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.     

Variability in terrigenous sedimentation processes off northwest Africa and its relation to climate changes: Inferences from grain-size distributions of a Holocene marine sediment record

Variations in deposition of terrigenous fine sediments and their grain-size distributions from a high-resolution marine sediment record offshore northwest Africa (30°51.0′N; 10°16.1′W) document climate changes on the African continent during the Holocene. End-member grain-size distributions of the terrigenous silt fraction, which are related to fluvial and aeolian dust transport, indicate millennial-scale variability in the dominant transport processes at the investigation site off northwest Africa as well as recurring periods of dry conditions in northwest Africa during the Holocene. The terrigenous record from the subtropical North Atlantic reflects generally humid conditions before the Younger Dryas, during the early to mid-Holocene, as well as after 1.3 kyr BP. By contrast, continental runoff was reduced and arid conditions were prevalent at the beginning of the Younger Dryas and during the mid- and late Holocene. A comparison with high- and low-latitude Holocene climate records reveals a strong link between northwest African climate and Northern Hemisphere atmospheric circulation throughout the Holocene. Due to its proximal position, close to an ephemeral river system draining the Atlas Mountains as well as the adjacent Saharan desert, this detailed marine sediment record, which has a temporal resolution between 15 and 120 years, is ideally suited to enhance our understanding of ocean-continent-atmosphere interactions in African climates and the hydrological cycle of northern Africa after the last deglaciation.     

Quaternary climates: a perspective for global warming

This brief review provides an Earth Science perspective on present climate change (global warming) using evidence from past ice ages with details from the Quaternary ice age. It places the present (Quaternary) ice age in the context of Earth history and outlines possible causes of ice ages and the scale and style of ice age climate. Milankovitch climate forcing is described and explained as the cause of relatively predictable climatic variations within an ice age (and at other times), and this is followed by an outline of the factors likely to be responsible for short and rapid sub-Milankovitch climate variations that are superimposed on the predictable changes. Finally the anomalous, relatively constant climate of the last 11.5 ka (Holocene) is highlighted and explained in terms of human input of greenhouse gasses into the Earth's atmosphere.     

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。     

Several distinct wet periods since 420 ka in the Namib Desert inferred from U-series dates of speleothems

The scarcity of numerical dates of the arid areas in southern Africa is a challenge for reconstructing paleoclimate. This paper presents a chronological reconstruction in the central part of the Namib Desert, Namibia, for the last 420,000 yr. It is based on ²³⁰Th/U dates (TIMS) from a large stalagmite and a thick flowstone layer in a small cave located in the hyper-arid central Namib Desert. The results provide for the first time evidence of three or possibly four succeeding wet periods of decreasing intensity since 420 ka through which speleothem deposited at approximately 420–385 ka, 230–207 ka and 120–117 ka following the 100-ka Milankovitch cycle. Speleothem growth was not recorded for the Holocene. These wet periods interrupted the predominantly dry climate of the Namib Desert and coincided with wet phases in deserts of the northern hemisphere in the Murzuq Basin, Sahara, the Negev, Israel, the Nafud Desert, Saudi Arabia, and the arid northern Oman, Arabian Peninsula.     

Climate and vegetation: Simulating the African humid period

The outputs of the climate simulated by two General Circulation Models (GCMs), (IPSL and UGAMP) have been used to force a vegetation model (LPJ-GUESS) to analyze the Holocene African humid period (AHP) and related vegetation changes over the 18°W-35°E, 5°S-25°N region. At the continental scale, simulations with the two models confirm the intensified African monsoon during the Holocene as compared to now, and the early but gradual termination of the AHP in eastern regions as compared to western regions. At the regional scale, the two GCMs results present important differences in the timing of the AHP, its spatial extent and the summer rainfall amplitude. Consequently, the vegetation model simulates changes that are globally in agreement with pollen data, but with large differences according to the region and the model considered. During the AHP, the IPSL climate induced proper vegetation changes in the eastern Sahara and in the Sahel, whereas the UGAMP climate induced correct changes in the western Sahara and in the equatorial zone.     

Mid‐ and late‐Holocene climatic changes: a test of periodicity and solar forcing in proxy‐climate data from blanket peat bogs

A wide range of palaeoenvironmental evidence from the Holocene has suggested periodicities in the Earth's climate of 10s to 1000s of years. Identifying these millennial‐, century‐ and decadal periodicities, and their impacts, is critical in developing a fuller understanding of natural climate variability. Any solar‐induced climatic change needs to be distinguished from other causes of natural climate variability and from short‐term catastrophic events induced either by external or internal processes. Such events might themselves generate a periodicity, or in combination with other forcing factors they may contribute towards a periodicity (and so spuriously imply a universal and continuing periodicity in the climate record), or they may resonate with a solar‐induced periodicity. Here, evidence from peat records for periodicity in climate change over the mid to late Holocene is reviewed and this is followed by a test of the replicability of claimed periodicities using blanket peat data covering the past 2000 yr from four sites in the British Isles. Results suggest that the mires studied do go through phases of being responsive to periodic forcing factors, with ca. 200, ca. 80 and 60–50 yr wavelengths reflected in some data sets. However, the patterns shown are not consistent. This could be the result of local conditions at individual mires (human impact, sensitivity and vegetation succession) or of changes in the strength or nature of global forcing factors. Assessing a solar–mire link remains difficult because the century‐scale variations of the Sun show different intervals between solar minima, the durations of which are themselves unequal, and because the proxy‐climate data‐sets from peat profiles may themselves not be dated with sufficient precision and/or accuracy. Copyright © 2001 John Wiley & Sons, Ltd.     

Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period

The Holocene climate evolution in Northern Africa is studied in a 9000-yr-long transient simulation with a coupled atmosphere–ocean–vegetation model forced by changes in insolation and atmospheric greenhouse gas concentrations. The model simulates in the monsoonal domains a significant decrease in precipitation under influence of the orbitally forced reduction in summer insolation. In the Western Sahara region, the simulated mid-Holocene transition from humid to arid conditions (the termination of the African Humid Period) is highly non-linear with the occurrence of centennial-scale climate fluctuations due to the biogeophysical feedback between precipitation and vegetation cover. This result is in agreement with proxy data from the Western Sahara region. The other monsoonal regions experience a more gradual climate evolution that linearly follows the insolation forcing, which appears in disagreement with available lake level records.     

Late Pleistocene snowline fluctuations at Nevado Coropuna (15°S), southern Peruvian Andes

Deposits preserved on peaks in the southern Peruvian Andes are evidence for past glacial fluctuations and, therefore, serve as a record of both the timing and magnitude of past climate change. Moraines corresponding to the last major expansion of ice on Nevado Coropuna date to 20‐25 ka, during the last glacial maximum. We reconstructed the snowline at Coropuna for this period using a combined geomorphic‐numeric approach to provide a first‐order estimate of the magnitude of late‐Pleistocene climate change. Our reconstructions show that snowline was approximately 550‐770 m lower during the last glacial maximum than during the late Holocene maximum, which ended in the 19^th century, and ～750 m lower than today. While these values are similar to data from nearby Nevado Solimana, reconstructions from the neighbouring peak of Nevado Firura reveal a smaller snowline depression, suggesting the glacial response to climate forcing in the tropics is strongly influenced by non‐climatic factors. These data constitute some of the first directly dated palaeo‐snowline data from the arid tropics and suggest that the magnitude of the last glaciation in at least parts of the tropical Andes was similar to late‐Pleistocene events at higher latitudes. Copyright © 2011 John Wiley & Sons, Ltd.     

Simulating the transient evolution and abrupt change of Northern Africa atmosphere–ocean–terrestrial ecosystem in the Holocene

We present the first synchronously coupled transient simulation of the evolution of the northern Africa climate-ecosystem for the last 6500 years in a global general circulation ocean–atmosphere–terrestrial ecosystem model. The model simulated the major abrupt vegetation collapse in the southern Sahara at about 5 ka, consistent with the proxy records. Local precipitation, however, shows a much more gradual decline with time, implying a lack of strong positive vegetation feedback on annual rainfall during the collapse. The vegetation change in northern Africa is driven by local precipitation decline and strong precipitation variability. In contrast, the change of precipitation is dominated by internal climate variability and a gradual monsoonal climate response to orbital forcing. In addition, some minor vegetation changes are also simulated in different regions across northern AfricaThe model also simulated a gradual annual mean surface cooling in the subtropical North Atlantic towards the latest Holocene, as well as a reduced seasonal cycle of SST. The SST response is caused largely by the insolation forcing, while the annual mean cooling is also reinforced by the increased coastal upwelling near the east boundary. The increased upwelling results from a southward retreat of the North Africa monsoon system, and, in turn, an increased northeasterly trade wind. The simulated changes of SST and upwelling are also largely consistent with marine proxy records, albeit with a weaker magnitude in the model.The mismatch between the collapse of vegetation and gradual transition of rainfall suggests that the vegetation collapse is not caused by a strong positive vegetation feedback. Instead, it is suggested that the Mid-Holocene collapse of North African vegetation is caused mainly by a nonlinear response of the vegetation to a precipitation threshold in the presence of strong climate variability. The implication to the modeling and observations is also discussed.     

Two decadally resolved records from north‐west European peat bogs show rapid climate changes associated with solar variability during the mid–late Holocene

Two ¹⁴C accelerator mass spectrometry (AMS) wiggle‐match dated peat sequences from Denmark and northern England record changes in mire surface wetness reconstructed using plant macrofossil and testate amoebae analyses. A number of significant mid–late Holocene climatic deteriorations (wet shifts) associated with declines in solar activity were recorded (at ca. 2150 cal. yr BC, 740 cal. yr BC, cal. yr AD 930, cal. yr AD 1020, cal. yr AD 1280–1300, cal. yr AD 1640 and cal. yr AD 1790–1830). The wet shifts identified from ca. cal. yr AD 930 are concurrent with or lag decreases in solar activity by 10–50 years. These changes are replicated by previous records from these and other sites in the region and the new records provide improved precision for the ages of these changes. The rapidly accumulating (up to 2–3 yr cm^?1, ～1310 yr old, 34 ¹⁴C dates) Danish profile offers an unprecedented high‐resolution record of climate change from a peat bog, and has effectively recorded a number of significant but short‐lived climate change events since ca. cal. yr AD 690. The longer time intervals between samples and the greater length of time resolved by each sample in the British site due to slower peat accumulation rates (up to 11 yr cm^?1, ～5250 yr old, 42 ¹⁴C dates) acted as a natural smoothing filter preventing the clear registration of some of the rapid climate change events. Not all the significant rises in water table registered in the peat bog archives of the British and Danish sites have been caused by solar forcing, and may be the result of other processes such as changes in other external forcing factors, the internal variability of the climate system or raised bog ecosystem. Copyright © 2008 John Wiley & Sons, Ltd.     

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。     

Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary

Both Atlantic and Indian Ocean climate dynamics exert influence over tropical African hydroclimate, producing complex patterns of convergence and precipitation. To isolate the Indian Ocean influence on African paleohydrology, we analyzed the deuterium/hydrogen ratio of higher plant leaf waxes (δD_wax) in a 25 000-year sediment record from Lake Challa (3° S, 38° E) in the easternmost area of the African tropics. Whereas both the seismic record of inferred lake level fluctuations and the Branched and Isoprenoidal Tetraether (BIT) index proxy record changes in hydrology within the Challa basin, δD_wax, as a proxy for the isotopic composition of precipitation (δD_P) is interpreted as a tracer of large-scale atmospheric circulation that integrates the history of the moisture transported to the Lake Challa area. Specifically, based on modern-day isotope–rainfall relationships, we argue that Lake Challa δD_wax reflects the intensity of East African monsoon circulation. The three hydrological proxy records show generally similar trends for the last 25 000 years, but there are important differences between them, primarily during the middle Holocene. We interpret this deviation of δD_wax from local hydrological history as a decoupling of East African monsoon intensity – which heavily influences the isotopes of precipitation in East Africa today – from rainfall amount in the Challa basin. In combination, the hydrological proxy data from Lake Challa singularly highlight zonal gradients in tropical African climate that occur over a variety of timescales, suggesting that the Congo Air Boundary plays a fundamental role in controlling hydroclimate in the African tropics.     

千百年尺度气候快速变化及其数值模拟研究进展

千百年尺度全球气候快速变化是古气候研究中的一个重要内容。研究发现，末次冰期和全新世都存在着千年、百年尺度的快速气候变化，其变化幅度可以达到典型的地质变化或天文因子所造成的冰期/间冰期的气候振荡幅度，同时这些古气候事件具有全球性。对冰期和全新世气候变化的数值模拟揭示了气候系统对地球轨道参数变化的响应以及海洋、植被、冰盖、温室气体等反馈因子的重要性,其中大洋温盐环流对北大西洋淡水注入的敏感性与末次冰期和全新世气候快速变化密切相联。利用中等复杂程度的气候模式（EMIC）CLIMBER 2模拟了末次冰期典型时段（60~20 ka BP）D/O和Heinrich事件以及东亚气候的响应过程。模拟研究揭示了全新世青藏高原冰雪环境对亚洲—非洲季风气候的显著影响。今后的古气候模拟研究将在改进模式分辨率、结合古气候代用资料确定更加符合历史时期边界条件以进一步改善气候模式的基础上,更加注重气候突变机制的研究以及加强全球变化背景下的区域气候的长期变化研究。     

Effect of aridity and rainfall seasonality on vegetation in the southern tropics of East Africa during the Pleistocene/Holocene transition

Fossil pollen analyses from northern Lake Malawi, southeast Africa, provide a high-resolution record of vegetation change during the Pleistocene/Holocene transition (~ 18–9 ka). Recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the Younger Dryas (YD). The Lake Malawi record tracks regional vegetation changes and allows comparison with other tropical African records identifying vegetation opening and local forest maintenance during the YD. Our record shows a gradual decline of afromontane vegetation at 18 ka. Around 14.5 ka, tropical seasonal forest and Zambezian miombo woodland became established. At ~ 13 ka, drier, more open formations gradually became prevalent. Although tropical seasonal forest taxa were still present in the watershed during the YD, this drought-intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways. The establishment of drought-tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after ~ 11.8 ka. The onset of the driest, most open vegetation type was coincident with a lake low stand at the beginning of the Holocene. This study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution.     

Holocene climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54° S), Patagonia

Recent advances in the chronology and the palaeoclimatic understanding of Antarctic ice core records point towards a larger heterogeneity of latitudinal climate fluctuations than previously thought. Thus, realistic palaeoclimate reconstructions rely in the development of a tight array of well‐constrained records with a dense latitudinal coverage. Climatic records from southernmost South America are critical cornerstones to link these Antarctic palaeoclimatic archives with their South American counterparts. At 54° S on the Island of Tierra del Fuego, Lago Fagnano is located in one of the most substantially and extensively glaciated regions of southernmost South America during the Late Pleistocene. This elongated lake is the largest (～110 km long) and non‐ice covered lake at high southern latitudes. A multi‐proxy study of selected cores allows the characterisation of a Holocene sedimentary record. Detailed petrophysical, sedimentological and geochemical studies of a complete lacustrine laminated sequence reveal variations in major and trace elements, as well as organic content, suggesting high variability in environmental conditions. Comparison of these results with other regional records allows the identification of major known late Holocene climatic intervals and the proposal for a time for the onset of the Southern Westerlies in Tierra del Fuego. These results improve our understanding of the forcing mechanisms behind climate change in southernmost Patagonia. Copyright © 2009 John Wiley & Sons, Ltd.     

Holocene vegetation and land‐use changes in response to climatic changes in the forelands of the southwestern Alps,Italy

The Holocene sediment of Lago Piccolo di Avigliana (Piedmont, Italy, 356 m a.s.l.) was dated by ¹⁴C and analysed for pollen to reconstruct the vegetation history of the area. The early‐ and mid‐Holocene pollen record shows environmental responses to centennial‐scale climatic changes as evidenced by independent palaeoclimatic proxies. When human impact was low or negligible, continental mixed‐oak forests decreased at ca. 9300 BC in response to the early‐Holocene Preboreal climatic oscillation. Abies alba expanded in two phases, probably in response to higher moisture availability at ca. 6000 and ca. 4000 BC , while Fagus expanded later, possibly in response to a climatic change at 3300 BC . During and after the Bronze Age five distinct phases of intensified land use were detected. The near synchroneity with the land‐use phases detected in wetter regions in northern and southern Switzerland points to a common forcing factor in spite of cultural differences. Increasing minerogenic input to the lake since 1000 BC coincided with Late Bronze—Iron Age technical innovations and probably indicate soil erosion as a consequence of deforestation in the lake catchment. The highest values for cultural indicators occurred at 700–450 and at 300–50 BC , coinciding with periods of high solar activity (inferred from Δ¹⁴C). This suggests that Iron Age land use was enhanced by high solar activity, while re‐occupation of partly abandoned areas after crises in earlier periods match better with the GRIP stable isotope record. On the basis of our data and comparison with independent palaeoclimatic proxies we suggest that precipitation variation was much more important than temperature oscillations in driving vegetation and societal changes throughout the Holocene. Copyright © 2006 John Wiley & Sons, Ltd.     

New evidence for a reduced water balance in East Africa during the Last Glacial Maximum: implication for model-data comparison

Some syntheses of lake-level data for the Last Glacial Maximum (LGM) in East Africa (10°N and 30°S, East of 25°E) show apparently wetter conditions than present for some basins, whereas palaeovegetation reconstruction indicates a generally dry climate. PMIP GCM simulations for the LGM support both scenarios for this region when run under different boundary conditions. Here, we compare three new records from lakes in the data-poor southern part of East Africa; Lake Malawi, Lake Massoko, and Lake Rukwa. We also re-assess previously published lake-level data and apply a salinity transfer function to the diatom record from Lake Manyara. Our results show that in contrast to previous interpretations, these lakes were at least as low as today at the LGM and are thus in agreement with the palaeovegetation data. Relative drought across East Africa is best simulated by GCMs that use computed SSTs rather than the higher CLIMAP values. Lower SSTs and the presence of the Northern Hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern African tropics.     

Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama (northern Chile) coastal deserts during the last 120,000 yr

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we ‘unmix’ into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean.     

Holocene Dune Mobility and Forcing Mechanisms at the Northern Margin of the East Asian Monsoon

Dune fields at the northern margin of the East Asian monsoon (EAM), are mosaics of mobile and vegetation-stabilized aeolian dunes. These sand dunes are highly sensitive to environmental change, thus the distribution and the timing of their development may provide important clues to past environmental dynamics. Due to the strong wind erosion and dune migration, long and continuous stratigraphic records are seldom preserved. Synthesizing a large body of events, ultimately producing a relatively complete and high-resolution record, may be a proper method to investigate the dune development history and climate change. In this study, we synthesized a large body of luminescence ages for aeolian deposits from the Mu Us, Otindag, Horqin dune fields at the northern margin of the EAM. The results show that these dune fields, as a whole experienced a most extensive mobility during the early Holocene, followed by a widespread shift toward limited mobility and soil development in the mid-Holocene, and widespread reactivation occurred during late Holocene. The dune developments are directly linked to the effective moisture change controlled by the EAM changes, which respond to the low latitude summer insolation variation. The increased subsidence at the margin contrary to the core EAM, the delay from the feedback of the soil-vegetation-air coupled system, the increased evaporation due to the high temperature all play partial role in the lag of the margin EAM effective moisture change to the low latitude summer insolation. The asynchronous end of the wetter mid-Holocene mainly responds to the southeastwardly shift of the precipitation belt, while the regional sensitivity, response speed and internal feedback also contributed. The correspondence between dune records and North Atlantic drift-ice records of the rapid climate changes implies a close relationship between North Atlantic climate and the frequent dune activity at the northern margin of EAM.