A new method to estimate ice age temperatures

On glacial time scales, the waxing and waning of the Eurasian and North American ice sheets depend largely on variations in atmospheric temperature. As global sea level is primarily determined by the volume of these ice sheets, there is a direct (yet complex) relation between global sea level and the northern hemispheric (NH) temperature. This relation is essentially represented by a model of the NH ice sheets. We use a thermomechanical ice-sheet–ice shelf–bedrock model in conjunction with an inverse method to deduce a time series of NH temperature (from 120 kyr BP until present) that is consistent with the observed global sea level record. The advantage of this method is that it provides the annual mean surface air temperature averaged over the NH continents north of 40°N. The results reveal that ice age temperatures were 4–10°C lower than today, which agrees with other temperature reconstructions. However, reconstructed temperatures are comparitively low during the early stages of the glacial, a feature that is consistent with the rapid growth of the ice sheets. The sensitivity of the results for uncertainties in precipitation rate, in observed sea level and in some other model parameters is examined to quantify the error in reconstructed temperature. During the glacial period (120–15 kyr BP), surface air temperatures in the NH (north of 40°N) were 7.2±1.5°C lower than todays (interglacial) temperatures.     

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

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

The greening of the McGill Paleoclimate Model. Part I: Improved land surface scheme with vegetation dynamics

The formulation of a new land surface scheme (LSS) with vegetation dynamics for coupling to the McGill Paleoclimate Model (MPM) is presented. This LSS has the following notable improvements over the old version: (1) parameterization of deciduous and evergreen trees by using the models climatology and the output of the dynamic global vegetation model, VECODE (Brovkin et al. in Ecological Modelling 101:251–261 (1997), Global Biogeochemical Cycles 16(4):1139, (2002)); (2) parameterization of tree leaf budburst and leaf drop by using the models climatology; (3) parameterization of the seasonal cycle of the grass leaf area index; (4) parameterization of the seasonal cycle of tree leaf area index by using the time-dependent growth of the leaves; (5) calculation of land surface albedo by using vegetation-related parameters, snow depth and the models climatology. The results show considerable improvement of the models simulation of the present-day climate as compared with that simulated in the original physically-based MPM. In particular, the strong seasonality of terrestrial vegetation and the associated land surface albedo variations are in good agreement with several satellite observations of these quantities. The application of this new version of the MPM (the green MPM) to Holocene millennial-scale climate changes is described in a companion paper, Part II.

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Atmospheric circulation anomalies due to 115 kyr BP climate forcing dominated by changes in the North Pacific Ocean

Climate at the time of inception of the Laurentide Ice Sheet (LIS) at ~115 kyr BP is simulated with the fully coupled NCAR Community Climate System Model (CCSM3) and compared to a simulated preindustrial climate (circa 1870) in order to better understand land surface and atmospheric responses to orbital and greenhouse cooling at inception. The interaction between obliquity and eccentricity produces maximum decrease in TOA insolation in JJA over the Arctic but increases occur over the tropics in DJF. The land surface response is dominated by widespread summer cooling in the Northern Hemisphere (NH), increases in snowfall, and decreases in melt rates and total precipitation. CCSM3 responds to the climate forcing at 115 kyr BP by producing incipient glaciation in the areas of LIS nucleation. We find that the inception of the LIS could have occurred with atmospheric circulation patterns that differ little from the present. The location of the troughs/ridges, mean flow over the Canadian Arctic and dominant modes of the atmospheric circulation are all very similar to the present. Larger changes in mean sea level pressure occur upstream of the inception region in the North Pacific Ocean and downstream in Western Europe. In the North Pacific region, the 115 kyr BP anomalies weaken both the Pacific high and Aleutian low making NH summers look more like the PREIND winters and vice versa. The occurrence of cold JJA anomalies at 115 kyr BP favors outbreaks of cold air not in the winter as in contemporary climates but during the summer instead and reinforces the cooling from orbital and GHG reductions. Increased poleward eddy transport of heat and moisture characterizes the atmospheric response in addition to reduced total cloud cover in the Arctic.     

Transient simulation of the last glacial inception. Part I: glacial inception as a bifurcation in the climate system

We study the mechanisms of glacial inception by using the Earth system model of intermediate complexity, CLIMBER-2, which encompasses dynamic modules of the atmosphere, ocean, biosphere and ice sheets. Ice-sheet dynamics are described by the three-dimensional polythermal ice-sheet model SICOPOLIS. We have performed transient experiments starting at the Eemiam interglacial, at 126 ky BP (126,000 years before present). The model runs for 26 kyr with time-dependent orbital and CO₂ forcings. The model simulates a rapid expansion of the area covered by inland ice in the Northern Hemisphere, predominantly over Northern America, starting at about 117 kyr BP. During the next 7 kyr, the ice volume grows gradually in the model at a rate which corresponds to a change in sea level of 10 m per millennium. We have shown that the simulated glacial inception represents a bifurcation transition in the climate system from an interglacial to a glacial state caused by the strong snow-albedo feedback. This transition occurs when summer insolation at high latitudes of the Northern Hemisphere drops below a threshold value, which is only slightly lower than modern summer insolation. By performing long-term equilibrium runs, we find that for the present-day orbital parameters at least two different equilibrium states of the climate system exist—the glacial and the interglacial; however, for the low summer insolation corresponding to 115 kyr BP, we find only one, glacial, equilibrium state, while for the high summer insolation corresponding to 126 kyr BP only an interglacial state exists in the model.

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Holocene climate evolution of the Ugii Nuur basin,Mongolia

In order to evaluate the Holocene palaeoenvironmental evolution of the Ugii Nuur basin, central Mongolia, investigations on chemical and mineralogical properties of lacustrine sediments were carried out on a 630 cm sediment core from lake Ugii Nuur. The interpretation of the record is based on a principal component analysis （PCA） of the elemental composition of the samples. The results show that lacustrine deposition started at 10.6 kyr BP. Low lake level conditions were identified during the Early Holocene （10.6-7.9 kyr BP）. The Mid Holocene （7.9-4.2 kyr BP） was characterized by generally higher lake levels and thus higher moisture supply, but it experienced strong climatic fluctuations. Arid conditions prevailed from 4.2-2.8 kyr BP and were followed by a stable, more humid phase until today.     

A survey of anthropogenic vegetation changes in West Africa during the last century — climatic implications

The extent of albedo change resulting from anthropogenic modification of the vegetation cover over the last century has been investigated in West Africa. The climatic implications of these changes are briefly discussed.West Africa spans a suite of vegetation zones ranging latitudinally northward from tropical rainforest to desert scrub, and comprises environmental problems from extremely rapid deforestation of the tropical forests in Ivory Coast or Ghana to desertification in the Sahel.Historical vegetation changes have been digitized on a 1° × 1° grid map based on a literature survey of government censuses, forestry and agricultural reports, supplemented by atlases, and other historical, economic and geographic sources.The principal processes of land cover modification during the last century include clearing of the natural vegetation for agriculture, grazing, logging, and degradation of marginal semi-arid to arid ecosystems by excessive grazing or cultivation. Forestry surveys for West Africa suggest clearance of around 56% of the forest zone; estimated losses for Ivory Coast, Ghana, and Liberia range between 64% and 70%. Estimates of total land conversion range between 88 million ha, from the digitized land use map (Figure 4) to 122.8 million ha, from extrapolation of forestry data (Section 3.1).The change in albedo corresponding to the land use modification is relatively small, using conservative estimates for desertification amounting to an increase of around 0.4% regionally over 100 yr and 0.5% since agriculture began. Thus 4/5 of the total albedo may have occurred within the last century. Additional assumptions regarding desertification and a lower albedo value for tropical forest compensate for each other and do not significantly alter the result of the initial calculation. The maximum zones of increased albedo are concentrated in the forest zone (4°–8° N) and savanna-southern sahel (10°–12°) which correspond to zones of maximum agricultural and population growth. Between 13° N and 17° N, the albedo change is small or negative due to both less intensive land utilization and replacement of scattered vegetation on exposed sandy soil by lower albedo irrigated crops.These estimates may represent a lower limit, particularly if desertification is more extensive than initially assumed. Under an extreme assumption that the entire Sahel zone between 14°–20° N has been desertified, the regional mean albedo could increase by as much as 4%. This represents an upper limit to likely historical anthropogenic disturbances of the land surface.Although historical climate records show three major droughts during the 20th century (1910–1920, 1940's, 1969–1975, possibly continuing into the 1980's; Nicholson, 1980a; Hare, 1983), and stream flow fluctuations which correlate well with precipitation (Faure and Gac, 1981;Palutikof et al., 1981), these records do not appear to indicate a regional secular decrease in precipitation as suggested by several climate models. Evidence for apparent desiccation or desert creep (= desertification) may be attributed, in large part, to adverse changes in soil and stream hydrology caused by anthropogenic disruption of the vegetation cover.     

Sea-ice anomalies observed in the Greenland and Labrador seas during 1901–1984 and their relation to an interdecadal Arctic climate cycle

Two independent ice data sets from the Greenland and Labrador Seas have been analyzed for the purpose of characterizing interannual and decadal time scale sea-ice extent anomalies during this century. Sea-ice concentration data for the 1953–1984 period revealed the presence of a large positive anomaly in the Greenland Sea during the 1960s which coincided with the great salinity anomaly, an upper-ocean low-salinity water mass that was observed to travel cyclonically around the northern North Atlantic during 1968–1982. This ice anomaly as well as several smaller ones propagated into the Labrador Sea and then across to the Labrador and east Newfoundland coast, over a period of 3 to 5 years. A complex empirical orthogonal function analysis of the same data also confirmed this propagation phenomenon. An inverse relation between sea-ice and salinity anomalies in the Greenland-Labrador Sea region was also generally found. An analysis of spring and summer ice-limit data obtained from Danish Meteorological Institute charts for the period 1901–1956 indicated the presence of heavy ice conditions (i.e., positive ice anomalies) in the Greenland Sea during 1902–1920 and in the late 1940s, and generally negative ice anomalies during the 1920s and 1930s. Only limited evidence of the propagation of Greenland Sea ice anomalies into the Labrador Sea was observed, however, probably because the data were from the ice-melt seasons. On the other hand, several large ice anomalies in the Greenland Sea occurred 2–3 years after large runoffs (in the early 1930s and the late 1940s) from northern Canada into the western Arctic Ocean. Similarly, a large runoff into the Arctic during 1964–1966 preceded the large Greenland Sea ice anomaly of the 1960s. These facts, together with recent evidence of climatic jumps in the Northern Hemisphere tropospheric circulation, suggest the existence of an interdecadal self-sustained climate cycle in the Arctic. In the Greenland Sea, this cycle is characterized by a state of large sea-ice extent overlying an upper layer of cool, relatively fresh water that does not convectively overturn, which alternates every 10–15 years with a state of small sea-ice extent and relatively warm saline surface water that frequently overturns.Dedicated to Robert W. Stewart on the occasion of his retirement     

Regional climate and atmospheric circulation controls on the discharge of two British rivers, 1974–97

Summary ¶The dependence of the discharge (Q) of two contrasting UK rivers (Itchen, Ewe) on concurrent and lagged regional climate (RC) and atmospheric circulation (AC) variations was assessed over the period 1974–97. RC variables used were temperature and precipitation; the AC indicators used were 850hPa water vapour flux anomalies (VF) at five western European stations, and the Arctic (AOI) and North Atlantic Oscillation (NAOI) indices. Correlation analyses were performed to assess Q-RC and Q-AC relationships before two sets of multiple linear regression models were developed to specify monthly Q values from RC and AC. Q-RC associations were generally stronger and more seasonally consistent than Q-AC relationships, with the flow of the Itchen (southern England) and Ewe (northern Scotland) being most sensitive to temperature (TEMP) and precipitation (PPT) respectively. In most months, discharge values of both rivers were positively associated to zonal and vector VF anomalies over the British Isles and northern France, but inversely related to vector VF over Iceland. The AOI and NAOI were significantly related to the Ewes flow only; relationships were strongest in the winter half-year. Monthly AC regression models explained 14.8–81.0% (25.0–90.9%) of the discharge variability of the Itchen (Ewe). Strong AC forcing of the Itchens discharge is confined to the winter (DJF), since the Itchens direct meteorological signal is attenuated by groundwater dynamics in other seasons. Analysis of anomalous flow periods (e.g. 1988–92 and 1995–7) revealed that discharge does not always respond in the same manner to a given RC/AC forcing, as the relationships themselves vary inter-annually as well as between the two rivers.     

Ice-free glacial northern Asia due to dust deposition on snow

During the Last Glacial Maximum (LGM, 21 kyr BP), no large ice sheets were present in northern Asia, while northern Europe and North America (except Alaska) were heavily glaciated. We use a general circulation model with high regional resolution and a new parameterization of snow albedo to show that the ice-free conditions in northern Asia during the LGM are favoured by strong glacial dust deposition on the seasonal snow cover. Our climate model simulations indicate that mineral dust deposition on the snow surface leads to low snow albedo during the melt season. This, in turn, caused enhanced snow melt and therefore favoured snow-free peak summer conditions over almost the entire Asian continent during the LGM, whereas perennial snow cover is simulated over a large part of eastern Siberia when glacial dust deposition is not taken into account.     

Trend analysis of air temperature time series in Greece and their relationship with circulation using surface and satellite data: 1955–2001

Summary In this study, trends of annual and seasonal surface air temperature time series were examined for 20 stations in Greece for the period 1955–2001, and satellite data for the period 1980–2001. Two statistical tests based on the least square method and one based on the Mann-Kendall test, which is also capable of detecting the starting year of possible climatic discontinuities or changes, were used for the analysis. Greece, in general, shows a cooling trend in winter for the period 1955–2001, whereas, summer shows an overall warming trend, however, neither is statistically significant. As a result, the overall trend of the annual values is nearly zero. Comparison with corresponding trends in the Northern Hemisphere (NH) shows that temperatures in Greece do not follow the intense warming trends. Satellite data indicate a remarkable warming trend in mean annual, winter and summer in Greece for the period 1980–2001, and a slight warming trend in annual, spring and autumn for the NH. Comparison with the respective trends detected in the surface air temperature for the same period (1980–2001) shows they match each other quite well in both Greece and the NH. The relationship between temperature variability in Greece and atmospheric circulation was also examined using correlation analysis with three circulation indices: the well-known North Atlantic Oscillation Index (NAOI), a Mediterranean Oscillation Index (MOI) and a new Mediterranean Circulation Index (MCI). The MOI and MCI indices show the most interesting correlation with winter temperatures in Greece. The behaviour of pressure and the height of the 500hPa surface over the Mediterranean region supports these results.     

Analysis of rainfall characteristics of the Madden–Julian oscillation using TRMM satellite data

Rainfall characteristics of the Madden–Julian oscillation (MJO) are analyzed primarily using tropical rainfall measuring mission (TRMM) precipitation radar (PR), TRMM microwave imager (TMI) and lighting imaging sensor (LIS) data. Latent heating structure is also examined using latent heating data estimated with the spectral latent heating (SLH) algorithm.The zonal structure, time evolution, and characteristic stages of the MJO precipitation system are described. Stratiform rain fraction increases with the cloud activity, and the amplitude of stratiform rain variation associated with the MJO is larger than that of convective rain by a factor of 1.7. Maximum peaks of both convective rain and stratiform rain precede the minimum peak of the outgoing longwave radiation (OLR) anomaly which is often used as a proxy for the MJO convection. Stratiform rain remains longer than convective rain until 4000 km behind the peak of the mature phase. The stratiform rain contribution results in the top-heavy heating profile of the MJO.Associated with the MJO, there are tri-pole convective rain top heights (RTH) at 10–11, 7 and 3 km, corresponding to the dominance of afternoon showers, organized systems, and shallow convections, respectively. The stratiform rain is basically organized with convective rain, having similar but slightly lower RTH and slightly lags the convective rain maximum. It is notable that relatively moderate (7 km) RTH is dominant in the mature phase of the MJO, while very tall rainfall with RTH over 10 km and lightning frequency increase in the suppressed phase. The rain-yield-per flash (RPF) varies about 20–100% of the mean value of 2–10 × 10⁹ kg fl⁻¹ over the tropical warm ocean and that of 2–5 × 10⁹ kg fl⁻¹ over the equatorial Islands, between the convectively suppressed phase and the active phase of MJO, in the manner that RPF is smaller in the suppressed phase and larger in the active phase.     

Paleo-evolution of the Black Sea watershed: sea level and water transport through the Bosporus Straits as an indicator of the Lateglacial–Holocene transition

The paleo-evolution of the Black Sea level during the Lateglacial–Holocene transition is still unclear, which motivates us to provide new estimates for that period based on the analysis of water budget. Hydrological conditions in the Black Sea catchment area are reconstructed here using water balance equation, available data, and constraints based on simple theory relating the runoff ratio with climatic characteristics. In order to estimate the impact of the aridity of climate we consider two alternative scenarios: (1) climate change under constant in time gradient in precipitation and evaporation over land and sea, and (2) climate change accounting for changes in the horizontal distribution of precipitation and evaporation. Hydrological data are compiled from available present-day data and paleo-observations. A number of sensitivity experiments is carried out revealing that the hydrological conditions in the Black Sea watershed should have evolved towards a very arid climate (similar to the present-day climate in the Caspian Sea area) in order to initiate a drop of sea level of ∼100 m below the sill depth of the Bosporus Straits, as speculated in some recent research. Estimates of sea level changes reveal a qualitative agreement with the coast-line evolution inferred from paleo-observations. The Younger Dryas draw down of the Black Sea starts at about 13.3 to 13 kyr BP, with a maximum low-stand of 104 m at 11.5 kyr BP. In an arid climate scenario the sea level reaches the outlet at about 8.8 to 7.4 kyr BP. Approximately at that time, Mediterranean sea level was about 10 m higher, making flooding events of the Black Sea possible. However, the nature and exact timing of the Holocene reconnection depends also on other (not well known) factors; one of them is the Bosporus sill depth, motivating further research in this field. Estimates of the water transport through the Bosporus Straits are also provided for the time of Lateglacial–Holocene transition.     

Seychelles coral record of changes in sea surface temperature bimodality in the western Indian Ocean from the Mid-Holocene to the present

We report fossil coral records from the Seychelles comprising individual time slices of 14–20 sclerochronological years between 2 and 6.2 kyr BP to reconstruct changes in the seasonal cycle of western Indian Ocean sea surface temperature (SST) compared to the present (1990–2003). These reconstructions allowed us to link changes in the SST bimodality to orbital changes, which were causing a reorganization of the seasonal insolation pattern. Our results reveal the lowest seasonal SST range in the Mid-Holocene (6.2–5.2 kyr BP) and around 2 kyr BP, while the highest range is observed around 4.6 kyr BP and between 1990 and 2003. The season of maximum temperature shifts from austral spring (September to November) to austral autumn (March to May), following changes in seasonal insolation over the past 6 kyr. However, the changes in SST bimodality do not linearly follow the insolation seasonality. For example, the 5.2 and 6.2 kyr BP corals show only subtle SST differences in austral spring and autumn. We use paleoclimate simulations of a fully coupled atmosphere–ocean general circulation model to compare with proxy data for the Mid-Holocene around 6 kyr BP. The model results show that in the Mid-Holocene the austral winter and spring seasons in the western Indian Ocean were warmer while austral summer was cooler. This is qualitatively consistent with the coral data from 6.2 to 5.2 kyr BP, which shows a similar reduction in the seasonal amplitude compared to the present day. However, the pattern of the seasonal SST cycle in the model appears to follow the changes in insolation more directly than indicated by the corals. Our results highlight the importance of ocean–atmosphere interactions for Indian Ocean SST seasonality throughout the Holocene. In order to understand Holocene climate variability in the countries surrounding the Indian Ocean, we need a much more comprehensive analysis of seasonally resolved archives from the tropical Indian Ocean. Insolation data alone only provides an incomplete picture.     

Remotely forced variability in the tropical Atlantic Ocean

An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950–1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Niño event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Niño event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Niño in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season.

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Effects of soil moisture on the sensitivity of a climate model to earth orbital forcing at 9000 yr BP

The sensitivity of climate to orbitally-related changes in solar radiation at 9000 yr BP (before present) is examined using fixed and interactive soil moisture versions of a low resolution general circulation model. In both versions of the model increased solar radiation for June–August at 9000 yr BP (compared to present) produced enhanced northern monsoons and warmer continental interiors in comparison to present whereas decreased solar radiation at 9000 yr BP in December–February produced weaker southern monsoons. The increased rainfall in the northern tropics in summer increased soil moisture and runoff at 9000 yr BP in the interactive model; in the southern hemisphere decreased rainfall in summer led to decreased soil moisture and runoff. Conditions in summer became drier (decreased soil moisture and runoff) at 9000 yr BP in the northern extratropics.The experiments showed that the magnitude (but not the sign) of model sensitivity to 9000 yr BP radiation is altered by the effects of interactive soil moisture. Decreased soil moisture (about 20%) over northern Eurasia in the interactive model led to smaller evaporative increases, greater temperature increases and greater reduction of precipitation than for the model with fixed soil moisture. Over northern tropical lands, slightly smaller temperature increases and greater evaporation and precipitation increases in the interactive model are linked to the simulation of increased soil moisture at 9000 yr BP. The differences in sensitivity between the two versions of the model over northern Eurasia are statistically significant at the 95% level whereas those for the tropics are not.Overall, the results of the simulations are generally supported by the geologic evidence for 9000 yr BP; however, the evidence lacks sufficient precision and the model resolution is too coarse for detailed model/data comparisons and for assessment of the relative accuracy of the two 9000 yr BP experiments.The computed sensitivities of temperature and soil moisture to 9000 yr BP radiation differ from those simulated under equilibrium conditions in the various general circulation model experiments for increased atmospheric concentration of CO₂. In contrast to the effects of the enhanced seasonal cycle of solar radiation at 9000 yr BP, a CO₂ increase causes a broad warming of both the ocean and land with little modification of land/ocean temperature difference. The experiments for 9000 yr BP indicate a clearer signal for summer drying than is obtained in the experiments for increased CO₂. The results suggest that the 9000 yr BP climate may be of limited utility as an analog to future warm climates.     

Climatic Changes, Successions of Peatlands and Zonal Vegetation, and Peat Accumulation Dynamics in the Holocene (the West-Siberia Peat Profile ‘Vodorasdel’)

The developmental history of peatland and dry land vegetation within the Ob-Vasugan watershed of Western Siberia was characterized according to features of the plant communities and climatic changes which were revealed by stratigraphic, spore-pollen and C¹⁴ (carbon) data obtained from a vertical peat profile Vodorasdel. Changes in the paleoecological environment over the last 10000 years were divided into five periods. The climate was characterized in the Holocene according to these periods. At the watershed studied, peatland-forming processes started about 9510 years ago resulting in 550 cm of peat accumulation. The rate of peat accumulation within the watershed decreased over time from 1.9–0.3 mm year^{– 1}.     

Vertical distribution of dimethylsulfide,sulfur dioxide,aerosol ions,and radon over the Northeast Pacific Ocean

Dimethylsulfide (DMS), sulfur dioxide (SO₂), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO₄ ^2–), sodium (Na⁺), ammonium (NH₄ ⁺), and nitrate (NO₃ ^–) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3–12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4–8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3–1.2 and 0.25–0.31 nmol m^–3 (STP) respectively in the mixed layer, decreasing to 0.01–0.12 and 0.03–0.13 nmol m^–3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period.The atmospheric SO₂ concentrations always increased with altitude from <0.16–0.25 to 0.44–1.31 nmol m^–3 (STP). The nonsea-salt sulfate (ns-SO₄ ^2–) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO₂ and nss-SO₄ ^2– from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a sea-salt inversion layer was reflected in the profiles of those aerosol components, e.g., Na⁺ and NO₃ ^–, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently remote northern hemispheric regions.     

Quantitative estimates of full glacial temperatures in equatorial Africa from palynological data*

This paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial Africa (3°28S, 29°34E, 2240 m). It has been achieved with an extended modern African reference data set by adding spectra from 228 new sites and using another selection for pollen taxa. The purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues. The results are in good agreement with those previously published, reinforcing the validity of the method. The mean standard error is reduced by 0.3°C. The mean temperature for the Holocene appears + 1.4°C warmer than the present and the last glacial maximum (25-18 kyrs BP) cooling is better specified at – 3 ± 1.9° C, a conservative value, more consistent with reconstructed sea surface temperature in the equatorial ocean.Contribution to Clima Locarno - Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

Chrysophyte cysts from lake sediments reveal the submillennial winter/spring climate variability in the northwestern Mediterranean region throughout the Holocene

In the last decade, much effort was dedicated to the reconstruction of past climate at high temporal resolution. Here, we show the suitability of chrysophyte cysts from lake sediments for revealing continental climate variability when used in sensitive sites, such as those in high mountains. We demonstrate that altitude is a main factor influencing the present distribution of chrysophytes and develop a transfer function to evaluate the local altitude anomaly on a lake site throughout time. Based on our knowledge of chrysophyte ecology, the altitude anomalies are interpreted as winter/spring climate signatures. The method was applied to a Holocene record from a lake in the Pyrenees showing submillennial climatic variability in this northwestern Mediterranean zone. A warming trend was present from the early Holocene to 4 kyear BP. Comparison with pollen-based reconstructions of summer temperatures denoted a contrasting decrease in continentality between the two parts of the Holocene. Oscillations of 1 cycle per ca. 2,000 years appeared throughout the record. The warmest Holocene winters were recorded during the Medieval Warm Period at ca. AD 900 and 450 and the Roman Warm Period (2.7–2.4 kyear BP). Winters in the period AD 1,050–1,175 were inferred to be as cold as in the Little Ice Age. The period between 3 and 7 kyear BP showed lower intensity in the fluctuations than in early and late Holocene. The cold event, 8,200 years ago, appeared embedded in a warm fluctuation. Another cold fluctuation was recorded around 9 kyear BP, which is in agreement with Irish and Greenland records.This revised version was published online in January 2005 with corrections to the background of figures 9 and 12.