The importance of solar insolation on temperature variations for the past 110 kyr on the Chinese Loess Plateau

The Chinese Loess Plateau is a large source of agriculture and home to the Yellow River, and thus plays an important role in Chinese civilization. Understanding how weather patterns and climate change affect the area is key to being able to prevent any major changes in the future. Since the 1960s data have been compiled on the monsoon patterns in the area, however there are few data on temperature changes. This gap is important to close if models and complete records are to be produced. Gao et al. (2012) collected temperature proxies in the Lanjian region of the Loess Plateau, using tetraether lipids from bacteria. Their findings suggest that insolation is the main driver behind temperature changes for the past 110 thousand years (ky). Their data records match up with other local records, as well as with global forcing records. The data the authors compiled will aid in creating more accurate models for understanding possible affects of climate change on the Loess Plateau. –Mathew Harreld
Gao, L., Nie, J., Clemens, S., Liu W., Sun, J., Zech, R., Huang, Y. 2012. The importance of solar insolation on temperature variations for the past 110 kyr on the Chinese Loess Plateau. Palaeogeography, Palaeoclimatology, Palaeoecology 317–318, 128–133.

            Local and global monsoon patterns are important indicators of climate shifts. On the Chinese Loess Plateau there are some of the best archives of monsoon profiles in the world, allowing scientists to recreate the East Asian Monsoon pattern for the past few million years. This information allows for the recreation and understanding of past local and global climates. Furthermore, the data collected have important implications for predicting future monsoon rainfall variations, especially in global warming scenarios. The Chinese Loess Plateau has a rich archive of surface soil magnetic properties, oxygen–18, and grain sizes, which all contribute to creating rainfall records. Because of the abundance of these proxies the rainfall patterns in the Chinese Loess Plateau are well understood. However, the temperature variations throughout the same periods are not very well documented. Gao et al. attempt to address the issue of missing temperature data by using tetraether lipids from bacteria in conjunction with known climatic forcings. Temperature plays one of the most important roles in understanding a climate, through the use as the primary source to parameterize climate models, through its influence on other proxies, and through its insight on local mountain glacier activities.
            Gao et al. compiled data from the Lantian region of the Chinese Loess Plateau. To optimize their data’s accuracy they compared it to recently published, high-resolution data sources. The differences between their data and the other sources are mostly due to general location, and thus different weather affects, and different modelling techniques.
            Variation in the Lantian region’s temperature matches closely with Northern Hemisphere absolute insolation maximum at 35°N. Thus, it seems likely that the insolation forcing of the sun and earth’s orbit drives temperature changes in this region. Maximum (94, 72, 22.5 ka) and minimum (105, 81, 58, 10.8 ka) insolations match with Lantian temperature maximum and minimum. The authors propose that insolation has such a large impact because of the postive feed back loop of monsoon intensities being increased greatly by high summer insolations, which then trap more heat, resulting in even higher temperatures. However, increased monsoon strength does not always occur along with higher temperatures because changes in insolation strengths don’t match up with monsoons changes.
            Another potential influence on temperature is changes in atmospheric CO2 levels. Increased temperatures coincide with high CO2 levels, and vice versa. This is to be expected on larger time scales, however on the short term it is more difficult to get a accurate understanding. The results found by Gao et al. further suggest the relationship between CO2variation and large scale temperature variations on the Chinese Loess Plateau.
            Glacial records from the nearby Tibetan mountain ranges show maximum local glacial advance earlier than global values. This is fairly common throughout the world, and is most likely due to abundant moisture availability and local cold temperatures. And the data compiled by the authors is consistent with other data compiled in the region. Furthermore, the lowest temperatures in the record are around 30 and 22.5 ka, which designates the local last glacial maximum. Temperatures increased rapidly from 22.5 ka years on, increasing about 9°C. This warming in earlier than global values, but this is also not unusual for local climates.
            Compiling a record of temperatures in the Lanjian region will greatly enhance understanding of monsoon changes in the past, as well as aiding in creating climate models for the region so we can begin to better understand future changes under climate change. It is clear that insolation is the main drive in temperature change in the Chinese Loess Plateau, and therefore any insolation maximum combined with monsoon changes and other climate change effects might vastly change the Chinese Loess Plateau region.

The Formation of Supercontinents, Old and New, based on the Orthoversion Model

The formation of supercontinents is a relatively unknown phenomenon, especially since humans have never observed it. But in understanding the processes of supercontinent formation, we may be able to predict future changes in Earth’s surface and mantle for millions of years to come, while also aiding in our understanding of Earth processes. Two theories predominate as to how supercontinents form: introversion and extroversion. Introversion postulates that a younger and interior body of water will close, forming a supercontinent in its place. Today this would be the closing of the Atlantic Ocean, about where Pangaea was located. Extroversion postulates that an older and exterior body of water will close, forming a supercontinent in its place. Today this would be the closing of the Pacific Ocean, opposite of where Pangaea was located. However, both of these theories have gapping holes in them, especially when compared to geological evidence of mantle movement, both in more recent times and on the million-year timescale. Mitchell et al. (2012) have proposed a new model, orthoversion. Orthoversion is the formation of supercontinent in the downwelling girdle of subduction (today the “ring of fire” in the Pacific) orthogonal (90°) to the predecessor supercontinent. Using seafloor spreading patterns and changes in paleomagnetic poles, the authors were able to recreate potential centers of former supercontinents Pangaea (~200 million years ago (Mya)), Rodinia (~600 Mya), and Nuna (~1000 Mya). Then using their orthoversion model, they were able to recreate the movement of continents from one supercontinent to the next. The significance of their model is that it clears up the gaps present in the other models. The orthoversion model provides the missing link in Pangaean formation, and is the only model that matches ocean records in showing that the Pacific Ocean rift has only existed since the creation of Pangaea. By creating and presenting the orthoversion model, Mitchell et al. have shown that orthoversion explains the connection between supercontinent decline and growth, and many other gaps in the other two. –Mathew Harreld
Mitchell, R.N., Kilian, T.M., Evans, D.A.D. 2012. Supercontinent cycles and the calculation of        absolute palaeolongitude in deep time. Nature 482, 208–211.

            It is now well-known that millions of years ago a “supercontinent,” named Pangaea, dominated Earth’s landscape. Since its formation about 200 million years ago (Mya), the continents, as we know them now, have been shifting. In the following millions of years the continents will continue to shift, once again forming a supercontinent, already named “Amasia”. Understanding the processes of how Amasia might form requires discovering how the supercontinents of the past have formed. Two general theories are prevalent in today’s scientific literature: introversion, extroversion. The introversion model is based on the idea that young interior oceans stop forming and then begin to close, creating a collision between two landmasses. In today’s terms this would mean the closing of the Atlantic Ocean, and the collision of the Americas, Europe, and Africa. This will mean Amasia will form where Pangaea once was. Extroversion is the process where the relatively older, exterior ocean closes, forming the supercontinent in its place as two landmasses collide. In today’s terms this would be the closing of the Pacific as Asia, Australia, and the Americas collide, forming Amasia opposite of where Pangaea formed. However, both of these theories have deep gaps in their explanations, especially when compared to geological evidence. Mitchell et al. have developed a new theory called orthoversion. Orthoversion is the prediction that a supercontinent will form orthogonal (90°) to its predecessor, in a region of downwelling, and in the subduction girdle formed by its predecessor. Today this would result in the closing of the Arctic Ocean, as North America and Northern Europe meet to form Amasia. Amasia would be formed in the Pacific region, based around the Pacific’s “ring of fire”, and eventually close the Atlantic Ocean and the Caribbean Sea. Each of the models shows that Amasia will be formed with Asia at its center.
            To show that any one of these models is more accurate than the others in forecasting the future, the models must first recreate the past. Using their orthoversion model, Mitchel et al. are able to show that previous supercontinents have orthoverted from one to the next, all the way back to 1000 Mya. By creating a model based on geological records of landmass centers and supercontinent centers, the authors were able to recalculate their movement, based on of the orthoversion model. The landmass and supercontinent data are acquired from seafloor-spreading records that allow for the precise calculation of Pangaea’s center. Then by working backwards they are able to recreate the older supercontinents, Rodinia (~600 Mya) and Nuna (~1000 Mya).
            Their results match what little understanding we do have of what these older supercontinents looked like. What is more important, however, is that authors’ method for deriving these supercontinents match movements being observed in the Earth’s mantle, and movements of the mantle in the past that have been reconstructed. Over a period of 800 Mya, Mitchell et al. are able to show the movement of Earth’s continents as they form one supercontinent into another, while also arriving at a picture that looks like Earth today. The orthoversion model also helps explain the movement of Australia, India, and Arabia. Each of those landmasses have moved eastward into the subduction girdle of post-Pangaea, and then moved northward. Only the orthoversion model can explain this movement. The orthoversion model also provides the missing link in Pangaean formation, and is the only model that matches ocean records in showing that the Pacific Ocean rift has only existed since the creation of Pangaea. By creating and presenting the orthoversion model, Mitchell et al. have shown that orthoversion explains the connection between supercontinent decline and growth, and many other gaps in the other two theories.
            

Recent contributins of glaciers and ice caps to sea level rise

Yearly glacier and ice cap melt contributes sea level changes. How the rate of this contribution to sea level changes is very important in understanding changes in Earth’s climate. Jacob et al. (2012) use the Gravity Recovery and Climate Experiment (GRACE) satellite to calculate changing masses of glaciers and ice caps around the world from 2003 to 2010. These values allow the authors to calculate the changes in the sea level. Their findings show that there are increasing rates of loss in most areas of the world that are ice-covered. The full contribution of ice mass loss around the world to sea level change is about 1.48±0.26 millimeters per year. However, their data did show difference from previously published work in some regions, specifically in High Mountain Asia (Northern India and the Himalayas). Looking at their data and the previously published data in more detail revealed that their data were most likely accurate, as there were little reasons for GRACE to miscalculate the data in the region. Furthermore, the final rate of change is sea level rise is nearly identical (difference of 0.2±0.6 millimeters per year, which is not significantly different from zero) to other recent studies that focus on physical calculations of sea level change. This paper demonstrates the firm understanding of glacier and ice cap contribution to sea level change in the scientific community. –Mathew Harreld
Jacob, T., Wahr, J., Pfeffer, W.T., Swenson, S. 2012. Recent contributins of glaciers and ice caps    to sea level rise. Nature (Online) 1-5.

            A major concern in a changing climate is its the impact on sea level changes. The melting of glaciers and ice caps throughout the world mostly drives the changes in sea level. Rising global mean temperature could mean a faster rate of glacier and ice cap melting, which in turn will increase sea level. The big question, however, is how much. Many studies have been done recently to calculate the changing sea level due to glacial melt, but Jacob et al. reevaluate those studies with one of their own. Using the Gravity Recovery and Climate Experiment (GRACE) satellite to calculate changing sea levels due to changing masses of glaciers and ice caps in the 8 year period from 2003 to 2010.
            GRACE observes monthly, global gravity field changes, allowing the authors to calculate changes in mass on Earth’s surface. Using data compiled from GRACE the authors calculated the rate of loss or gain of glaciers and ice caps from around the world, and converted that data into rates of sea level rise. The data from GRACE were split into 175 small arbitrarily defined regions of Earth, called “mascons”. The 175 mascons were then grouped into 20 regions based on location on the Earth’s surface. GRACE does not have high enough resolution to separate Greenland and Antarctic ice sheets from their peripheral glaciers and ice caps, and therefore the main part of the study focused on results without the peripheral glaciers and ice caps. The authors, however, used a different source for the peripheral glaciers and ice caps for the sake of completeness in their final results.
            The results without the peripheral glaciers and ice caps showed increasing rates of loss in most areas of the world. The authors do note that certain areas show positive increases in rate, but that the increases are not significantly different from zero. The total mass of glacier and ice cap rate loss between 2003 and 2010, with the peripherals,was calculated to be about –536±93 gigatons per year. The peripheral glacier and ice caps contribute about –236 gigatons to the total amount, a significant amount. Since the peripheral glacier and ice cap amounts were not derived from GRACE, the amounts could be brought into question. The authors calculated that the –536 gigaton per year loss of glacier and ice caps contributed to an increased rate of 1.48±0.26 millimeters per year of the sea surface between 2003 and 2010. The peripheral glaciers and ice caps contributed about 1.06 mm per year to the total.
            The findings of this paper were similar to the results of other papers, except for the High Mountain Asia region (Northern India and the Himalayas), which the authors calculated to have a much lower rate of loss than reported in another paper. A paper used GRACE to evaluated glacier and ice cap changes between 2002 and 2009 and determined that the same region rate of loss was around –55 gigatons per year, whereas Jacob et al. determined it to be around –4 gigatons per year. Due to this large discrepancy the authors decided to evaluate the region in more detail. It is possible that changes in the tectonic process under the region could be causing Jacob et al.’s difference in data, but this seems to be unlikely for a number of reasons. The amount of tectonic uplift needed to cause the large discrepancy is unlikely to work on such a short time period, or occur. For GRACE to not pick up these changes is even for unlikely, for the broad spatial changes must occur on a hundred to thousand year timescale. Another possibility is the absorption of melt water by the ground, effectively showing no change in mass in the studied area, when in fact the glaciers have decreased in size. But this too seems unlikely to contribute a large enough offset to result in such drastically different data.
            The authors determined that their derived data were accurate enough to add validity to changes in sea level rise. Their final results suggest that ice-covered regions are contributing an increase in sea level rise at a rate of 1.48±0.26 millimeters per year. Their findings differ from previous findings based on water-based measurements by 0.2±0.6 millimeters per year, which is not significantly different from zero. This suggests we have an accurate understanding of sea level changes due to glacial and ice cap melt. Future work needs to work on consolidating the data, and region changes.

The Weakening of the North Atlantic Current During the Late Pliocene and Early Pleistocene

The role of the North Atlantic Current (NAC) in regulating the Northern Hemisphere sea surface temperature (SST) gradient and the air temperature is huge. If the current were to weaken or stop, the effects could be drastic. Roughly 2.6 Ma that is exactly what happened, resulting in a major glacial event. Naafs et al. (2010) address how big an impact the NAC had on changing the climate, and show how the NAC may have weakened. Using coring samples from the middle of the Atlantic Ocean, Naafs et al. reproduce SST and productivity, to show how both change over time, and what those changes mean for the NAC. The shift of the Arctic Front southward, and the slow weakening of the NAC led to a major glacial event, changing the face of the planet. The findings of the authors highlight the long-term nature of change in Earth systems, while also showing how drastically they can be changed. –Mathew Harreld

Naafs, B.D.A., Stein, R., Hefter, J., Khélifi, N., Schepper, S.D., Haug, G.H. 2010. Late Pliocene changes in the North Atlantic Current. Earth and Planetary Science Letters 298, 434-442.

The North Atlantic Current (NAC) is hugely important in regulating the northern hemisphere climate. Warmer surface waters are pulled northward toward the Arctic Ocean, and some is downwelled into the deeper ocean, while some is sent back southward. This explains why England is more temperate than the Eastern Seaboard of the United States even though it is at a higher latitude. It is the warmer tropical waters from the south that keep the northern Atlantic Ocean at temperate temperatures. A topic of debate in current research is how might the slowing and stopping of this current affect local temperatures, and how might the NAC slow down. Naafs et al. raise this question in a previous epoch of Earth’s history. During the early Pliocene epoch, roughly 5 million years ago (Ma), there were warmer sea surface temperatures (SST) in the Northern Atlantic Ocean than there are today by nearly 10°C. This higher temperature meant that the NAC was more powerful than it is presently. However we also know that around 3.6 Ma an intense Northern Hemisphere glacial and interglacial cycle period began. Around 2.6 Ma, at the beginning of the Pleistocene epoch, the Earth fell into its first major glacial period that exhibited large Norther Hemisphere ice sheets.

During this shift into a deep glaciation the NAC is thought to have shifted dramatically. The extent of of the Arctic Front extends into lower latitudes, as ice sheets expand, and therefore there must be some change in NAC, which is carrying warmer waters that would otherwise melt the forming ice. In order for the ice to be expanding, and temperatures to be falling the NAC must have shifted. Naafs et al. hypothesize that that the NAC flattens out at a lower latitude, possibly even shrinking, while the Arctic Front expands southward, weakening the NAC. To test their hypothesis Naafs et al. use Deep Sea Drilling Project core samples from a site in the middle of the Atlantic Ocean on a latitude of 41°00’N. The core samples are used to remodel ocean productivity, which acts as a proxy for global average to temperature, and SST. They purposely chose a site that is currently right in the middle of the NAC because they believed it would show a greater effect of change in glacial conditions.

Having acquired the core samples, the authors used oxygen-18 data and organic compound data to remap SST and productivity. Oxygen-18 allowed the authors to remap SST because in colder temperatures there is more oxygen-18 present in oceans because oxygen-16 (a lighter isotope than oxygen-18) is frozen in the ice. The organic compounds allowed the authors to see levels of productivity because a higher level of organic compounds meant there were more organisms living in that area at that time. It should also be noted that there are higher levels of productivity when there are lower SSTs because that means there is less influence from the NAC, which would allow more nutrient-rich waters to stay in the area. Using both of these variables the authors were able to put together changes in SST and productivity, giving a better understanding of how the NAC might have changed over the million-year period being studied.

During mid/late Pliocene there were generally low levels of oxygen-18 in the oceans, reflecting high SST. This combined with the generally low amounts of organic compounds meant that there was a highly intense NAC affecting the coring area about 3.6 to 3.4 Ma. During this time, however, there are also periods of lowering SST and increasing productivity, but not enough to suggest any major shift.

In the period between 3.4 and 3.29 Ma there began a shift toward lower SSTs, and lower productivity. This suggests that the NAC was weakening of this time, but not enough to allow for nutrients to flow back into the area. This period also saw the beginning of the move of the Arctic Front southwards, and the initial build up of glaciers.

The next 0.5 million years saw the ocean return to a warmer climate. SSTs rose and productivity decreased, suggesting an increased influence of the NAC and a northern Arctic Front. This period did see a gradual shift downward of SST, but nothing like what was seen between 3.4 and 3.29 Ma.

Between 2.9 and 2.45 Ma SSTs plummeted and an increase in productivity was seen. The authors mention that there may be many factors that may have influenced an increase in productivity, but that such a large increase—more than 10 times—in productivity could only be attributed to the weakening of the NAC. This period at the end of Pliocene saw a significantly reduced NAC and a much lower latitude Arctic Front. The changes observed in the core sample are remarkable, and are supported by other findings in other core samples in the Northern Atlantic region.

The change that is expected in the NAC during a glaciation period is exactly what the authors predicted. The combination of increased ice albedo (reflectivity) and eastward movement of the NAC could have brought on the glaciation period. However there are still some variables that are not well understood about how the NAC could change so drastically. The authors suggest that perhaps changes in wind forcings may have played an important role, as might have orbital forcings. Either way it seems certain that the slowing and weakening of the NAC, pushing it southward, played a pivotal role in producing ice sheet growth southward, and drastic temperature drops in the higher and mid latitudes. The article also highlights the long-term nature of the changes that affect our planet greatly. And while it may be possible to predict such changes in their beginning stages, the full effects will not be observed for thousands or millions of years.

CO2 has a Major Role in Driving the Antarctic Glaciation 33.7 Million Years Ago

The Antarctic glaciation that occurred about 33.7 million years ago is a major turning point in Earth’s history. Pagani et al. (2011) look back to this glaciation to see what may have caused it, with a focus on CO2. Previous work done on CO2 levels during this time period have shown that CO2 was increasing, which contradicts greenhouse gas theory. However, Pagani. et al. determine that CO2 was overestimated, and that, in fact, CO2 was decreasing well before and during the Antarctic glaciation. This finding is pivotal in our understanding of how CO2 and the glaciation cycle are interlinked. –­Mathew Harreld
Pagani, M., Huber, M., Liu, Z., Bohaty, S.M., Henderiks, J., Sijp, W., Krishnan, S., DeConto, R.M., 2011. The role of carbon dioxide during the onset of Antarctic glaciation. Science 334, 1261–1265

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            About 33.7 million years ago a major glaciation in the Antarctic shifted the climate towards what it is today. The onset of this Antarctic glaciation has been studied intensively because CO2 levels at the time of its occurrence seem counter intuitively high. Previous papers have suggested that CO2 was increasing along with the new glaciation, but this is counter to current theory and understanding of the relationship between temperature and CO2. Pagani et al. revaluate the alkenone (a substrate found in phytoplankton)-based record to determine why this counter intuitive finding may be occurring. A more recent paper did, in fact, model global CO2 levels during the period to be decreasing, but used boron records to develop CO2 levels. Pagani et al. explored this curious contradiction, and looked to use alkenone records to correctly model CO2 during the Antarctic glaciation 33.7 million years ago.
            To evaluate the CO2 during the glaciation period, the authors used the same coring samples used by previous papers that used alkenone. The drilling sites were spread throughout the Atlantic, mostly near South America and the Southern Ocean, and one near New Zealand. Two of these drilling sites show evidence of increasing CO2 levels throughout the glaciation period, however the quality of the cores is very poor.
            The authors used the carbon-13 values from the alkenones from six drilling sites, allowing for a wide range of environments and algal-growth patterns. The carbon-13 is derived from methyl ketone, which is found in the coring samples. The authors also used carbon isotopes levels to determine algal growth. Using various testing methods the authors compiled the algal growth and carbon-13 values. Their analysis provided evidence for lower carbon-13 and algal growth levels in the Antarctic region than in northern regions. However the results here do not show lower levels than reported previously.
            The authors than ran tests on how cell size may affect the levels of CO2 in the oceans, but determined that the changes in cell size, which varied by location, had more to do with the available nutrient cycle than that of CO2. The authors also attempted to remodel possible ocean dynamics, which revealed that there were indeed lower levels of nutrients in the study region, as well as differing levels of other key materials used to study and calculate CO2. Many of the sites initially picked for this study are not of use because of this finding. The uncertainty of temperature and nutrient cycles in lower latitude regions makes it very difficult to determine CO2 levels, so the authors refocused their work on two sites at higher Southern latitudes, which are less susceptible to effects of the variables.
            The results from these two sites revealed that CO2 declined about two million years before the rapid glaciation, and that the decrease continues into the event. There are possible places where CO2 increased, but full evidence is still lacking. This paper’s results determined that previous papers did not take full account of pre-glaciation values, and thus were left with poorer results. The results from this paper and from boron-based testing match with model estimates of this Antarctic glaciation. The authors concluded that there was a certain CO2 decline during the period, but cannot yet determine the absolute values.
            This paper highlights the important role CO2 may have on affecting climate, and that CO2 decrease is critical for global cooling to occur and for the evolution of Earth’s cryosphere. The decrease in CO2 had a great effect on the earth’s climate 33.7 million years ago, but it didn’t act alone. The combination of long-term decreasing CO2 as well as many other important factors pushed the Earth’s climate into a long glaciation.

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.

No Correlation Between Yield Production and Biodiversity in Large-Scale Farming

There is a growing concern about the amount of food being produced and the growth of the human population. In an effort to meet the demands of population growth it is feared that many natural habitats will have to be replaced by agricultural land. This would be hugely detrimental to biodiversity, especially in tropical regions, which are home to most of the world’s biodiversity and 13% of human agriculture. Although more wildlife-friendly agriculture practices have been put forward, they are rarely used on a large scale because it is believed they decrease the total yield. Clough et al. (2011) explore this argument in Indonesian cocoa agroforestry plantations. They discovered that there is no correlation between biodiversity and agricultural yield, opening up many possibilities in large-scale wildlife friendly agricultural practices. They also explored possible ways to benefit yield and biodiversity in trees and birds, giving an example for a new way of thinking about farming and biodiversity conservation. While their findings do not suggest that wildlife friendly farming practices will end the depletion in biodiversity, as primary forests still are home to many more species than any other area on earth, it is a step forward as we attempt to feed our growing population and conserve the planet and the other species on it. –Mathew Harreld
Clough, Y., Barkmann, J., Juhrbandt, J., Kessler, M., Wanger, T.C., Anshary, A., Buchori, D., Cicuzza, D., Darras, K., Putra, D.D., Erasmi., S., Pitopang, R., Schmidt, C., Schulze, C.H., Seidel, D., Steffan-Dewenter, I., Stenchly, K., Vidal, S., Weist, M., Wielgoss, A.C., Tscharntke, T. 2011. Combining high biodiversity with high yields in tropical agroforests. PNAS 108:20, 8311–8316.

       The importance of food and agriculture in our culture is unquestionable, and the global importance of agriculture will continue to grow in the years to come. As human populations grow there is an increasing demand for food. Agriculture, however, is one of the main threats to global biodiversity. As farms try to increase yields to meet increasing demands, removing natural habitats and increasing farmland is often the action taken. If wild species are to survive then a balance must be found between agriculture and biodiversity. There is the potential for biodiversity-friendly farming. This farming method is often criticized for decreasing the yield quantity because of the focus on biodiversity preservation. Clough et al. explore this argument in Indonesian cacao farms. The argument that wildlife friendly farming practices is ineffective on larger scales is put to the test. The authors evaluated the possibility of combining high species diversity and high yields and where this might be done.
       The authors chose to focus on tropical regions in Indonesia because of the high biodiversity combined with the high human populations density. Furthermore, agriculture in tropic regions compasses 13% of the total agricultural system globally, and thus is an important area for future agricultural output.
       Clough et al. broke the study into two parts: a field study and a survey study. The field study consisted of data collected on yield and species richness in nine different taxonomic groups, during a two-year period. The authors used the land of 43 smallholder cacao agroforestry system in Sulawesi, Indonesia. Only mature plots ten to twenty years old were chosen for the study. Also, the authors studied the possible relationship between biodiversity and yield as distance to a natural forest changed.
       The survey portion of the study focused on 60 cacao plantations that were run only by the owners, and were not affected by the study. Only tree species were used as a measurement of biodiversity, however other agronomic data were recorded. These data were used mostly for a better understanding of general yield patterns throughout the regions.
       The authors discovered a surprising result once their data were collected, there was little to no correlation between species richness and yield. This general lack of correlation could potentially have large impacts on farming methods, but first the authors explored what does affect species richness and yield.
       They found that differences in region and altitude had large impacts on species richness, but it was mainly associated with distance from forests and shade by trees. There was a clear negative relationship between distance from forest and species richness. Plots with high levels of trees and shade had more bird species, but had fewer light-dependent species, such as herbs and butterflies.
       Yield was mainly negatively affected by the amount of shade a plot received, however distance to forest had a small positive effect. Other variables proved to have little effect on yield. Through the survey portion of the study, it was determined that labor and pesticide use were the largest determinants of yield.
       In an attempt to find a possible method of sustaining high yields and high biodiversity the authors focused on the effects of trees, yield, and birds. They found that birds were more dependent on tree height than total amount of shade, whereas yield was affected by shade. Therefore it may be possible to increase bird habitat with taller, but fewer trees, resulting in less shade.
          This study reveals that it is possible to have wildlife friendly farms that produce high yields. These findings suggest that wildlife friendly policies can be, and should be, implemented in agriculture without fear of yield loss. The authors also caution that their findings may be inaccurate because there may be a lag in time between the presence of farms and their effect on biodiversity. This is why the authors chose the most established and mature agroforestry sites. More data need to be collected to continue to record the effects on the practices of wildlife friendly agricultural on biodiversity and yield. At first glance, though, the findings of Clough et al. are quite stunning and exciting, leaving the potential for new wildlife friendly farming practices that produce high yields for a growing human population.