Search

This paper describes the flood risks faced by Surat, one of India’s most successful and also most flood-prone cities. The city is located on the Tapi River and faces flood risks not only from heavy precipitation in and around the city but also from heavy precipitation upstream and from high tides downstream. Reducing the risks from upstream depends on better water management in a water catchment area and dam reservoir located far outside the city authority’s jurisdiction and in another state. The paper also reviews measures being taken to reduce flood risks – and how climate change is likely to affect such risks. It suggests that part of the city’s response needs to be a greater ability to live with floods, while minimizing the costs these usually bring in terms of loss of life, damage to homes and disruption to businesses. [ABSTRACT FROM PUBLISHER]

Over recent decades, analyses of the structure and impact of atmospheric teleconnections have substantially increased our understanding of the climate system and the role of climate variability. Moving beyond simple correlations between teleconnection indices and temperature and precipitation anomalies, synoptic climatology has been able to provide insight on the spatiotemporal manifestation of teleconnection anomalies, as well as further understanding in terms of teleconnection-related anomalies in circulation pattern frequencies that can lead to extreme events over individual areas. In this progress report, we focus on a number of recent papers that, broadly defined, assess two realms of teleconnections: the North Atlantic circulation, largely focusing on the North Atlantic Oscillation; and the north and tropical Pacific circulation, as manifest in El Niño–Southern Oscillation, the Pacific North American pattern, and the Pacific Decadal Oscillation. In each of these two regions, we highlight the major goals and results of recent synoptic research. [ABSTRACT FROM PUBLISHER]

This paper adopts a technique common in the dynamical climate modelling literature, that of pattern scaling, and applies it to previously available statistically downscaled station level data for Ireland for two climatically relevant variables, that of temperature and precipitation. This technique allows for the rapid development of climate scenarios for additional emissions scenarios not previously available from the GCM modelling centres. Having derived the end of century (2080s) change in both these variables for four marker emissions scenarios (A1FI, A2, B2, B1), regional response rates, or the regional rate of warming per °C global warming at each station, were calculated. The estimated ranges in regional responses at each station were then compared to regional response rates for the Irish ‘grid box’ derived from a larger sample of 14 GCMs, in order to determine if the calculated response rates were illustrative of a wider suite of GCMs. A Monte Carlo (MC) resampling approach was then employed to sample regional response rates for selected stations and for different estimates of future warming. On the basis of the MC approach, probability distribution functions (pdfs) of simulated changes in temperature and precipitation were constructed and compared to the original statistically downscaled data. The methodology and results presented represent a significant contribution to the traditional approach of statistical downscaling through the development of associated likelihoods, rather than just a change in the mean value. While the methodology presented should enable the rapid development of probabilistic based climate projections, based on a limited availability of downscaled climate scenarios, caution needs to be exercised in the interpretation of the results. While they provide a basis for risk or policy assessment, estimates of the level of risk are not independent of the method employed. [ABSTRACT FROM AUTHOR]

Flooding is the most common natural hazard and third most damaging globally after storms and earthquakes. Anthropogenic climate change is expected to increase flood risk through more frequent heavy precipitation, increased catchment wetness and sea level rise. This paper reviews steps being taken by actors at international, national, regional and community levels to adapt to flood risk from tidal, fluvial, surface and groundwater sources. We refer to existing inventories, national and sectoral adaptation plans, flood inquiries, building and planning codes, city plans, research literature and international policy reviews. We distinguish between the enabling environment for adaptation and specific implementing measures to manage flood risk. Enabling includes routine monitoring, flood forecasting, data exchange, institutional reform, bridging organizations, contingency planning for disasters, insurance and legal incentives to reduce vulnerability. All such activities are ‘low regret’ in that they yield benefits regardless of the climate scenario but are not cost-free. Implementing includes climate safety factors for new build, upgrading resistance and resilience of existing infrastructure, modifying operating rules, development control, flood forecasting, temporary and permanent retreat from hazardous areas, periodic review and adaptive management. We identify evidence of both types of adaptation following the catastrophic 2010/11 flooding in Victoria, Australia. However, significant challenges remain for managing transboundary flood risk (at all scales), protecting existing property at risk from flooding, and ensuring equitable outcomes in terms of risk reduction for all. Adaptive management also raises questions about the wider preparedness of society to systematically monitor and respond to evolving flood risks and vulnerabilities. [ABSTRACT FROM AUTHOR]

Palaeoproxy records alone are seldom sufficient to provide a full assessment of regional palaeoclimates. To better understand the possible changes in the Mediterranean climate during the Holocene, a series of palaeoclimate integrations for periods spanning the last 12 000 years have been performed and their results diagnosed. These simulations use the HadSM3 global climate model, which is then dynamically downscaled to approximately 50 km using a consistent regional climate model (HadRM3). Changes in the model’s seasonal-mean surface air temperatures and precipitation are discussed at both global and regional scales, along with the physical mechanisms underlying the changes. It is shown that the global model reproduces many of the large-scale features of the mid-Holocene climate (consistent with previous studies) and that the results suggest that many areas within the Mediterranean region were wetter during winter with a stronger seasonal cycle of surface air temperatures during the early Holocene. This precipitation signal in the regional model is strongest in the in the northeast Mediterranean (near Turkey), consistent with low-level wind patterns and earlier palaeosyntheses. It is, however, suggested that further work is required to fully understand the changes in the winter circulation patterns over the Mediterranean region. [ABSTRACT FROM AUTHOR]

This paper provides a synthesis of available palynological data from central Mediterranean lacustrine records, studying the causes of the important vegetation changes which occurred in the mid Holocene. They are illustrated by seven well-dated records, lakes Accesa, Mezzano, Vico, and Pergusa in Italy, Maliq, Voulkaria and Malo J. in the Balkans. A good tool for disentangling climate change and human impact is the combined interpretation of pollen percentage and concentration/influx data. Pollen concentration/influx drops are an indication of increased erosion in the catchment, mainly ascribable to forest reduction. There is no doubt that major synchronous changes could not have been caused by humans alone. Many southern European records show substantial differences between early- and late-Holocene vegetation, suggesting a general evolution from wetter to drier climatic conditions, in agreement with arid phases recognised by other, independent palaeoclimatic methods. Other pollen records show a different trend. The role of changing seasonality seems important for this region of the Mediterranean. Precipitation seasonality increased during the early to mid Holocene with winter precipitation attaining a maximum, and summer precipitation a minimum. At least three rapid climate events with changes in plant biomass are in evidence: an abrupt and short change around 8200 yr BP, another one centred around 6000 yr BP, and one soon after 3000 yr BP. From the beginning of the Bronze Age (c. 4400 yr BP in this region) human impact overlapped with a climate change, probably bipartite, towards dryness. Our results show that this aridification trend began around 8000 yr BP, and culminated around 4000 yr BP. [ABSTRACT FROM AUTHOR]

Postglacial expansion of deciduous oak woodlands of the Zagros-Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros-Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros-Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions. [ABSTRACT FROM AUTHOR]

Saunders KM, Grosjean M, Hodgson DA (2013) A 950 yr temperature reconstruction from Duckhole Lake, southern Tasmania, Australia. The Holocene 23(6) 771–783. DOI: 10.1177/0959683612470176.Page 771: AbstractThe calibration model was based on a negative, not positive, relationship between trough area (R570/630) and temperature. The corrected model is: R = −0.56, pauto < 0.05, root mean squared error of prediction (RMSEP) = 0.21°C, five-year filtered data, calibration period 1911–2000.Page 774 ‘Reflectance spectroscopy’The correlations in this paragraph are negative (not positive as stated in the original paper). The paragraph should read as follows:VIS-RS data are summarized in Figure 3. R570/630 and R660/670 fluctuated throughout the core with a slight overall decreasing trend towards the surface. R570/630 was positively correlated to R660/670 (R = 0.5, p < 0.05). R650;700 and RABD660;670 were highly significantly correlated (R = −0.85, p < 0.05). R650;700 was weakly correlated to R660/670 (R = 0.36). R650;700, RABD660;670 and R660/670 were significantly correlated with water content (R = −0.83, 0.60 and –0.53, respectively, p < 0.05). Once corrected for water content, the negative correlation between R650;700 and R660/670 increased (R = −0.41, p < 0.05), while there was no longer a correlation between RABD660;670 and R660/670.Page 775 ‘Calibration with meteorological data’The R values referred to should be negative:All VIS-RS-derived proxies (R650;700, RABD660;670, R660/670 and R570/630) were calibrated to monthly and annual temperature (mean, maximum and minimum) and precipitation (total) for the period AD 1911–2000. R650;700 was most highly and significantly correlated to mean annual (January–December) temperature (R = −0.84, pauto < 0.05, pauto after detrending < 0.05).The correlation between R650;700 and annual temperature was highly significant after applying a five-year triangular filter (non-detrended: R = −0.85, pauto < 0.001; Figure 4a) and remained significant when detrended (R = −0.29, pauto < 0.05; Figure 4b).Consequently, as R650;700 (reflecting chlorophylls and derivatives) correlated best to temperature, it was used to develop an annual temperature calibration model for Duckhole Lake: R = −0.56, pauto < 0.05, RMSEP = 0.21°C, where both R and p were adjusted for autocorrelation.Page 778 Figure 4The y axes of the lower panels in 4(a) and 4(b) were not correct. The corrected figure is above: Comparison of R650;700 and annual mean temperature: (a) non-detrended R650;700 and mean annual temperature; (b) detrended data. All data are five-year triangular filtered. Note: reversed y axes in the lower panels (proxy data) of (a) and (b). [ABSTRACT FROM AUTHOR]

Holocene fluctuations of a small outlet glacier from the ice cap Høgtuvbreen at 65° N in coastal northern Norway are reconstructed based on distal glacier-fed lake sediments, complemented by a moraine sequence dated by lichenometry. Glaciers respond to changes in accumulation-season precipitation, ablation-season temperature and redistribution of snow by wind. Hence, reconstructions of glacier fluctuations based on distal glacier-fed lakes may give detailed information about past climate at a potentially high temporal resolution. Yet, the importance of any of these climate components is often difficult to solve. Here, we apply the ‘Liestøl-relationship’, which expresses the relationship between ablation-season temperature and annual accumulation of snow at the equilibrium line altitude (ELA), to the reconstructed local temperature–precipitation–wind ELA (TPW-ELA) to infer the relative importance of winter-balance and ablation-season temperature as causes of reconstructed glacier variation. The reconstructions show a large glacier readvance corresponding with the 8.2-ka cold event and a sequence of eight distinct glacier advances and retreats during the Neoglacial time period bracket between 4300 ± 40 cal. yr BP and AD 1900. The glacier reached its Holocene maximum position in AD 1773 ± 29, subsequently followed by an ongoing unprecedented retreat, interrupted only by some minor halts and readvances. Based on a detailed comparison of our results with similar studies of both continental and maritime glaciers, as well as independent temperature proxy records across Scandinavia, we argue that significant and consistent deviations in ELA fluctuations between continental and maritime glaciers in the region are caused by a north–south migration of the arctic polar front. Additionally, we suggest that deviations in ELA fluctuations between Scandinavian maritime and continental glaciers around 7150, 6560, 6000, 5150, 3200 and 2200 cal. yr BP reflect the different response of continental and maritime glaciers to drops in total solar irradiance (TSI). [ABSTRACT FROM AUTHOR]

At a global scale, there is no evidence for synchronous multi-decadal warm (‘Medieval Warm Period’, MWP) or cold (‘Little Ice Age’, LIA) periods in the late Holocene. On the other hand, there is good correspondence globally in the timing of MWP or LIA and phases of glacier retreat and advance, respectively, with local exceptions mainly explained by the precipitation regime. Antarctica exhibits contrasting patterns, both regarding the existence of these two historical climatic periods and the glacial responses to climatic forcing. Here, we present evidence for glacial retreat corresponding to the MWP and a subsequent LIA advance at Rothera Point (67°34′S; 68°07′W) in Marguerite Bay, western Antarctic Peninsula. Deglaciation started at ca. 961–800 cal. yr BP or before, reaching a position similar to or even more withdrawn than the current state, with the subsequent period of glacial advance commencing between 671 and 558 cal. yr BP and continuing at least until 490–317 cal. yr BP. Based on new radiocarbon dates, during the MWP, the rate of glacier retreat was 1.6 m yr−1, which is comparable with recently observed rates (~0.6 m yr−1 between 1993 and 2011 and 1.4 m yr−1 between 2005 and 2011). Moreover, despite the recent air warming rate being higher, the glacial retreat rate during the MWP was similar to the present, suggesting that increased snow accumulation in recent decades may have counterbalanced the higher warming rate. [ABSTRACT FROM AUTHOR]

It is currently suggested that climate change on the northeastern Tibetan Plateau (TP) was influenced alternately by the monsoon and the Westerlies. However, the mechanisms driving Holocene climate change on the TP remain unclear, since the extent of the influence of individual atmospheric circulation systems has not yet been clearly defined because of the shortage of high-quality paleoclimatic records. This is especially true in the central TP, where only a few ice core and paleolimnological records are available. Here, we present a decadal-resolution temperature record from Dagze Co in the central TP for the past 2000 years, based on the unsaturation index of long-chain alkenones, using an updated temperature calibration, and a record of precipitation isotopes from compound-specific isotope ratios of leaf waxes. The centennial-scale variation of the temperature and precipitation isotope records captures well-known climatic events over the past 1000 years, for example, the ‘Little Ice Age’, which was cooler and drier than the ‘Medieval Warm Period’. However, the relationship between temperature and the precipitation isotope records differed during the interval at 2000–1000 cal. yr BP compared to the past 1000 years, probably because of changes in precipitation seasonality and the additional influence of the Westerlies on the central TP. In addition, the temperature records exhibit a prominent 210-year cyclicity, suggesting a possible influence of solar radiation on temperature variability. [ABSTRACT FROM AUTHOR]

Atmospheric circulation in the oceans surrounding southern Africa plays an important role in determining its precipitation. This study uses wind information recorded in ships’ logbooks in order to statistically reconstruct summer and winter season precipitation at four southern African weather stations from 1796 to 1854. The reconstruction was obtained by first relating gridded 8° × 8° NCEP-DOE reanalysis seasonal mean wind vectors in the adjacent oceans to station precipitation. Over a 30-year calibration period (1979–2008), significant correlations between wind and precipitation at Cape Town, Mthatha and Royal National Park showed particular correspondence with those areas with the greatest concentration of logbook observations. Principal component regression was used to assess the potential of the dominant patterns of variability in the wind vectors as predictors to reconstruct precipitation. Cross-validation in the calibration period gave confidence that precipitation could be reconstructed at several stations across South Africa, meaning the regression relationships derived in the calibration period could be applied to the gridded seasonal mean logbook data to produce reconstructions of precipitation from 1796 to 1854. The reconstructions show a degree of correspondence with other regional data sets. For instance, the decade beginning in 1810 was the wettest of the period at Mthatha and Royal National Park, while the 1820s were the driest. At Cape Town, the 1820s were the wettest decade, with drier conditions observed in the 1830s. An index of west–east circulation in the summer season revealed correspondence with two documentary reconstructions of El Niño events and increased westerliness, although this did not always result in drier conditions. Attention is also drawn to the remaining 3000 yet to be digitised English East India Company logbooks which would provide a high-resolution picture of atmospheric circulation back to 1700 in the region under consideration. [ABSTRACT FROM PUBLISHER]

The relation between climate change and historical rhythms has long been discussed; however, this type of study still faces the lack of high-resolution data of long-term socio-economic processes. In this study, we collected 1101 items of direct and proffered evidence from 24 Chinese fiscal and economic history books written by leading contemporary scholars. By analysing the semantics of words, we reconstructed a 2130-year-long fiscal sequence with decadal resolution to express the phase transition of fiscal soundness for approximately 21 dynasties in China. Our reconstruction shows four relatively sufficient stages and three relatively deficient stages, with several secondary stages lasting 60 or 160 years on average within each large stage, from 220 bc to ad 1910. Then, the fiscal association with climate change was analysed. The results show that the fiscal crisis was more likely to occur in cold-dry climatic scenarios. Both temperature and precipitation displayed more significant effects on the fiscal fluctuation within the long term, particularly for temperature. In short, fiscal decline had a 10-year lag in response to decreasing precipitation, whereas influence of decreasing temperature on the fiscal balance might be through amplifying already deteriorating fiscal conditions. Regional differences existed in the influence of precipitation on the fiscal balance. The precipitation in the Northern China Plain and in the Jiang-Huai area had more significant effects on the fluctuation of the dynastic fiscal balance than on that in the Jiang-Nan area. In particular, the dynastic finance was more sensitive to the change in the precipitation in the Northern China Plain because of its special geographical, economic and social backgrounds. [ABSTRACT FROM AUTHOR]

This study empirically explores climate change-related impacts on rice farming by using global circulation models together with crop modelling. The approach uses a bottom-up strategy which focuses on agricultural production loss under various climatic conditions following observational records of precipitation and warming climatic factors up to the year 2090. The study provides a wide range of potential climate outcomes and assesses the comparative advantages of both mitigation and adaptation alternatives concerning prioritizing requirements to reduce scenario vulnerabilities in rice agriculture. Utilizing scenario vulnerabilities, this study visualizes alternative guidelines and some of the potential paths policymakers could take with their planning strategies. The issue considered in the present study could be used as a reference for developing climate change and global warming-related agricultural policies in Malaysia and other places with similar ecological conditions. [ABSTRACT FROM AUTHOR]

The mean precipitation fields for eastern South America from eight mid-Holocene (6 ka) simulations, part of the third phase of Palaeoclimate Modeling Intercomparison Project (PMIP3) and the fifth phase of the Coupled Model Intercomparison Project (CMIP5), are evaluated. These simulations were compared to a new multiproxy compilation of 120 previously published records of changes in South American paleoclimate. Results show that when compared with modern conditions, mid-Holocene proxy data point to a drier Southern Brazil and South Atlantic Convergence Zone (SACZ), but a wetter/similar Northeastern Brazil. This suggests a weaker South American Monsoon System during the mid-Holocene when compared with its modern strength. All analyzed model simulations indicate a similar pattern, with a southward shift of the Intertropical Convergence Zone during the mid-Holocene, related to a weaker South Atlantic subtropical high, and negative annual precipitation anomalies over the SACZ area. Regional differences between the analyzed models were clearly detected. [ABSTRACT FROM AUTHOR]

Our pollen-based climate reconstruction shows that both temperature and moisture were low during the early Holocene from ~10,000 to ~8000 cal. a BP and gradually climbed from ~8000 to ~6000 cal. a BP. The last ~6000 years have been generally warm and wet with high-amplitude fluctuations. The high-moisture conditions for the past ~6000 years are not only corroborated by high arboreal pollen/non-arboreal (AP/NAP; trees and shrubs over herbs) pollen ratio and high pollen concentration but also by high organic matter content and diatom data. We suggest that lower winter insolation and remaining ice covers in the early Holocene might have slowed down temperature rise in the continental interiors and that the resulted low temperature might have suppressed regional evaporation that was a major water vapor source for precipitation. At the same time, icemelt-water injection into the North Atlantic Ocean might have also suppressed water vapor supplies to the downwind areas, including the northern Mongolian Plateau. The increase in winter insolation and the final melting of ice sheets, together with the increased atmospheric CO2 concentration and the associated increase in atmospheric water vapor, could be responsible for the higher temperature during the past ~6000 years. The icemelting and the associated rise of sea level during the past ~6000 years might be responsible for the weakening of the winter monsoon in the interior of Asian continent, thus alluring farther eastward penetration of the relatively warm and wet Atlantic air masses and consequently bringing more precipitation to the northern Mongolian Plateau. [ABSTRACT FROM AUTHOR]

This study focuses on the reconstructions of vegetation and climate changes during the past ~11,000 years on the basis of 222 fossil pollen samples from Qigai Nuur (core QGN-2004) in the southern part of the Mongolian Plateau. Plant Functional Type – Modern Analogue Technique was used for quantitative climatic reconstructions from fossil pollen data. Our results showed that regional vegetation experienced dramatic shifts between steppe forest and dry steppe before 9200 cal. yr BP in the Qigai Nuur region. The reconstructed mean annual temperature and especially mean annual precipitation also showed fluctuations at that time. From ~9200 to ~2800 cal. yr BP, the vegetation was dominated by steppe forests, and the reconstructed climate was generally warm and wet, but with two cold and dry intervals at ~7400 to ~6000 cal. yr BP and at ~4000 to ~2800 cal. yr BP. From ~2800 to ~850 cal. yr BP, dry steppe vegetation dominated the landscape under slightly cooler and much drier climate conditions. During the past ~850 years, the vegetation was characterized by abundant herbs and the climate showed rising temperature and fluctuating precipitation. The comparison of the reconstructed precipitation from the Qigai Nuur core with monsoon-strength records from southern China shows that the first-order variation in the precipitation was most likely controlled by the summer insolation-dictated Inter-Tropical Convergence Zone movements (i.e. N–S shifts). The comparison of the reconstructed precipitation from Qigai Nuur core with surface sea temperature in the Western Tropical Pacific Ocean suggests that the second-order variation in the precipitation was most likely modulated by the El Niño-Southern Oscillation dynamics (i.e. E–W shifts). [ABSTRACT FROM PUBLISHER]

Understanding the mid-Holocene dynamics of the East Asian monsoon (EAM) is integral to improving models of the Holocene development of the global climate system. Here we reconstruct the mid-Holocene EAM history from the Pearl River estuary, southern China, using bulk organic carbon isotopes (δ13C), total carbon to total nitrogen (C/N) ratios and total organic carbon (TOC) concentration. Sedimentary δ13C, C/N and TOC are potentially good indicators of changes in monsoonal precipitation strength. Sediments buried during a period of high precipitation exhibit a high proportion of terrigenous material, and have low δ13C and high C/N, and vice versa during a period of low precipitation. Results suggest a general decreasing trend in monsoonal precipitation from 6650 to 2150 cal. yr BP because of the weakening Northern Hemisphere insolation most likely related to the current precession circle. Superimposed on this trend are apparent dry–wet oscillations at centennial to millennial timescales most likely in response to solar activity. Mismatches between our δ13C record and results from the Dongge Cave in southern China at millennial timescales may indicate that the δ13C from the Pearl River estuary reveals changes in precipitation over a broader area than the δ18O from Dongge Cave. [ABSTRACT FROM AUTHOR]

Based on published pollen data from 44 sites in the Loess Plateau and surrounding areas, we recalculate values of Picea abundance digitized from the original diagrams and construct six isopoll maps to reconstruct the distribution of spruce forest at 2000 yr intervals during the Holocene. Spruce forest gradually expanded from about 10 ka BP ago and reached its peak distribution around 8 ka BP. It retained its maximum extent between 8 ka BP and 6 ka BP, broadly covering the subalpine area and stretching to the valleys of the western Loess Plateau with high densities and even reaching the edges of the adjacent desert regions. After 6 ka BP, spruce forest began to retreat to higher elevations. This trend was accelerated after 4 ka BP. After 2 ka BP, it disappeared completely from the Loess Plateau and most of its surrounding areas. Precipitation strongly dominated by the Asian summer monsoon was the primary controlling factor in the distribution of spruce before 2 ka BP. After about 2 ka BP, the increasing intensity of human activity became the primary factor in the disappearance of spruce in the Loess Plateau, resulting in an enormous impact on the present landscape. [ABSTRACT FROM AUTHOR]

We used a network of eight pedunculate oak (Quercus robur L.) sites (ntrees = 70) and one Scots pine (Pinus sylvestris L.) site (ntrees = 53) to develop drought-sensitive master chronologies for the two areas in southern Scandinavia: a SW-area centred on 57°N 12.7°E and a NE-area centred on 58.8°N 18.2°E. The ratio of actual to equilibrium evapotranspiration (AET/EET) was used as a measure of drought during the growing season defined as the period with average daily temperatures above 9°C. Instrumental data were used to parameterize the relationship between tree-ring data and the drought index (DI) over 1922—2000 for the SW area and over 1922—1995 for the NE area. The DI reconstructions explained 29.7% (SW area) and 43.7% (NE area) of the variance in the observed DI index in the calibration period, and were extended back to AD 1770 for the SW area and to AD 1750 for the NE area. Reconstructed drought dynamics suggested strong decadal- and century-scale temporal variability and limited regional synchronicity over 1770—2000. Large variations in DI were observed in both regions in the second half of the 1700s. Dry conditions were synchronously reconstructed in both sub-regions during 1781—1784, 1853—1855, and, to a lesser degree, during 1974—1978. Over the 1945—1975 period the SW area exhibited a trend towards drier growing seasons, whereas no such trend could be identified for the NE area. Analysis of correlation maps indicated that regional DI dynamics reflected two different climate regimes, associated with the Kattegat area (SW reconstruction) and southeastern Swedish coast of the Baltic sea (NE reconstruction). [ABSTRACT FROM AUTHOR]

Past seasonality changes are often poorly represented by Quaternary proxies because one season, or one factor, dominates the reconstructed signal. During the early Holocene in New Zealand, mean annual temperatures were at least 1.5°C warmer than present. However, treelines were lower, suggesting summers were cooler. Here we use a forest ecosystem process model, LINKNZ, to explore past precipitation and temperature seasonality changes in an intermontane basin of the Southern Alps, New Zealand. Pollen and macrofossils from the basin show that during an early-Holocene warm event (11 500 to 9500 cal. yr BP) podocarp and broadleaved species dominated. In exploratory modelling runs, mean annual temperatures were increased by up to 2°C, precipitation was reduced by 20—30%, and temperature seasonality reduced. When mean annual temperature was increased by 1.0—2.0°C, LINKNZ reconstructed wet forest associations, very different to those in the early-Holocene fossil assemblages. Acceptable matches were made with the early-Holocene vegetation using elevated temperature scenarios with up to 30% lower annual precipitation and decreased temperature seasonality. Longer growing seasons apparently compensate for cooler summer temperatures. We suggest that during the early Holocene in this area, westerly wind flow over the Southern Alps to the west of the basin was less, reducing spill-over rainfall and vapour pressure deficits. Warm oceans generated milder, cloudier climates reducing seasonal contrasts. Inverse modelling is recommended as a tool for exploring past climate scenarios when proxies fail to reconstruct important climate variables. [ABSTRACT FROM AUTHOR]

This study presents pollen-based climate reconstructions of Holocene temperature and precipitation seasonality for two high-resolution pollen sequences from the central (Lake Accesa, central Italy) and eastern Mediterranean (Tenaghi Philippon, Greece) regions. The quantitative climate reconstruction uses multiple methods to provide an improved assessment of the uncertainties involved in palaeoclimate reconstructions. The multimethod approach comprises Partial Least Squares regression, Weighted Average Partial Least Squares regression, the Modern Analogues Technique, and the Non-Metric-Multidimensional Scaling/Generalized Additive Model method. We find two distinct climate intervals during the Holocene. The first is a moist period from 9500 to 7800 cal. BP characterised by wet winters and dry summers, resulting in a strongly seasonal hydrological contrast (stronger than today) that is interrupted by a short-lived event around 8200 cal. BP. This event is characterised by wet winters and summers at Accesa whereas at Tenaghi Philippon the signal is stronger, reversing the established seasonal pattern, with dry winters and wet summers. The second interval represents a later aridification phase, with a reduced seasonal contrast and lower overall precipitation, lasting from 7800 to 5000 cal. BP. Present-day Mediterranean conditions were established between 2500 and 2000 cal. BP. Many studies show the Holocene to have a complex pattern of climatic change across the Mediterranean regions. Our results confirm the traditional understanding of an evolution from wetter (early Holocene) to drier climatic conditions (late Holocene), but highlight the role of changing seasonality during this time. Our data yield new insights into the aspect of seasonality changes, and explain the apparent discrepancies between the previously available climate information based on pollen, lake-levels and isotopes by invoking changes in precipitation seasonality. [ABSTRACT FROM AUTHOR]

The study of Caribbean climate pre-1990 focused almost exclusively on attempts to link spatial patterns in climatic variables to physical processes. Much of this research assumed a 'simple' regional climate, warm year round with a wet season dominated by tropical cyclones, but researchers soon found that a precipitation regionalization of the Caribbean was not as straightforward and simple. Consequently, a satisfactory understanding of the regional precipitation climate has eluded researchers for much of the second half of the twentieth century. Recently, with the increased availability and quality of satellite and precipitation data, researchers have begun to use gridded data sets to identify the spatial boundaries of the bimodal precipitation region and the atmospheric processes associated with the two maxima and minimum in precipitation. The findings of these most recent studies can be combined to construct a five part (North Atlantic high pressure, low level Caribbean jet, subsidence caused by Central America convection, basin wide increased wind shear, and divergence around Jamaica) conceptual Caribbean precipitation model that begins to address spatial variability in the bimodal structure of annual rainfall and the development of the midsummer minimum in precipitation. Such a regional precipitation climate model provides hypotheses to be tested and investigated in future research. Further, researchers must work towards a more effective and clear communication of the bimodal nature of Caribbean precipitation and the associated summer decrease in precipitation, integrate upper air analysis into the current working hypotheses, and further examine the interannual to interdecadal variability of the Caribbean midsummer drought for prediction purposes. [ABSTRACT FROM AUTHOR]

A detailed understanding of decadal to millennial-scale climate changes requires seasonal-scale (summer-winter) reconstructions of past precipitation and temperature fluctuations. Comparing seasonally resolved varve records with pollen-based sum of growing degree-days (GDD) reconstructions from Lake Nautajärvi, we examined the intra-annual nature of climate variability in central southern Finland during the Holocene. The organic varve record and the GDD reconstruction show roughly comparable trends supporting the interpretation that both proxies predominantly reflect summer temperatures in the study area. The records suggest low but rising early-Holocene (9500 to 8500 cal. yr BP) summer temperatures. The Holocene Thermal Maximum (HTM) in the GDD record dates to about 7500 to 4500 cal. yr BP, but the organic varve record along with reconstructed changes in vegetation composition, notably a peak of Tilia pollen percentages, indicate that during the HTM there was a trend towards a more continental climate with maximum mid-summer temperatures reached at 6500 to 4500 cal. yr BP. Both records reflect the start of the post-HTM cooling at about 4500 cal. yr BP, simultaneously with an increase of the amount of catchment erosion and mineral matter influx into the lake, suggesting gradually colder and/or longer winters with high net accumulation of snow. The organic varve record and the GDD record start to diverge at 2000 cal. yr BP, possibly owing to the human influence on catchment processes. The reconstructed mid-Holocene summer temperature peak deviates from the regional climate model outputs, which suggest highest summer temperatures during the early Holocene. [ABSTRACT FROM AUTHOR]

The oxygen and carbon isotope composition of sedimentary carbonates formed in Lake Hayq, a closed-basin lake in northern Ethiopia, suggests marked variation in the balance between precipitation and evaporation during the last 2000 years. Diatom stratigraphy shows that the lake remained deep enough to support planktonic taxa, and was never saline. The record suggests that rainfall was higher than that of the twentieth century during most of this time, especially during two short intervals centred on AD 700 and AD 1300. Climate was drier only around AD 800, and from AD 1750–1900. Similar, but slightly moister climate than today, with high interdecadal variability, prevailed from AD 800 to AD 1200, equivalent to the European ‘Mediaeval Warm Period’. A period of high effective precipitation followed, from AD 1200 to AD 1700, during the ‘Little Ice Age’. Pollen evidence suggests that increased forest cover from AD 1400 to AD 1750 was a response to higher rainfall; forest cover may have increased evaporative loss from the catchment, contributing to a subsequent trend of increasing lakewater δ18O values. The record shows similarities in the timing and direction of changes in proxy-climatic records from Lake Naivasha (Kenya) and from the Nile. [ABSTRACT FROM AUTHOR]

Compared with the southwest monsoon, the mechanisms and history of northeast monsoon (NEM) variability over South Asia are poorly known. The NEM nevertheless contributes >/50% of rainfall to semi-arid southeast India, and has underpinned the success, over the last 2000 years, of a widespread indigenous water resource management system based on rainwater harvesting and storage in man-made lakes. Based on a power spectrum analysis of 60- to 100-yr instrumental rainfall records, we show that NEM rainfall is both chaotic in the high-frequency time domain and spatially unpredictable, but that the sustainability of the agricultural water-harvesting infrastructure is tied to excess rain from NEM cyclones. Cyclonic activity has declined sharply since 1977. Based on twentieth-century analogues, we propose a qualitative model in which storminess decreases when NEM wind intensities increase and Eurasian winter temperatures are anomalously low. Extrapolating to the Holocene time frame, model expectations for the palaeorainfall record correlate with the independently established chronologies of Indian runoff-irrigation development and the ‘Mediaeval Warm Period’—‘Little Ice Age’ oscillations, and further suggest that NEM and SWM rainfall anomalies have covaried with the same sign from seasonal to millennial timescales. The fortunes of the reservoir system are therefore tangibly climate-driven. [ABSTRACT FROM AUTHOR]

A high-resolution pollen record, c. 50 yr/sample, from terminal lake sediments in the Shiyang River drainage basin on the present margin of the summer monsoon was used to reconstruct vegetation and climate history during the Holocene. Forest trees from mountainous areas of the drainage, including Sabina, Picea and Pinus, dominated pollen assemblages in the early Holocene (11.6–7.1 cal. ka). In the mid-Holocene (7.1–3.8 cal. ka) desert and steppe shrubs and herbs around the lake, including Nitraria, Poaceae, Compositae and Artemisia, were dominant. The late Holocene (3.8–0 cal. ka) was again dominated by alternation of Pinus–Sabina tree pollen and desert-steppe pollen. The early Holocene forest expansion in the mountains and subsequent increase in the river transport of tree pollen corresponds with maximum precipitation during the East Asian summer monsoon maximum. The timing of these changes in our record from arid China is different from that of East China, where the Holocene monsoon maximum appeared in the middle Holocene. This difference indicates that the extent and development of summer monsoon circulation in the Holocene was complex. Changes in the pollen record appear to show pervasive and persistent centennial- to millennial-scale oscillations throughout both wet and dry periods of the Holocene. Our results imply the continental interior was sensitive to changing moisture conditions and responsive to Holocene climatic events. [ABSTRACT FROM AUTHOR]

On Bylot Island, a field of tundra polygons at the margin of a glacial outwash plain contains a well-preserved syngenetic permafrost sequence of ground ice and alternating loess and organic layers that was accumulated during the late Holocene. Periods of increased deposition of loess alternated with periods of growth of bryophytes during the last 3500 years. These shifts in soil accretion regime are interpreted in terms of significant shifts of the summer surface wind conditions and active layer moisture regime (Precipitation–Evaporation or P–E), in response to regional climatic variations and recurrent changes of atmospheric circulation. There was a high level of variability and large amplitude of the P–E regime and summer surface wind conditions on a decennial and secular timescale in general. However, according to the Greenland GISP2 bi-decennial oxygen isotopes data, there was a low variability and amplitude (by a few degrees centigrade or less) of the regional mean annual air temperature. From 2950 to 2750 cal. BP, the summer climate was warmer and had the strongest and most frequent northwesterly surface winds of the late Holocene. Shifts to a weaker northwesterly summer surface wind activity preceded the dryer episodes that occurred from 2750 to 2450 and around 1850 cal. BP. Major wetter episodes occurred from 2450 to 2350, around 2050, from 1750 to 1550, from 1350 to 1150 and from 550 to 250 cal. BP. There is no clear relationship between P–E or summer surface wind regimes and air temperatures. Shifts of late Holocene summer aeolian regime can probably be better explained by the recurrence of particular synoptic circulation types in response to changes in the position of the atmospheric eastern Canadian Polar Trough. [ABSTRACT FROM AUTHOR]

Holocene sedimentary records from two lakes in White Pass, British Columbia, show that the onset of Neoglacial sedimentation began c. 2 ka cal. BP. Prior to this time, the lakes accumulated organic-rich sediments, with properties and sedimentation rates similar to adjacent lakes that have not had glacial influences during the Holocene. Neoglaciation at White Pass was coincident with the increased delivery of exotic Tsuga heterophylla (western hemlock) pollen. These results, together with regional indications for increased delivery of hemlock pollen at 2 ka BP by a deepened low pressure in the northeastern Pacific Ocean, suggest that increased moisture was important for the expansion of glaciers in the region in addition to longer-term late Holocene cooling. [ABSTRACT FROM AUTHOR]

Decadal to multidecadal fluctuations between drought and relatively more moist conditions are a defining characteristic of precipitation variability in the western USA. In an attempt to better understand the forcing mechanisms responsible for producing this variability, relationships between proxy indices of tropical climatic variability and precipitation-related reconstructions in the western USA are examined. The tropical proxies include the δ18O record from the Quelccaya ice cap (southern subtropical Andes) and the lacustrine sedimentation record from Laguna Pallcacocha (near-equatorial Andes). The precipitation-related records are tree-ring reconstructions of river flow, drought severity and precipitation over the contiguo USA. One new finding is a consistent relation between decadal variability in El Niño activity and precipitation-related proxies through much of the western USA during the past 1200 years. Surprisingly, the proxy records do not show a systematic link between inferred El Niño activity and precipitation in the interior Southwest USA. A second new result is the existence of relationships at decadal- to multidecadal timescales between fluctuations in Quelccaya δ18O, precipitation-related proxies from the western USA, and proxy records from the Pacific Basin. This finding links the Quelccaya record to the transhemispheric Pacific decadal-scale climatic variability documented from proxy records by Villalba et al. (2001), and suggests that similar changes in the large-scale circulation changes accompany both the decadal and multidecadal signals. A composite constructed using gridded proxy precipitation constructed on the basis of decadal-scale fluctuations in inferred-El Niño activity shows a pattern rather different to Niño-based teleconnection maps for the instrumental record. In the proxy-based composite, positive values cover much of the extreme western, west central and southeastern USA, with negative values in the central Mississippi Valley. The interior Southwest shows essentially no response. A second composite based on multidecadal variability in Quelccaya δ18O shows a well-developed dipole pattern across the western USA with opposing centres in the interior Southwest and Northwest. It is suggested that this dipole pattern results from multidecadal variability in tropical Pacific SSTs and resulting changes in the strength of the North-east Pacific High and the latitudinal distribution of cool season westerlies across the eastern North Pacific and western USA. [ABSTRACT FROM AUTHOR]

A pollen record is presented from Laguna de Aculeo (33°50′S, 70°55′W, 360m a.s.l.) that documents important vegetation changes over the last 2500 cal. yr in Central Chile. Grasses, composites, trees, paludal and aquatic taxa dominated the Aculeo watershed between 2500 and 100cal. yr BP under a humid climate. Large amplitude fluctuations of pollen and microalgal accumulation rates and numerous turbidite layers during this interval, however, suggest high precipitation variability probably linked to El Niño Southern Oscillation (ENSO) events. Although the expansion of warm sclerophyllous forest taxa over the last 100 years could be interpreted as an onset of a drier and warmer climate, this trend was more likely linked to human activities in the watershed. High accumulation rates of microscopic charcoal particles, exotic pollen taxa, and a shift from oligo-mesotrophic to hypereutrophic indicators would seem to back the latter hypothesis. [ABSTRACT FROM AUTHOR]