Simulation of the global bio-geophysical interactions during the Last Glacial Maximum

 The bio-geophysical feedbacks during the Last Glacial Maximum (LGM, 21 000 y BP) are investigated by use of an asynchronously coupled global atmosphere-biome model. It is found that the coupled model improves on the results of an atmosphere-only model especially for the Siberian region, where the inclusion of vegetation-snow-albedo interaction leads to a better agreement with geological reconstructions. Furthermore, it is shown that two stable solutions of the coupled model are possible under LGM boundary conditions. The presence of bright sand desert at the beginning of a simulation leads to more extensive subtropical deserts, whereas an initial global vegetation cover with forest, steppe, or dark desert results in a northward spread of vegetation of up to some 1000 km, mainly in the western Sahara. These differences can be explained in the framework of Charney’s theory of a “self-induction” of deserts through albedo enhancement. Moreover, it is found that the tropical easterly jet is strengthened in the case of the “green” Sahara, which in turn leads to a modification of the Indian summer monsoon. Received: 20 June 1997/Accepted: 14 January 1998     

Influence of vegetation changes during the Last Glacial Maximum using the BMRC atmospheric general circulation model

 The influence of different vegetation distributions on the atmospheric circulation during the Last Glacial Maximum (LGM, 21 000 years before present) is investigated. The atmospheric general circulation model of the Bureau of Meteorology Research Center was run using a modern vegetation and in a second experiment with a vegetation reconstruction for the LGM. It is found that a change from conifer to desert and tundra causes an additional LGM cooling of 1–2 °C in Western Europe, up to −4 °C in North America and −6 °C in Siberia. An expansion of dryland vegetation causes an additional annual cooling of 1–2 °C for Australia and northern Africa. On the other hand, an increase of temperature (2 °C) is found in Alaska due to changes in circulation. In the equatorial region the LGM vegetation leads to an increased modelled temperature of 0.5–1.5 °C and decreased precipitation (30%) over land due to a reduction of the tropical rainforest, mainly in Indonesia, where the reduction of precipitation over land is associated with an increase of precipitation of 30% over the western Pacific. Received: 15 December 1999 / Accepted: 10 January 2001     

我国对末次冰期冰盛期东亚区域气候模拟的研究

末次冰期冰盛期（或称为末次盛冰期）,为第四纪更新世最后一个冰期的鼎盛时期,该时期的气候与当代气候迥然不同。近年来,中国科研工作者已就末次冰期冰盛期东亚区域气候开展了一些数值模拟工作。结果表明：该时期中国大陆地表气温降低,中东部地区降水显著减少,东亚冬季风增强、夏季风显著减弱。在国际古气候模拟比较计划（PMIP）标准试验的基础上,进一步指出东亚植被的反馈作用、青藏高原可能冰川的反馈作用、以及西太平洋表面温度的作用能够引起额外的气候效应,可在一定程度上改进PMIP标准试验的模拟效果。     

Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum

Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled to the dynamical vegetation model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal opening induced by the reduction in atmospheric CO₂ concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental temperature and precipitation mainly results from regional interactions with vegetation. In Eurasia, particularly Siberia and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation. Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport associated with it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of the Asian summer monsoon surface flow. The pattern of moisture convergence is slightly altered, with moist convection weakening in the western tropical Pacific and strengthening north of Australia.     

Hydro-climatic Characteristics of Yarlung Zangbo River Basin since the Last Glacial Maximum

Global climate changes significantly impact the water condition of big rivers in glacierized high mountains. However,there is a lack of studies on hydrological changes within river basins caused by climate changes over a geological timescale due to the impossibility of direct observations. In this study, we examine the hydro-climatic variation of the Yarlung Zangbo River Basin in the Tibet Plateau since the Last Glacial Maximum(LGM) by combining δ18 O proxy records in Indian and Omani caves with the simulated Indian summer monsoon, surface temperature, precipitation, evapotranspiration and runoff via the Community Climate System Model and the reconstructed glacier coverage via the Parallel Ice Sheet Model. The mean river runoff was kept at a low level of 145 billion cubic meters per year until an abrupt increase at a rate of 8.7 million cubic meters per year in the B?lling-Aller?d interval(BA). The annual runoff reached a maximum of 250 billion cubic meters in the early Holocene and then reduced to the current value of 180 billion cubic meters at a rate of 6.4 million cubic meters per year. The low runoff in the LGM and Heinrich Stadial 1(HS1) is likely attributed to such a small contribution of precipitation to runoff and the large glacier cover. The percentage of precipitation to runoff was only 20%during the LGM and HS1. Comparison of glacier area among different periods indicates that the fastest deglaciation occurred during the late HS1, when nearly 60% of glacier area disappeared in the middle reach, 50% in the upper reach,and 30% in the lower reach. The rapid deglaciation and increasing runoff between the late HS1 and BA may have accelerated widespread ice-dam breaches and led to extreme outburst flood events. Combining local geological proxy records and regional simulations could be a useful approach for the study of paleo-hydrologic variations in big river basins.     

GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica

 The LMDz variable grid GCM was used to simulate the Last Glacial Maximum (LGM, 21 ky Bp.) climate of Greenland and Antarctica at a spatial resolution of about 100 km.The high spatial resolution allows to investigate the spatial variability of surface climate change signals, and thus to address the question whether the sparse ice core data can be viewed as representative for the regional scale climate change. This study addresses primarily surface climate parameters because these can be checked against the, limited, ice core record. The changes are generally stronger for Greenland than for Antarctica, as the imposed changes of the forcing boundary conditions (e.g., sea surface temperatures) are more important in the vicinity of Greenland. Over Greenland, and to a limited extent also in Antarctica, the climate shows stronger changes in winter than in summer. The model suggests that the linear relationship between the surface temperature and inversion strength is modified during the LGM. The temperature dependency of the moisture holding capacity of the atmosphere alone cannot explain the strong reduction in snowfall over central Greenland; atmospheric circulation changes also play a crucial role. Changes in the high frequency variability of snowfall, atmospheric pressure and temperature are investigated and possible consequences for the interpretation of ice core records are discussed. Using an objective cyclone tracking scheme, the importance of changes of the atmospheric dynamics off the coasts of the ice sheets, especially for the high frequency variability of surface climate parameters, is illustrated. The importance of the choice of the LGM ice sheet topography is illustrated for Greenland, where two different topographies have been used, yielding results that differ quite strongly in certain nontrivial respects. This means that the paleo-topography is a significant source of uncertainty for the modelled paleoclimate. The sensitivity of the Greenland LGM climate to the prescribed sea surface conditions is examined by using two different LGM North Atlantic data sets. Received: 23 October 1997 / Accepted: 17 March 1998     

Synoptic-scale perturbations in AGCM simulations of the present and Last Glacial Maximum climates

 The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes at the Last Glacial Maximum (LGM, 21 000 years ago) are very different from the present ones: this period is characterised by a general cooling of the extra-tropics, with massive ice sheets over the Northern Hemisphere continents and sea-ice extending very far south over the North Atlantic. The present work uses regression analysis to study the characteristics of the synoptic eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (AGCM) of the UK Universities' Global Atmospheric Programme (UGAMP). In the LGM experiment, the structure of the Pacific eddies is similar to the present-day (PD) situation, but they are weaker. On the other hand, the Atlantic eddies show an increased zonal wavelength and a much shallower structure in the temperature and vertical wind perturbations. To understand the changes of these characteristics from present-day to LGM, we compare them to those computed for the most unstable modes of the corresponding mean flows, determined using a dry primitive equation model. A normal-mode stability analysis is carried both on zonally symmetric and asymmetric flows for each of the Northern Hemisphere storm-tracks. The changes in the most unstable normal modes found by both these analyses give a good account of changes in the structure of the perturbations as retrieved from the AGCM, suggesting that changes in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of the present-day Atlantic storm-track, the growth rate of these modes is found to be very low compared to the other cases. A complementary analysis evaluates the importance of non-modal growth, in the form of downstream development of perturbations, for each of the storm-tracks. This type of growth is found to be especially important in the case of the present-day Atlantic storm-track. Received: 29 September 1999 / Accepted: 17 November 1999     

Sensitivity of the Northern Hemisphere climate of the Last Glacial Maximum to sea surface temperatures

The role of prescribing sea surface temperature in paleoclimate atmospheric simulations has been investigated by comparing Last Glacial Maximum AGCMs experiments using different SSTs data sets as well as coupled atmosphere/oceanic mixed layer models. Changes in the SSTs and sea-ice margin generate different patterns of zonal asymmetries in the atmospheric circulation that are responsible for reorganisation of heat and moisture transport, leading to important variations of Northern Hemisphere regional climates, particularly in winter. Additional sensitivity experiments have been carried out to isolate the individual role of North Pacific and North Atlantic SSTs anomalies. We found that changes in North Pacific SSTs have a much stronger impact over all the northern continental surfaces, including Europe and Siberia, than changes in the North Atlantic SSTs. As these SSTs anomalies are of the order of the typical errors generated by coupled ocean-atmosphere models, this suggests that these more complete models will likely still have problems in simulating the regional climate change at the LGM. Received: 11 October 1999 / Accepted: 9 June 2000     

关于末次盛冰期青藏高原大范围冰盖存在可能性的再研究

通过4组数值敏感性试验,利用BIOME3生态模式对末次盛冰期中国大陆植被分布状况进行了数值模拟,再次研究了该时期青藏高原大范围冰盖存在的可能性问题.结果表明,末次盛冰期大陆植被发生了大规模的变化,总体呈退化趋势;在降温5℃、降水减少10%、二氧化碳下降145×10-6和地球轨道参数轻微变化的情况下,青藏高原约1/2的地区已经被冰覆盖;进一步的降温和干燥则会加大高原冰的覆盖面积和大陆其余地区植被的退化.     

Insolation, Moisture Balance and Climate Change on the South American Altiplano Since the Last Glacial Maximum

Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt.%C_org, %CaCO₃, and % biogenicsilica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the ¹³C of modern lacustrine organic sources and the ¹³C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000–13,500 yr BP) and lake water was freshunder the associated outflow conditions. The Pleistocene/Holocene transition (13,500–7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP.Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system.     

Changes in fire regimes since the Last Glacial Maximum: an assessment based on a global synthesis and analysis of charcoal data

Fire activity has varied globally and continuously since the last glacial maximum (LGM) in response to long-term changes in global climate and shorter-term regional changes in climate, vegetation, and human land use. We have synthesized sedimentary charcoal records of biomass burning since the LGM and present global maps showing changes in fire activity for time slices during the past 21,000 years (as differences in charcoal accumulation values compared to pre-industrial). There is strong broad-scale coherence in fire activity after the LGM, but spatial heterogeneity in the signals increases thereafter. In North America, Europe and southern South America, charcoal records indicate less-than-present fire activity during the deglacial period, from 21,000 to ∼11,000 cal yr BP. In contrast, the tropical latitudes of South America and Africa show greater-than-present fire activity from ∼19,000 to ∼17,000 cal yr BP and most sites from Indochina and Australia show greater-than-present fire activity from 16,000 to ∼13,000 cal yr BP. Many sites indicate greater-than-present or near-present activity during the Holocene with the exception of eastern North America and eastern Asia from 8,000 to ∼3,000 cal yr BP, Indonesia and Australia from 11,000 to 4,000 cal yr BP, and southern South America from 6,000 to 3,000 cal yr BP where fire activity was less than present. Regional coherence in the patterns of change in fire activity was evident throughout the post-glacial period. These complex patterns can largely be explained in terms of large-scale climate controls modulated by local changes in vegetation and fuel load. The readers are requested to refer to the section “List of contributors” for the complete list of author affiliation details.     

A coupled climate model simulation of the Last Glacial Maximum,Part 2: approach to equilibrium

The climate of the last glacial maximum (LGM) is simulated with a coupled climate model. The simulated climate undergoes a rapid adjustment during the first several decades after imposition of LGM boundary conditions, as described in Part 1, and then evolves toward equilibrium over 900 model years. The climate simulated by the coupled model at this period is compared with observationally-based LGM reconstructions and with LGM results obtained with an atmosphere-mixed layer (slab) ocean version of the model in order to investigate the role of ocean dynamics in the LGM climate. Global mean surface air temperature and sea surface temperature (SST) decrease by about 10 °C and 5.6 °C in the coupled model which includes ocean dynamics, compared to decreases of 6.3 and 3.8 °C in slab ocean case. The coupled model simulates a cooling of about 6.5 °C over the tropics, which is larger than that of the CLIMAP reconstruction (1.7 °C) and larger than that of the slab ocean simulation (3.3 °C), but which is in reasonable agreement with some recent proxy estimates. The ocean dynamics of the coupled model captures features found in the CLIMAP reconstructions such as a relative maximum of ocean cooling over the tropical Pacific associated with a mean La Niña-like response and lead to a more realistic SST pattern than in the slab model case. The reduction in global mean precipitation simulated in the coupled model is larger (15%) than that simulated with the slab ocean model (~10%) in conjunction with the enhanced cooling. Some regions, such as the USA and the Mediterranean region, experience increased precipitation in accord with proxy paleoclimate evidence. The overall much drier climate over the ocean leads to higher sea surface salinity (SSS) in most ocean basins except for the North Atlantic where SSS is considerably lower due to an increase in the supply of fresh water from the Mississippi and Amazon rivers and presumably a decrease in salt transport by the weakened North Atlantic overturning circulation. The North Atlantic overturning stream function weakens to less than half of the control run value. The overturning is limited to a shallower depth (less than 1000 m) and its outflow is confined to the Northern Hemisphere. In the Southern Ocean, convection is much stronger than in the control run leading to a stronger overturning stream function associated with enhanced Antarctic Bottom Water formation. As a result, Southern Ocean water masses fill the entire deep ocean. The Antarctic Circumpolar Current (ACC) transport through the Drake Passage increases by about 25%. The ACC transport, despite weaker zonal winds, is enhanced due to changes in bottom pressure torque. The weakening of the overturning circulation in the North Atlantic and the accompanying 30% decrease in the poleward ocean heat transport contrasts with the strengthening of the overturning circulation in the Southern Ocean and a 40% increase in heat transport. As a result, sea ice coverage and thickness are affected in opposite senses in the two hemispheres. The LGM climate simulated by the coupled model is in reasonable agreement with paleoclimate proxy evidence. The dynamical response of the ocean in the coupled model plays an important role in determining the simulated, and undoubtedly, the actual, LGM climate.     

Last Glacial Maximum climate of the former Soviet Union and Mongolia reconstructed from pollen and plant macrofossil data

 An improved concept of the best analogues method was used to reconstruct the Last Glacial Maximum (LGM) climate from a set of botanical records from the former Soviet Union and Mongolia. Terrestrial pollen and macrofossil taxa were grouped into broad classes – plant functional types (PFTs), defined by the ecological and climatic parameters used in the BIOME1 model. PFT scores were then calibrated in terms of modern climate using 1245 surface pollen spectra from Eurasia and North America. In contrast to individual taxa, which exhibit great variability and may not be present in the palaeoassemblages, even in suitable climates, PFTs are more characteristic of the vegetation types. The modified method thus allows climate reconstruction at time intervals with partial direct analogues of modern vegetation (e.g. the LGM). At 18 kBP, mean temperatures were 20–29 °C colder than today in winter and 5–11 °C colder in summer in European Russia and Ukraine. Sites from western Georgia show negative, but moderate temperature anomalies compared to today: 8–11 °C in January and 5–7 °C in July. LGM winters were 7–15 °C colder and summers were 1–7 °C colder in Siberia and Mongolia. Annual precipitation sums were 50–750 mm lower than today across northern Eurasia, suggesting a weakening of the Atlantic and Pacific influences. Reconstructed drought index shows much drier LGM conditions in northern and mid-latitude Russia, but similar to or slightly wetter than today around the Black Sea and in Mongolia, suggesting compensation of precipitation losses by lower-than-present evaporation. Received: 11 May 1998 / Accepted: 25 September 1998