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1.
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   

2.
Peatlands play a major role in the global carbon cycle but are largely overlooked in current large-scale vegetation mapping efforts. In this study, we investigated the potential of the Moderate Resolution Imaging Spectroradiometer (MODIS) to capture the extent and distribution of peatlands in the St. Petersburg region of Russia by analyzing the relationships between peatland cover fractions derived from reference maps and  1-km resolution MODIS Nadir BRDF-Adjusted Reflectance (NBAR) data from year 2002.First, we characterized and mapped 50 peatlands from forest inventory and peat deposit inventory data. The peatlands represent three major nutritional types (oligotrophic, mesotrophic, eutrophic) and different sizes (0.6–7800 ha). In addition, parts of 6 peatlands were mined for peat and these were mapped separately. The reference maps provided information on peatland cover for 1105 MODIS pixels. We performed regression analysis on 50% of the pixels and reserved the remainder for model validation. Canonical correlation analysis on the MODIS reflectance bands and the peatland cover fractions produced a multi-spectral peatland cover index (PCI), which served as the predictor in a reduced major axis (RMA) regression model. The results suggest a high potential for mapping peatlands with MODIS. The RMA regression models explained much of the variance in the PCI (r2 = 0.74 for mined and r2 = 0.81 for unmined peatlands). Model validation showed high correlation between observed versus predicted peatland cover (mined: r = 0.87; unmined: r = 0.92). We used the models to derive peatland cover estimates for the St. Petersburg region and compared the results to current MODIS land cover maps.  相似文献   

3.
Because the Khumbu Himal of the Nepal Himalayas lacks long-term climate records from weather stations, mountain permafrost degradation serves as an important indicator of climate warming. In 1973, the permafrost lower limit was estimated to be 5200–5300 m above sea level (ASL) on southern-aspect slopes in this region. Using ground-temperature measurements, we examined the mountain permafrost lower limit on slopes with the same aspect in 2004. The results indicate that the permafrost lower limit was 5400–5500 m ASL in 2004. The permafrost lower limit was estimated to be 5400 to 5500 m on slopes with a southern aspect in the Khumbu Himal in 1991 using seismic reflection soundings. Thus, it is possible that the permafrost lower limit has risen 100–300 m between 1973 and 1991, followed by a stable limit of 5400 to 5500 m over the last decade. An increase in mean annual air temperature of approximately 0.2 to 0.4 °C from the 1970s to the 1990s has indicated a rise in the permafrost lower limit of 40 to 80 m at the Tibetan Plateau. The rise in the mountain permafrost lower limit in the Khumbu Himal exceeds that of the Tibetan Plateau, suggesting the possibility of greater climate warming in the Khumbu Himal.  相似文献   

4.
Climatic changes over the Mediterranean basin in 2031–2060, when a 2 °C global warming is most likely to occur, are investigated with the HadCM3 global circulation model and their impacts on human activities and natural ecosystem are assessed. Precipitation and surface temperature changes are examined through mean and extreme values analysis, under the A2 and B2 emission scenarios. Confidence in results is obtained via bootstrapping. Over the land areas, the warming is larger than the global average. The rate of warming is found to be around 2 °C in spring and winter, while it reaches 4 °C in summer. An additional month of summer days is expected, along with 2–4 weeks of tropical nights. Increase in heatwave days and decrease in frost nights are expected to be a month inland. In the northern part of the basin the widespread drop in summer rainfall is partially compensated by a winter precipitation increase. One to 3 weeks of additional dry days lead to a dry season lengthened by a week and shifted toward spring in the south of France and inland Algeria, and autumn elsewhere. In central Mediterranean droughts are extended by a month, starting a week earlier and ending 3 weeks later. The impacts of these climatic changes on human activities such as agriculture, energy, tourism and natural ecosystems (forest fires) are also assessed. Regarding agriculture, crops whose growing cycle occurs mostly in autumn and winter show no changes or even an increase in yield. In contrast, summer crops show a remarkable decrease of yield. This different pattern is attributed to a lengthier drought period during summer and to an increased rainfall in winter and autumn. Regarding forest fire risk, an additional month of risk is expected over a great part of the basin. Energy demand levels are expected to fall significantly during a warmer winter period inland, whereas they seem to substantially increase nearly everywhere during summer. Extremely high summer temperatures in the Mediterranean, coupled with improved climate conditions in northern Europe, may lead to a gradual decrease in summer tourism in the Mediterranean, but an increase in spring and autumn.  相似文献   

5.
Climate changes and recent glacier behaviour in the Chilean Lake District   总被引：1，自引：1，他引：0  
Atmospheric temperatures measured at the Chilean Lake District (38°–42°S) showed contrasting trends during the second half of the 20th century. The surface cooling detected at several meteorological stations ranged from − 0.014 to − 0.021 °C a− 1, whilst upper troposphere (850–300 gpm) records at radiosonde of Puerto Montt (41°26′S/73°07′W) revealed warming between 0.019 and 0.031 °C a− 1. Regional rainfall data collected from 1961 to 2000 showed the overall decrease with a maximum rate of − 15 mm a− 2 at Valdivia st. (39°38′S/73°05′W). These ongoing climatic changes, especially the precipitation reduction, seem to be related to El Niño–Southern Oscillation (ENSO) phenomena which has been more frequent after 1976. Glaciers within the Chilean Lake District have significantly retreated during recent decades, in an apparent out-of-phase response to the regional surface cooling. Moreover, very little is known about upper troposphere changes and how they can enhance the glacier responses. In order to analyse their behaviour in the context of the observed climate changes, Casa Pangue glacier (41°08′S/71°52′W) has been selected and studied by comparing Digital Elevation Models (DEMs) computed at three different dates throughout the last four decades. This approach allowed the determination of ice elevation changes between 1961 and 1998, yielding a mean thinning rate of − 2.3 ± 0.6 m a− 1. Strikingly, when ice thinning is computed for the period between 1981 and 1998, the resulting rate is 50% higher (− 3.6 ± 0.6 m a− 1). This enhanced trend and the related area loss and frontal retreat suggests that Casa Pangue might currently be suffering negative mass balances in response to the upper troposphere warming and decreased precipitation of the last 25–30 yr, as well as debris cover would not prevent the glacier from a fast reaction to climate forcing. Most of recent glaciological studies regarding Andean glaciers have concentrated on low altitude changes, namely frontal variations, however, in order to better understand the regional glacier changes, new data are necessary, especially from the accumulation areas.  相似文献   

6.
Elevation dependency of climate change signals has been found over major mountain ranges such as the European Alps and the Rockies, as well as over the Tibetan Plateau. In this study we examined the temporal trends in monthly mean minimum temperatures from 116 weather stations in the eastern Tibetan Plateau and its vicinity during 1961–2006. We also analyzed projected climate changes in the entire Tibetan Plateau and its surroundings from two sets of modeling experiments under future global warming conditions. These analyses included the output of the NCAR Community Climate System Model (CCSM3) with approximately 150 km horizontal resolution for the scenario of annual 1% increase in atmospheric CO2 for future 100 years and physically-based downscaling results from the NCAR CAM3/CLM3 model at 10' × 10' resolution during three 20-year mean periods (1980–1999, 2030–2049 and 2080–2099) for the IPCC mid-range emission (A1B) scenario. We divided the 116 weather stations and the regional model grids into elevation zones of 500 m interval to examine the relationship of climatic warming and elevation. With these corroborating datasets, we were able to confirm the elevation dependency in monthly mean minimum temperature in and around the Tibetan Plateau. The warming is more prominent at higher elevations than at lower elevations, especially during winter and spring seasons, and such a tendency may continue in future climate change scenarios. The elevation dependency is most likely caused by the combined effects of cloud-radiation and snow-albedo feedbacks among various influencing factors.  相似文献   

7.
We obtained the high-resolution record of terrestrial biomarkers (C29 and C31 n-alkanes) for the last 26,000 years from Oki Ridge in the south Japan Sea that enabled us to discuss millennial scale climate changes. Our sampling resolution for the biomarker during the major deglaciation period (10–19.5 cal ka BP) is 300 years and for the elemental analyses (total organic carbon and total nitrogen) is as good as ca 200 years. The estimated mass accumulation rate of these molecules during the last glacial period is substantially higher than during the Holocene. They also exhibited two distinct peaks at 17.6 cal ka BP and 11.4 cal ka BP, which are coincident with Heinrich Event 1 and the latest stage of the Younger Dryas, respectively. The unique oceanographic setting of the Japan Sea tends to preferentially preserve organic material of aeolian origin. The nature of our biomarker record in fact suggests a strong aeolian signal, and hence their flux to the Japan Sea potentially reflects the climate conditions of the dust source regions and transport intensity. Our results are consistent with previously reported monsoon variations based on other proxies that is indicative of a strong linkage between North Atlantic climate and Asian monsoon intensity.  相似文献   

8.
We present results of the first middle Miocene climate modelling study using the latest NCAR Community Atmosphere Model (CAM v.3.1) and Community Land Model (CLM v.3.0) coupled to a slab ocean. We examine the sensitivity of the middle Miocene climate to varying concentrations of atmospheric carbon dioxide (180, 355 and 700 ppm). Model simulations are forced with realistic Miocene boundary conditions for continental geometry, topography and vegetation. Global annual mean surface temperature increases by 2.2 °C with each successive doubling of CO2 which is consistent with climate sensitivity of previous paleoclimate studies and estimates for future climate. In addition to growing evidence that tropical sea surface temperatures were higher than suggested by proxy-data, our understanding of middle to high latitude warming mechanisms is still incomplete. We compare our results to the late Miocene study of Steppuhn et al. [Steppuhn, A., Micheels, A., Bruch, A., Uhl, D., Utescher, T., Mosbrugger, V., 2007. The sensitivity of ECHAM4/ML to a double CO2 scenario for the Late Miocene and the comparison to terrestrial proxy data. Global and Planetary Change, 57, 189–212] to explore the dependence of paleoclimate model sensitivities on different software systems and boundary conditions. Our comparison shows climate sensitivity to be overall quite robust — this is as significant, as it is often unclear to what extent simulation behaviour and outputs are dependent on a particular model implementation and initial/boundary conditions. Some distinct differences in model outputs, such as our reduced latitudinal surface temperature gradient and stronger Asian monsoon system, compared to the late Miocene study of Steppuhn et al. [Steppuhn, A., Micheels, A., Bruch, A., Uhl, D., Utescher, T., Mosbrugger, V., 2007. The sensitivity of ECHAM4/ML to a double CO2 scenario for the Late Miocene and the comparison to terrestrial proxy data. Global and Planetary Change, 57, 189–212] are shown to be closely linked to the choice of topography, vegetation and ocean heat flux.  相似文献   

9.
Climate change may affect the sediment generation and transportation processes and the consequent sediment flux in a river. The sensitivity of suspended sediment flux to climate change in the Longchuanjiang catchment is investigated with Artificial Neural Networks (ANNs). ANNs were calibrated and validated using sediment flux data from 1960 to 1990 during which the influence from human activities was relatively stable. The established ANN is used to predict the responses of sediment flux to 25 hypothetical climate scenarios, which were generated by adjusting the baseline temperature up to − 1, 1, 2 and 3 °C and by scaling the baseline precipitation by +/− 10% and +/− 20%. The results indicated when temperature remains unchanged, an increase in rainfall will lead to a rise in sediment flux; when rainfall level remains unchanged, an increase in temperature is likely to result in a decrease in sediment flux. Same percentage of changes in rainfall and temperature are likely to trigger higher responses in wetter months than in drier months. However, it is the combination of the change in temperature and rainfall that determines the change of sediment flux in a river. Higher sediment flux is expected to appear under wetter and warmer climate, when higher transport capacity is accompanied by higher erosion rate.  相似文献   

10.
The dynamic climate in the Northern Hemisphere during the early Holocene could be expected to have impacted on the global carbon cycle. Ice core studies however, show little variability in atmospheric CO2. Resolving any possible centennial to decadal CO2 changes is limited by gas diffusion through the firn layer during bubble enclosure. Here we apply the inverse relationship between stomatal index (measured on sub-fossil leaves) and atmospheric CO2 to complement ice core records between 11,230 and 10,330 cal. yr BP. High-resolution sampling and radiocarbon dating of lake sediments from the Faroe Islands reconstruct a distinct CO2 decrease centred on ca. 11,050 cal. yr BP, a consistent and steady decline between ca. 10,900 and 10,600 cal. yr BP and an increased instability after ca. 10,550 cal. yr BP. The earliest decline lasting ca. 150 yr is probably associated with the Preboreal Oscillation, an abrupt climatic cooling affecting much of the Northern Hemisphere a few hundred years after the end of the Younger Dryas. In the absence of known global climatic instability, the decline to ca. 10,600 cal. yr BP is possibly due to expanding vegetation in the Northern Hemisphere. The increasing instability in CO2 after 10,600 cal. yr BP occurs during a period of increasing cooling of surface waters in the North Atlantic and some increased variability in proxy climate indicators in the region.The reconstructed CO2 changes also show a distinct similarity to indicators of changing solar activity. This may suggest that at least the Northern Hemisphere was particularly sensitive to changes in solar activity during this time and that atmospheric CO2 concentrations fluctuated via rapid responses in climate.  相似文献   

11.
We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gállego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 ± 5 ka, 64 ± 11 ka, and 36 ± 3 ka (from glacial till) and 20 ± 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 ± 21 ka, 97 ± 16 ka, 61 ± 4 ka, 47 ± 4 ka, and 11 ± 1 ka, and in the Gállego River valley at 151 ± 11 ka, 68 ± 7 ka, and 45 ± 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and Heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 ± 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 ± 4 ka) and Gállego (68 ± 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to 1) global climate changes controlled by insolation, 2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and 3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian Peninsula. Our scenario of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.  相似文献   

12.
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   

13.
New temperature logs in wells located in the grassland ecozone in the Southern Canadian Prairies in Saskatchewan, where surface disturbance is considered minor, show a large curvature in the upper 100 m. The character of this curvature is consistent with ground surface temperature (GST) warming in the 20th century. Repetition of precise temperature logs in southern Saskatchewan (years 1986 and 1997) shows the conductive nature of warming of the subsurface sediments. The magnitude of surface temperature change during that time (11 years) is high (0.3–0.4°C). To assess the conductive nature of temperature variations at the grassland surface interface, several precise air and soil temperature time series in the southern Canadian Prairies (1965–1995) were analyzed. The combined anomalies correlated at 0.85. Application of the functional space inversion (FSI) technique with the borehole temperature logs and site-specific lithology indicates a warming to date of approximately 2.5°C since a minimum in the late 18th century to mid 19th century. This warming represents an approximate increase from 4°C around 1850 to 6.5°C today. The significance of this record is that it suggests almost half of the warming occurred prior to 1900, before dramatic build up of atmospheric green house gases. This result correlates well with the proxy record of climatic change further to the north, beyond the Arctic Circle [Overpeck, J., Hughen, K., Hardy, D., Bradley, R., Case, R., Douglas, M., Finney, B., Gajewski, K., Jacoby, G., Jennings, A., Lamourex, S., Lasca, A., MacDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A., Zielinski, G., 1997. Arctic environmental change of the last four centuries, Science 278, 1251–1256.].  相似文献   

14.
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   

15.
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   

16.
Influence of upper air conditions on the Patagonia icefields   总被引：1，自引：1，他引：0  
Upper-air conditions archived in the NCEP-NCAR Reanalysis have been used to investigate changes in precipitation and snowfall over the Patagonia icefields during 1960–99. Apparently, whereas total precipitation has not changed, warming has caused a decrease in the amount falling as snow. Precipitation at a site is taken to be proportional to the product of the relative humidity and the component of the wind in a particular critical direction, both at 850 hPa ( 1400 m) at a point over the ocean to the west of the icefields; whether it falls as rain or snow is assumed to depend on whether the temperature at the elevation of the site is above or below + 2 °C. The critical direction is assumed to be 270°, which is perpendicular to the north–south trending Andes and is also the prevailing wind direction in this zone of strong westerlies. Because of the scarcity of precipitation records on or near the icefields, the constant of proportionality cannot be determined, so the investigation is limited to examining relative changes in those upper air variables. Warming at 850 hPa has been 0.5 °C over the 40 years, both winter and summer, with the effects that it has: (1) shifted from snow to rain 5% of the precipitation, the total of which has changed little, and (2) increased annual melt in the ablation areas by 0.5 m w.e. The icefields have been losing mass since at least 1870, so this 40-year trend represents only an acceleration of the longer-term trend of adjusting to climate change since the Little Ice Age.  相似文献   

17.
A palynological study of oil exploration wells in the Gippsland Basin southeastern Australia has provided a record of southern high latitude climate variability for the last 12 million years of the Cretaceous greenhouse world. During this time, the vegetation was dominated by a cool to temperate flora of Podocarpaceae, Proteaceae and Nothofagidites spp. at a latitude of 60°S. Milankovitch forced cyclic alternations from drier to wetter climatic periods caused vegetation variability from 72 to 77 Ma. This climate change was probably related to the waxing and waning of ephemeral (100 ky) small ice sheets in Antarctica during times of insolation minima and maxima. Drying and cooling after 72 Ma culminated from 68 to 66 Ma, mirroring trends in global δ18O data. Quantitative palynofloral analyses have the potential to provide realistic proxies for small-scale climate variability in the predominantly ice-free Late Cretaceous.  相似文献   

18.
To evaluate the consequences of possible future climate changes and to identify the main climate drivers in high latitudes, the vegetation and climate in the East Siberian Arctic during the last interglacial are reconstructed and compared with Holocene conditions. Plant macrofossils from permafrost deposits on Bol'shoy Lyakhovsky Island, New Siberian Archipelago, in the Russian Arctic revealed the existence of a shrubland dominated by Duschekia fruticosa, Betula nana and Ledum palustre and interspersed with lakes and grasslands during the last interglacial. The reconstructed vegetation differs fundamentally from the high arctic tundra that exists in this region today, but resembles an open variant of subarctic shrub tundra as occurring near the tree line about 350 km southwest of the study site. Such difference in the plant cover implies that, during the last interglacial, the mean summer temperature was considerably higher, the growing season was longer, and soils outside the range of thermokarst depressions were drier than today. Our pollen-based climatic reconstruction suggests a mean temperature of the warmest month (MTWA) range of 9–14.5 °C during the warmest interval of the last interglacial. The reconstruction from plant macrofossils, representing more local environments, reached MTWA values above 12.5 °C in contrast to today's 2.8 °C. We explain this contrast in summer temperature and soil moisture with a combination of summer insolation higher than present and climatic continentality in arctic Yakutia stronger than present as result of a considerably less inundated Laptev Shelf during the last interglacial.  相似文献   

19.
Late Glacial to Holocene ice retreat was investigated along a 120 km long fjord system, reaching from Gran Campo Nevado (GCN) to Seno Skyring in the southernmost Andes (53°S). The aim was to improve the knowledge on regional and global control on glacier recession with special emphasis on latitudinal shifting of the westerlies. The timing of ice retreat was derived from peat and sediment cores, using mineralogical and chemical characteristics, and pollen as proxies. Stratigraphy was based on 14C-AMS ages and tephrochronology. The ice retreat of the Seno Skyring Glacier lobe is marked by an ice rafted debris layer which was formed around 18,300 to 17,500 cal. yr B.P. Subsequently, fast glacier retreat occurred until around 15,000 to 14,000 cal. yr B.P. during which around 84% of Skyring Glacier were lost. This fast recession was probably also triggered by an increase of the Equilibrium Line Altitude (ELA) from 200 to 300 m. Subsequently, the ice surface was lowered below the ELA in an area that previously made up more than 50% of the accumulation area. Much slower retreat and glacier fluctuations of limited extent in the fjord channel system northeast of GCN occurred between around 14,000 to 11,000 cal. yr B.P. during both the Antarctic Cold Reversal and the Younger Dryas. This slow down of retreat indicates a decline in the general warming trend and/or increased precipitation, due to a southward migration of the westerlies. After around 11,000 cal. yr B.P. pollen distribution shows evolved Magellanic Rainforest and similar climate as at present, which lasted throughout most of the Holocene. Only Late Neoglacial moraine systems were formed in the period 1220–1460 AD, and subsequently in the 1620s AD, and between 1870 and 1910 AD. The results indicate that the Gran Campo Nevado ice cap has reacted more sensitive and partly distinct to climate change, compared to the Patagonian Ice Field.  相似文献   

20.
A previous study of Fox [Fox, A.N. 1993. Snowline altitude and climate at present and during the Last Pleistocene Glacial Maximum in the Central Andes (5°–28°S). Ph.D. Thesis. Cornell University.] showed that for a fixed 0 °C isotherm altitude, the equilibrium-line altitude (ELA) of the Peruvian and Bolivian glaciers from 5 to 20°S can be expressed based on a log–normal expression of local mid-annual rainfall amount (P). In order to extrapolate the function to the whole Andes (10°N to 55°S) a local 0 °C isotherm altitude is introduced. Two applications of this generalised function are presented. One concerns the space evolution of mean inter-annual ELA for three decades (1961–1990) over the whole South American continent. A high-resolution data set (grid data: 10′ for latitude/longitude) of mean monthly air surface temperature and precipitation is used. Mean annual values over the 1961–1990 period were calculated. On each grid element, the mean annual 0 °C isotherm altitude is determined from an altitudinal temperature gradient and mean annual temperature (T) at ground level. The 0 °C isotherm altitude is then associated with the annual precipitation amount to compute the ELA. Using computed ELA and the digital terrain elevation model GTOPO30, we determine the extent of the glacierised area in Andean regions under modern climatic conditions. The other application concerns the ELA time evolution on Zongo Glacier (Bolivia), where inter-annual ELA variations are computed from 1995 to 1999. For both applications, the computed values of ELA are in good agreement with those derived from glacier mass balance measurements.  相似文献   

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