The Mathematical Proof that Obliquity and Precession Drive Glaciation Cycles Together

Glaciation cycles having been driving the Earth’s climate system for millions of years, changing the face of the planet every hundred thousand years or so. How these cycles began, why they began, how they continue, and how they might affect us in the future are large questions being asked by scientists today. The timing of glaciations is driven largely by orbital forcings of the Earth, in theory. Huybers (2011) attempts to quantify how two of these forcings, obliquity and precession, drive glaciations. Using oxygen-18 records Huybers is able to create a model and equation that shows how obliquity and precession cycles come together about every 10,000 years to drive an interglaciation period, that correlates with the oxygen-18 record available. Huybers’s work is an important step it turning a theory into mathematical understanding. –Mathew Harreld

Huybers, P., 2011. Combined obliquity and precession pacing of late Pleistocene deglaciations. Nature 480, 229-232.

The earth system is driven by many factors, but there are a few factors that dominate the pattern of climate change. One of these major factors is the glaciation cycle that has existed for the past million years. How these cycles began, why they began, and how they are changing are key questions to understanding our greater earth system. Early in the 20th century a scientist named Milutin Milankovich proved that the major drivers of the Pleistocene era glaciations were orbital forcings, and calculated how those orbital forcings changed. Milankovich hypothesized that changes in orbital eccentricity, changes in Earth’s obliquity, and precession, changes in the orientation of the spin axis with respect to Earth’s orbit affected the northern hemisphere insolation (solar radiation) levels. He determined that these affects came together to drive the glaciation cycles over first 40,000 year cycles (40 ky) and then 100 ky cycles. However, the technology and data available to Milankovich kept him from ever proving that obliquity and precession actually worked together in changing the Earth’s climate.

The understanding of how obliquity affects earth climate systems is now well understood, but many gaps remain in understanding precession and how the two might work together if at all. Huybers (2011) attempts to show that precession does in fact play a key role, and does so in conjunction with obliquity, while suggesting a larger role in southern hemisphere climate changes than previously believed.

The difficulty of proving the effects precession has on the earth’s climate comes down to its shorter cycle compared to that of obliquity. Obliquity changes on a 41 ky year cycle, however precession changes on a 26 ky cycle, making the mathematical proof sensitive to timing errors. Huybers takes the first steps forward in proving the affects of the precession cycle by using new statistical tests that nearly remove the timing errors.

Using oxygen-18 dating of layers in ice cores taken from ice sheets scientists have long been able to map the changes of glaciations for the past million years. The changes in these cycles from glaciations to interglaciations occur approximately on a 100 ky cycle. Huybers evaluated the combined affects of obliquity and precession to determine if they correlated with Milankovich maximums, the deglaciations. His first step was to estimate the time of the terminations (the maximums) using delta oxygen-18 records, as well as geomagnetic reversals. Second, Huybers defined insolation using a generic and broad formula that takes into account obliquity, precession, time, perihelion (point closest to the sun on earth’s orbit), and aphelion (point farthest from the sun on earth’s orbit). The equation allowed Huybers to model Milakovich’s hypothesis about warmer and longer Northern Hemisphere summer by setting certain parameters. Lastly, Huyber calculated a median value of the forcing maxima corresponding with termination points (Milakovich maximums). This method is key to Huybers work because it greatly detaches his results from timing errors. Median values are less sensitive to timing errors because only wrong forcing cycles could cause them and because median values’ outliers have little effect on the result.

What Huybers found was that both precession and obliquity are important in determing the patterns of glacial terminations.

Both obliquity and precession play important roles in affecting insolation during the periods of termination, and thus both affect the glaciation cycle. Also, the known cycle of obliquity and precession match up so that precession achieves a maximum during every above-average obliquity period. However, more research must be done to determine if precession affects the precise timing of terminations, especially during the early Pleistocene. Huybers also notes that when the summer solstice occurs near a perihelion for the northern hemisphere, the southern hemisphere also has longer summers at the aphelion, potentially releasing more CO2, and potentially aiding in the rate of deglaciation. This relationship highlights the complexity of the earth system, as well as the deep interconnectedness shared between the planet, the local forcings, and the sun.