Glaciation Cycles

Milankovitch theory postulates that Earth’s orbital variations are key to the nature of the glacial cycles. Milankovitch believed that 100 thousand year (kyr) forcings drove the glaciation cycle, however that 100 kyr marker is missing at the high latitudes of the Northern Hemisphere’s summer insolation. Earth’s orbit eccentricity does cycle nearly every 100 kyr, so there may be a possible link. Ganopolski and Calov (2011) use ice sheet models coupled with less advanced global climate models to calculate the glacial cycles of the past 800 kyr, in an effort to determine if there is a direct affect on ice sheet changes to 100 kyr eccentricity cyclicity. The authors tested their model until they were confident in its accuracy. They then tested whether holding CO2  constant created a change in glaciation patterns. The results suggested that eccentricity 100 kyr forcings are stronger below CO2 concentrations of 260 ppm. In these scenarios not only do ice volumes differ, but also glacial terminations occur along the 100 kyr cycle. Next, the authors held certain global forcings constant. When obliquity was fixed it was found to have weakened affects on many termination events, making results less robust during periods of low eccentricity. Fixing eccentricity resulted in the model failing in eccentricity levels below 0.02 and then resulting in being dominated by precession intervals when above 0.05. At eccentricity levels of 0.03 and 0.04 with low CO2 concentrations the model correctly aligned with reconstructed records, and also showed accurate termination periods. Overall, the authors determined that there is an underlying forcing of 100 kyr, but are open to the idea that there are many factors that drive glaciation cycles. –Mathew Harreld
Ganopolski A. and Calov, R. 2011. The roloe of orbital forcing, carbon dioxide, and regolith in 100 kyr glacial cycles. Climate of the Past 7, 1415–1425.

            Milankovitch theory postulates that Earth’s orbital variations are key to the nature of the glacial cycles. Milankovitch believed that 100 thousand year (kyr) forcings drove the glaciation cycle, however that 100 kyr marker is missing at the high latitudes of the Northern Hemisphere’s summer insolation. Earth’s orbit eccentricity does cycle nearly every 100 kyr. It has been calculated that the affect of this change is very miniscule, especially compared to other orbital forcings. Eccentricity is also driven at a 400 kyr cycle, but this forcing is completely missing from available ice records. It has been proposed that many factors come into play to drive this 100 kyr cycle, and that those factors hide the true 100 kyr pattern. Ganopolski and Calov use ice sheet models coupled with less advanced global climate models to calculate the glacial cycles of the past 800 kyr, in an effort to determine if there is a direct affect on ice sheet changes to 100 kyr eccentricity cyclicity.
            The model used by the authors was started at 860 kyr to allow for a 60 kyr calibration time. 860 kyr was chosen because it most closely resembled current interglacial levels. If they had started at 800 kyr they would have begun the experiment near the last glacial maximum, making it difficult to start calibration. Using the current condition data the model for each experiment was run from 860 kyr to 800 kyr, and for each experiment the 60 kyr spin-up data was not recorded.
            The first experiment run was their baseline, running realistic changes in all parameters. The significance of this experiment was to test the accuracy of the model over the 800 kyr, comparing it to reconstructed data from the field. The model, overall, supplied accurate changes over the 800 kyr compared to the reconstructed data. Possible issues with the model were found, but were thought to be insignificant in answering their question. The model’s individual parameters were also tested, such as oxygen-18, CO2, and ice volume. Each parameter matched well with reconstructed data, and also matched cyclical peaks found in reconstructed data. Glacier extent maxima matched strongly with current theory, showing that North American glacier extent is dominant and is by a 100 kyr cycle. However, they also suggest that the model underestimates European glacial extents because of incorrectly estimating European glacial sensitivity to orbital forcings.
        The next experiment run was to hold CO2 levels constant to see the change in affect of 100 kyr cycles. CO2 concentrations were run from 200 to 280 parts per million (for every 20 ppm). The results found in this experiment suggest that eccentricity 100 kyr forcings are stronger below CO2 concentrations of 260 ppm. In these scenarios not only do ice volumes differ, but also glacial terminations occur along the 100 kyr cycle. However, it is important to note that glacial terminations can only occur if the deposition of glaciogenic , glacier generated, dust is taken into account. At times of glacial termination dust deposits in North America greatly increase, decreasing ice albedo and increasing ablation, increasing the rate of glacial retreat.
            In the next set of experiments orbital forcings were kept constant by removing each orbital forcing, while all other variables were allowed to change. When obliquity was fixed it was found to have weakened affects on many termination events, making results less robust during periods of low eccentricity. Fixing eccentricity resulted in the model failing in eccentricity levels below 0.02 and then resulting in being dominated by precession intervals when above 0.05. At eccentricity levels of 0.03 and 0.04 with low CO2 concentrations the model correctly aligned with reconstructed records, and also showed accurate termination periods.
            The next step in the study showed the importance of regolith (global sediment) in changing glacial cycles from 40 kyr to 100 kyr. The authors ran one model at set CO2 concentrations while also setting the distribution of terrestrial sediments to a higher level then present. The model supported the hypothesis that the removal of sediments from the North American continent resulted in longer term glaciation cycles—from 40 kyr to 100 kyr. The more terrestrial sediment present means albedo can be reduced sooner and to a greater extent, resulting in shorter glaciation cycles.
            The results presented by Ganopolski and Calov give support to a system that is dominated by 100 kyr eccentricity cycles. However, the authors remain open to the idea that the glaciation cycles are driven by many factors, such as CO2, regolith, other orbital forcings, and other factors. More simulations need to be built upon the findings of this study to further gain understanding in the long term forcings behind glaciations.

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