Earth Orbital Parameters Research Articles

Sensitivity of paleoclimate simulation results to season definitions

According to the Milankovitch theory, slow variations of the Earth's orbital parameters change the amplitude of the seasonal cycle of insolation and are considered to be the main forcing mechanism of glacial‐interglacial cycles. Because of the precession and changes in eccentricity the length of seasons also varies. No absolute phasing is then possible between the insolation curves of two different periods. Various solutions to compare different periods have been given either for astronomical computations [e.g., Berger and Loutre, 1991; Laskar et al., 1993] or for model simulations [e.g., Kutzbach and Otto‐Bliesner, 1982; Mitchell et al., 1988], but the sensitivity of model results to the different possible solutions has never been quantified. Our results, based on simulations of the last interglacial climate, 126 kyr B.P., where changes in the length of the seasons are large, clearly show that phase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the Milankovitch forcing. Our main conclusions are that (1) when comparing various model simulations, the date of the vernal equinox (i.e., the phasing of the seasonal cycle of insolation) as well as the definition of seasons must be the same for all models in order to avoid artificial differences; (2) seasons based on astronomical positions are preferred to seasons defined with the same lengths as today, since they better account for the phasing of insolation curves. However, insolation is not the only forcing in most atmospheric general circulation model simulations. We also discuss the impact of the calendar hidden behind the definition of the seasonal cycle of the other boundary conditions, such as sea ice or sea surface temperatures.

Long-term variations in insolation and their effects on climate, the LLN experiments

Used to test the Milankovitch theory over the last glacial-interglacial cycles, the Louvain-la-Neuve two-dimension Northern Hemisphere climate model shows that orbital and CO2 variations induce, in the climate system, feedbacks sufficient to generate the low frequency part of the climatic variations over the last 200 kyr. Initiation and termination of glacial cycles cannot indeed be explained without invoking both the fast feedbacks associated with atmospheric processes (water vapor, cloud, snow and sea ice) and the slower feedbacks associated with coupling to other parts of the climate system, in particular the land ice-sheet buildup and disintegration. This model shows that longterm changes in the Earth's orbital parameters lead to variations in the amount of solar radiation received at the top of the atmosphere, which in turn act as a pacemaker for climatic variations at the astronomical frequencies, through induced albedo-temperature and greenhouse gases-temperature feedbacks. Spectral analysis of the Northern Hemisphere global ice volume variations simulated under both insolation and CO2 forcings reproduces correctly the relative intensity of the peaks at the orbital frequencies as seen in SPECMAP data. Except for variations with time scales shorter than 5 kyr, the simulated long-term variations of total ice volume are comparable to that reconstructed from deep-sea cores. For example, the model simulates glacial maxima of similar amplitudes at 134 kyr BP and 15 kyr BP, followed by abrupt deglaciations.

Vegetation and soil feedbacks on the response of the African monsoon to orbital forcing in the early to middle Holocene

FOSSIL pollen, ancient lake sediments and archaeological evidence from Africa indicate that the Sahel and Sahara regions were considerably wetter than today during the early to middle Holocene period, about 12,000 to 5,000 years ago1–4. Vegetation associated with the modern Sahara/Sahel boundary was about 5° farther north, and there were more and larger lakes between 15 and 30° N. Simulations with climate models have shown that these wetter conditions were probably caused by changes in Earth's orbital parameters that increased the amplitude of the seasonal cycle of solar radiation in the Northern Hemisphere, enhanced the land-ocean temperature contrast, and thereby strengthened the African summer monsoon5–7. However, these simulations underestimated the consequent monsoon enhancement as inferred from palaeorecords4. Here we use a climate model to show that changes in vegetation and soil may have increased the climate response to orbital forcing. We find that replacing today's orbital forcing with that of the mid-Holocene increases summer precipitation by 12% between 15 and 22° N. Replacing desert with grassland, and desert soil with more loamy soil, further enhances the summer precipitation (by 6 and 10% respectively), giving a total precipitation increase of 28%. When the simulated climate changes are applied to a biome model, vegetation becomes established north of the current Sahara/Sahel boundary, thereby shrinking the area of the Sahara by 11% owing to orbital forcing alone, and by 20% owing to the combined influence of orbital forcing and the prescribed vegetation and soil changes. The inclusion of the vegetation and soil feedbacks thus brings the model simulations and palaeovegetation observations into closer agreement.

Role of orbitally induced changes in tundra area in the onset of glaciation

THE link between glacial–interglacial cycles and changes in insolation due to variations in the Earth's orbital parameters is well established1–4. But of the attempts to simulate incipient glaciation using three-dimensional general circulation models (GCMs) driven by orbital forcing alone5–10, only one10 has been successful. GCM experiments7,11 show that reduced summer insolation 115,000 years ago (during an interglacial-to-glacial climate shift) produces sufficient high-latitude cooling to cause expansion of tundra at the expense of boreal forest11, which in turn can induce more cooling11–14. Here we show, using a global climate model, that the increase in surface albedo (under snow-covered conditions) that results from a biome model estimate11 of tundra expansion 115,000 years ago is sufficient to induce glaciation over extreme-northeastern Canada. If the additional cooling from this estimated tundra expansion induces further expansion, then widespread glaciation occurs at latitudes above 65° N. These results suggest that the climate feedback from high-latitude tundra expansion might have contributed to the onset of the most recent glaciation.

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Pulsars and solar-system ephemerides

The analysis of pulsar time-of-arrival data is intimately bound up with planetary ephemerides. Highly accurate ephemerides are required for Earth and Moon and, to a lesser degree, for the other planets, in order to make full use of the timing data for millisecond-class pulsars. These data, in turn, present an opportunity for improving planetary ephemerides in a variety of ways. Fitting the Earth and Moon orbital parameters to the timing data is the obvious first step, though it is less valuable in the short term for many applications than using the current accumulation of spacecraft-tracking and lunar laser ranging data. By themselves, the pulsar timing data convey no information on the orientation of Earth's orbit, since each pulsar's position on the sky must be determined from those same data. However, independent pulsar position measurements by VLBI, in combination with the timing-derived positions, can serve to fix the orientation of Earth's orbit with respect to the radio reference frame and thereby link the planetary and radio frames. In the long run, the acquisition of timing data over increasing time spans and with improving precision should prove to be an important factor in determining the shape, as well as the orientation, of Earth's orbit. In addition, pulsar timing over a sufficiently long span can directly measure a planet mass through the reaction of the rest of the solar system. The effect must be observed for a major fraction of the orbital period of the planet in question so that the signature can be separated from that of the ordinary spin-down of each pulsar. Finally, pulsar timing can be used to probe gravitational physics, a field with far-reaching consequences and a basic part of the framework for constructing the ephemerides.

Correlation of the magnetic susceptibility stratigraphy of Chinese loess and the marine oxygen isotope record: chronological and palaeoclimatic implications

The thick deposits of wind-blown silt of the Loess Plateau in central China constitute arguably the most important terrestrial record of the late Pliocene and Pleistocene climate. Magnetic susceptibility stratigraphy has proven extremely useful for quantifying the palaeoclimatic record of Chinese loess, with intervals of high magnetic susceptibility corresponding to warm, humid interglacials and intervals of low susceptibility to cooler, drier glacials. Several workers have noted that the magnetic susceptibility stratigraphy strongly resembles the marine oxygen isotope record, for which a high resolution chronology is available from tuning to the frequencies of Earth orbital parameters. Attempts have been made to correlate the two records statistically and we use this approach to generate a new magnetic susceptibility time series for the central Chinese Loess Plateau. Our results suggest that the two records are sufficiently similar over the interval ca. 0–1.5 My BP for this approach to yield a useful age model for loess accumulation. Spectral analysis of the resulting magnetic susceptibility time series clearly shows that the growth of 100 ky variance, which occurs in the marine oxygen isotope record after about 1 My BP, also occurs synchronously in the magnetic susceptibility record, reinforcing recent suggestions of a strong linkage between the high latitude northern hemisphere and the Asian climate. We also show that a significant improvement in the strength of the loess magnetic susceptibility-marine oxygen isotope correlation occurs after about 1.5 My BP, which corresponds to a significant increase in the rate of sediment accumulation across the Loess Plateau.

Continental climate response to orbital forcing from biogenic silica records in Lake Baikal

CHANGES in insolation caused by periodic changes in the Earth's orbital parameters provide the primary forcing for global ice ages1–6. But it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets. Here we investigate this question using the record of biogenic silica in Lake Baikal as a proxy for climate change in this high-latitude mid-continental region. We find a good correlation between this record and that of marine oxygen isotopes4. Over the past 250 kyr the Baikal record exhibits both a strongly nonlinear component (manifested in a 100-kyr periodicity) and weaker direct-insolation components (manifested in the 41-kyr (obliquity) and 23- and 19-kyr (precession) orbital cycles). These results show that even though extreme continental climates such as this are influenced directly by insolation variations, they are dominated by the nonlinear rhythm of the oceans and ice sheets.

The sensitivity of the terrestrial biosphere to climatic change: A simulation of the Middle Holocene

A process‐based ecosystem model, DEMETER, is used to simulate the sensitivity of the terrestrial biosphere to changes in climate. In this study, DEMETER is applied to the two following climatic regimes: (1) the modern observed climate and (2) a simulated mid‐Holocene climate (6000 years before present). The mid‐Holocene climate is simulated using the GENESIS global climate model, where shifts in the Earth's orbital parameters result in warmer northern continents and enhanced monsoons in Asia, North Africa, and North America. DEMETER simulates large differences between modern and mid‐Holocene vegetation cover: (1) mid‐Holocene boreal forests extend farther poleward than present in much of Europe, Asia, and North America, and (2) mid‐Holocene North African grasslands extend substantially farther north than present. The simulated patterns of mid‐Holocene vegetation are consistent with many features of the paleobotanical record. Simulated mid‐Holocene global net primary productivity is approximately 3% larger than present, largely due to the increase of boreal forest and tropical grasslands relative to tundra and desert. Global vegetation carbon is higher at 6 kyr B.P. compared to present by roughly the same amount (4%). Mid‐Holocene global litter carbon is larger than present by 10%, while global soil carbon is approximately 1% less. Despite the regional changes in productivity and carbon storage the simulated total carbon storage potential of the terrestrial biosphere (not including changes in peat) does not change significantly between the two simulations.

Spectral analysis of the diatom paleotemperature records at DSDP sites 579 and 580 near the subarctic front in the western North Pacific

Maximum entropy spectral analyses and a fitting test to find the best suitable curve for the modified time series based on the non-linear least squares method for Td (diatom temperature) values were performed for the Quaternary portion of the DSDP Sites 579 and 580 in the western North Pacific. The sampling interval averages 13.7 kyr in the Brunhes Chron (0–780 ka) and 16.5 kyr in the later portion of the Matuyama Chron (780–1800 ka) at Site 580, but increases to 17.3 kyr and 23.2 kyr, respectively, at Site 579. Among dominant cycles during the Brunhes Chron, there are 411.5 kyr and 126.0 kyr at Site 579, and 467.0 kyr and 136.7 kyr at Site 580 correspond to 413 kyr and 95 to 124 kyr of the orbital eccentricity. Minor cycles of 41.2 kyr at Site 579 and 41.7 kyr at Site 580 are near to 41 kyr of the obliquity (tilt). During the Matuyama Chron at Site 580, cycles of 49.7 kyr and 43.6 kyr are dominant. The surface-water temperature estimated from diatoms at the western North Pacific DSDP Sites 579 and 580 shows correlation with the fundamental Earth's orbital parameters during Quaternary time.

Synchronous changes in seawater strontium isotope composition and global climate

THE 87Sr/86Sr ratio of sea water has increased gradually over the past 40 Myr, suggesting a concomitant increase in global chemical weathering rates1–6. Recently, Dia et al.7 analysed a 250-kyr 87Sr/86Sr record, and found superimposed on this gradual increase higher-frequency 87Sr/86Sr variations which appeared to follow a 100-kyr cycle; this periodicity corresponds to one of the prominent cycles in the Earth's orbital parameters, which are known to modulate the patterns of solar insolation and hence climate8–10. The resolution of this record was, however, insufficient to establish the phase relationship between the 87Sr/86Sr variations and global climate cycles. Here we present a high-resolution seawater 87Sr/86Sr record spanning the past 450 kyr. We find that maxima and minima in 87Sr/86Sr coincide with minima and maxima, respectively, in continental ice volume (from the SPECMAP oxygen isotope record20), apparently suggesting that there was less chemical weathering in arid glacial periods than in the more humid interglacials. During glacial–interglacial transitions, however, seawater 87Sr/86Sr changes at a rate of ∼ 1 p.p.m. kyr−1, approximately three times that evaluated by Dia et al.7. Mass-balance calculations illustrate that simple changes in modern chemical weathering regimes cannot fully account for such rapid changes, suggesting that we need to revise current ideas about strontium reservoirs and the mechanisms for exchange between them.

Initial results of U.S.‐Soviet paleoclimate study of Lake Baikal

Lake Baikal, a Miocene‐age rift lake in southeastern Siberia, is an especially promising site for paleoclimate studies. Its high‐latitude location (52°–56°N) makes it particularly sensitive to changes in solar insolation due to long‐period variations in the Earth's orbital parameters. These variations are widely believed to be the main forcing functions of climate change in the Quaternary [Hays et al., 1976; Imbrie et al., 1984. The extreme continentality of the climate in southeastern Siberia makes Baikal an ideal location to study temporal changes in seasonality. Baikal is also one of the few high‐latitude lakes that has not been glaciated during the last 1–2 million years [Grosswald, 1980], although a record of glaciation in its drainage basin is preserved in the lake sediments. Finally, Lake Baikal is the largest (23,000 km3), the deepest (1640 m), and one of the oldest extant lake systems in the world. The sedimentary section in the Baikal depression is more than 7 km thick and probably spans more than 15 million years [Hutchinson et al., 1992]. Accordingly, Lake Baikal sediments represent one of the longest and most complete continental climate records available anywhere in the world.

Frequency variations of the Earth's obliquity and the 100-kyr ice-age cycles

VARIATIONS in the Earth's orbital parameters modulate the seasonal distribution of solar radiation and thereby induce changes in the Earth's climate1. Periodicities in the geological climate record with cycles of 100, 41 and 23 kyr have been linked with changes in eccentricity, obliquity and precession of the equinoxes, respectively2,3. But although the eccentricity does vary with a 100-kyr period, the effect on the incoming solar radiation is rather weak relative to the signals from obliquity and precession variations. The 100-kyr signal in the climate record should therefore be of negligible intensity, yet it is observed instead to dominate the record. Internal, nonlinear processes within the climate system have been proposed to account for this fact4–14. In contrast, I show here that variations in the frequency of the obliquity cycle can give rise to strong 100-kyr forcing of climate.

Past, Present and Future Levels of Greenhouse Gases in the Atmosphere and Model Projections of Related Climatic Changes

Ice core analyses of polar ice reveal a high correlation between climatic change and variations in the atmospheric concentrations of greenhouse gases (carbon dioxide and methane) over the last 160 000 years. Although the resolution of the data is not sufficient to determine the phase relationship between the respective variations, it is generally believed that climate change occurred first as a result of the quasi-periodic variations of the Earth's orbital parameters. However, data and model results are consistent with the hypothesis that climate and atmospheric concentrations of greenhouse gases interact via a positive feedback loop

Orbital signals in the diurnal cycle of radiation

Time of day plays a critical role in determining the dominance of the Earth's orbital parameters involving eccentricity, precession, and obliquity. Extension of orbital radiation data to the diurnal cycle permits the recognition of well‐defined, temporally separate signals corresponding to all three major orbital parameters and to combinations of these parameters. Past research involving computed radiation changes at the daily and seasonal scales has not shown significant eccentricity cycles. Results of this study show: (1) at the solstices, a marked change in dominance from precession (for the solar elevation classes associated with early morning/late afternoon) to obliquity (for the elevation class containing solar noon) occurs in solar radiation; (2) at the equinoxes, cycles corresponding to those associated with eccentricity are evident in the radiation data at all individual time intervals of the day. Past studies have shown that the daily radiative total reflects only the influence of precession. A third set of cycles at the equinoxes centered near 11 kyr corresponds to a combination of orbital signals. These results imply that paleoclimatic proxy data displaying strong spectral peaks corresponding to eccentricity (100 kyr) and obliquity (41 kyr), such as records of global ice volume, may be related to radiation changes occurring in the solar elevation class containing solar noon, while paleoclimatic proxy data displaying strong spectral peaks corresponding to precession, for example, sea‐surface temperatures, appear to be showing the dominance of precession on the daily total radiation.

Calculating the parameters of the Earth's orbit

Derives a simple analytic expression for the equation of time, which expresses the difference between sundial time and clock time, in terms of the Earth's orbital parameters. The expression is then applied to calculate these parameters, using as input data only a table of sunrise and sunset times. This simple calculation yields the dates of the Earth's perihelion passage and vernal equinox to accuracies of one or two days, and the eccentricity of the Earth's elliptical orbit and the inclination of its axis to a few per cent. An appendix explains the well known phenomenon whereby the earliest sunrise, latest sunset and longest day do not coincide.

Reversion of Forest to Tundra in the Central Yukon

Pollen and plant macrofossil analyses of sediments from three sites in the central Yukon that are presently in shrub tundra provided a record of former forest establishment. Shrub tundra with groves and gallery forest of balsam poplar occupied the region between 10 000 and 8000 BP. At 9400 BP white spruce (Picea glauca) populations expanded, and open white spruce woodlands persisted until 6500 BP when black spruce (Picea mariana) and green alder (Alnus viridis) populations increased, resulting in open spruce woodlands with a distribution of species probably similar to that commonly found today in the northern boreal forest: white spruce on drier south—facing slopes and on alluvial sites with balsam poplar, and black spruce on colder, wetter sites on north—facing slopes and valley bottoms. At 5000 BP forest began to revert to shrub tundra, abruptly at first but then more gradually. The modern groves of spruce in the region, which are mostly white spruce, therefore are probably relict populations surviving in favorable microsites. These results support the conclusion from other recent studies that Alaska and northwest Canada experienced warmer summers than today from as early as 10 000 BP to °6000 BP in response to increased summer insolation arising from changes in the earth's orbital parameters as predicted by the Astronomic Theory of climate change. The decline of forest after 5000 BP is not associated with any significant changes in the abundance of shrub or herb pollen types, suggesting that the ground vegetation has behaved independently of the tree populations.

二酸化炭素濃度と気候変動史

On the geological scale, earth history yields vital information concerning past environments and climates. This highlights the urgency for increased collaborative international response and research into solutions of problems posed by current climatic changes, whether natural or induced by the activities of man. Climatic changes occur on variety of timescales, ranging from catastrophic volcanic eruptions (minutes or days), through gradual changes in Earth's orbital parameters (104-106 years) to tectonically driven changes (106-108 years). It seems likely that the long-term fluctuations of climates (less than the time-scale of planetary evolution), which give rise to glacial epochs, are largely determined by continental plate distributions and movement, whilst the shorter-term fluctuations which produce glacial and inter-glacial periods in glacial epochs reflect variations in coming solar radiation.Attention is focussed on the possible existence of an anoxygenic, primeval atmosphere and on the history of atmospheric CO2. Five great biologic revolutions have occurred through earth history that have fundamentally shaped the modern geochemical picture. It is significant that organisms have controlled their own environment. Organic CO2 and calcium budget have played critical roles.

Greenhouse Warming: Is the Mid-Holocene a Good Analogue?

Abstract The mid-Holocene period (from approximately 9000 to 6000 years before present) is often suggested as an analogue for enhanced greenhouse warming. The changes in net radiative forcing at the top of the atmosphere are very different; increases in greenhouse gases producing a small annual mean warming of little seasonal or latitudinal variation, whereas during the Holocene the annual mean did not change but there were large seasonal and latitudinal variations. Two climate model experiments, one in which CO2 amounts are doubled and the other in which the value of the earth's orbital parameters are altered to those appropriate to 9000 years before present (BP), are compared. Any similarity in the simulated response is found to be limited to the northern continents and, even them, the mechanisms producing the changes differ between the two experiments. Assuming that the gross behavior of the model is realistic, the Holocene is not a good analogue for a “greenhouse” warming. Furthermore, as the mechanis...

Palaeoclimatic information from ice cores: the Vostok records

ABSTRACTIce deposits from Greenland and Antarctic ice sheets have stored over long periods of time information about the climate and environment of our planet. Attention will be focused on the 2083 m Vostok Antarctic ice core which represents a unusually long record (160 000 ka) due to the low accumulation rate (∼2 g cm−2a−1) and the rather uniform conditions of ice flow. This ice core provides a unique opportunity to obtain several palaeo-data such as temperature, accumulation (precipitation), aerosol loading, CO2 and trace gases over a full glacial-interglacial climatic cycle.The Vostok temperature, deduced from the interpretation of the deuterium content, and the CO2 records show a large 100 ka signal with a change of the order of 10°C and 70 ppmv respectively. The two records are closely correlated and both display shorter periodicities characteristic of the earth orbital parameters. CH4 concentrations also show variations from about 0·35 to about 0·65 ppmv linked with the glacial-interglacial warming. These features suggest a fundamental link between the climatic system and the carbon cycle and stress the role of radiatively active gases in climatic changes.The accumulation (precipitation) record appears to be governed by temperature with values during the coldest stages reduced by a factor of 2 with respect to the present rate. Ice deposited during these coldest stage is also characterised by high concentration of marine and terrestrial aerosols; these peaks probably reflect strengthened sources and meridional transport during full glacial conditions, linked to higher wind speed, more extensive arid areas on surrounding continent and a greater exposure of continental shelves.