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1.
《Quaternary Science Reviews》2007,26(13-14):1818-1837
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.  相似文献   

2.
Two marine pollen diagrams for sediments off the southwest coast of India provide a 20,000-yr history of the nearby continental vegetation, determined by the monsoon climate, within the framework of the isotopic stratigraphy. Two important phases of the evolution of the monsoon climate are a very arid period about 22,000–18,000 yr B.P. and a very humid period culminating at 11,000 yr B.P. The very arid period corresponds to the lowest pollen representation of mangrove vegetation, which is used here as an indirect indicator of monsoonal runoff. This aridity is due to a very weak southwest airflow, a great reduction of summer monsoonal rainfall, and reduced runoff of the western Ghats rivers. The extension of the monsoon over India after the last glacial maximum has been a gradual process following the northward progression of the Intertropical Convergene Zone. The very humid period, corresponding to the highest pollen representation of mangrove vegetation and the heaviest summer rainfall, is a climatic response to the maximum summer insolation of the Northern Hemisphere at 11,000 yr B.P.  相似文献   

3.
The environmental history of the Northern Rocky Mountains was reconstructed using lake sediments from Burnt Knob Lake, Idaho, and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large-scale climate changes during the Holocene. Vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold-dry conditions. From 14,000 to 12,000 cal yr B.P., abundant Pinus pollen suggests warmer, moister conditions than the previous period. Most sites record the development of a forest with Pseudotsuga ca. 9500 cal yr B.P. indicating warm dry climate coincident with the summer insolation maximum. As the amplification of the seasonal cycle of insolation waned during the middle Holocene, Pseudotsuga was replaced by Pinus and Abies suggesting cool, moist conditions. The fire reconstructions show less synchroneity. In general, the sites west of the continental divide display a fire-frequency maximum around 12,000–8000 cal yr B.P., which coincides with the interval of high summer insolation and stronger-than-present subtropical high. The sites on the east side of the continental divide have the highest fire frequency ca. 6000–3500 cal yr B.P. and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late Holocene. This study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher-than-present fire frequency occurred in the past.  相似文献   

4.
The northeastern Qinghai-Tibetan Plateau(QTP) of China is located at the triple junction of the Asian winter and summer monsoons and the westerlies, where paleoclimatic evolution has an important scientific significance for recognizing the spatial-temporal pattern of Asian monsoons in the past and predicting environmental change in the future. Nevertheless, the framework of the Holocene moisture variation and related mechanisms remain controversial, owing to complex hydroclimatic conditions triggered by the landform of the large mountain-arid basin. Here, we employed geochemical proxies from typical aeolian sand-palaeosol sequences in the Gonghe Basin, northeastern QTP, together with Optically Stimulated Luminescence(OSL) dating, to reconstruct the pattern of effective moisture variation and associated mechanisms in this region. Our results indicate that the regional effective moisture was at its lowest until 9–8 ka, and approached a maximum during 8–4/3 ka of the middle Holocene. Afterwards, the climate became relatively dry in general, but with a transient humid interval around 2–1 ka. Our geochemical evidence indicates that the dry early Holocene probably can be attributed to a strong winter monsoon forced by remnant ice sheet, combined with the high evaporation caused by solar insolation. Also, shifts of humid-dry are closely linked to the Asian summer monsoonal strength and therefore the balance of evaporation-precipitation in the middle and late Holocene. Thus, the pattern of the Holocene effective moisture variation is characterized as the ‘monsoon model' in a closed intermontane arid and semi-arid basin near the western Asian monsoonal limit.  相似文献   

5.
石正国  刘晓东 《第四纪研究》2009,29(6):1025-1032
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。  相似文献   

6.
High-resolution paleomonsoon proxy records from peat and eolian sand–paleosol sequences at the desert–loess transition zone in China denote a rapid oscillation from cold–dry conditions (11,200–10,60014C yr B.P.) to cool–humid conditions (10,600–10,20014C yr B.P.), followed by a return to cold–dry climate (10,200–10,00014C yr B.P.). Variations in precipitation proxies suggest that significant climatic variability occurred in monsoonal eastern Asia during the Younger Dryas interval. Late-glacial climate in the Chinese desert–loess belt that lies downwind from Europe was strongly influenced by cold air from high latitudes and from the North Atlantic via the westerlies. The inferred precipitation variations were likely caused by variations in the strength of the Siberian high, which influenced the pressure gradient between land and ocean and therefore influenced the position of the East Asian monsoon front.  相似文献   

7.
6kaB.P.东亚区域气候模拟及其变化机制探讨   总被引：17，自引：12，他引：5  
中全新世是地质历史上最近的温暖时期。文章利用包含较详细陆面过程的区域气候模式,通过分别加入现代植被和根据花粉化石资料转化的东亚地区古植被,模拟了6kaB.P.东亚季风气候,并研究了植被变化对东亚气候的影响。由区域气候模拟得到的较高分辨率的气候演变图像表明6kaB.P.太阳辐射季节循环增大,高纬度地区积雪、海冰减少,极地海洋升温,导致冬季大陆冷高压减弱,使中国冬季温度降低幅度大大减小。模拟中加入恢复的古植被造成地表反照率减小,使得冬季进一步升温,这不但突破了PMIP众多模拟的东亚6kaB.P.冬季降温的局限,而且使模拟温度变化的季节特征与古地质资料更为吻合。中全新世东亚大陆全年升温导致东亚夏季风强盛、冬季风减弱;降水及有效降水增加,降水带向西、向北扩张。与PMIP模拟相比,由于模式分辨率的提高,该工作模拟出了中国东部区域性降水增加的特征并得到了较为精细的气候变化空间分布。  相似文献   

8.
Climate and vegetation: Simulating the African humid period   总被引：1，自引：0，他引：1  
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.  相似文献   

9.
Ten sites near the Snake River Plain have consistent differences in their climatic histories. Sites at low elevation reflect the “early Holocene xerothermic” of the Pacific Northwest, whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later, ca. 6000 yr ago. This elevational contrast in climatic histories is duplicated at three sites from the central Snake River Plain. For sites in such close proximity, the different chronologies cannot be explained by changes in atmospheric circulation during the late Quaternary. Rather, the differences are best explained by the autecology of the plants involved and the changing seasonal climate. The seasonal climatic sequence predicted by multiple thermal maxima explains the high- and low-elevation chronologies. During the early Holocene, maximum insolation and intensified summer drought in July forced low-elevation vegetation upward. However, moisture was not a limiting factor at high elevation, where vegetation moved upward in response to increased length of growing season coincident with maximum September insolation 6000 yr ago.  相似文献   

10.
Climate impact of high northern vegetation: Late Miocene and present   总被引：1，自引：0，他引：1  
The Late Miocene belongs to the late phase of the Cenozoic. Climate at that time was still warmer and more humid as compared to today, especially in the high latitudes. Corresponding to the climate situation, palaeobotanical evidences support that vegetation in the high northern latitudes changed significantly from the Late Miocene until today. To quantify the climate impact of this vegetation change, we analyse how vegetation in the high northern latitudes contribute to climate evolution. For that, we perform climate modelling sensitivity experiments for the present and for the Late Miocene (Tortonian, 11–7 Ma). For our present-day sensitivity experiment, we introduce the Tortonian vegetation in the high northern latitudes. For our Tortonian sensitivity experiment, we introduce the modern vegetation on the same grid cells. In the Tortonian and in the present, the modern vegetation leads to a strong cooling of the northern extratropics (up to −4°C). Nevertheless, the meridional heat transports remain nearly unchanged in both cases. In general, the vegetation impact on climate is similar in the Tortonian and in the present. However, some exceptions occur. Due to the Tethys Ocean in the Tortonian, temperatures decline only weakly in eastern Europe and western Asia. In the Tortonian climate, temperatures on the Sahara realm rise (up to +1.5°C), while the temperatures do not change remarkably in the present-day climate. This different behaviour is caused by a stronger and more sensitive hydrological cycle on the Sahara region during the Tortonian.  相似文献   

11.
The Mediterranean climate and its variability depend on global-scale climate patterns. Close correlations appear when comparing Holocene palaeoenvironmental data (lake levels, fluvial activity, Mediterranean surface temperature and salinity, marine sedimentation) with the main stages of the history of the circum-Mediterranean vegetation. They indicate an evolution of the Mediterranean biome controlled by the climate and emphasize the teleconnections between the climate of the Mediterranean area and the global climatic system. In the circum-Mediterranean area, the Holocene can be divided into three periods: a lower humid Holocene (11 500–7000 cal BP) interrupted by dry episodes; a transition phase (7000–5500 cal BP) during which occurred a decrease in insolation as well as the installation of the present atmosphere circulation in the northern hemisphere; and an upper Holocene (5500 cal BP—present) characterized by an aridification process. Throughout the Holocene, humans used and modified more or less strongly the environment but the climatic changes were the determining factors of the evolution of the Mediterranean biome. Societies had to adapt to natural environmental variations, their impact on the environment increasing the ecological consequences of the global changes.  相似文献   

12.
文章选取了参加国际古气候模拟比较计划(Paleoclimate Modeling Intercomparison Project,简称PMIP)的6个耦合气候模式的模拟结果,着重对中全新世中东亚干旱区以及东亚季风区的夏季气候变化(气温、降水)进行分析,探讨了植被反馈作用对这3个区域中全新世气候变化的影响.由于各个模式之...  相似文献   

13.
The low-latitude hydrological cycle is a key climate parameter on different timescales, as it contributes to various feedback processes. Modelling studies suggest that the interhemispheric insolation contrast is the major factor controlling the cycle, although the influence of glacial conditions and the phase relationships relative to insolation forcing remain undetermined. In this work, we studied precipitation variability over Papua New Guinea (PNG, 3°S) for the past 400 ka using terrigenous fractions transported by the Sepik River to the Western Pacific Warm Pool (WPWP). A multi-decadal to centennial resolution of the elemental content was obtained using X-ray fluorescence scanning of a marine sediment core using an age model based on 14C dates and benthic foraminiferal δ18O. Indicators of the coarse river particulate fraction (bulk and CaCO3-free basis Ti concentrations, the log intensity ratios of Ti/K and Ti/Ca) displayed a dominant 23 ka periodicity without a clear glacial–interglacial trend. Our precipitation records showed a tight relationship with local summer insolation (3°S, January) with time-dependent lag of 0 to 4 ka. They were generally in anti-phase for U–Th dated Chinese speleothem δ18O records. Based on an analogy to modern climate, we propose that precipitation over PNG was primarily determined by interhemispheric insolation contrast, and the contribution of austral fall/winter precipitation added second-order variability that formed the lags. For the last four climate cycles, the WPWP hydrological cycle was closely associated with the eastern Asian monsoon, and the influence of glacial conditions on the low-latitude hydrological cycle was estimated to be limited.  相似文献   

14.
In the source regions of the Yangtze and Yellow Rivers of China, glaciers, frozen ground, the hydrological system, and alpine vegetation have changed over the past decades years. Climatic causes of these variations have been analyzed using mean monthly air temperature and monthly precipitation between 1956 and 2000, and monthly evaporation from φ20 evaporation pans between 1961 and 1996. In the source region of the Yangtze River, lower temperature and plentiful precipitation during the 1960s and continuing into the early 1980s triggered a glacier advance that culminated in the early 1990s, while a robust temperature increase and precipitation decrease since 1986 has forced glaciers to retreat rapidly since 1995. Permafrost degradation is another consequence of the climatic warming. The variations in the hydrological system and alpine vegetation are controlled mainly by the climate during the warm season. Warmer and drier summer climate is the major cause of a degradation of the vegetation, desiccation of the high-cold marshland, a decrease in the areas and numbers of lakes and rivers in the middle and north source regions of the Yangtze and Yellow Rivers, and a reduction in surface runoff in the source region of the Yangtze River for the last 20 years. The causes of eco-environmental change in Dari area, near the outlet from the source area of the Yellow River, are different from those elsewhere in the study area. A noticeable reduction in runoff in the source region of the Yellow River and degradation of alpine vegetation in Dari area are closely related to the permafrost degradation resulting from climate warming.  相似文献   

15.
At the 41,000-period of orbital tilt, summer insolation forces a lagged response in northern ice sheets. This delayed ice signal is rapidly transferred to nearby northern oceans and landmasses by atmospheric dynamics. These ice-driven responses lead to late-phased changes in atmospheric CO2 that provide positive feedback to the ice sheets and also project ‘late’ 41-K forcing across the tropics and the Southern Hemisphere. Responses in austral regions are also influenced by a fast response to summer insolation forcing at high southern latitudes.At the 22,000-year precession period, northern summer insolation again forces a lagged ice-sheet response, but with muted transfers to proximal regions and no subsequent effect on atmospheric CO2. Most 22,000-year greenhouse-gas responses have the ‘early’ phase of July insolation. July forcing of monsoonal and boreal wetlands explains the early CH4 response. The slightly later 22-K CO2 response originates in the southern hemisphere. The early 22-K CH4 and CO2 responses add to insolation forcing of the ice sheets.The dominant 100,000-year response of ice sheets is not externally forced, nor does it result from internal resonance. Internal forcing appears to play at most a minor role. The origin of this signal lies mainly in internal feedbacks (CO2 and ice albedo) that drive the gradual build-up of large ice sheets and then their rapid destruction. Ice melting during terminations is initiated by uniquely coincident forcing from insolation and greenhouse gases at the periods of tilt and precession.  相似文献   

16.
Spatial patterns of Holocene glacier advance and retreat in Central Asia   总被引：1，自引：0，他引：1  
Glaciers in the southern Himalayas advanced in the early Holocene despite an increase in incoming summer solar insolation at the top of the atmosphere. These glacier advances are in contrast to the smaller alpine glaciers in the western and northern regions of Central Asia. Two different glacier mass-balance models are used to reconcile this Holocene glacier history with climate by quantifying the change in equilibrium-line altitudes (ELA) for simulated changes in Holocene climate. Both ELA models clearly show that the lowering of ELAs in the southern Himalayas is largely due to a decrease in summer temperatures, and that an increase in monsoonal precipitation accounts for less than 30% of the total ELA changes. The decrease in summer temperatures is a dynamic response to the changes in solar insolation, resulting in both a decrease in incoming shortwave radiation at the surface due to an increase in cloudiness and an increase in evaporative cooling. In the western and northern zones of Central Asia, both ELA models show a rise in ELAs in response to a general increase in summer temperatures. This increase in temperatures in the more northern regions is a direct radiative response to the increase in summer solar insolation.  相似文献   

17.
《Quaternary Science Reviews》2007,26(17-18):2042-2066
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 δ18Oplanktonic 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.  相似文献   

18.
Western European loess sequences of the last glaciation (100,000–15,000 years BP) exhibit strong, cyclic variations of the sedimentation rate, which are coeval to the Greenland stadial/interstadial cycles and the Heinrich events. These North-Atlantic rapid climate changes appear, thus, as a potential cause for the sedimentation variations, via changes in dust intensity cycle. Here we make a first step in testing this hypothesis, by modelling the impact of the North-Atlantic abrupt climate variations on dust emission. Our dust emission calculations use meteorological fields generated by the LMDZ atmospheric general circulation model at a resolution down to 60 km over Western Europe. Three numerical experiments are run, representing a Greenland stadial, an interstadial and a Heinrich event. Orbital parameters and ice-sheet configuration correspond to conditions from Marine Isotope Stage 3 (60,000–25,000 years BP), a period characterized by strong millennial-scale climate variability. The only differences we impose in the boundary conditions regard the North-Atlantic surface temperature and sea-ice cover in the latitudinal band 30°–63°N. The changes in wind, precipitation, soil moisture and snow cover from one simulated state to another result in small differences in dust emission intensity. In contrast, when the inhibition of the aeolian erosion by vegetation is taken into account, the dust fluxes for the cold climate states (Greenland stadial and Heinrich event) become generally more than twice higher than those for the relatively warmer Greenland interstadial, in agreement with the loess data. These results support the hypothesis that the North-Atlantic millennial-scale variability is imprinted in Western European loess profiles, and point to vegetation changes as the main factor responsible for millennial-scale sedimentation variations. An analysis for the English Channel and southern North Sea areas, major potential dust sources, shows that the seasonality of dust emission is not controlled by the wind speed, as in modern large deserts, but by the surface conditions. Consequently, the dusty season lasts from late winter to early summer, with maximum activity in April–May, and is shifted towards summer when the climate is colder.  相似文献   

19.
Palynological and sedimentological data from Lake Telmen, in north-central Mongolia, permit qualitative reconstruction of relative changes in moisture balance throughout the mid to late Holocene. The climate of the Atlantic period (7500–4500 yr ago) was relatively arid, indicating that Lake Telmen lay beyond the region of enhanced precipitation delivered by the expanded Asian monsoon. Maximum humidity is recorded between 4500 and 1600 cal yr B.P., during the Subboreal (4500–2500 yr ago) and early Subatlantic (2500 yr–present) periods. Additional humid intervals during the Medieval Warm Epoch (1000–1300 A.D. or 950–650 ago) and the Little Ice Age (1500– 1900 A.D. or 450–50 yr B.P.) demonstrate the lack of long-term correlation between temperature and moisture availability in this region. A brief aridification centered around 1410 cal yr B.P. encompasses a decade of cold temperatures and summer frost between A.D. 536 and 545 (1414–1405 yr B.P.) inferred from records of Mongolian tree-ring widths. These data suggest that steppe vegetation of the Lake Telmen region is sensitive to centennial- and decadal-scale climatic perturbations.  相似文献   

20.
A high-resolution pollen and Pediastrum record, spanning 12,500 yr, is presented for Lake Bayanchagan (115.21°E, 41.65°N, and 1355 m a.s.l.), southern Inner Mongolia. Individual pollen taxa (PT-MAT) and the PFT affinity scores (PFT-MAT) were used for quantitative climatic reconstruction from pollen and algal data. Both techniques indicate that a cold and dry climate, similar to that of today, prevailed before 10,500 cal yr B.P. The wettest climate occurred between 10,500 and 6500 cal yr B.P., at which time annual precipitation was up to 30–60% higher than today. The early Holocene increases in temperature and precipitation occurred simultaneously, but mid-Holocene cooling started at approximately 8000 cal yr B.P., 1500 yr earlier than the drying. Vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type. The results suggest that this region was dominated by steppe vegetation throughout the Holocene, except for the period 9200 to 6700 cal yr B.P., when forest patches were relatively common. Inner Mongolia is situated at the limit of the present East Asian monsoon and patterns of vegetation and climate changes in that region during the Holocene probably reflect fluctuations in the monsoon's response to solar insolation variations. The early to middle Holocene monsoon undoubtedly extended to more northern latitudes than at present.  相似文献   

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