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In February 1963, a huge landslide (ca. 1,950,000 m3) blocked the Visočica River and, thus, formed Zavoj Lake. The primary objective of this research was to investigate the importance of snowmelt in relation to landslide occurrence and to define the critical climatic conditions that may trigger massive winter landslides. We used monthly precipitation and average monthly maximum temperature data from meteorological and precipitation stations in the Visočica River basin (Dojkinci) and in the immediate proximity of Lake Zavoj (Pirot, Dimitrovgrad and Topli Do) as data inputs to the Snow-Melt Landslide (SML) index. It considers the summed monthly precipitation for previous months that continuously have an average maximum temperature below 0 °C. According to this method, the event at Zavoj Lake stands out among all other precipitation and snowmelt values for the past 72 years. After applying the SML index, all stations showed values of >300 mm for February 1963, which we consider as the threshold value for potential landslides appearance. In addition to meteorological data, we applied the SML index to data from the Coordinated Regional Downscaling Experiment (CORDEX) regional climate model outputs for the region from 2022 to 2100. As expected, climate change will have influenced the temperature values, especially during the winter. Conversely, the study area is experiencing drastic changes in land use caused by depopulation, leading to a reduced risk of winter landslides in the Visočica basin. We suggest that future climatic conditions in the area will make it more likely to experience extreme summer precipitation events, which might trigger large landslides. The SML method can be implemented for all landscapes that experience snowy winters, providing information in a timely manner so that local residents can react properly when the probability of landslide occurrence rises. The SML index, grounded in essential meteorological principles, provides a tailor-made, data-driven methodology applicable across varied geographical settings. Its utility extends to mitigating hydro-meteorological hazards on scales ranging from local to national scales, offering diverse and effective early warning solutions. [ABSTRACT FROM AUTHOR]

Understanding how El Niño‐Southern Oscillation (ENSO) responds to natural variability is of key importance for future climate projections under a warming climate. However, there is no clear consensus on what drives ENSO's variability on centennial timescales. Here, we find that the epikarst in southeastern Alaska is effective at filtering ENSO and solar irradiance signals from the Aleutian Low regional climate, which are subsequently recorded in speleothem proxy data. By applying a correlation test, we find that ENSO was significantly influenced by solar irradiance over the past ∼3,500 years. This relationship dissolved after ∼1970 CE, with ENSO now being dominated by anthropogenic forcing. This implies a new ENSO mean state that will need to be incorporated into future climate projections. Plain Language Summary: Although El Niño‐Southern Oscillation (ENSO) is one of the most important climate phenomena globally, it is currently unknown how ENSO responds to natural variability on timescales over 100 years. Natural variability refers to periodic changes in climate due to either solar forcing, the circulation of the atmosphere and ocean, volcanic eruptions, and other factors, irrespective of human intervention. Here, we help solve this problem by studying deposits in caves (speleothems) from Alaska. The speleothems are excellent at capturing atmospheric conditions over the past 3,500 years, and show that solar forcing was significantly controlling ENSO variability. However, the speleothems also show that humans have altered this natural variability, with ENSO entering a new mean state after ∼1970 CE. Key Points: Speleothems from southeastern Alaska capture El Niño‐Southern Oscillation (ENSO) variability through a teleconnection with the Aleutian LowENSO variability was strongly influenced by solar forcing before ∼1970 CEENSO significantly changed properties at ∼1970 CE, likely due to anthropogenic forcing [ABSTRACT FROM AUTHOR]

The Red Sea is a deep marine basin often considered as small‐scale version of the global ocean. Hydrographic observations and ocean‐atmosphere modeling indicate Red Sea deep water was episodically renewed by wintertime open‐ocean deep convections during 1982–2001, suggesting a renewal time on the order of a decade. However, the long‐term pacing of Red Sea deep water renewals is largely uncertain. We use an annually resolved coral oxygen isotope record of winter surface water conditions to show that the late twentieth century deep water renewals were probably unusual in the context of the preceding ~100 years. More frequent major events are detected during the late Little Ice Age, particularly during the early nineteenth century characterized by large tropical volcanic eruptions. We conclude that Red Sea deep water renewal time is on the order of a decade up to a century, depending on the mean climatic conditions and large‐scale interannual climate forcing. Plain Language Summary: The Red Sea is a deep marine basin often considered as small‐scale version of the global ocean. Hydrographic observations and ocean‐atmosphere modeling indicate Red Sea deep water was episodically renewed by wintertime open‐ocean deep convections in the northern Red Sea during 1982–2001, suggesting a deep water renewal time on the order of a decade. However, the long‐term pacing of Red Sea deep water renewals is largely uncertain, due to lack of hydrographic observations. By using an annually resolved coral oxygen isotope record of winter surface water conditions in the northern Red Sea we show that the late twentieth century deep water renewals were probably unusual in the context of the preceding ~100 years. Our results suggest an absence of major deep water formation events until the 1883 Krakatau volcanic eruption. More frequent major events are detected during the late Little Ice Age, particularly during the early nineteenth century characterized by large tropical volcanic eruptions. From our long‐term perspective we conclude that Red Sea deep water renewal time is on the order of a decade up to a century, depending on the mean climatic conditions and large‐scale interannual climate forcing, which should be considered in management strategies of its unique ecosystems. Key Points: Observed frequent late twentieth century Red Sea deep water renewals were probably unusual in the context of the preceding ~100 yearsCoral proxy record suggests absence of major deep water formation events until the 1883 Krakatau volcanic eruptionMore frequent major Red Sea deep water formation events are detected during the early nineteenth century and late Little Ice Age [ABSTRACT FROM AUTHOR]

Abstract The increasing trend curve of global surface temperature against time since the 19th century is the icon for the considerable influence humans have on the climate since the industrialization. The discourse about the curve has spread from climate science to the public and political arenas in the 1990s and may be characterized by terms such as "hockey stick" or "global warming hiatus". Despite its discussion in the public and the searches for the impact of the warming in climate science, it is statistical science that puts numbers to the warming. Statistics has developed methods to quantify the warming trend and detect change points. Statistics serves to place error bars and other measures of uncertainty to the estimated trend parameters. Uncertainties are ubiquitous in all natural and life sciences, and error bars are an indispensable guide for the interpretation of any estimated curve—to assess, for example, whether global temperature really made a pause after the year 1998. Statistical trend estimation methods are well developed and include not only linear curves, but also change-points, accelerated increases, other nonlinear behavior, and nonparametric descriptions. State-of-the-art, computing-intensive simulation algorithms take into account the peculiar aspects of climate data, namely non-Gaussian distributional shape and autocorrelation. The reliability of such computer age statistical methods has been testified by Monte Carlo simulation methods using artificial data. The application of the state-of-the-art statistical methods to the GISTEMP time series of global surface temperature reveals an accelerated warming since the year 1974. It shows that a relative peak in warming for the years around World War II may not be a real feature but a product of inferior data quality for that time interval. Statistics also reveals that there is no basis to infer a global warming hiatus after the year 1998. The post-1998 hiatus only seems to exist, hidden behind large error bars, when considering data up to the year 2013. If the fit interval is extended to the year 2017, there is no significant hiatus. The researcher has the power to select the fit interval, which allows her or him to suppress certain fit solutions and favor other solutions. Power necessitates responsibility. The recommendation therefore is that interval selection should be objective and oriented on general principles. The application of statistical methods to data has also a moral aspect. [ABSTRACT FROM AUTHOR]

Speleothem δO records provide valuable information about past continental environmental and climatic conditions, although their interpretation is often not straightforward. Here we evaluate a compilation of late Holocene speleothem δO records using a Monte Carlo based Principal Component Analysis (MC-PCA) method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each δO record. The MC-PCA approach permits not only the identification of temporally coherent changes in speleothem δO; it also facilitates their graphical depiction and evaluation of their spatial coherency. The MC-PCA method was applied to 11 Holocene speleothem δO records that span most of the European continent (apart from the circum-Mediterranean region). We observe a common (shared) mode of speleothem δO variability that suggests millennial-scale coherency and cyclicity during the last 4.5 ka. These changes are likely caused by variability in atmospheric circulation akin to that associated with the North Atlantic Oscillation, reflecting meridionally shifted westerlies. We argue that these common large-scale variations in European speleothem δO records are in phase with changes in the North Atlantic Ocean circulation indicated by the vigour of the Iceland Scotland Overflow Water (ISOW), the strength of the subpolar gyre (SPG) and an ocean stacked North Atlantic ice rafted debris (IRD) index. Based on a recent modelling study, we conclude that these changes in the North Atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies. However, the mechanisms that ultimately force the westerlies remain unclear. [ABSTRACT FROM AUTHOR]

The Walker Circulation (WC) is an east-west trending band of atmospheric circulation cells along the equator and the predominant controller of heat and moisture transport in the tropics. Its variability is closely linked to the sea-surface temperature (SST) changes across the Pacific, the Indian and the Atlantic Oceans and can have pronounced effects on the humidity regimes of the adjacent continents. In recent years, the evolution of the WC during the Plio- and Pleistocene epochs has been intensely studied in the context of the effectiveness of the tropics in modulating global climate change (e.g., the intensification of Northern Hemisphere glaciation). However, the onset of the modern WC pattern as well as its global impact during the Plio- and Pleistocene is controversially assessed in the literature. For its onset, previous studies have suggested dates ranging between 2.4 and 0.8 million years ago (Myr), while its argued impact ranges from – crucially influencing the increase of Northern Hemisphere ice sheet growth by channelling heat and moisture from the tropics into the high latitudes to – having no effect on global ice volume changes. In order to achieve a comprehensive understanding of the spatiotemporal evolution of the WC during this time frame, we statistically analysed 30 globally distributed SST records covering the low and high latitudes between 3.5 and 1.5 Myr, encompassing the Late Pliocene to Early Pleistocene. We utilized a statistical change-point regression model to determine significant change points in the SST evolution of the (sub)-tropics and high latitudes that potentially relate to changes in the WC. We find that the WC experienced a multifaceted evolution between the Late Pliocene and the Early Pleistocene with significant transitional steps at ∼2.7 and ∼2.1 - Ma. Our results suggest after the Late Pliocene, a pre-modern WC set in, which was characterized by a progressively strengthened Pacific Walker Cell alongside a weakened Indian Ocean Walker Cell. This change was potentially triggered by the constriction of the Indonesian seaway, an important transmitter between the Pacific and Indian Ocean. The ensuing mode of the WC intensified until ∼2.1 Myr, when SST values around the global scale signalled a progressive strengthening of the Indian Walker Cell in phase with the progressive strengthening of the Pacific and Atlantic Cells. Our findings indicate that a shift from a pre-modern to a modern-like WC potentially only occurred during the mid-Pleistocene. • Crucial evolution steps of the Walker Circulation during the Plio-Pleistocene at 2.75 and 2.1 Ma. • Neither synchronous nor geographical uniform response of low latitudes to Walker Circulation changes. • Modern Walker Circulation emerged potentially after 1.5 Ma. • Gateway and insolation changes drove Walker Circulation changes during the Plio-Pleistocene. [ABSTRACT FROM AUTHOR]

One obstacle on the way to a comprehensive spatial reconstruction of regional temperature changes over the past centuries is the sparseness of long winter temperature records. This paper reconstructs a proxy record of April and November-December temperatures in south-central Finland for the interval from 1836 to 1872 from breakup and freeze-up dates and ice-cover duration of a lake. Emphasis is on detecting the suitable winter months and quantifying the calibrations with measured temperatures (1873-2002). The calibration slope for the breakup date (0.158°C/day) is larger than for freeze-up date (0.119°C/day) or duration (0.090°C/day). A comparison with results from other proxy records shows that the slope may depend also on the geographical site. Trend analyses of the full temperature records (1836-2002) indicate the existence of minor change-points at around 1867 (April temperature) and 1874 (November-December temperature), with warming rates thereafter of 1.67°C per century (April) and 1.16°C per century (November-December). Spectral analyses reveal peaks in the band between 2 and 5 year period, which may point to influences of the North Atlantic Oscillation, and less power in the decadal band (up to 42 year period). [ABSTRACT FROM AUTHOR]

Abstract: Two-sample experiments (paired or unpaired) are often used to analyze treatment effects in life and environmental sciences. Quantifying an effect can be achieved by estimating the difference in center of location between a treated and a control sample. In unpaired experiments, a shift in scale is also of interest. Non-normal data distributions can thereby impose a serious challenge for obtaining accurate confidence intervals for treatment effects. To study the effects of non-normality we analyzed robust and non-robust measures of treatment effects: differences of averages, medians, standard deviations, and normalized median absolute deviations in case of unpaired experiments, and average of differences and median of differences in case of paired experiments. A Monte Carlo study using bivariate lognormal distributions was carried out to evaluate coverage performances and lengths of four types of nonparametric bootstrap confidence intervals, namely normal, Student''s t, percentile, and BCa for the estimated measures. The robust measures produced smaller coverage errors than their non-robust counterparts. On the other hand, the robust versions gave average confidence interval lengths approximately 1.5 times larger. In unpaired experiments, BCa confidence intervals performed best, while in paired experiments, Student''s t was as good as BCa intervals. Monte Carlo results are discussed and recommendations on data sizes are presented. In an application to physiological source–sink manipulation experiments with sunflower, we quantify the effect of an increased or decreased source–sink ratio on the percentage of unfilled grains and the dry mass of a grain. In an application to laboratory experiments with wastewater, we quantify the disinfection effect of predatory microorganisms. The presented bootstrap method to compare two samples is broadly applicable to measured or modeled data from the entire range of environmental research and beyond. [Copyright &y& Elsevier]

Extreme river floods have been a substantial natural hazard in Europe over the past centuries, and radiative effects of recent anthropogenic changes in atmospheric composition are expected to cause climate changes, especially enhancement of the hydrological cycle, leading to an increased flood risk. For the past few decades, however, observations from Europe do not show a clear increase in flood occurrence rate. Here we present longer-term records of winter and summer floods in two of the largest rivers in central Europe, the Elbe and Oder rivers. For the past 80 to 150?yr, we find a decrease in winter flood occurrence in both rivers, while summer floods show no trend, consistent with trends in extreme precipitation occurrence. The reduction in winter flood occurrence can partly be attributed to fewer events of strong freezing-following such events, breaking river ice at the end of the winter may function as a water barrier and enhance floods severely. Additionally, we detect significant long-term changes in flood occurrence rates in the sixteenth to nineteenth centuries, and conclude that reductions in river length, construction of reservoirs and deforestation have had minor effects on flood frequency. [ABSTRACT FROM AUTHOR]

Paleoclimatic time series are often unevenly spaced in time, making it difficult to obtain an accurate estimate of their red-noise spectrum. A Fortran 90 program (REDFIT) is presented that overcomes this problem by fitting a first-order autoregressive (AR1) process, being characteristic for many climatic processes, directly to unevenly spaced time series. Hence, interpolation in the time domain and its inevitable bias can be avoided. The program can be used to test if peaks in the spectrum of a time series are significant against the red-noise background from an AR1 process. Generated and paleoclimatic time series are used to demonstrate the capability of the program. [Copyright &y& Elsevier]

The last interglacial (LIG; Marine Isotope Substage 5e, ~127–117 ka) experienced globally warmer than modern temperatures; however, profound differences in regional climate occurred that are relevant to the assessment of future climate change scenarios. Tropical Atlantic sea surface temperature (SST) and hydrology are intrinsic to the spatiotemporal evolution of past and future climate. We present eight monthly resolved coral Sr/Ca and δ18O records (130–118 ka) to reconstruct mean western tropical Atlantic SST and seawater δ18O changes during the LIG. Cooler and fresher than modern surface waters are indicated for the middle of the LIG at ~126 ka. This was followed by a rapid transition to modern‐like SSTs and salinities that characterized the remaining part of the LIG. Our results, which account for differences found among corals, proxies, and SST calibration uncertainties, agree with western tropical Atlantic sediment records. Together, they suggest that an oceanic regime existed that differed from today. Plain Language Summary: The last interglacial is a period of time that occurred approximately ~127 to 117 thousand years ago and experienced globally warmer than modern temperatures, similar to those predicted by computer simulations of future climate change. However, little is known about the temperature and hydrology of the tropical oceans at this time. We analyzed fossil Caribbean corals that lived between 130 and 118 thousand years ago and record within their structures the properties of the seawater they inhabited. From these we reconstructed snapshots of past mean sea surface temperatures and the changing influences of regional ocean currents. Surprisingly, we found cooler and fresher than modern surface waters occurred within the tropical Atlantic at ~126 thousand years ago, a time usually associated with peak global warming. These anomalous conditions were followed by a rapid transition to modern‐like sea surface temperatures and salinities that went on to define the remaining part of the LIG until ~118 ka. Our results agree with other reconstructions of last interglacial climate derived from tropical Atlantic sediment records. Together, they suggest changes in ocean currents that transport waters into the Caribbean and we highlight the complementary use of widely different marine archives to assess past climate change. Key Points: Last interglacial (LIG) coral Sr/Ca indicates 2.1 ± 0.7 °C cooler than modern tropical Atlantic sea surface temperatures at ~126 kaPaired coral Sr/Ca and δ18O records indicate that fresher tropical Atlantic surface waters also occurred at ~126 kaCoral Sr/Ca and δ18O records complement lower resolution less precisely dated marine sedimentary records of tropical Atlantic LIG climate [ABSTRACT FROM AUTHOR]

Cross-spectral analysis is commonly used in climate research to identify joint variability between two variables and to assess the phase (lead/lag) between them. Here we present a Fortran 90 program (REDFIT-X) that is specially developed to perform cross-spectral analysis of unevenly spaced paleoclimate time series. The data properties of climate time series that are necessary to take into account are for example data spacing (unequal time scales and/or uneven spacing between time points) and the persistence in the data. Lomb–Scargle Fourier transform is used for the cross-spectral analyses between two time series with unequal and/or uneven time scale and the persistence in the data is taken into account when estimating the uncertainty associated with cross-spectral estimates. We use a Monte Carlo approach to estimate the uncertainty associated with coherency and phase. False-alarm level is estimated from empirical distribution of coherency estimates and confidence intervals for the phase angle are formed from the empirical distribution of the phase estimates. The method is validated by comparing the Monte Carlo uncertainty estimates with the traditionally used measures. Examples are given where the method is applied to paleoceanographic time series. [ABSTRACT FROM AUTHOR]

Abstract: The instrumental record of Indian Summer Monsoon (ISM) precipitation displays two complete manifestations of an inferred multidecadal cycle. Hitherto, no precipitation-sensitive proxy record from the Indian subcontinent has had the necessary resolution and length to adequately assess whether this observed feature is an inherent aspect of the ISM system on longer timescales. Here we present compelling evidence for persistence of this cycle using a millennial length (AD 600–1550) and sub-annually resolved speleothem oxygen isotope record (δ18O) from Dandak Cave in east-central India. The record displays a high degree of correlation with a speleothem-based Asian monsoon reconstruction from Wanxiang Cave in north-central China on annual to decadal timescales showing the regional significance of these findings. The observed period in our monsoon reconstruction is similar to that associated with the Gleissberg solar cycle and multidecadal sea surface temperature variability in the north Atlantic (AMO), both of which are often cited as the prominent pacemakers of ISM variability on multidecadal timescales. We document transient coherence between ISM precipitation amount and solar variability that persists exclusively in the century prior to and during the Medieval Climate Anomaly (nominally, AD 950–1300). The non-stationary nature of the SFV–monsoon relationship presented here may be evidence of the time-varying influence of tropical ocean–atmosphere dynamics on the solar–monsoon link; however, it is not possible to show with significance that this period of coherence is anything more than an artifact of two timeseries with similar spectra. We therefore, are inclined to interpret our record as evidence of a minimal role of solar variability in driving persistent multidecadal variability of the ISM. Multidecadal SST variability in the North Atlantic remains as the likely alternative driver for persistence of this cycle. Regardless of the causative mechanism(s), the amplitude and regional signature of the observed cycle in ISM precipitation highlight its societal importance with respect to forecasting ISM precipitation on multidecadal timescales. [Copyright &y& Elsevier]

Abstract: We analyzed published records of terrigenous dust flux from marine sediments off subtropical West Africa, the eastern Mediterranean Sea, and the Arabian Sea, and lake records from East Africa using statistical methods to detect trends, rhythms and events in Plio-Pleistocene African climate. The critical reassessment of the environmental significance of dust flux and lake records removes the apparent inconsistencies between marine vs. terrestrial records of African climate variability. Based on these results, major steps in mammalian and hominin evolution occurred during episodes of a wetter, but highly variable climate largely controlled by orbitally induced insolation changes in the low latitudes. [Copyright &y& Elsevier]

The detection of periodic fluctuations is important in the quest for a deeper understanding of the drivers of past climates in the interests of being better able to understand the climate changes which are likely in the decades to come. Paleoclimatological information derived from natural archives is typically accompanied by chronological uncertainty and variable temporal resolution, both of which complicate the analysis of their time series and both are often ignored. This has, however, changed in recent years with the development of statistical tools supporting, e.g. spectral analysis tasks, which aim to take these problems into account. In cases where the original data is no longer available, it may not be possible to assess the reliability of published reports of periods detected. In this study we aim to test/model whether or not a signal for a given period can be robustly detected from a sedimentary proxy record considering its mean sampling resolution and degree of chronological uncertainty. To achieve this aim, annually sampled time series free of gaps and timescale error were modeled with white or red noise, resampled in a controlled way to simulate different time resolutions with timescale uncertainty, ultimately mimicking a real-life sedimentary record. In fact, an ensemble of potential timescales was retrieved, and their spectral characteristics explored. It was found that: (i) although sampling frequency (i.e. temporal spacing) is limiting from the side of the smallest-period, (ii) at higher mean sampling resolutions, it can ameliorate the detectability of periodic signals even in the presence of timescale error; furthermore, (iii) the increase in mean sampling resolution is less influential in an autocorrelated time series, since more information is retained due to the phenomenon of persistence. An online tool called CUSP was also developed, which gives a suggestion whether a given period can be considered to be present in a robust way utilizing our test results. [Display omitted] • Pseudoproxy time series modeled with chronological uncertainty and variable temporal spacing. • Time series derived from white noise and red noise periodic signals separately. • Each chronological uncertainty and mean sampling resolution pair considered. • Threshold determined above which periodic signals cannot be robustly detected. • Developed tool helps testing the robustness of periodic signals in sedimentary records. [ABSTRACT FROM AUTHOR]

Calcitic stalagmites from caves in the Sauerland, Germany, prove the existence of sub-Milankovitch cycles in precipitation during the last 6000 yr. The δ18O record dated with Th/U is interpreted as an indicator of paleohumidity. Spectral analysis of δ18O from 6000 a BP up to the recent top of a stalagmite from the Atta cave yields statistically significant peaks at 1450, 117, 64 and 57 a. Additionally we find a good correlation of the stalagmite’s δ18O and Δ14C from European tree rings. The 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by Bond et al. [Science 278 (1997) 1257–1266; 294 (2001) 2130–2136] derived from the amount of ice rafted debris in deep sediments from the North Atlantic. Our results suggest that the centennial to millennial shifts observed in the North Atlantic are accompanied by synchronous shifts of the climate in Northern and Central Europe, which most probably can be attributed to solar irradiation variations. [Copyright &y& Elsevier]

A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4 to 0.4 thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After ∼8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity. [ABSTRACT FROM AUTHOR]

This paper presents a computational program named BINCOR (BINned CORrelation) for estimating the correlation between two unevenly spaced time series. This program is also applicable to the situation of two evenly spaced time series not on the same time grid. BINCOR is based on a novel estimation approach proposed by Mudelsee (2010) for estimating the correlation between two climate time series with different timescales. The idea is that autocorrelation (e.g. an AR1 process) means that memory enables values obtained on different time points to be correlated. Binned correlation is performed by resampling the time series under study into time bins on a regular grid, assigning the mean values of the variable under scrutiny within those bins. We present two examples of our BINCOR package with real data: instrumental and paleoclimatic time series. In both applications BINCOR works properly in detecting well-established relationships between the climate records compared. [ABSTRACT FROM AUTHOR]

Climate signal maps can be used to identify regions where robust climate changes can be derived from an ensemble of climate change simulations. Here, robustness is defined as a combination of model agreement and the significance of the individual model projections. Climate signal maps do not show all information available from the model ensemble, but give a condensed view in order to be useful for non-climate scientists who have to assess climate change impact during the course of their work. Three different ensembles of regional climate projections have been analyzed regarding changes of seasonal mean and extreme precipitation (defined as the number of days exceeding the 95th percentile threshold of daily precipitation) for Germany, using climate signal maps. Although the models used and the scenario assumptions differ for the three ensembles (representative concentration pathway (RCP) 4.5 vs. RCP8.5 vs. A1B), some similarities in the projections of future seasonal and extreme precipitation can be seen. For the winter season, both mean and extreme precipitation are projected to increase. The strength, robustness and regional pattern of this increase, however, depends on the ensemble. For summer, a robust decrease of mean precipitation can be detected only for small regions in southwestern Germany and only from two of the three ensembles, whereas none of them projects a robust increase of summer extreme precipitation. [ABSTRACT FROM AUTHOR]

The Andes Cordillera acts as regional 'Water Towers' for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0° isotherm, and changes in regional streamflow regimes. The Temperate-Mediterranean transition (TMT) zone of the Andes (35.5°-39.5°S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657 years and captures interannual to decadal scales of variability in late spring-early summer PDSI. These changes are related to the north-south oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprecedented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Niño-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross-Amundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twentieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demonstrates the highly significant hindcast skill of A. chilensis as an aridity proxy. [ABSTRACT FROM AUTHOR]

The Citarum river basin of western Java, Indonesia, which supplies water to 10 million residents in Jakarta, has become increasingly vulnerable to anthropogenic change. Citarum's streamflow record, only ~45 years in length (1963-present), is too short for understanding the full range of hydrometeorological variability in this important region. Here we present a tree-ring based reconstruction of September-November Citarum streamflow (AD 1759-2006), one of the first such records available for monsoon Asia. Close coupling is observed between decreased tree growth and low streamflow levels, which in turn are associated with drought caused by ENSO warm events in the tropical Pacific and Indian Ocean positive dipole-type variability. Over the full length of record, reconstructed variance was at its weakest during the interval from ~1905-1960, overlapping with a period of unusually-low variability (1920-1960) in the ENSO-Indian Ocean dipole systems. In subsequent decades, increased variance in both the streamflow anomalies and a coral-based SST reconstruction of the Indian Ocean Dipole Mode signal the potential for intensified drought activity and related consequences for water supply and crop productivity in western Java, where much of the country's rice is grown. [ABSTRACT FROM AUTHOR]

Abstract: Proxy reconstructions of precipitation from central India, north-central China, and southern Vietnam reveal a series of monsoon droughts during the mid 14th–15th centuries that each lasted for several years to decades. These monsoon megadroughts have no analog during the instrumental period. They occurred in the context of widespread thermal and hydrologic climate anomalies marking the onset of the Little Ice Age (LIA) and appear to have played a major role in shaping significant regional societal changes at that time. New tree ring-width based reconstructions of monsoon variability suggest episodic and widespread reoccurrences of monsoon megadroughts continued throughout the LIA. Although the El-Niño Southern Oscillation (ENSO) plays an important role in monsoon variability, there is no conclusive evidence to suggest that these megadroughts were associated with anomalous sea surface temperature anomalies that were solely the result of ENSO-like variability in the tropical Pacific. Instead, the causative mechanisms of these megadroughts may reside in protracted changes in the synoptic-scale monsoon climatology of the Indian Ocean. Today, the intra-seasonal monsoon variability is dominated by ‘active’ and the ‘break’ spells – two distinct oscillatory modes of monsoon that have radically different synoptic scale circulation and precipitation patterns. We suggest that protracted locking of the monsoon into the “break-dominated” mode – a mode that favors reduced precipitation over the Indian sub-continent and SE Asia and enhanced precipitation over the equatorial Indian Ocean, may have caused these exceptional droughts. Impetus for periodic locking of the monsoon into this mode may have been provided by cooler temperatures at the extratropical latitudes in the Northern Hemisphere which forced the mean position of the Inter-Tropical Convergence Zone (ITCZ) further southward in the Indian Ocean. [Copyright &y& Elsevier]

We investigated changes in wildfire risk over the 1901−2002 (ad) period with an analysis of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested ecoregions of the North American and Eurasian continents. The MDC is an estimate of the net effect of changes in evapotranspiration and precipitation on cumulative moisture depletion in soils, and is well correlated with annual fire statistics across the circumboreal (explaining 25–61% of the variance in regional area burned). We used linear trend and regime shift analyses to investigate (multi-) decadal changes in MDC and percentage area affected by drought, and kernel function for analysis of temporal changes in the occurrence rates of extreme drought years. Our analyses did not reveal widespread patterns of linear increases in dryness through time as a response to rising Northern Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought severity changes that were inherent to the nonuniformly distributed impacts of climate change on dryness. Notably, significant trends toward increasing summer moisture in southeastern and southwestern boreal Canada were detected. The diminishing wildfire risk in these regions is coherent with widely reported decreases in area burned since about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely, we found evidence for increasing percentage area affected by extreme droughts in Eurasia (+0.57% per decade; P<0.05) and occurrence rates of extreme drought years in Eurasian taiga (centered principally on the Okhotsk–Manchurian taiga, P=0.07). Although not statistically significant, temporal changes in occurrence rates are sufficiently important spatially to be paid further attention. The absence of a linear trend in MDC severity, in conjunction with the presence of an increase in the occurrence rate of extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could be shifting toward being increasingly pulse dependent. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution oxygen isotope (δ18O) profiles of Holocene stalagmites from four caves in Northern and Southern Oman and Yemen (Socotra) provide detailed information on fluctuations in precipitation along a latitudinal transect from 12°N to 23°N. δ18O values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the ITCZ and dynamics of the Indian summer monsoon (ISM). During the early Holocene rapidly decreasing δ18O values indicate a rapid northward displacement in the mean latitudinal position of the summer ITCZ and the associated ISM rainfall belt, with decadal- to centennial-scale changes in monsoon precipitation correlating well with high-latitude temperature variations recorded in Greenland ice cores. During the middle to late Holocene the summer ITCZ continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation, a trend, which is also recorded in other monsoon records from the Indian and East Asian monsoon domains. Importantly, there is no evidence for an abrupt middle Holocene weakening in monsoon precipitation. Although abrupt monsoon events are apparent in all monsoon records, they are short-lived and clearly superimposed on the long-term trend of decreasing monsoon precipitation. For the late Holocene there is an anti-correlation between ISM precipitation in Oman and inter-monsoon (spring/autumn) precipitation on Socotra, revealing a possible long-term change in the duration of the summer monsoon season since at least 4.5kaBP. Together with the progressive shortening of the ISM season, gradual southward retreat of the mean summer ITCZ and weakening of the ISM, the total amount of precipitation decreased in those areas located at the northern fringe of the Indian and Asian monsoon domains, but increased in areas closer to the equator. [Copyright &y& Elsevier]

High-resolution stable isotope profiles of three contemporaneously deposited stalagmites from a shallow cave in Southern Oman provide an annually resolved record of Indian Ocean monsoon rainfall variability for the past 780 years. Uranium-series age dating and counts of annual growth bands enable an excellent age calibration. Although modern speleothems do not grow in perfect isotopic equilibrium, oxygen isotope ratios (δ18O) are a proxy for the amount of monsoon rainfall. This is supported by the statistically significant correlation between δ18O and the thickness of annual bands, whereas δ18O is inversely correlated with annual band thickness. Additionally, overlapping δ18O profiles are very similar in pattern and range, indicating that sample specific noise did not blur the climatic signal. The longest oxygen isotope profile, derived from stalagmite S3, clearly shows the transition at ∼1320 AD from a generally wetter Medieval Warm Period to a drier Little Ice Age that lasted from approximately AD 1320–1660 in Southern Oman. The decrease in monsoon rainfall since the 1960s is also obvious in meteorological records from Northern Africa and India, indicating that our speleothem-based rainfall records do not only reflect local monsoon rainfall variability. [Copyright &y& Elsevier]

The hypothesis of a connection between the onset (or intensification) of Northern Hemisphere Glaciation, the stepwise increase in African aridity (and climate variability), and an important mammalian (including hominin) species turnover is a textbook example of the initiation of a scientific idea and its propagation in science. It is, however, also an example of the persistent popularity of a hypothesis despite mounting evidence against it. A critical review of key publications on the topic and statistical re-analysis of key records of global ice volume and African climate leads to three conclusions: (1) The Northern Hemisphere Glaciation was a gradual process occurring between ∼3.5 and 2.5 Ma, not a single event at ∼2.8 Ma or at any other time. (2) A consistent stepwise (+/−0.2 Ma) transition toward greater aridity in Africa at ∼2.8 Ma does not exist; instead, there are regionally different, gradual transitions partly in connection with the intensification of the Northern Hemisphere Glaciation, but above all with the establishment of the tropical Walker Circulation after ∼2 Ma. (3) Mammalian (including hominin) species turnovers at this time also appear to have been gradual, rather than stepwise. • We falsified the hypothesis of a connection between Northern Hemisphere Glaciation, African climate and human evolution. [ABSTRACT FROM AUTHOR]

River flooding is among the most destructive of natural hazards globally, causing widespread loss of life, damage to infrastructure and economic deprivation. Societies are currently under increasing threat from such floods, predominantly from increasing exposure of people and assets in flood‐prone areas, but also as a result of changes in flood magnitude, frequency, and timing. Accurate flood hazard and risk assessment are therefore crucial for the sustainable development of societies worldwide. With a paucity of hydrological measurements, evidence from the field offers the only insight into truly extreme events and their variability in space and time. Historical, botanical, and geological archives have increasingly been recognized as valuable sources of extreme flood event information. These different archives are here reviewed with a particular focus on the recording mechanisms of flood information, the historical development of the methodological approaches and the type of information that those archives can provide. These studies provide a wealthy dataset of hundreds of historical and palaeoflood series, whose analysis reveals a noticeable dominance of records in Europe. After describing the diversity of flood information provided by this dataset, we identify how these records have improved and could further improve flood hazard assessments and, thereby, flood management and mitigation plans. This article is categorized under: Science of Water > Water QualityEngineering Water > Planning WaterScience of Water > Methods Historical, botanical and geological archives offer unique insight into truly extreme flood events and their variability in space and time, thanks to the long timeframe they document. The evidence contained within these underutilized archives has the potential to improve flood hazard assessments that are crucial for the sustainable development of societies. [ABSTRACT FROM AUTHOR]