For example, in Norway, the Sun never completely descends beneath the horizon between late May to late July. The Milankovitch Cycles are among the arguments fielded by detractors of the Global Warming concept. Skip to content. Milankovitch cycles. Like this: Like Loading Next Post Next Famous Earthquakes. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat.
Second, Milankovitch cycles are just one factor that may contribute to climate change, both past and present. Even for Ice Age cycles, changes in the extent of ice sheets and atmospheric carbon dioxide have played important roles in driving the degree of temperature fluctuations over the last several million years. In just the past 20 years alone, carbon dioxide is up 11 percent. At that rate, Earth is expected to warm another half a degree Celsius almost a degree Fahrenheit as soon as and very likely by Air bubbles trapped in ice cores also provide a direct record of the past chemical composition of the atmosphere, particularly CO 2.
Carbon isotope ratios of shells in marine cores are equally valuable for estimates of water circulation and atmospheric CO 2 concentrations. Eolian dust preserved in both marine sediment and ice cores has been correlated with climate and environmental conditions in the dust's source region, specifically as a proxy for aridity. Continuous ice cores from Greenland record back to over , years ago Bender et al.
Thus, these records are relevant to the later members of the genus Homo , such as H. Documenting a much longer timescale, marine sediment cores have been collected across the globe, and composite records have been compiled that extend beyond the Cenozoic , thus covering the entire duration of the Primate fossil record Zachos et al. Peaks in most records depicted and odd MIS numbers indicate interglacial phases while troughs and even MIS numbers indicate glacial phases. All rights reserved.
Figure 2: Global deep-sea oxygen and carbon isotope records. There are a variety of other important high-resolution paleoclimate records relevant to hominin evolutionary history, but these are temporally or spatially restricted compared to marine cores. For example, the variation in thickness and grain size in Chinese loess deposits are related to extensive periods of cold, dry, winter Asian monsoon winds stretching back over the last 7 million years An Speleothems found in caves are also a rich archive of local paleoclimate information and, combined with uranium-thorium dating , can provide high-resolution records back to , years ago.
Carbon and oxygen isotopic analysis as well as relative growth band thickness of speleothems have provided proxy data for local temperature, rainfall, aridity, and overlying vegetation C 3 vs.
Similar to the study of marine cores, an extensive arsenal of analytical methods have been applied to the study of lake cores, which serve as long, continuous archives of terrestrial climate change at annual to decadal scale for individual basins or watersheds. Existing lake cores in close proximity to paleoanthropological sites are typically restricted to the Holocene e.
Additional scientific drilling initiatives are exploring thick lacustrine deposits directly associated with Plio-Pleistocene paleoanthropological sites Cohen et al.
The pattern of incident solar radiation insolation received on the planet at a given place and time is an important factor in understanding both directional trends and variability observed in many paleoclimatic records, particularly those related to Quaternary ice ages Hays et al.
Changes in insolation are, in turn, driven by Earth's natural orbital oscillations, termed Milankovitch cycles. The three elements of Milankovitch cycles are eccentricity, obliquity, and precession Figure 3.
Eccentricity describes the degree of variation of the Earth's orbit around the Sun from circular to more elliptical. Obliquity describes the tilt of the Earth's axis in relation to its orbital plane, which ranges from Precession describes the motion of the Earth's axis of rotation, which does not point towards a fixed direction in the sky through time.
Instead, the axis of rotation describes a clockwise circle in space, like the spinning of a wobbling top, with a periodicity of 19,—23, years Animation 1. Courtesy of NASA. While we now know that there is no single "magic bullet" that is responsible for the multitude of anatomical and behavioral changes documented in the hominin record, the concept that certain changes in the human lineage may have evolved in open habitat settings has persisted.
The "Turnover Pulse Hypothesis" championed by paleontologist Elisabeth Vrba Vrba , proposed that a synchronous change in hominins, such as the origins of the genus Homo , and other African mammalian lineages, particularly speciation and extinction events in bovids , was caused by a shift from warm, moist conditions to cooler, drier, and more open habitats associated with a sharp transition in the marine oxygen isotope record associated with the onset of NHG Figure 4.
Other studies have since indicated that the record at specific East African hominin sites show either no faunal turnover at this time e. A seminal study of terrigenous dust in marine cores off the coast of Africa by paleoceanographer Peter deMenocal suggested that subtropical African climate oscillated between markedly wetter and drier conditions, paced by Earth's orbital variations, with step-like increases in climate variability and aridity near 2.
These steps were coincident with changes in the dominant orbital cycles from precession to obliquity to eccentricity, and with the onset and intensification of high-latitude glacial cycles, respectively. Compared to the African fossil and geological record, these time periods also coincided with proposed diversification points in the hominin lineage 2.
In addition to unidirectional shifts, deMenocal also highlighted the importance of "variability packets" of high- and low-amplitude paleoclimatic variability lasting 10, to , years in duration, paced by the orbital eccentricity modulation of precession Figure 3a. These alternating periods of relative paleoclimatic stability low eccentricity and instability high eccentricity as a mechanism for introducing genetic variance to natural selection are a key component of the "Variability Selection Hypothesis" Potts , which proposes that the wide variability in adaptive settings over time ultimately favored complex adaptations that were responsive to novel conditions i.
Figure 5: Summary diagram of important paleoclimatic and hominin evolution events during the Plio-Pleistocene. Gray bands indicate periods when African climate became progressively more arid after step-like shifts near 2. Approximate first and last appearance datums and possible relationships among hominin taxa. Soil carbonate carbon isotopic data from East African hominin localities documenting a progressive shift from woodland to grassland vegetation.
Composite benthic foraminfera oxygen isotope record illustrating the evolution of high-latitude glacial cycles and dominant periodicity of glacial variability. Studies of East African lake records by geologist Martin Trauth and colleagues have also focused on critical intervals near 2. African monsoon intensity correlates with precession-paced insolation, and increased polar ice-volume acts to accentuate the pole-Equator thermal gradient, which leads to a north-south compression of the Intertropical Convergence Zone ITCZ , the major control of monsoonal precipitation patterns in Africa.
Associated with major glacial events near 2. They propose that these occurred during periods of eccentricity maxima every , years since 2. While some East African lake records provide strong evidence for this pattern e. Ultimately, this hypothesis proposes that periods of dramatic climatic oscillations between 2.
Figure 6: Summary diagram of global climate transition, East African lake occurrences and soil carbonate records, and hominin evolution.
East African lake occurrences are suggested to cluster during eccentricity maxima prior to 2. Note that lake phases do not occur during all eccentricity maxima and that some occur during eccentricity minima. Bovids : Members of the family Bovidae that includes antelopes, oxen, goat and sheep. Unbranched horns made up of a layer of keratin surrounding a bony core are one of the defining characteristics.
C 4 plants include warm-climate grasses and grains, and are advantageous under conditions of high heat and light, and low carbon dioxide levels. Environmental determinism : The view that changes in physical, abiotic environmental factors are the dominant influence on evolution, as opposed to stochastic i.
External and internal forcings : External forcing mechanisms involve agents acting from outside the climate system e. Internal mechanisms operate within the climate system itself e. Foraminifera : A large and diverse group of, single-celled aquatic organisms mainly marine that construct their shells from calcium carbonate. Intertropical Convergence Zone ITCZ : The equatorial region where the trade winds of both hemispheres come together and are associated with high precipitation.
As the ITCZ is tethered to the zone of maximum solar insolation, its location migrates north and south of the equator with the seasons. Isotope : Variants of a particular element that have the same number of protons, but different number of neutrons. Milankovitch : Milutin Milankovitch — , a Serbian mathematician who proposed that climatic changes, particularly ice ages, were the result of variations in the Earth's orbital elements.
Monsoon : A wind system whose direction changes with the seasons. Often associated with seasonal precipitation. Radiolaria : A large and diverse group of single-celled marine organisms that construct their shells from silica. Seasonality : Changes in timing, duration, or intensity of the within-year distribution of climatic elements, but not in total annual amounts e.
Speleothem : A mineral deposit, typically calcium carbonate, that precipitates from solution in a cave e. Uranium-thorium dating : An absolute dating technique based on the natural radioactive decay of uranium to thorium. An, Z. The history and variability of the East Asian paleomonsoon climate.
Quaternary Science Reviews 19, Bar-Matthews, M. Sea-land oxygen isotopic relationships from planktonic foraminifera and speleothems in the Eastern Mediterranean region and their implication for paleorainfall during interglacial intervals. Geochimica et Cosmochimica Acta 67,
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