) J.D Hays, John Imbrie, and N.J. Shackleton, (1976) “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages,” Science, 194, no. 4270, 1121-1132. http://www.sciencemag.org/content/194/4270/1121
3) Spencer Weart, “Past Climate Cycles and Ice Ages,” The Discovery of Global Warming, http://www.aip.org/history/climate/cycles.htm
4) John Imbrie and Katherine Palmer Imbrie, (1986) Ice Ages: Solving the Mystery. Rev. Ed. Cambridge, MA: Harvard University Press.