Computation of the space and time evolution of equilibrium-line altitudes on Andean glaciers (10°N–55°S)

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.     

Climate warming and growth of high-elevation inland lakes on the Tibetan Plateau

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.     

Past and recent changes in air and permafrost temperatures in eastern Siberia

Air and ground temperatures measured in Eastern Siberia has been compiled and analyzed. The analysis of mean annual air temperatures measured at 52 meteorological stations within and near the East-Siberian transect during the period from 1956 through 1990 demonstrates a significant and statistically significant (at 0.05 level) positive trend ranging from 0.065 to 0.59 °C/10 yr. A statistically significant (at 0.05 level) positive trend was also observed in mean annual ground temperatures for the same period. The permafrost temperature reflects changes in air temperature on a decadal time scale much better than on an interannual time scale. Generally, positive trends in mean annual ground temperatures are slightly smaller in comparison with trends in mean annual air temperatures, except for several sites where the discordance between the air and ground temperatures can be explained by the winter snow dynamics. The average trend for the entire region was 0.26 °C/10 yr for ground temperatures at 1.6 m depth and 0.29 °C/10 yr for the air temperatures. The most significant trends in mean annual air and ground temperatures were in the southern part of the transect, between 55° and 65° N. Numerical modeling of ground temperatures has been performed for Yakutsk and Tiksi for the last 70 yr. Comparing the results of these calculations with a similar time series obtained for Fairbanks and Barrow in Alaska shows that similar variations of ground temperatures took place at the same time periods in Yakutsk and Fairbanks, and in Tiksi and Barrow. The decadal and longer time scale fluctuations in permafrost temperatures were pronounced in both regions. The magnitudes of these fluctuations were on the order of a few degrees centigrade. The fluctuations of mean annual ground temperatures were coordinated in Fairbanks and Yakutsk, and in Barrow and Tiksi. However, the magnitude and timing of these fluctuations were slightly different for each of the sites.     

Modeled impacts of changes in tundra snow thickness on ground thermal regime and heat flow to the atmosphere in Northernmost Alaska

Seasonal snow covers the tundra surface for up to nine months of each year on the Alaskan North Slope. Variations in the snow thickness could strongly influence the thermal regime of the underlying soil and permafrost, and the surface energy balance. The impacts of increases and decreases in the tundra snow thickness on the thermal regime of snow surface, active layer, and permafrost, and on the conductive heat flow to the atmosphere were investigated numerically, by using an improved surface energy balance approach based one-dimensional heat transfer model. The baseline inputs for the numerical model are mean daily meteorological data and surface albedos collected at Barrow, Alaska from 1995 through 1999. Based on a study for the long-term mean daily maximum and minimum snow thickness distributions at Barrow in the snow season of 1948 through 1997, a snow thickness factor was defined and five simulation cases were run for the snow season of 1997–1998 by changing the snow thickness factor. The modeled results indicate that changes in snow thickness have significant impacts on ground thermal regimes and conductive heat flow to the atmosphere. Decreasing the snow thickness by 50% led to the maximum ground temperature decrease of 1.48 °C at 0.29 m depth, and 0.72 °C at 3.0 m depth; the magnitude of the mean conductive heat flow to the atmosphere for December increase of 4.3 Wm^− 2. Increasing the snow thickness by 50% resulted in the maximum ground temperature increase of 1.44 °C at 0.29 m depth, and 0.66 °C at 3.0 m depth; the magnitude of the mean conductive heat flow to the atmosphere for December decrease of 1.57 W m^− 2. On an annual basis, variation in the snow thickness by 50%, the ground temperature variations of more than 0.25 °C occurred as deep as 8.0 m below the ground surface. The modeled results also show that changes in snow thickness have a relatively small influence on the snow surface temperature.     

Permafrost warming in the Tien Shan Mountains, Central Asia

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.     

Climatic implications of correlated Upper Pleistocene glacial and fluvial deposits on the Cinca and Gállego Rivers (NE Spain) based on OSL dating and soil stratigraphy

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.     

Recent evolution and mass balance of Cordón Martial glaciers, Cordillera Fueguina Oriental

Past and present glacier changes have been studied at Cordón Martial, Cordillera Fueguina Oriental, Tierra del Fuego, providing novel data for the Holocene deglaciation history of southern South America and extrapolating as well its future behavior based on predicted climatic changes. Regional geomorphologic and stratigraphic correlations indicate that the last glacier advance deposited the ice-proximal (“internal”) moraines of Cordón Martial, around 330 ¹⁴C yr BP, during the Late Little Ice Age (LLIA). Since then glaciers have receded slowly, until 60 years ago, when major glacier retreat started. There is a good correspondence for the past 100 years between the surface area variation of four small cirque glaciers at Cordón Martial and the annual temperature and precipitation data of Ushuaia. Between 1984 and 1998, Martial Este Glacier lost 0.64 ± 0.02 × 10⁶ m³ of ice mass (0.59 ± 0.02 × 10⁶ m³ w.e.), corresponding to an average ice thinning of 7.0 ± 0.2 m (6.4 ± 0.2 m w.e), according to repeated topographic mapping. More detailed climatic data have been obtained since 1998 at the Martial Este Glacier, including air temperature, humidity and solar radiation. These records, together with the monthly mass balance measured since March 2000, document the annual response of the Martial Este Glacier to the climate variation. Mass balances during hydrological years were positive in 2000, negative in 2001 and near equilibrium in 2002. Finally, using these data and the regional temperature trend projections, modeled for different future scenarios by the Atmosphere-Ocean Model (GISS-NASA/GSFC), potential climatic-change effects on this mountain glacier were extrapolated. The analysis shows that only the Martial Este Glacier may survive this century.     

Provincialism in trends and high frequency changes in the northwest North Atlantic during the Holocene

In the present paper, we report on micropaleontological (dinocysts) and isotopic (¹⁸O and ¹³C in foraminifers) analyses performed in Holocene sediments from fifteen cores raised from the central and northwest North Atlantic. Sea-surface temperature (SST), sea-surface salinity (SSS), thus potential density, and sea-ice cover are reconstructed based on dinocyst assemblages. After proper calibration, oxygen isotope data on the mesopelagic foraminifer Neogloboquadrina pachyderma left coiled (Npl) are converted into potential density values deeper in the water column, thus allowing documentation of vertical density gradients and identification of intervals favourable for winter convection to occur with formation of intermediate Labrador Sea Water (LSW). The most important findings from this study include: (1) the existence of an early-mid Holocene thermal optimum with positive anomalies up to 6 °C above present along the main SW–NE axis of the North Atlantic Current, but no significant SST maximum at most sites along eastern Canadian margins; (2) the evidence for larger than present amplitude of annual SSTs during the early Holocene, thus for a stronger seasonality; (3) minimum sea-ice cover from 11 500 to 6000 cal years BP, and a slight increase of sea-ice variability, and average seasonal duration of 0.5 to 1 month per year afterwards; (4) variable SSS during the entire Holocene, suggesting changes in the routing and rates of freshwater–meltwater discharges from the Arctic and eastern Canada; (5) the setting of conditions compatible with LSW production after 8 ka only, and likely a more steady production during the late Holocene; (6) an overall trend for a potential density increase of the Labrador Sea, throughout the Holocene, matching a decreasing trend eastward, thus suggesting a progressive enhancement of the western branch of the Atlantic Meridional Overturning with respect to its northeastern route; and (7) indication of maximum production and fast dispersal of LSW in the entire North Atlantic during recent times only, as suggested by linearly-converging δ¹⁸O-values of Npl from all sites, towards its modern relatively homogeneous composition ( 2.5/2.6‰). The overall picture of the Holocene North Atlantic arising from this study is that of a basin marked by a strong regionalism with large discrepancies in hydrographical trends and high frequency oscillations, at least partly controlled by freshwater–meltwater routes and rates of export from the Arctic.     

Biomization and quantitative climate reconstruction techniques in northwestern Mexico—With an application to four Holocene pollen sequences

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.     

Ice elevation and areal changes of glaciers from the Northern Patagonia Icefield, Chile

High thinning rates (up to − 4.0 ± 0.97 m a^− 1) have been measured at Campo de Hielo Patagónico Norte (CHN) or Northern Patagonia Icefield, Chile between 1975 and 2001. Results have been obtained by comparing a Digital Elevation Model (DEM) derived from regular cartography compiled by Instituto Geográfico Militar of Chile (IGM) based upon 1974/1975 aerial photographs and a DEM generated from Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) satellite images acquired in September 2001. A complete cloud-free Landsat ETM+ satellite image mosaic acquired in March 2001 was used to update the available glacier inventory of the CHN, including all glaciers larger than 0.5 km² (48 new glaciers). A new delineation of ice divides was also performed over the accumulation areas of glaciers sharing the high plateau where the existing regular cartography exhibits poor coverage of topographic information. This updated glacier inventory produced a total ice area for 2001 of 3953 km², which represents a decrease of 3.4 ± 1.5% (140 ± 61 km² of ice) with respect to the total ice area of the CHN in 1979 calculated from a Landsat MSS satellite image. Almost 62% of the total area change between 1979 and 2001 took place in glaciers located at the western margin of the CHN, where the maximum area loss was experienced by Glaciar San Quintín with 33 km². At the southern margin, Glaciar Steffen underwent the largest ice-area loss (12 km² or 2.6% of the 1979 area), whilst at the eastern margin the greatest area loss took place in Glaciares Nef (7.9 km², 5.7% of the 1979 area) and Colonia (9.1 km², 2.7% of the 1979 area). At the northern margin of the CHN the lower debris-covered ablation area of Glaciar Grosse collapsed into a new freshwater lake formed during the late 1990s. The areal changes measured at the CHN are much larger than previously estimated due to the inclusion of changes experienced in the accumulation areas. The CHN as a whole is contributing melt water to global sea level rise at rates  25% higher than previous estimates.     

The exceptionally hot summer of 2007 in Athens, Greece — A typical summer in the future climate?

Summer 2007 was abnormally warm for many areas of southeastern Europe, the Balkan peninsula and parts of Asia Minor with departures from the seasonal means exceeding 4 °C in some areas but also distinct periods of extremely hot weather. Greece experienced very likely the warmest summer of its instrumental history with record breaking temperatures being observed at a number of stations. The historical air temperature record of the National Observatory of Athens (NOA), extending back to the 19th century, was used in order to highlight the rarity of the event. Seasonal (June to August) temperature anomalies at NOA exceeded 3 °C corresponding to more than 3 standard deviations with respect to the 1961–1990 reference period. The record value of 44.8 °C was observed at NOA on 26 June 2007 (previous record 43 °C in June 1916) during the first and most intense heat wave that affected the area. The study places summer 2007 in the climatology of the previous century and also examines whether the statistics of summer 2007 have similarities with Mediterranean summers of the future. An ensemble of regional climate model simulations undertaken for the European domain indicate that summer 2007 reflects the daily maximum temperatures that are projected to occur in the latter part of the 21st century. The analysis of temperature data from other less urbanized stations indicates that the urban heat effect in Athens contributed positively to the anomalies of the nocturnal temperatures. The abnormally hot summer of 2007 is perhaps not the proof but a strong indicator of what eastern Mediterranean summers could resemble in future.     

Changes in the lower limit of mountain permafrost between 1973 and 2004 in the Khumbu Himal, the Nepal Himalayas

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.     

The 2003 November 14 occultation by Titan of TYC 1343-1865-1: II. Analysis of light curves

We observed a stellar occultation by Titan on 2003 November 14 from La Palma Observatory using ULTRACAM with three Sloan filters: u^′, g^′, and i^′ (358, 487, and 758 nm, respectively). The occultation probed latitudes 2° S and 1° N during immersion and emersion, respectively. A prominent central flash was present in only the i^′ filter, indicating wavelength-dependent atmospheric extinction. We inverted the light curves to obtain six lower-limit temperature profiles between 335 and 485 km (0.04 and 0.003 mb) altitude. The i^′ profiles agreed with the temperature measured by the Huygens Atmospheric Structure Instrument [Fulchignoni, M., and 43 colleagues, 2005. Nature 438, 785–791] above 415 km (0.01 mb). The profiles obtained from different wavelength filters systematically diverge as altitude decreases, which implies significant extinction in the light curves. Applying an extinction model [Elliot, J.L., Young, L.A., 1992. Astron. J. 103, 991–1015] gave the altitudes of line of sight optical depth equal to unity: 396±7 and 401±20 km (u^′ immersion and emersion); 354±7 and 387±7 km (g^′ immersion and emersion); and 336±5 and 318±4 km (i^′ immersion and emersion). Further analysis showed that the optical depth follows a power law in wavelength with index 1.3±0.2. We present a new method for determining temperature from scintillation spikes in the occulting body's atmosphere. Temperatures derived with this method are equal to or warmer than those measured by the Huygens Atmospheric Structure Instrument. Using the highly structured, three-peaked central flash, we confirmed the shape of Titan's middle atmosphere using a model originally derived for a previous Titan occultation [Hubbard, W.B., and 45 colleagues, 1993. Astron. Astrophys. 269, 541–563].     

Salinity changes in the western tropical South Atlantic during the last 30 kyr

The hydrographic changes in the western tropical South Atlantic during the last 30 kyr were reconstructed based in the faunal and isotopic analyses of planktonic foraminifera of three cores taken along the Brazilian Continental Margin between 14°S and 25°S. The application of the SIMMAX–MAT method on faunal counts data provided the sea surface temperature estimates. Sea surface salinity estimates were based on the oxygen isotope composition of Globigerinoides ruber (white). Additionally, the abundance record of the planktonic foraminifera Globorotalia truncatulinoides (right) was used as a proxy for vertical mixing of surface waters. Sea surface temperature estimates suggest a relative stability of the area during the last 30 kyr. However, significant changes in the isotopic composition of G. ruber (white) suggest that the isotopic signal is dominated by the influence of sea surface salinity changes. The observed salinity changes are related to both the local hydrological balance and global circulation. Orbital forcing and sea surface salinity changes were responsible for considerable changes in the stability of the upper water column and consequently in the depth of the mixed layer, as indicated by the abundance record of G. truncatulinoides (right).     

Sediment flux sensitivity to climate change: A case study in the Longchuanjiang catchment of the upper Yangtze River, China

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.     

Steric and mass-induced Mediterranean sea level trends from 14 years of altimetry data

Long-term series of almost 14 years of altimetry data (1992–2005) have been analysed along with Sea Surface Temperature (SST) and temperature and salinity profiles to investigate sea level trends over the Mediterranean Sea. Although sea level variations are mainly driven by the steric contribution, the mass-induced component plays some role in modulating its oscillation. A spatially averaged positive trend of 2.1 ± 0.6 mm/year has been observed, but a change in sign in 2001 seems to appear. Steric effects (mainly on thermal origin) account for  55% of sea level trend. Although Mediterranean Sea is a semi-enclosed basin, this value is comparable to that reported for the global ocean. Sea level rise is particularly important in the Levantine basin south of Crete with values up to 10 ± 1 mm/year. Other areas of sea level rise are localised throughout the Levantine basin and in the Adriatic and Alboran Seas, with more moderate values. Sea level drop areas are localised in the Algerian basin, between the Balearic Islands and the African coasts and, particularly, in the Ionian basin. In this area, negative trends as high as − 10 ± 0.8 mm/year are detected mainly due to the mass-induced contribution, which suggests decadal changes of surface circulation. The inferred sea level trends have been correlated with North Atlantic Oscillation (NAO) indices and a low but significant correlation has been detected between sea level in the Levantine and Balearic basins and NAO index.     

Rapid responses of the prairie-forest ecotone to early Holocene aridity in mid-continental North America

The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka = 1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (< 300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position.     

Continental climate in the East Siberian Arctic during the last interglacial: Implications from palaeobotanical records

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.     

The 8.2 ka event: Evidence for seasonal differences and the rate of climate change in western Europe

Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods.     

Climate changes and recent glacier behaviour in the Chilean Lake District

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.