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.

Dust Particle Accumulation Shows Glaciations and Orbital Forcing Theory Disproved

The Northern Hemisphere Glaciation (about 2.7 Ma) is one of the more important changes in Earth’s climate. The characteristics first demonstrated during that time have influenced the future patterns of glaciations. Thus understanding that time period could be pivotal in understanding the climate today. Naafs et al. (2012) attempt to evaluate dust particle levels using sedimentary cores in the northern North Atlantic. Dust may play an important role during glaciations, and has been seen to increase during glacial cycles in other parts of the world. However, no work has been done in the North Atlantic and North American region. The authors also want to evaluate the origin of the dust particles, and do so using long-chained alkanes to ensure it is coming from North America. Long-chained alkanes originate from organic plant material, which gets carried off by wind gusts. The alkanes are predominantly found in C3 plants, which represent most of the foliage in North America. C4plants dominate other potential sources of dust, specifically Africa and Eastern Asia. This allows the authors to confirm that North America is the main source of dust at their drill site in the North Atlantic.
Having determined that dust particle levels greatly increase for the first time during the Northern Hemisphere Glaciation, and that the particles originate from North America, the authors test an orbital forcing hypothesis for the first time. It has been proposed that precession forcings drive glacial cycles on a 23 and 19 thousand year cycle (ky), but the affects are cancelled out because of the effect it has on both hemispheres. Using the data collected at the drilling site (i.e. dust build up and oxygen-18, both proxies for glaciation), Naafs et al. tested to see the effect precession had on glaciation. They discovered that in the North Atlantic the frequency of change was driven mostly by a 41 ky cycle, or the obliquity forcing. Naafs et al. call for a new hypothesis to be proposed to help explain why current theory suggests precession has a larger forcing on glaciations. –Mathew Harreld
Naafs, B.D.A., Stein, R., Hefter, J., Acton, G., Haug, G.H., Martínez-Garcia, A., Pancost, R., Stein, R. 2012. Strengthening of North American dust sources during the late Pliocene (2.7 Ma). Earth and Planetary Science Letters 317-318, 8–19.

            Changes in the earth’s climate toward the Northern Hemisphere Glaciation (NHG) during the Pliocene-Pleistocene period (2–4 Ma) were heavily driven by changes in the North Atlantic Current. It has also been noted that large amounts of dust particles (i.e. organic matter collected by winds) are present in glacial stages than in interglacial stages since the NHG. Work in Antarctica has revealed a trend in large amounts of dust deposits during glacials, but virtually zero work has been done in the northern North Atlantic region to evaluate this possible trend. The presence of dust particles may have a large impact in the understanding of glacial cycles, and help understand the natural cycles that drive such changes. Changes in dust concentrations may have a large effect on the presence of CO2 in water, the formation of clouds, and ice albedo (reflectivity). Naafs et al. evaluate dust levels in the northern North Atlantic in an attempt to better understand the NHG. Improving our understanding of how dust might play a role glaciations will greatly increase our ability to potential predict future changes in our current climate. In previous work, Naafs presented the evidence for the change in the North Atlantic Current during and before the NHG, but now explores why this change may have occurred.
            Naafs et al. used a drilling site in the middle of the Atlantic Ocean for their data, the same drilling data used in previous papers. Using four different drilling holes, the dust data were acquired and evaluated with a variety of methods. Oxygen-18 records compiled from previous work (Naafs et al. 2010, 2011) were used by the current authors to measure dust age and quantity. Naafs et al. also established a method for calculating if dust particles were from two types of plants, C3 plants and C4 plants, using long-chained n-alkanes and n-alkan-1-ols, substances that make of plant tissue material. Due to the different methods of photosynthesizing Naafs et al. were able to determine whether the dust collected originated from C3 plants or C4 plants by the level of n-alkanes and n-alkan-1-ols found in the sediment cores. The importance of this determination is in the geographic location of the two types of plants. C3 plants tend to be in grassier and wetter areas—like temperate North America—whereas C4 plants tend to be in drier areas. The accumulation rates of the n-alkanes and the n-alkan-1-ols were also calculated, allowing the authors to test the variability of dust presence and glaciation cycles.
            The dust collected in the coring samples was almost certainly from the North American Continent. Using Carbon Preference Index (CPI) Naafs et al. were able to determine that the dust contained in the cores was closest matched by dust collected in North America, and the closest match was found in Bermuda. Furthermore, the dust carbon-13 levels suggested C3 plants were most continuous and common source, which rules out dust from Africa or East Asia, since both are dominated by C4 plants.
            The results showed a low accumulation rate of dust during the late Pliocene (i.e. before glaciation). About 2.7 Ma there begins a significantly large increase in dust accumulation, right in accordance with the beginning of major Northern Hemisphere glaciation. However, during the interglacial periods the amount of dust greatly decreased, as compared to glacial periods. This seems to show a great correlation between dust and glaciations. Furthermore, the comparison of the North Atlantic dust record to the Southern Ocean record shows the two are greatly similar in trend, differing only in variation of total amounts. This shows further support for the relationship between glacials and increased dust and, thus, wind levels.
            The suggestion that wind levels must have increased however is misleading. An increased wind force during glacials does not support the necessary amount of dust seen in the data. Instead Naafs et al. suggest that the 30-fold increase of dust during the glacial periods may be explained by the increased glacier margins on continents resulting in more land erosion, meaning more dust is brought out to sea. The larger variation in ice extent on North America during glacial and interglacial events may have driven the larger increase in dust sources during glacial events.
            Having determined that dust sources during glacial periods only began around 2.7 Ma, and that the pattern follows for the rest of the cycles, as well as the cycles observed in the higher southern latitudes, Naafs et al. decided to take the opportunity to test a relatively untested hypothesis. A few scientists decided to try and explain Milankovitch cycles, which stated that climate variation was mainly driven by orbital precession. However, during the Pleistocene, ice volume varied on a cycle closer to the obliquity period. The scientists proposed that precession did in fact drive changes in climate, but that it affected both hemispheres, and thus cancelled each other out. Naafs et al. were able to test this hypothesis for the first time because of their work on creating a long and continuous climate record for North Hemisphere ice-sheet variability, using a cross-spectral analysis of oxygen-18 and dust concentrations. The results of the cross-spectral analysis showed that changes in ice-variability were on the scale of 41,000 years (41 ky), or obliquity cycle, which corresponds with the data they collected at their site. If precession cycles drove ice volume then evolutionary change of the n-alkanes should be similar to the precession driven ice volume, since more n-alkanes were present as dust during glaciations. However, Naafs et al. found that the n-alkanes varied on a 41 ky scale, not the predicted 23 and 19 ky scale of precession. This finding suggests that precession cycles have little to no effect on ice-sheet volume during the Pleistocene. This finding is also supported by their other data, pushing the authors to call for a new hypothesis to be developed. 

A Model for Future Protection and Restoration of Biodiversity in Warming Tropics

Global climate change will greatly affect the niche habitats of endemic and specialized species in the tropical rain forests. Most primary endemic and specialized species inhabit the coolest regions of a rainforest, putting them at risk of the rising temperatures accompanying global climate change. To protect them it is important to preserve remaining rainforest and to restore lost cooling regions. Shoo et al. (2011) developed and tested a new model for determining the overall temperatures of tropical regions using elevation, proximity to coast, foliage extent, and other variables. This model will aid in determining what areas are most in need of protection and restoration. The authors discovered a way to quantify natural mechanisms that regulate temperature, allowing them to create a model that will aid conservation efforts in rain forests globally. —Mathew Harreld
Shoo, L. P., Storlie, C., VanderwalL, J., Little, J. and Williams, S. E., 2011. Targeted protection and restoration to       conserve tropical biodiversity in a warming world. Global Change Biology 17, 186–193.

          The change in global temperature that is causing global climate change has a major impact on species niches and their surrounding environments. Other studies have already determined that their is a large cost to delaying study of and action in preservation of endangered areas (Hannah et al. 2007), and therefore Shoo et al. concluded that a quantitative model was needed to target key climate refuges for future conservation in rain forests. By observing the preferred temperature niches of local species and then finding the mean and range of temperatures for the key regions, Shoo et al. believed they could prioritize the protection and restoration of tropical refuges to maximize future biodiversity in the wake of climate change. Using key geographic traits of areas (i.e. elevation, latitude, distance to stream, distance to coast, foliage cover, clear-sky radiation, cloud offset, and wind exposure) Shoo et al. were able to generate detailed predications over large spatial scales of temperature variations in the desired region. Shoo et al. ran a trial of this quantitative model on the tropical northeastern Australian rainforest.
          Shoo et al. began by measuring local temperature ranges using 12 permanent open air weather stations which recorded the 24 hour maximum and minimum temperatures and 23 weather stations deployed by their team which recorded the temperature in 15 minute intervals from January 2007 to the end of December 2008. Using independent predictor models the authors developed a model of spatial surfaces for the region. They modeled elevation, latitude, distance to stream, distance to coast, foliage cover, clear-sky radiation, cloud offset, and wind exposure. Then using a linear regression approach for each independent month of recorded data, Shoo et al. developed three equations. The first equation was a base set of predictor variables which included elevation, latitude, distance to stream, distance to coast, foliage cover, clear-sky radiation. The second added on cloud offset, and the third added wind exposure instead of cloud offset. The two extensions to the base model were developed because the authors wanted to see if cloud offset or wind exposure affected the overall data in a significant way, but neither did.
          The results of their work showed the utility of a quantitative model to determine future restoration and protection sights for vital cool tropical areas. The three largest determining factors of maximum temperature were elevation, distance to coast, and foliage cover. It is the foliage cover of the rainforest that is the main focus for future conservation efforts. With the model the authors found that in the 8738 km2of rainforest the average temperature is 27.55°C with a range of 17.32°C to 35.01°C. Shoo et al discovered that 30% of the vertebrates (n=152) in the area live in the first temperature quartile (<25.88°C) and 39% in the second temperature quartile, both with cooler temperatures than for the rest of the vertebrates studied. The vertebrates in the lowest temperature quartile were also the most specialized, and therefore most in danger of climate and temperature shifts. The 62 endemic vertebrates species studied were broken down to 45%, 39%, 16% and 0% for the first, second, third, and forth quartiles respectively, further showing the need for cooler temperature in the region. The first quartile alone covers 2109 km2 of the rainforest, and 85% of this is already considered protected area. Another 26% of the rainforest has been cleared or degraded since pre-European settlement. To discover the importance of this lost 26% of the rainforest played had in the regions’ temperature, Shoo et al. modeled the “precleared” temperatures. The average temperature was 29.45°C with a range of 22.55°C to 33.70°C. The team estimated that about 139 km2 of the lost forest was once habitat for the lowest temperature quartile of vertebrate. This leaves a total of 189 km2 of unprotected land, including unprotected land and cleared land.
          The rainforest of northeast Australia is a poor model for conservation work because so much of the deforestation has avoided the cooler, more vital regions of the rainforest. But Shoo et al. clearly demonstrate the success of this models ability to be a guide for protection and restoration of tropical regions in an effort to preserve biodiversity. Shoo et al. estimate that if 80% of the “precleared” land was restored to its rainforest state that it would dramatically help lower temperatures in that region, aiding nearly half of the species that are home in the coolest regions of rainforest. What is most important about this study is the development of a model that can successfully guide future preservation and restoration projects around the world. In areas of Africa, South America, and Latin America where tropical regions are in danger of human development, the model could prove to be very helpful to determine what areas to protect and restore. The authors helped to reiterate the importance of cool temperature for the diverse, specialized, endemic species in rain forests, and presented a model to aid in their protection. As the world faces further climate change due to human activity this developed model could aid the protection and restoration of biodiversity in tropical regions around the world.