Orbital insolation,ice volume,and greenhouse gases

The SPECMAP models of orbital-scale climate change (Imbrie et al., Paleoceanography 7 (1992) 701, Paleoceanography 8 (1993) 699) are the most comprehensive to date: all major climatic observations were analyzed within the framework of the three orbital signals. Subsequently, tuning of signals in Vostok ice to insolation forcing has fixed the timing of greenhouse-gas changes closely enough to permit an assessment of their orbital-scale climatic role. In addition, evidence from several sources has suggested changes in the SPECMAP δ¹⁸O time scale. This new information indicates that the timing of CO₂ changes at the periods of precession and obliquity does not fit the 1992 SPECMAP model of a “train” of responses initiated in the north, propagated to the south, and later returning north to force the ice sheets. In addition, analysis of the effects of rectification on 100,000-year climatic signals reveals that all have a phase on or near that of eccentricity. This close clustering of phases rules out the long time constants for 100,000-year ice sheets required by the 1993 SPECMAP model.A new hypotheses presented here revives elements of an earlier CLIMAP view (Hays et al., Science 194 (1976a) 1121) but adds a new assessment of the role of greenhouse gases.As proposed by Milankovitch, summer (mid-July) insolation forces northern hemisphere ice sheets at the obliquity and precession periods, with an ice time constant derived here of 10,000 years. Changes in ice volume at 41,000 years drive ice-proximal signals (SST, NADW, dust) that produce a strong positive CO₂ feedback and further amplify ice-volume changes. At the precession period, July insolation forces ice sheets but it also drives fast and early responses in CH₄ through changes in tropical monsoons and boreal wetlands, and variations in CO₂ through southern hemisphere processes. These CH₄ and CO₂ responses enhance insolation forcing of ice volume.Climatic responses at 100,000 years result from eccentricity pacing of forced processes embedded in obliquity and precession cycles. Increased modulation of precession by eccentricity every 100,000 years produces 23,000-year CO₂ and CH₄ maxima that enhance ablation caused by summer insolation and drive climate deeper into an interglacial state. When eccentricity modulation decreases at the 100,000-year cycle, ice sheets grow larger in response to obliquity forcing and activate a 41,000-year CO₂ feedback that drives climate deeper into a glacial state. Alternation of these forced processes because of eccentricity pacing produces the 100,000-year cycle. The 100,000-year cycle began 0.9 Myr ago because gradual global cooling allowed ice sheets to survive during weak precession insolation maxima and grow large enough during 41,000-year ice-volume maxima to generate strong positive CO₂ feedback.The natural orbital-scale timing of these processes indicates that ice sheets should have appeared 6000–3500 years ago and that CO₂ and CH₄ concentrations should have fallen steadily from 11,000 years ago until now. But new ice did not appear, and CO₂ and CH₄ began anomalous increases at 8000 and 5000 years ago, respectively. Human generation of CO₂ and CH₄ is implicated in these anomalous trends and in the failure of ice sheets to appear in Canada.     

米兰科维奇冰期旋回理论:挑战与机遇

米兰科维奇理论认为,北半球高纬夏季太阳辐射变化是驱动第四纪冰期旋回的主因。这个理论的核心是单一敏感区的触发驱动机制,即北半球高纬气候变化信号被放大、传输进而影响全球。最近,由于大量高分辨率及精确定年的气候变化记录的获得,从以下4个方面构成了对米氏理论的挑战:1)一些低纬地区并没有明显的10万年冰量周期,而是以2万年岁差周期为主,表明北半球冰盖的扩张、收缩变化并没有完全控制低纬区的气候变化;2)在最近几次冰消期时,南半球和低纬区的温度增高,要早于北半球冰盖的融化,表明冰消期的触发机制并非是北半球高纬夏季太阳辐射;3)大气CO₂浓度在第2冰消期的增加同南极升温相一致,表明该时大气CO₂浓度增加亦有可能早于北半球冰盖消融;4)南半球的末次冰盛期有可能早于北半球。这就说明单一敏感区触发驱动机制已难以圆满解释所有观察事实,天文因素控制下轨道尺度气候变化机制研究正面临理论突破的新需求和新机遇。     

Sensitivity of the Australian summer monsoon to tilt and precession forcing

The response of the Australian summer monsoon to orbital forcing is studied using a coupled General Circulation Model (GCM) with the focus on the relative roles of tilt and precession on the forcing of the northern Australian summer monsoon. It was found that unlike the Northern Hemisphere monsoons, which are dominated by precession forcing, the Australian monsoon can be enhanced significantly not only by precession forcing, but also by tilt forcing coupled to oceanic feedback. The new insights obtained from a series of experiments with differing tilt-precession configurations allow an interpretation of the Australian Late Quaternary monsoon record in which insolation forcing plays a significant role.     

Climate: Is the past the key to the future?

 The climate of the Holocene is not well suited to be the baseline for the climate of the planet. It is an interglacial, a state typical of only 10% of the past few million years. It is a time of relative sea-level stability after a rapid 130-m rise from the lowstand during the last glacial maximum. Physical geologic processes are operating at unusual rates and much of the geochemical system is not in a steady state. During most of the Phanerozoic there have been no continental ice sheets on the earth, and the planet’s meridional temperature gradient has been much less than it is presently. Major factors influencing climate are insolation, greenhouse gases, paleogeography, and vegetation; the first two are discussed in this paper. Changes in the earth’s orbital parameters affect the amount of radiation received from the sun at different latitudes over the course of the year. During the last climate cycle, the waxing and waning of the northern hemisphere continental ice sheets closely followed the changes in summer insolation at the latitude of the northern hemisphere polar circle. The overall intensity of insolation in the northern hemisphere is governed by the precession of the earth’s axis of rotation, and the precession and ellipticity of the earth’s orbit. At the polar circle a meridional minimum of summer insolation becomes alternately more and less pronounced as the obliquity of the earth’s axis of rotation changes. Feedback processes amplify the insolation signal. Greenhouse gases (H₂O, CO₂, CH₄, CFCs) modulate the insolation-driven climate. The atmospheric content of CO₂ during the last glacial maximum was approximately 30% less than during the present interglacial. A variety of possible causes for this change have been postulated. The present burning of fossil fuels, deforestation, and cement manufacture since the beginning of the industrial revolution have added CO₂ to the atmosphere when its content due to glacial-interglacial variation was already at a maximum. Anthropogenic activity has increased the CO₂ content of the atmosphere to 130% of its previous Holocene level, probably higher than at any time during the past few million years. During the Late Cretaceous the atmospheric CO₂ content was probably about four times that of the present, the level to which it may rise at the end of the next century. The results of a Campanian (80 Ma) climate simulation suggest that the positive feedback between CO₂ and another important greenhouse gas, H₂O, raised the earth’s temperature to a level where latent heat transport became much more significant than it is presently, and operated efficiently at all latitudes. Atmospheric high- and low-pressure systems were as much the result of variations in the vapor content of the air as of temperature differences. In our present state of knowledge, future climate change is unpredictable because by adding CO₂ to the atmosphere we are forcing the climate toward a “greenhouse” mode when it is accustomed to moving between the glacial–interglacial “icehouse” states that reflect the waxing and waning of ice sheets. At the same time we are replacing freely transpiring C3 plants with water-conserving C4 plants, producing a global vegetation complex that has no past analog. The past climates of the earth cannot be used as a direct guide to what may occur in the future. To understand what may happen in the future we must learn about the first principles of physics and chemistry related to the earth’s system. The fundamental mechanisms of the climate system are best explored in simulations of the earth’s ancient extreme climates. Received: 7 November 1996/Accepted: 23 January 1997     

The impact of varying ice sheets and greenhouse gases on the intensity and timing of boreal summer monsoons

Climate models, forced only with insolation, indicate that boreal summer monsoons respond to orbital forcing with a zero phase both at the precession and obliquity bands. Discrepancies exist among data with respect to the timing of the response. Some late Pleistocene monsoon records show small lags of 2–3 kyr, close to model results, while many others show considerably longer lags of 5–8 kyr. It has been hypothesized that such lags arise from factors that were, up till now, not included in the modelling experiments, namely variations in glacial-age boundary conditions.Here we address this issue using long, time-dependent climate simulations that do include varying ice sheets and greenhouse gas concentrations. Inclusion of these additional forcings introduces a small peak in the monsoon spectra at the 100 kyr period, while monsoon variance remains dominated by precession with a smaller contribution from obliquity. At the precession band orbital forcing remains the dominant control, with lags close to zero. At the obliquity band varying ice sheet and greenhouse gases explain most of the simulated African and Indian monsoon variance, with orbital forcing playing a minor role. For the East Asian monsoon orbital forcing remains dominant. As a result the simulated obliquity phase of different monsoon systems lies between summer insolation maxima and ice minima/greenhouse gas maxima, with a lag that varies with distance to the Eurasian ice sheet.     

更新世冰盖与大洋碳储库相互作用的箱式模型模拟

全球大洋深海有孔虫碳同位素(δ13C)记录中广泛发现40万年周期,这一周期可能与偏心率长周期的轨道驱动有关.1.6 Ma以来,δ13C的这一长周期拉长到50万年,且重值期不再与偏心率低值对应.目前对δ13C 40万年周期的成因及其周期拉长的机制还不明确.这里使用了包含9个箱体的箱式模型,用于研究热带过程与冰盖相互作用及其对大洋碳循环的影响.模拟结果显示当北半球高纬海区海冰迅速增大时冰盖迅速融化,进入冰消期,而当海冰快速消失后,冰盖则重新缓慢增长.冰盖变化具有冰期长,间冰期短的非对称形态.在季节性太阳辐射量的驱动下冰盖变化具有10万年冰期-间冰期旋回.当冰盖融化速率受北半球高纬夏季太阳辐射量控制时,冰盖变化的岁差周期明显加强,相位与地质记录一致,说明轨道驱动可以通过非线性相位锁定机制使冰盖变化与其在相位上保持一致.海冰的阻隔效应使大气中CO₂在冰消期时增多.冰期时大洋环流减弱使大气中CO₂逐渐减少.当模型只有ETP驱动的风化作用而不考虑冰盖变化时,模拟的δ13C记录显示极强的40万年周期,体现了大洋碳储库对热带风化过程的响应.当同时考虑冰盖变化和风化作用时,模拟的δ13C结果中40万年周期减弱而10万年周期加强,并且40万年周期上碳储库与偏心率的相位与不考虑冰盖变化时的相位也存在差异,反映了冰盖变化引起的洋流改组压制了大洋碳循环对热带过程的响应.      

Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the pretoria saltpan

Late Pleistocene variations in rainfall in subtropical southern African are estimated from sediments preserved in the Pretoria Saltpan, a 200000 year-old closed-basin crater lake on the interior plateau of South Africa. We show that South African summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession. As predicted by orbital precession geometry (Berger, 1978), this South African record is out of phase with North African palaeomonsoon indices (Street and Grove, 1979; Rossignol-Strick, 1983; McIntyre et al., 1989); the amplitude of the rainfall response to insolation forcing agrees with climate model estimates (Prell and Kutzbach, 1987). These results document the importance of direct orbital insolation forcing on both subtropical North and South African climate as well as the predicted antiphase sensitivity to precessional insolation forcing.     

http://dx.doi.org/10.1016/j.gsf.2016.04.004

We present here a simple and novel proposal for the modulation and rhythm of ice-ages and interglacials during the late Pleistocene. While the standard Milankovitch-precession theory fails to explain the long intervals between interglacials, these can be accounted for by a novel forcing and feedback system involving CO_2, dust and albedo. During the glacial period, the high albedo of the northern ice sheets drives down global temperatures and CO_2 concentrations, despite subsequent precessional forcing maxima. Over the following millennia more CO_2 is sequestered in the oceans and atmospheric concentrations eventually reach a critical minima of about 200 ppm, which combined with arid conditions,causes a die-back of temperate and boreal forests and grasslands, especially at high altitude. The ensuing soil erosion generates dust storms, resulting in increased dust deposition and lower albedo on the northern ice sheets. As northern hemisphere insolation increases during the next Milankovitch cycle, the dust-laden ice-sheets absorb considerably more insolation and undergo rapid melting, which forces the climate into an interglacial period. The proposed mechanism is simple, robust, and comprehensive in its scope, and its key elements are well supported by empirical evidence.     

Cold climate during the closest Stage 11 analog to recent Millennia

The early part of marine isotopic Stage 11 near 400,000 years ago provides the closest analog to Holocene insolation levels of any interglaciation during the era of strong 100,000-year climatic cycles. The CH₄ concentration measured in Vostok ice fell to ∼450 ppb, and CO₂ values to ∼250 ppm. These natural decreases contrast with the increases in recent millennia and support the early anthropogenic hypothesis of major gas emissions from late-Holocene farming. During the same interval, δD values fell from typical interglacial to nearly glacial values, indicating a major cooling in Antarctica early in Stage 11. Other evidence suggests that new ice was accumulating during the closest insolation analog to the present day: a major increase in δ¹⁸O_atm at Vostok, a similar increase in marine δ¹⁸O values, and re-initiation of ice rafting in the Nordic Sea. The evidence permits extended (>20,000 year) intervals of Stage 11 interglacial warmth in the Antarctic and North Atlantic, yet it also requires that this warmth ended and a new glacial era began when insolation was most similar to recent millennia. The Holocene CO₂ anomaly was produced only in part by direct anthropogenic emissions; over half of the anomaly resulted from the failure of CO₂ values to fall as they had during previous interglaciations because of natural responses, including a sea-ice advance in the Antarctic and ice-sheet growth in the northern hemisphere.     

Obliquity and precessional forcing of continental snow fall and melt: implications for orbital forcing of Pleistocene ice ages

Milankovitch theory posits that Earth's orbital cycles were the primary forcing of Pleistocene ice-age cycles through their strong influence on summer insolation at high latitudes. Accordingly, Milankovitch theory predicts ice volume should vary at both obliquity and precessional periods. However, early Pleistocene global ice volume varied mainly at the obliquity period with weak variability at the precessional period suggesting that Milankovitch theory is not sufficient to explain the ice-age cycles. Here we describe the results from a series of coupled ocean-atmosphere general circulation model experiments, using the Fast Ocean Atmosphere Model, that systematically investigate the influence of precession and obliquity on continental snowfall and potential ablation.Our model results identify three factors that magnify the influence of obliquity forcing on the global ice volume: First, high-latitude snowfall variability is dominated by changes in Earth's axial tilt. Second, hemispheric changes in net snowfall due to Earth's precession are out-of-phase, and largely cancel to produce a very small global snowfall change. Third, snowmelt variability over Antarctica responds greatly to changes in obliquity that intensify accumulation over obliquity cycle. We discuss the implications of these factors for existing hypotheses that account for the variability in the ice volume record.     

我国气候变化将比模式预期的小吗？谁驱动了亚洲季风的演化：一个瞬变模拟试验的启示

亚洲季风演化受到地球轨道参数强迫,尤其是岁差所引起日射变化的显著影响,但关于其驱动机制的争议仍然存在,且集中在“零相位”和“南半球潜热”两种假说上。两个假说都得到了部分地质证据的支持,因此亟需相应的数值模拟,尤其是长期瞬变试验的检验。长期瞬变模拟试验可以对气候的连续演变进行模拟,并能与地质证据进行对比,有助于深入认识亚洲季风系统演化的内在物理机制。但由于计算能力的匮乏,过去的古季风数值模拟多为“时间片”模拟,这使得季风变迁机理研究受到限制。文章通过一个海－气耦合模式的长期瞬变试验,讨论了轨道日射的变化特征,证明过去280ka亚洲夏季风降水对日射有十分显著的响应,且与北半球初夏日射变化相位接近,部分支持了“零相位”假说。同时,模拟结果还揭示了随意选取日射参考标尺会导致缺乏内在物理机制的位相关系,合理选择日射参考以及明确地质记录的气候学意义在古季风强迫－响应机制研究中十分重要。     

格陵兰冰芯记录的大气环流变化及其与南极冰芯的比较(英)

The GISP2, central Greealand, glaciochemical series (sodium, potassium, ammonium,calcium, magnesium, sulfate, nitrate and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over much of the Northern Hemisphere. Interpretation of this record reveals the controls on both high and low frequency climate events of the last 110 000 years.Changes in insolation on the order of the major orbital cycles control the long-term behavior of atmospheric circulation patterns through changes in ice volume (sea level) and related positive feedbacks.Events such as the Heinrich events (massive discharges of icebergs first identified in the marine record)are found to operate on a 6 100 year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics Rapid climate change events (massive reorganizations of atmospheric circulation) are demonstrated to operate on 1 450 year cycle possibly in response to internal oscillations in the ocean-atmosphere system or due to changes in solar output. Changes in insolation and associated positive feedbacks related to ice sheets assist in explaining favorable time periods and controls on the amplitude of these massive rapid climate change events.Comparison of the GISP2 glaciochemical series with an ice record from Taylor Dome in Antarctica indicates considerable similarity suggesting that both polar regions experience marked changes in climate. While preliminary evidence points to similar phasing of several major climate events in the two polar regions exact phasing cannot as yet be determined, because dating of Antarctic ice core records is not as well-established as the dating for Greenland ice cores.     

石笋初始234 U/238 U值的冰量周期特征及其环境意义 ——以湖北三宝洞为例

石笋氧同位素记录具有明显的2万年周期,其他记录中广泛存在的10万年周期是否在石笋中有所表现目前还鲜有报道。通过对湖北三宝洞20支石笋的铀同位素数据的分析研究发现,石笋初始234U/238U值在序列连续性较好的640.3~299.6 ka B.P.时间段有强烈的10万年周期特征。在间冰期和冰期时,初始234U/238U值分别呈增大和减小状态。初始234U/238U值的10万年周期与全球冰量、黄土磁化率、黄土平均粒度和大气CO₂变化有良好的对应关系。这些对应关系表明全球冰量、大气CO₂对喀斯特区地球化学元素富集和迁移作用有重要影响。石笋氧同位素的显著岁差周期独立于石笋微量元素、高纬冰量和全球温室气体变化,暗示了太阳辐射变化对中低纬水汽环流的直接影响。石笋初始234U/238U与氧同位素、太阳辐射在冰消期时的对应变化支持北半球太阳辐射能量变化对冰期-间冰期旋回的调控作用。     

Global ice-sheet system interlocked by sea level

Denton and Hughes (1983, Quaternary Research 20, 125–144) postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during late Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75°N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet, thus fitting the concept of a globally interlocked ice-sheet system. But recent atmospheric modeling results (Manabe and Broccoli, 1985, Journal of Geophysical Research 90, 2167–2190) suggest that factors other than areal changes of the grounded Antarctic Ice Sheet strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate (Shackleton and Pisias, 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In “The Carbon Cycle and Atmospheric CO₂: Natural Variations Archean to Present” (E. T. Sundquest and W. S. Broecker, Eds.), pp. 303–318. Geophysical Monograph 32, American Geophysical Union, Washington, D.C.), but another potential influence was high-frequency climatic oscillations (2500 yr). It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm high-frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 ¹⁴C yr B.P. This permitted renewed formation of North Atlantic Deep Water, which could well have controlled atmospheric carbon dioxide (W. S. Broecker, D. M. Peteet, and D. Rind, 1985, Nature (London) 315, 21–26). Combined melting and consequent sea-level rise from the three warming factors initiated irreversible collapse of the interlocked global ice-sheet system, which was at its largest but most vulnerable configuration.     

Paleoclimate of Southwestern China for the Past 50,000 yr Inferred from Lake Sediment Records

Long sediment cores (12.5 and 13.5 m) from two lakes in Yunnan Province were used to infer the paleoclimate of southwest China over the past 50,000 yr. During the Holocene and marine isotope stage (MIS 3), bio-induced carbonate precipitation and organic matter (OM) production was high, suggesting warm temperatures and high primary productivity. In contrast, sediment inorganic carbon (IC) and organic carbon (OC) concentrations were low in last glacial deposits from 38,000 to 12,000 cal yr B.P., indicating cool temperatures and low productivity. The 50,000-yr record has alternating peaks of carbonate and coarse-grain (>38 μm) quartz that reflect warm, moist interglacial or interstadial conditions alternating with cold, dry glacial or stadial conditions, respectively. Spectral analysis of the carbonate and quartz signals reveals power concentrated at periods of 7200 and 8900 cal yr, respectively, that may reflect a nonlinear climate response to precessional forcing at a time of reduced eccentricity modulation (McIntyre and Molfino, 1996). Oxygen isotope values of calcite from Yunnan lake cores indicate the summer monsoon was weak during the last glaciation from 50,000 to 12,000 cal yr B.P. The summer monsoon intensified between 12,000 and 8000 cal yr B.P., but weakened gradually in response to insolation forcing during the mid-to-late Holocene. Our results support the Overpeck et al. (1996) model that posits a weak summer monsoon during the last glaciation that responded nonlinearly to insolation forcing when its intensity was affected by Eurasian snow cover and ice-sheet extent. The summer monsoon intensified and responded linearly to seasonal insolation forcing in the Holocene when ice volume diminished.     

Stomatal evidence for a decline in atmospheric CO2 concentration during the Younger Dryas stadial: a comparison with Antarctic ice core records

A recent high‐resolution record of Late‐glacial CO₂ change from Dome Concordia in Antarctica reveals a trend of increasing CO₂ across the Younger Dryas stadial (GS‐1). These results are in good agreement with previous Antarctic ice‐core records. However, they contrast markedly with a proxy CO₂ record based on the stomatal approach to CO₂ reconstruction, which records a ca. 70 ppm mean CO₂ decline at the onset of GS‐1. To address these apparent discrepancies we tested the validity of the stomatal‐based CO₂ reconstructions from Kråkenes by obtaining further proxy CO₂ records based on a similar approach using fossil leaves from two independent lakes in Atlantic Canada. Our Late‐glacial CO₂ reconstructions reveal an abrupt ca. 77 ppm decrease in atmospheric CO₂ at the onset of the Younger Dryas stadial, which lagged climatic cooling by ca. 130 yr. Furthermore, the trends recorded in the most accurate high‐resolution ice‐core record of CO₂, from Dome Concordia, can be reproduced from our stomatal‐based CO₂ records, when time‐averaged by the mean age distribution of air contained within Dome Concordia ice (200 to 550 yr). If correct, our results indicate an abrupt drawdown of atmospheric CO₂ within two centuries at the onset of GS‐1, suggesting that some re‐evaluation of the behaviour of atmospheric CO₂ sinks and sources during times of rapid climatic change, such as the Late‐glacial, may be required. Copyright © 2002 John Wiley & Sons, Ltd.     

Impact of climate and CO2 on a millennium-long tree-ring carbon isotope record

We present one millennium-long (1171-year), and three 100 year long annually resolved δ¹³C tree-ring chronologies from ecologically varying Juniperus stands in the Karakorum Mountains (northern Pakistan), and evaluate their response to climatic and atmospheric CO₂ changes. All δ¹³C records show a gradual decrease since the beginning of the 19th century, which is commonly associated with a depletion of atmospheric δ¹³C due to fossil fuel burning. Climate calibration of high-frequency δ¹³C variations indicates a pronounced summer temperature signal for all sites. The low-frequency component of the same records, however, deviates from long-term temperature trends, even after correction for changes in anthropogenic CO₂. We hypothesize that these high-elevation trees show a response to both climate and elevated atmospheric CO₂ concentration and the latter might explain the offset with target temperature data. We applied several corrections to tree-ring δ¹³C records, considering a range of potential CO₂ discrimination changes over the past 150 years and calculated the goodness of fit with the target via calibration/verification tests (R², residual trend, and Durbin-Watson statistics). These tests revealed that at our sites, carbon isotope fixation on longer timescales is affected by increasing atmospheric CO₂ concentrations at a discrimination rate of about 0.012‰/ppmv. Although this statistically derived value may be site related, our findings have implications for the interpretation of any long-term trends in climate reconstructions using tree-ring δ¹³C, as we demonstrate with our millennium-long δ¹³C Karakorum record. While we find indications for warmth during the Medieval Warm Period (higher than today’s mean summer temperature), we also show that the low-frequency temperature pattern critically depends on the correction applied. Patterns of long-term climate variation, including the Medieval Warm Period, the Little Ice Age, and 20th century warmth are most similar to existing evidence when a strong influence of increased atmospheric CO₂ on plant physiology is assumed.     

Seven ambiguities in the Mediterranean palaeoenvironmental narrative

A review of seven outstanding issues on Mediterranean palaeoenvironments is presented. These are related to the dominant orbital pacing of climate variability, the length of the interglacial vegetation succession, the influence of the African summer monsoon, the seasonality of precipitation during boreal insolation maxima, the moisture balance during glacial maxima and the appearance of the mediterranean-type climate rhythm and evolution of mediterranean sclerophyllous plants. What emerges is that (1) marine δ¹⁸O_planktonic and SST records show that precession has been a fundamental tempo of Mediterranean climate change, representing both a low-latitude signal (runoff from North Africa) and the direct influence of insolation at Mediterranean latitudes, but high-latitude glacial effects (41-kyr and 100-kyr cycles) became superimposed after 2.8 Ma. Sapropel and dust deposition patterns in marine cores reveal that obliquity also has an effect on Mediterranean climate through dry–wet oscillations, which are independent of glacial–interglacial variability. (2) The temperate part of interglacial vegetation succession has a duration of approximately half a precession cycle. This persisted during the interval of obliquity-dominated glacial cycles (∼2.8–1 Ma), with distinct forest successions following the precessional cycles. However, these are not always separated by an open vegetation phase because of minimal ice growth, producing an impression of a prolonged interglacial forest interval. (3) The effect of an enhanced African monsoon during summer insolation maxima has been mainly indirect, in terms of Nile discharge and runoff along the North African coast, leading to increased freshwater input into the Mediterranean Sea, reduced deep-water ventilation and sapropel deposition. (4) The notion of an accentuated summer rain regime in the northern Mediterranean borderlands also contributing to a freshening of the Mediterranean Sea during boreal insolation maxima is not supported by the available evidence, which suggests increased summer aridity. (5) Recent improvements in chronological precision and data resolution point to an increase in aridity and decreased temperatures during the Last Glacial Maximum (21±2 ka), but suggest an increase in effective moisture during the immediately preceding interval of 24–27 ka. (6) The mediterranean-type climate is not exclusively a post-3.6 Ma phenomenon, but may have appeared intermittently during the course of the Tertiary (or before). (7) If that is the case, then the paradigm that the sclerophyllous evergreen habit represents a pre-adaptation to summer drought may need re-evaluation.     

The middle Pleistocene transition: characteristics, mechanisms, and implications for long-term changes in atmospheric pCO2

The emergence of low-frequency, high-amplitude, quasi-periodic (100-kyr) glacial variability during the middle Pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system. This middle Pleistocene transition (MPT) began 1250 ka and was complete by 700 ka. Its onset was accompanied by decreases in sea surface temperatures (SSTs) in the North Atlantic and tropical-ocean upwelling regions and by an increase in African and Asian aridity and monsoonal intensity. During the MPT, long-term average ice volume gradually increased by 50 m sea-level equivalent, whereas low-frequency ice-volume variability experienced a 100-kyr lull centered on 1000 ka followed by its reappearance 900 ka, although as a broad band of power rather than a narrow, persistent 100-kyr cycle. Additional changes at 900 ka indicate this to be an important time during the MPT, beginning with an 80-kyr event of extreme SST cooling followed by the partial recovery and subsequent stabilization of long-term North Atlantic and tropical ocean SSTs, increasing Southern Ocean SST variability primarily associated with warmer interglacials, the loss of permanent subpolar sea-ice cover, and the emergence of low-frequency variability in Pacific SSTs and global deep-ocean circulation. Since 900 ka, ice sheets have been the only component of the climate system to exhibit consistent low-frequency variability. With the exception of a near-universal organization of low-frequency power associated with marine isotope stages 11 and 12, all other components show an inconsistent distribution of power in frequency-time space, suggesting a highly nonlinear system response to orbital and ice-sheet forcing.Most hypotheses for the origin of the MPT invoke a response to a long-term cooling, possibly induced by decreasing atmospheric pCO₂. None of these hypotheses, however, accounts for the geological constraint that the earliest Northern Hemisphere ice sheets covered a similar or larger area than those that followed the MPT. Given that the MPT was associated with an increase in ice volume, this constraint requires that post-MPT ice sheets were substantially thicker than pre-MPT ice sheets, indicating a change in subglacial conditions that influence ice dynamics. We review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline Precambrian Shield bedrock became exposed by glacial erosion of a thick mantle of regolith. This exposure of a high-friction substrate caused thicker ice sheets, with an attendant change in their response to the orbital forcing. Marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the MPT. If this carbon came from terrigenous sources, an increase in atmospheric pCO₂ would be likely, which is inconsistent with evidence for widespread cooling, Apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith. A more likely source of organic carbon and nutrients (which would mitigate pCO₂ rise) is from shelf and upper slope marine sediments, which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the MPT. Modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long-term silicate weathering rates, in good agreement with marine Sr and Os isotopic records. We use a carbon cycle model to show that a post-MPT increase in silicate weathering rates would lower atmospheric pCO₂ by 7–12 ppm, suggesting that the attendant cooling may have been an important feedback in causing the MPT.     

地球气候变化的米兰科维奇理论研究进展

米兰科维奇理论是从全球尺度上研究日射量与地球气候之间关系的天文理论（以下简称为“米氏理论”）。该理论认为,地球轨道偏心率、黄赤交角及岁差等三要素变化引起的到达北半球中高纬度夏季日射量变化是造成冰期—间冰期旋回的根本原因。详细回顾了米氏理论的发展历程,并以南极东方站过去42万年大气和气候变化的历史资料为例,讨论了经典米氏理论中有待研究的若干问题。