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1.
The Gran Campo Nevado (GCN) forms an isolated ice cap on the Península Muñoz Gamero (PMG) located 200 km to the south of the Southern Patagonia Icefield (SPI). We present a glacier inventory of the GCN made up by 27 drainage basins (in total 199.5 km2) and other small cirque and valley glaciers of the southern part of PMG (in total 53 km2). The glacier inventory is based on a digital elevation model (DEM) and ortho-photos. Contour lines from maps, relief information derived from Landsat TM satellite imagery from 1986 and 2002 and stereoscopic data from aerial photos were combined in a knowledge-based scheme to obtain a DEM of the area. A digital ortho-photo map based on aerial photos from 1998 and several ortho-photos based on aerial photos from 1942 and 1984 could be produced from the initial DEM. A geographical information system (GIS) served to outline the extent of the present glaciation. All major glaciers of the GCN show a significant glacier retreat during the last 60 yr. Some of the outlet glaciers lost more than 20% of their total area during this period. Overall glacier retreat amounts to 2.8% of glacier length per decade and the glacier area loss is 2.4% per decade in the period from 1942 to 2002. We hypothesise that GCN glaciers may have reacted faster and more synchronously with the observed warming trend during recent decades when compared with the SPI.  相似文献   

2.
Climatic changes of the 20th century have altered the water cycle in the Andean basins of central Argentina. The most visible change is seen in the mountain glaciers, with loss of part of their mass due to decreasing thickness and a substantial recession in the last 100 years. This paper briefly describes the results of glacier mass balance research since 1979 in the Piloto Glacier at the Cajón del Rubio, in the headwaters of Las Cuevas River, presenting new results for the period 1997–2003. Very large interannual variability of net annual specific balance is evident, due largely to variations in winter snow accumulation, with a maximum net annual value of + 151 cm w.e. and a minimum value of - 230 cm w.e. Wet El Niño years are normally associated with positive net annual balances, while dry La Niña years generally result in negative balances. Within the 24-year period, 67% of the years show negative net annual specific balances, with a cumulative mass balance loss of - 10.50 m water equivalent (w.e.). Except for exceptions normally related to El Niño events, a general decreasing trend of winter snow accumulation is evident in the record, particularly after 1992, which has a strong effect in the overall negative mass balance values. The glacier contribution to Las Cuevas River runoff is analysed based on the Punta de Vacas River gauge station for a hypothetical year without snow precipitation (YWSP), when the snowmelt component is zero. Extremely dry years similar to a YWSP have occurred in 1968–1969, 1969–1970 and 1996–1997. The Punta de Vacas gauge station is located 62 km downstream from Piloto Glacier, and the basin contains 3.0% of uncovered glacier ice and 3.7% of debris-covered ice. The total glacier contribution to Las Cuevas River discharge is calculated as 82 ± 8% during extremely dry years. If glacier wastage continues at the present trend as observed during the last 2 decades, it will severely affect the water resources in the arid central Andes of Argentina.  相似文献   

3.
Permafrost warming in the Tien Shan Mountains, Central Asia   总被引：4，自引：0，他引：4  
The general features of alpine permafrost such as spatial distribution, temperatures, ice content, permafrost and active-layer thickness within the Tien Shan Mountains, Central Asia are described. The modern thermal state of permafrost reflects climatic processes during the twentieth century when the average rise in mean annual air temperature was 0.006–0.032 °C/yr for the different parts of the Tien Shan. Geothermal observations during the last 30 yr indicate an increase in permafrost temperatures from 0.3 °C up to 0.6 °C. At the same time, the average active-layer thickness increased by 23% in comparison to the early 1970s. The long-term records of air temperature and snow cover from the Tien Shan's high-mountain weather stations allow reconstruction of the thermal state of permafrost dynamics during the last century. The modeling estimation shows that the altitudinal lower boundary of permafrost distribution has shifted by about 150–200 m upward during the twentieth century. During the same period, the area of permafrost distribution within two river basins in the Northern Tien Shan decreased approximately by 18%. Both geothermal observations and modeling indicate more favorable conditions for permafrost occurrences and preservation in the coarse blocky material, where the ice-rich permafrost could still be stable even when the mean annual air temperatures exceeds 0 °C.  相似文献   

4.
This study simulates water resources in the Tien Shan alpine basins to forecast how global and regional climate changes would affect river runoff. The model employed annual mean values for the major characteristics of the water cycle: annual air temperature, precipitation, evapotranspiration and river runoff. The simulation was based on 304 hydro-meteorological stations, 23 precipitation sites, 328 high altitudinal points with glaciological measurements, 123 stream-gauges, and 54 evaporation sites, and it took into account topography. The findings were simulated over Tien Shan relief using a 1:500,000 scale 100 m grid resolution Digital Elevation Model. An applicable GIS-based distributed River Runoff Model was implemented in regional conditions and tested in the Tien Shan basins. The annual evapotranspiration exceeds the river runoff in the Tien Shan watersheds particularly up to 3700 m. Hypothetical climate-change scenarios in the Tien Shan predict that by 2100 river runoff will increase by 1.047 times with an increase in air temperature averaging 3 °C and an increase in precipitation averaging 1.2 times the current levels. Change in precipitation, rather than temperature, is the main parameter determining river runoff in the Tien Shan. The maximum ratio for predicted river runoff could reach up to 2.2 and the minimum is predicted to be 0.55 times current levels. This possibly dramatic change in river runoff indicates on non-linear system response caused mainly by the non-linear response of evapotranspiration from air temperature and precipitation changes. In the frame of forecasted possible climate change scenarios the probability of river runoff growth amounts 83–87% and probability of this decline is 17–13% by 2100 in the Tien Shan River basins.  相似文献   

5.
The depositional and erosional history of the Lapis Tiburtinus endogenic travertine located circa 25 km to the east of Rome, Central Italy, near the Colli Albani quiescent volcano, is interpreted through three-dimensional stratigraphy and uranium-series geochronology. Analyses of large exposures located in active quarries and of cores obtained from 114 industrial wells reveal that the travertine deposit is about 20 km2 wide and 60 m thick on average. The travertine thickness is over 85 m toward its western N–S-elongated side, where thermal springs and large sinkholes occur aligned over a seismically-active N-striking fault. The travertine age was calculated using the U/Th isochron method. Results constrain the onset and conclusion of travertine deposition at about 115 and 30 ka, respectively. The three-dimensional study of the travertine shows that this deposit is characterized by a succession of depositional benches grown in an aggradational fashion. The benches are separated by five main erosional surfaces, which are associated with paleosols, conglomerates, and karstic features. This evidence shows that the travertine evolution was mostly controlled by water table fluctuations. Chronological correlations between travertine evolution and paleoclimate indicators suggest that the travertine deposition was partly modulated by climate conditions. Other influencing factors may have been fault-related deformation and volcanic events.  相似文献   

6.
Here we present a crustal folding or buckling mechanism to explain the rootless 3–5 km high Alborz Mountains in northern Iran as well as  10 km of Late Miocene to recent subsidence in the south Caspian basin and  3–6 km of subsidence in the central Iranian basin in the context of the middle Miocene to recent Arabia–Eurasia collision. A key element of the mechanism is the presence of lateral and vertical lithospheric strength contrasts between the north Iranian continental and south Caspian oceanic crusts: when compression from the collision is applied across the region, the strong south Caspian oceanic crust, buried under > 10 km of premiddle Miocene sediment, interacts with the bottom of the mechanically strong continental upper crust of northern Iran, resulting in upward buckling of the continental crust and downward buckling of the oceanic crust. We test this mechanism using a finite-element numerical model with a Maxwell rheology and obtain results that are consistent with the geological and geophysical observations. The observations compiled here and the model results demonstrate the potential for using this region as a natural laboratory for studying the early stages of continent–oceanic collision, including processes like basin inversion, fault localization and, potentially, subduction initiation.  相似文献   

7.
The Upper Rhine Graben (URG), a Cenozoic intra-plate rift situated in the Alpine foreland, is presently characterised by relative slow tectonic deformation and low to medium seismicity. Concurrently, it is a region with a significant amount of ongoing subsidence in two recent depocentres (0.1 to 0.2 mm/a geological, 1 mm/a geodetical rate). In this paper, the recent kinematic behaviour of the URG is simulated using a 3D finite element model, containing three lithospheric layers (upper mantle, lower crust and upper crust) with different rheological properties. First order fault structures (e.g. border faults) are implemented as frictional contact surfaces within the upper crustal layer. The stresses generated by applying lateral displacements over a time period of 10 ka are insufficient to obtain a match between predicted and observed stress magnitudes. Therefore, a technique of “combined pre-stressing” has been developed to avoid unrealistic deformation and unrealistic stress magnitudes within the model. The stress magnitudes and stress directions predicted are calibrated against in-situ stress measurements and stress indicator data. For benchmarking of the modelling results, the vertical surface displacements predicted are compared to surface uplift derived from geological and geomorphological data. Furthermore, predicted fault slip rates are compared to available geological and geodetical data. Parameters derived from the calculated stress tensor, such as fracture potential and the regime stress ratio are also analysed in order to describe the possible kinematic behaviour of the URG. The modelling results suggest that the URG is currently being reactivated as a sinistral strike–slip system with the central segment of the URG forming a restraining bend and the two recent depocentres situated in releasing bend settings. The modelling results suggest that both sinistral shearing and mantle uplift are active mechanisms driving the recent kinematics of the URG and that the recent subsidence within the two depocentres is re-enforced by ongoing mantle uplift additionally.  相似文献   

8.
We utilize a regional climate model with detailed land surface processes (RegCM2) to simulate East Asian monsoon climates at 0 ka, 6 ka and 21 ka BP, and evaluate the changes in hydrology process, including vapor transportation, precipitation, evapotranspiration and runoff in the eastern and western China during these periods. Results indicate that the Tibetan Plateau climate presents a wet–cold status during the LGM while it exhibits a wet–warm climate at 6 ka BP. The LGM wetter climate over the Tibetan Plateau mainly results from the increased vapor inflow through its south boundary, while the increase in the vapor import over the Tibetan Plateau at 6 ka BP mostly sources from its west boundary. The increase in the LGM runoff over the Tibetan Plateau is mainly caused by the decrease in evapotranspiration, while the increase in runoff at the 6 ka BP mainly by the enhanced precipitation. Eastern China (including southern China) presents a dry status during the LGM, which precipitation and runoff decreases significantly due largely to weakened Asian summer monsoon that results in the decreased vapor inflow through the south boundary of eastern China. The variation pattern in the hydrological cycle in eastern China is contrary to that in western China during the LGM. The increase in precipitation and runoff at 6 ka BP in eastern China is tightly related to the strong Asian summer monsoon that leads to increased vapor import through the south boundary. Long term decrease trend in precipitation and runoff in northern China since the last 20 000 years may be attributed to the steady increase in vapor export through the east boundary as a result of the changes of East Asian monsoon and the adjustments of local atmospheric circulations in this area.  相似文献   

9.
We discuss the excitation of polar motion by earthquake displacement field. Instead of the usual static equilibrium equations in the literature, we use an improved set as given in /1/, which guarantee continuity at the core-mantle boundary. We take the parameter values of three earthquakes from /2/.To obviate the singularity at r = 0, we use asymptotic solutions by power series within a small sphere around the centre. Outisde this sphere, the equations are numerically integrated by the Runge-Kutta algorithm. Our equations /1/ gave polar shifts some 3 times larger than Dahlen's equations /2/.  相似文献   

10.
Earthquakes are one of the most destructive and harmful natural disasters, especially in recent years, the 2008/5/12 Wenchuan M7.9 earthquake, the 2011/3/11 Tohoku M9.0 earthquake and the 2012/4/11 Sumatra M8.6 earthquake have caused a significant impact to the human life. In this paper, we make a study of the temporal and spatial distribution of the Global Positioning System Total Electron Content (GPS TEC) anomalies prior to the three strong earthquakes by the method of statistical analysis. Our results show that the pre-earthquake ionospheric anomalies are mainly positive anomalies and take the shape of a double-crest structure with a trough near the epicenter. The ionospheric anomalies do not coincide with the vertical projection of the epicenter of the subsequent earthquake, but mainly localize in the near-epicenter region and corresponding ionospheric anomalies are also simultaneously observed in the magnetic conjugate region prior to the three earthquakes. In addition, the amplitude and scale-size of the ionospheric ΔTEC are different with the magnitude of the earthquake, and the horizontal scale-size of the greatest anomalies before the Tohoku M9.0 earthquake is ～30° in longitude and ～10° in latitude, with the maximum amplitude of TEC disturbances reaching ～20 TECu relative to the background. The peak of anomaly enhancement usually occurs in the afternoon to sunset (i.e. between 14:00 and 18:00 local time) which lasts for approximate 2 hours. Possible causes of these anomalies are discussed, and after eliminating the effect of solar activities and magnetic storms it can be concluded that the detected obvious and regular anomalous behavior in TEC within just a few days before the earthquakes is related with the forthcoming earthquakes with high probability.  相似文献   

11.
The earthquake produces a global static displacement field due to the dislocation in the epicenter. This displacement field in turn changes the Earth's inertia tensor, hence exciting the variation of both polar motion and length of day. In general, large earthquake produces greater displacement field, which has greater effects upon the earth rotational properties. In spite of scientists efforts in the last decades, it is found that the polar motion and the length of day excited by earthquake are at least two orders of magnitude less than those derived from observation. In the future with high observational accuracy the effect of earthquake on polar motion and length of day would be observable.Using the fault plane parameters, the location of epicenter and the expression given by Dahlen as well as the step function, the accumulative change of the axial and equatorial moments of inertia of the Earth earthquake occurring in period of 1977–1994 is estimated in this paper. Results have shown that change of pole axis direction induced by single earthquake is independent of the magnitude of the earthquake, which is random, but large earthquake contributes most to the accumulative change of direction of polar drift. The earthquake tends to make the drift of rotational pole towards the direction of 130–150 E. This direction is roughly different to that inferred from observations. Accumulative changes of both the two equatorial principal moments and the axial moment of inertia of the Earth present the strongest non-randomness and secular behaviour. The change depends upon the slip angle of the fault movement in a large extent.  相似文献   

12.
Modelling of uplift histories in the Upper and Middle Thames valleys has revealed an important difference, in the form of additional early post-Anglian uplift in the Middle Thames, attributed to an Anglian (~ 440 ka) glacio-isostatic effect. Terraces in the Upper Thames around Oxford seem unaffected by glacio-isostasy and their heights show regional uplift of ~ 35–40 m since the Anglian. The result of the glacio-isostasy is that Anglian terraces are significantly higher above the valley floor in the Middle Thames (up to 55 m) than in the Upper Thames. Recognition of this displacement of Middle Thames terraces has solved long-standing problems of correlation between this area and the Upper Thames: the pre-Anglian (Cromerian Complex) age of the Sugworth Channel deposits, indicated by biostratigraphy, is no longer a difficulty, whereas the Hanborough Terrace is now thought to be of Anglian age, albeit incorporating pre-Anglian faunal remains and perhaps with a significant early post-Anglian component. These findings have implications for the understanding of the effects of Middle Pleistocene glacio-isostasy and of landscape evolution on the periphery of glaciated regions.  相似文献   

13.
The Brazilian Northeast affords good opportunities for obtaining reliable timings and rates of landscape evolution based on stratigraphic correlations across a vast region. The landscape formed in the context of an episodically fluctuating but continuously falling base level since the Cenomanian. After formation of the transform passive margin in Aptian times, landscape development was further driven by a swell-like uplift with its crest situated  300 km from the coastline. The seaward flank of this swell or broad monocline between the interior Araripe and coastal Potiguar basins was eroded, and currently forms a deeply embayed plain bordered by a semi-circular, north-facing erosional escarpment. The post-Cenomanian uplift caused an inversion of the Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant Mesozoic sedimentary caprock, or to eroded stumps of syn-rift Cretaceous footwall uplands. Denudation in the last 90 My never exceeded mean rates of 10 m·My− 1 and exhumed a number of Cretaceous stratigraphic unconformities. As a result, some topographic surfaces at low elevations are effectively Mesozoic land surfaces that became re-exposed in Cenozoic times. The Neogene Barreiras Formation forms a continuous and mostly clastic apron near the coast. It testifies to the last peak of erosion in the hinterland and coincided with the onset of more arid climates at  13 Ma or earlier. The semi-circular escarpment is not directly related to the initial breakup rift flanks, which had been mostly eroded before the end of the Mesozoic, but the cause and exact timing of post-Cenomanian crustal upwarping are poorly constrained. It could perhaps have been a flexural response of the low-rigidity lithosphere to sediment loads on the margin, and thus a slowly ongoing process since the late Cretaceous. Uplift could instead be the consequence of a more discrete dynamic event related either to Oligocene magmatism in the region, or to continental-scale far-field stresses determined by Andean convergence.  相似文献   

14.
Subsidence analysis of 16 wells in the Austrian Molasse basin documents major spatial and temporal changes in tectonic subsidence as well as a late-stage surface uplift. The timing of the main phase of tectonic subsidence shifted from early Oligocene in the western part of the peripheral foreland to the early Miocene in the eastern part. These temporal and spatial changes in tectonic subsidence reflect a change from oblique dextral to sinistral convergence between the Alpine nappe stack and its foreland. The main phase of sediment accumulation was delayed to the early Miocene and led to the infill of the basin and a major second, sediment-load driven phase of basement subsidence. Sediment accumulation rates in the basin reflect the build-up of topography in the Alpine mountain chain. Since approximately 6 Ma a pronounced regional uplift of the entire Molasse basin has taken place, marking the transition from lateral extrusion to orthogonal contraction within the Alpine system and deep-seated changes in geodynamic boundary conditions, possibly due to delamination of previously thickened lithosphere. Surface uplift is contemporaneous with similar processes in extra-Alpine Central Europe, where it is interpreted to reflect intra-plate stress changes.  相似文献   

15.
The microcharcoal content (particles < 180 µm) of overlapping sedimentary sequences from two crater lake basins in central Turkey are used to reconstruct the regional fire history of the East Mediterranean oak–grass parkland zone from the Last Glacial Maximum to the present-day. These results are correlated with stable isotope and pollen data from the same cores in order to assess the changing role of climate, vegetation and human activity in landscape burning. This indicates that climatically-induced variation in biomass availability was the main factor controlling the timing of regional fire activity during the Last Glacial–Interglacial climatic transition, and again during Mid-Holocene times, with fire frequency and magnitude increasing during wetter climatic phases. Spectral analysis of the Holocene part of the record from Eski Acıgöl indicates significant cyclicity with a periodicity of ~ 1500 years that may be linked with large-scale climate forcing. Although proto-agricultural societies were established in this region as early as 10,000 years ago, it is only during the last two to three millennia that the pacing of wildfire cycles appears to have become decoupled from climate and linked instead to human-induced changes in land cover and fuel load availability.  相似文献   

16.
汶川地震前后的气象异常   总被引：1，自引：0，他引：1  
有研究认为局地气象异常与地震之间存在着一定的相关性,那么震惊中外的汶川大地震前后有没有气象异常呢？应用NCEP再分析资料,以1988—2007年20年间局地气象要素的平均值和标准偏差为基准,通过构建气象异常指数对2008年5月12日汶川发生的大地震前后的日平均气温、日最高气温、日最低气温、日平均降水强度和日平均气压等5项气象要素指标的异常状况进行了分析,结果表明在地震前2个月的时间内有异常状况出现,并且其变化幅度和持续时间都比较长,在地震过后这些异常并未立即消失;从5项气象要素异常空间分异来看,汶川是气象异常的中心地带,表征了汶川地震与局地气象之间存在着一定的相关性。尽管气象异常是否是地震发生的必然前兆尚待进一步考证,但出现在汶川特大地震前后的气象异常再次给我们一个提示,即气象异常与地震的关系值得深入研究。  相似文献   

17.
Rainfed tropical agriculture provides important avenue to ascertain the consequences of climate change. This is because reliability of rainfall accounts for much of the variation in agriculture in the region. In addition, the region is already hot and vulnerable from further warming. This study shows from a climate change experiment using Ricardian method in Cameroon that a 7% decrease in precipitation would cause net revenues from crops to fall US$2.86 billion and a 14% decrease in precipitation would cause net revenue from crops to fall US$3.48 billion. Increases in precipitation would have the opposite effect on net revenues. For a 2.5 °C warming, net revenues would fall by US$0.79 billion, and a 5 °C warming would cause net revenues to fall US$1.94 billion. This highlights that agriculture is not only limited by seasonality and magnitude of moisture availability, but also it is significantly impacted by climate change.  相似文献   

18.
A glaciological program has been undertaken since 1991 on Zongo glacier in Bolivia (6000–4850 m asl, 2.4 km2, 16°S). This program involves mass balance measurements, hydrological studies and energy balance investigations. On outer-tropical glaciers, melting and snow accumulation are both maximum in the wet season (austral summer), whereas the dry season (winter) is a period of low ablation. Errors on each term of the glaciological (stakes, snow-pits and integration method of the measurements) and hydrological (precipitation, discharge and runoff coefficient of free ice areas) methods are investigated to estimate the overall accuracy of the mass balance measurements. The hydrological budget is less than the glaciological one (mean difference: 60 cm w.e. per year), but both methods reproduce similar inter-annual variations. Errors in assessment of evaporation or water storage inside the glacier cannot explain the discrepancy. Errors using the glaciological method are large (around ± 40 cm w.e. per year), but no bias can explain the departure from the hydrological balance. Errors on discharge measurements are small and the uncertainty on the runoff coefficient has a minor effect on the mass balance. We concluded that hydrological budgets are too low due to the catch deficiency of rain gauges and absence of precipitation measurements at high altitudes, emphasizing the difficulty to assess snowfall distribution in high mountainous basins.  相似文献   

19.
New paleovegetation and paleoclimatic reconstructions from the Sierra Madre Occidental (SMO) in northwestern Mexico are presented. This work involves climate and biome reconstruction using Plant Functional Types (PFT) assigned to pollen taxa. We used fossil pollen data from four Holocene peat bogs located at different altitudes (1500‑2000 m) at the border region of Sonora and Chihuahua at around 28° N latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.; Ortega-Rosas, C.I., Peñalba, M.C., Guiot, J. Holocene altitudinal shifts in vegetation belts and environmental changes in the Sierra Madre Occidental, Northwestern Mexico. Submitted for publication of Palaeobotany and Palynology). The closest modern pollen data come from pollen analysis across an altitudinal transect from the Sonoran Desert towards the highlands of the temperate SMO at the same latitude (Ortega-Rosas, C.I. 2003. Palinología de la Ciénega de Camilo: datos para la historia de la vegetación y el clima del Holoceno medio y superior en el NW de la Sierra Madre Occidental, Sonora, Mexico. Master Thesis, Universidad Nacional Autónoma de México, México D.F.). An additional modern pollen dataset of 400 sites across NW Mexico and the SW United States was compiled from different sources (Davis, O.K., 1995. Climate and vegetation pattern in surface samples from arid western U.S.A.: application to Holocene climatic reconstruction. Palynology 19, 95–119, North American Pollen Database, Latin-American Pollen Database, personal data, and different scientific papers). For the biomization method (Prentice, I.C., Guiot, J., Huntley, B., Jolly, D., Cheddadi, R., 1996. Reconstructing biomes from paleoecological data: a general method and its application to European pollen data at 0 and 6 ka. Climate Dynamics 12, 185–194), we modified the pollen-PFT and PFT-biomes assignation of Thompson and Anderson (Thompson, R.S., Anderson, K.H., 2000. Biomes of western North America at 18,000; 6000 and 0 14C yr BP reconstructed from pollen and packrat midden data. Journal of Biogeography 27, 555–584) for a better representation of the modern vegetation of NW Mexico. The biome reconstruction method was validated with the modern pollen sites and applied to the fossil sites. Our results show that, during the early Holocene, a cool conifer forest extended at least down to 1700 m, while today this biome is present above 2000 m in the Chihuahua state. The Younger Dryas event was recorded in one site with cold and dry conditions. The reconstructed annual temperature for this period was 3°–6 °C colder than today, and annual precipitation was 250 mm lower than at present (900 mm/yr). The middle Holocene after 9200 cal yr BP was marked by a warming trend, reaching temperatures 2 °C warmer than today at 7000 cal yr BP, and by the installation of a warm mixed forest, the present day biome, at 1700 m elevation, while at higher elevations (1900 m) the cool conifer forest was still present. Summer precipitation was 200 mm/yr above the early Holocene values, suggesting that monsoon-like conditions strengthened since 9200 cal yr BP at this region. During the last 4000 yr, the same warm mixed forest was reconstructed below 1700 m and a conifer forest above 1700 m. A great variability of vegetation and climate patterns was recorded for the last 3000 yr particularly at high elevation sites, where warming and cooling trends would be coeval of the Medieval warm period and Little Ice Age, likely related to ENSO variability.  相似文献   

20.
The growth of two high-elevation inland lakes (at 4600 m) was analyzed using satellite imagery (2000–2005) and data were collected over the last decade (1997–2006) at a plateau meteorological station (at 4820 m) and stream gauging data from a station (at 4250 m) in central Tibet. We examined the lake water balance responses to meteorological and hydrological variables. The results show that the lake areas greatly expanded by a maximum of 27.1% (or 43.7 km2) between 1998 and 2005. This expansion appears to be associated with an increase in annual precipitation of 51.0 mm (12.6%), mean annual and winter mean temperature increases of 0.41 °C and 0.71 °C, and an annual runoff increase of 20% during the last decade. The changes point to an abrupt increase in the annual precipitation, mean temperature and runoff occurring in 1996, 1998 and 1997, respectively, and a decrease in the annual pan evaporation that happened in 1996. The timing of lake growth corresponds closely with abrupt increases in the annual precipitation and runoff and with the decrease in the annual evaporation since the mid-1990s. This study indicates a strong positive water balance in these permafrost highland lakes, and provides further evidence of lake growth as a proxy indicator of climate variability and change.  相似文献   

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