Hydroclimatic Changes Research Articles

Abrupt Drying on the Southeast Coast of China During the Mid‐to‐Late Holocene Transition

AbstractIt is becoming increasingly clear that China experienced significant hydrological changes during the mid‐to‐late Holocene transition—a period characterized by societal changes. However, the nature of the hydroclimate anomaly as well as the direct consequences on societies in Southeast China remains unclear. Here, we present a leaf wax record from the Shuizhuyang peat deposit in Southeast China spanning the Holocene. The δD values of C29 n‐alkane (δDC29) showed a large positive shift up to 24‰ from 4.7 to 3.8 ka, which changed independent of vegetation proxies and could not be solely explained by precipitation δD variations. It is thus most likely to reflect abrupt drying, which is probably shaped by a more El Niño‐like mean state in the tropical Pacific Ocean. We hypothesize that such a significant change in hydroclimate might have promoted the development of mixed rice and millet farming on the southeast coast of China.

Hydroclimatic variability across the international Lake of the Woods watershed: Implications for nutrient export and climate sensitivity

Characterizing streamflow and relationships with climate and watershed characteristics is an essential first step in the design of any monitoring program to assess basin response to changes in land use or climate. This is especially true for the international Lake of the Woods watershed, where recurrent algae blooms have been associated with nutrient inputs from the watershed and climate warming. Here, we present a basin-wide hydroclimatic analysis within the sparsely monitored Canadian portion of the basin. Spatial and temporal patterns in climate and runoff were assessed across the two major geo-zones: the Precambrian ‘Shield zone’, dominated by bedrock, forests and lakes, and the poorly drained ‘Agassiz zone’ where ditching and drainage for agriculture have substantially enhanced the hydrologic connectivity. While climate conditions were consistent across the watershed, Agassiz basins were flashy, highly variable, and more seasonal compared with Shield rivers, likely due to the moderating effect of lake storage in the Shield region. Temperatures increased across the basin (1910–2010), and there was more rainfall and runoff during the ice-covered months (Nov-Mar), suggesting a shift toward earlier snowmelt. Marked seasonality and large swings in flow extremes at the Agassiz rivers suggest this region is particularly sensitive to hydroclimatic change and that frequent monitoring is needed to capture important periods of nutrient export like spring runoff and storm events. In contrast, substantial storage within the Shield landscape suggests this zone is more hydrologically ‘resilient’ to climate extremes and that water quality and quantity measurements can be less frequent.

High- and low- latitude forcing on hydroclimate variability in southern China over the last two millennia

Knowledge of centennial-scale hydroclimate variability and underlying forcing mechanisms in southern China over the last two millennia remains limited. This study presents multiproxy results of the grain size, elemental ratios (Zr/Rb, and Rb/Sr), total organic carbon (TOC), total nitrogen (TN) and C/N ratios, together with high-precision 137Cs/210Pb and accelerator mass spectrometry (AMS) 14C ages of a sediment core from Lake Baxian, which is a small closed lake in Guangxi Province, southern China. Based on these high-resolution data, the hydroclimate change over the last two millennia is clearly revealed. Relatively high values of the mean grain size, Zr/Rb, TOC and TN, along with low values of Rb/Sr and C/N ratios during the Dark Ages Cold Period (DACP, 500–900 CE) and Little Ice Age (LIA, 1400–1850 CE), indicate a relatively wet climate during these intervals. Conversely, low values of the mean grain size, Zr/Rb, TOC and TN, along with high Rb/Sr ratios during the Roman Warm Period (RWP, 50–400 CE) and Medieval Climate Anomaly (MCA, 1000–1300 CE), suggest relatively dry conditions during these intervals. During the Current Warm Period (CWP, 1850-present), the values of the mean grain size and Zr/Rb increase abruptly and Rb/Sr ratios exhibit large fluctuations, which might be attributed to the impact of human activities. The general pattern of the reconstructed hydroclimate variations in this study is broadly consistent with the North Atlantic ice-rafted detritus and Indo-Pacific Warm Pool (IPWP) records, indicating that the hydroclimate variability on the centennial-scale in southern China has been controlled by both high- and low- latitude forcings over the last two millennia.

Kalimantan hydroclimate variability since the last glacial period

Changes in convective activity and hydroclimate over Northeastern Kalimantan are key features to understand glacial to interglacial climate evolution in the center of the West Pacific Warm Pool during the Late Pleistocene to Holocene. We use high-resolution X-ray fluorescence (XRF) scanner-derived elemental ratios in sediment Core SO217-18522 (1º 24.106’ N, 119º 4.701’E, 975 m water depth) recovered from the northern Makassar Strait to reconstruct changes in precipitation-related weathering and erosion over Northeastern Kalimantan over the last 50 kyr. Enhanced seasonality of rainfall and an extended dry season during Heinrich Stadials (HS4 to HS1) and the Younger Dryas (YD) suggest weakening of the tropical convection associated with a southward shift of the tropical rain belt and the annual mean position of the Intertropical Tropical Convection Zone. Increasing sediment discharge and intensification of convective activity occurred during the early to mid-Holocene during an interval of high Northern Hemisphere insolation, elevated atmospheric pCO2 levels and global warming. Our reconstructions in comparison with regional terrestrial and marine records highlight the high spatial variability of Kalimantan hydroclimate on millennial to glacial-interglacial timescales.

The Holocene environmental changes revealed from the sediments of the Yarkov sub-basin of Lake Chany, south-western Siberia

Lake Chany is the largest endorheic lake in Siberia whose catchment is entirely on the territory of Russia. Its geographical location on the climate-sensitive boundary of wet and dry landscapes provides an opportunity to gain more knowledge about environmental changes in the West Siberian interior during the Holocene and about the evolution of the lake itself. Sediment cores obtained from the Yarkov sub-basin of the lake in 2008 have been comprehensively studied by a number of approaches including sedimentology and AMS dating, pollen, diatom and chironomid analyses (with statistical interpretation of the results), mineralogy of authigenic minerals and geochemistry of plant lipids (biomarker analysis.). Synthesis of new results presented here and published data provides a good justification for our hypothesis that Lake Chany is very young, no older than 3.6 ka BP. Before that, between 9 and 3.6 ka BP, the Chany basin was a swampy landscape with a very low sedimentation rate; it could not be identified as a water body. In the early lake phase, between 3.6 and 1.5 ka BP, the lake was shallow, 1.2–3.5 m in depth, and it rose to its modern size, up to 6.5 m in depth, during the last millennium. Our data reveal important changes in the understanding of the history of this large endorheic lake, as before it was envisioned as a large lake with significant changes in water level since ca. 14 ka BP. In addition to hydrology, our proxies provide updates and details of the regional vegetation and climate change since ca. 4 ka BP in the West-Siberian forest-steppe and steppe. As evolution of the Chany basin is dependent on hydroclimatic changes in a large region of southern West Siberia, we compare lake-level change and climate-change proxies from the other recently and most comprehensively studied lakes of the region.

Leaf wax biomarkers of a high-mountain lake area in western iberia—Implications for environmental reconstructions

Leaf wax n-alkane biomarkers are widely used to infer past vegetation dynamics and hydroclimate changes. The use of these compounds strongly relies on the characterization of modern plants. However, few studies have explored leaf waxes of modern plants and their application to reconstructing climate and environmental changes in the Iberian Peninsula, a region known for its high vulnerability to climate change. In this study, we characterize the distributions and compound-specific isotopic compositions of the leaf waxes of dominant plants in the vegetation cover, soil, and surface sediment of the Lake Peixão area, a high-mountain glacial lake in Serra da Estrela (central Portugal). Our results show that the modern oro-Mediterranean (subalpine) vegetation of the study area is dominated by C3 grasses/herbs and shrubs that preferentially produce long-chain leaf waxes (≥C27). The C31n-alkane display the overall highest concentration, produced by some grasses and shrubs, but especially Erica sp (heather), which is highlighted as a major source for the total n-alkane pool in the lake sediments. C29 is the second-most abundant and the most equally produced n-alkane of the vegetation cover; C25 and C27 homologs are mainly associated with aquatic-related grasses/herbs, while C33 and C35 are particularly linked to cold-drought tolerant Juniperus sp. shrubs. Shrubs show higher but proportional values than grasses/herbs in the isotopic space, suggesting a directly proportional physiological adaptation of the two ecological forms to the prevailing climatic and environmental factors of the study area. C29 is pointed as the most representative (or less plant-biased) leaf wax n-alkane in the lake sediments. Thus, δD of C29n-alkane is interpreted as a robust terrestrial hydrological indicator (δDterr), which signal is believed to be strongly influenced by the mean air temperature and/or precipitation amount. Despite the sparse vegetation and small catchment area, the apparent hydrogen fractionation factor, determined from δDterr of the lake surface sediment, is in line with the modeled global mean values for the latitude of the study area. The different molecular and compound-specific signatures of the studied oro-Mediterranean species have the potential to support future interpretations of leaf wax biomarkers in the Iberian Peninsula.

Tropical Indian Ocean basin hydroclimate at the Mid- to Late-Holocene transition and the double drying hypothesis

The spatial pattern of Holocene climate anomalies is crucial to determining the mechanisms of change, distinguishing between unforced and forced climate variability, and understanding potential impacts on past and future human societies. The 4.2 ka event is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Yet outside the data-rich Northern Hemisphere mid-latitudes, the global pattern of climate anomalies is uncertain. In this study we investigate the spatial and temporal variability of the tropical Indian Ocean hydroclimate at the Mid- to Late-Holocene transition. We conducted Monte-Carlo principal component analysis, considering full age uncertainty, on ten high-resolution, precisely dated paleohydroclimate records from around the tropical Indian Ocean basin, all growing continuously or almost continuously between 5 and 3 kyr BP. The results indicate the dominant mode of variability in the region was a drying between 3.97 kyr BP (±0.08 kyr standard error) and 3.76 kyr BP (±0.07 kyr standard error) with dry conditions lasting for an additional 300 years in some records, and a permanent change in others. This drying in PC1, which we interpret as a proxy of summer monsoon variability, fits with a previously recognised tropic wide change in hydroclimate around 4.0 kyr BP. An abrupt event from 4.2 to 3.9 kyr BP is seen locally in individual records but lacks regional coherence.A lack of apparent 4.2 ka event in tropical Indian Ocean hydroclimate has ramifications for climate variability in the Indus valley, and for the Harappan civilization. Through a comparison of existing Indian subcontinent paleoclimate records, upstream climatic variability in the Indian Summer Monsoon and winter Westerly Disturbances source regions, and modern climatology, we present the “Double Drying hypothesis”. A winter rainfall drying between 4.2 and 3.9 kyr BP was followed by a summer rainfall drying between 3.97 kyr BP and at least 3.4 kyr BP. The Double Drying hypothesis provides more detailed climatic context for the Harappan civilization, resolves the cropping paradox, and fits the spatial-temporal pattern of urban abandonment. The consequences for the new Mid- to Late-Holocene Global Boundary Stratotype Section and Point in a stalagmite from Meghalaya are explored.

Quantitative reconstruction of the relative humidity by a coupled δ18O-δ2H approach during the Younger Dryas in central China

Paleoclimate records in central China reveal asynchronous hydroclimate changes to those from both northern and southern China during the Holocene. Whether such a spatial mode works for the Younger Dryas event (YD), a prominent abrupt climate change during the transition from the last glacial to the Holocene, is poorly constrained by quantitative records. In this study, we apply the novel coupled hydrogen isotope composition of leaf wax n-alkanes and oxygen isotope of hemicellulose (δ2Hn-alkane-δ18Osugar) paleohygrometer approach to reconstruct the paleo relative humidity (RH) on a peat layer spanning 13.1–10.0 cal ka BP in the Dajiuhu peatland, central China. The coupled δ2Hn-alkane-δ18Osugar approach reveals that the RH was relatively low in the period of 12.7–12.1 cal ka BP (RH avg. 79%, 1σ 4.9%) and then turned to relatively high values during 12.1–11.7 cal ka BP (RH avg. 88.6%, 1σ 3.9%). A dry interval happened from the end of the YD to 11.3 cal ka BP (RH< 80%). A general wetting trend during the YD period in central China contrasts with the prevailing drying in northern China and maybe also in southern China. The occurrence of a wetting trend during the YD period in central China probably results from the interplay between the westerlies and the East Asian summer monsoon under a context with weak summer monsoon intensity.

Meltwater Contribution to Lake Water Budget in Mid‐Latitude Asia During the Deglaciation and Early Holocene

AbstractContrasting moisture evolutions between westerlies‐dominated Central Asia and monsoonal Asia since the mid‐Holocene have been widely recognized. Yet, inconsistent hydrological records over the early Holocene from both regions hamper our understanding of hydroclimatic changes and associated mechanisms. Here we present isotopic and biomarker (alkenone) records from three Chinese lakes located at both regions to address a possible meltwater role in lake water budget. Our results show the occurrence of particularly depleted isotopic values of lake water associated with lake freshening within the deglaciation and early Holocene. Together with anomalously old 14C dates, which were also documented in many other Asian lakes, our observations likely indicate substantial meltwater contribution to mid‐latitude Asian lake water budget, along with the climatic warming. Hence, the potential meltwater effect should be considered when inferring Asian summer monsoon and westerlies behavior over this critical period, with important implications to Holocene hydrological reconstructions and future hydroclimatic projections.

Hydroclimatic record from an Altiplano cushion peatland (24°S) indicates large-scale reorganisation of atmospheric circulation for the late Holocene.

The hydroclimate of South America is characterized by the South American summer monsoon (SASM), a tropical atmospheric circulation that induces a summer precipitation regime, and the Southern Hemisphere Westerlies (SHW), an extratropical atmospheric circulation that induces a winter precipitation regime. Stretched between these two systems is a NW-SE-oriented region dominated by descending air masses, resulting in the South American subtropical dry zone (SASDZ), also known as the arid diagonal. We investigated the Cerro Tuzgle cushion peatland (CTP) located on the Argentine Altiplano, north of the present-day SASDZ. Previous work revealed that the CTP was consistently in the SASM regime during the last 2900 cal yr BP. Here, we extend the CTP record to the middle Holocene covering the last 7200 cal yr BP to gain further knowledge of the Holocene development of the SASM and potential modulations of the SASDZ. The prominent feature of the entire record is a distinct and lasting transition centred around 3100 cal yr BP characterized by declining minerogenic content, increasing organic carbon content, rising stable carbon isotope values of organic matter and cellulose, and increasing stable oxygen isotope values of cellulose. We interpret this specific proxy pattern as a hydroclimatic transition towards less arid conditions at the CTP after 3100 cal yr BP. The transition corresponds with the end of the continuous Holocene strengthening of the SASM between 3500 cal yr BP and 3000 cal yr BP indicated by proxy records from north and east of the CTP. The CTP does not reflect this strengthening of the SASM and rather exhibits a threshold response indicating the effective establishment of the SASM summer precipitation regime at 24°S. This suggests that moisture supply during a more arid middle Holocene was provided by isotopically depleted precipitation, while moisture supply after the transition originated from isotopically enriched SASM summer precipitation. Concurrent hydroclimatic changes in the SHW winter precipitation regime south of the SASDZ are documented in a distinct lake level rise of Laguna Aculeo (33°50´S) around 3200 cal yr BP. These coinciding hydrological changes of the SASM and the SHW precipitation regimes indicate larger scale reorganisations of atmospheric circulation components, potentially connected to major modulations of the SASDZ. Thus, our CTP record sheds light on the middle to late Holocene development of the SASM at its southern limit and corroborates connections between the tropical and extratropical hydroclimate of South America.

On the value of satellite remote sensing to reduce uncertainties of regional simulations of the Colorado River

Abstract. As the major water resource in the southwestern United States, the Colorado River is experiencing decreases in naturalized streamflow and is predicted to face severe challenges under future climate scenarios. To better quantify these hydroclimatic changes, it is crucial that the scientific community establishes a reasonably accurate understanding of the spatial patterns associated with the basin hydrologic response. In this study, we employed remotely sensed land surface temperature (LST) and snow cover fraction (SCF) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) to assess a regional hydrological model applied over the Colorado River Basin between 2003 and 2018. Based on the comparison between simulated and observed LST and SCF spatiotemporal patterns, a stepwise strategy was implemented to enhance the model performance. Specifically, we corrected the forcing temperature data, updated the time-varying vegetation parameters, and upgraded the snow-related process physics. Simulated nighttime LST errors were mainly controlled by the forcing temperature, while updated vegetation parameters reduced errors in daytime LST. Snow-related changes produced a good spatial representation of SCF that was consistent with MODIS but degraded the overall streamflow performance. This effort highlights the value of Earth observing satellites and provides a roadmap for building confidence in the spatiotemporal simulations from regional models for assessing the sensitivity of the Colorado River to climate change.

Last 10 millennial history of Indian summer monsoon in the Bengal region – a multi-proxy reconstruction from a lacustrine archive

We combine phytoliths, non-pollen palynomorphs, stable carbon isotopes of sediment organic matter, environmental magnetic parameters and sediment texture data of a lacustrine sedimentary archive from the Bengal region to understand the ecosystem response to past hydroclimatic changes. The region experienced a high Indian summer monsoon (ISM) rainfall during c.10.2–5.6 ka corresponding to the Holocene Climatic Optimum (HCO) when the lake level was high. Our study reveals that ISM weakened at c.4.3 ka with a decline in lake level, became strong between c.3.7 and 2.1 ka and then shifted towards a drier mode. However, this weakened phase was punctuated by a stable phase between c.1–0.8 ka, and a comparatively stronger phase between c.0.2–0.1 ka corresponding to the Medieval Warm Period (MWP) and Little Ice Age (LIA), respectively. An eutrophication of the lake began at c.3.7 ka which was completely in-filled during c.0.2–0.1 ka. A comparison of the present results with published records suggests millennial-scale coherence in the Holocene ISM variability across the Bay of Bengal (BoB) moisture source regions while, the finer-scale incongruities are attributable to differential proxy response and dating uncertainty. Our proxy-based observations are also compared with a few palaeo-model outputs for selected time-spans [such as, 1550–1850 CE (0.4–0.1 ka), 900–1300 CE (1–0.6 ka), 2100–2000 BCE (4.1 − 4 ka), and 4050–3350 BCE (6–5.3 ka)] from the Palaeo Modeling Intercomparison Project Phase 3–4 (PMIP3/PMIP4) experiments which support the assumption that changes in lake ecosystem may be strongly influenced by ISM rainfall, and the region surrounding northern BoB received higher precipitation than the other parts of India since the past 10.2 ka. The millennial-scale variations in the ISM rainfall may largely be attributed to gradual decline in orbital insolation and dynamics of Inter tropical Convergence Zone (ITCZ). However, centennial scale variations may be collectively triggered by forcings like NAO, ENSO and IOD.

Hydroclimatic changes since the Last Glacial Maximum recorded in mountain peat deposit on the southwestern margin of the Sichuan Basin, China

Knowledge of the hydroclimatic changes in Southwest China since the Last Glacial Maximum (LGM) is crucial for disentangling the long-term evolution of the Asia Monsoon and predicting the future fate of the mountain peat deposit in the Asia Monsoon region. In this study, we obtained a 530-cm-long peat core from the Ganchi wetland in Southwest China and analyzed its geochemical indices, including total nitrogen (TN), total organic carbon (TOC), stable carbon isotope composition of organics (δ13Corg), and the concentration of several major elements, to investigate the sedimentary and hydroclimate evolution since the LGM. We found that the peat strata in the Ganchi wetland have developed gradually from 13.7 cal kyr BP, which is likely ascribed to the warm climate during the Bølling-Allerød (B/A) period. TOC, δ13Corg, K/Ti, and Fe/Mn records showed notable paleoclimate shifts since the last deglaciation. The first warming period after the LGM was observed starting at 18.2 cal kyr BP, which is consistent with other records from Southwest China. The reconstruction results show that the western margin of the Sichuan Basin during the last deglaciation was most affected by the East Asia summer monsoon (EASM), and less affected by the Indian summer monsoon (ISM). The climate of the early Holocene (11.2–7.5 cal kyr BP) was affected by both the ISM and EASM, resulting in more complex local climatic features. The Holocene Megathermal period observed from 7.5 to 3.5 cal kyr BP, is consistent with the timing detected in other records of Southwest China.

Late Pleistocene to early Holocene ENSO-related hydroclimatic changes revealed by palynological records from the coastal Xixi Basin, eastern China

We report a palaeontological record for the last 37.0 cal kyr BP in the Xixi Basin, eastern China, to better understand the correlation between hydroclimatic change and vegetation history during the late Pleistocene to early Holocene. Principal component analysis based on the 25 main palynological taxa indicated that dominant taxa fall into three groups: conifers that preferred dry conditions on the left of leading principal component (PC) 1, ferns and wetland communities associated with a humid climate responding to the positive loading of PC1, and Quercus-dominated communities principally corresponding to PC2. Based on this analysis, hydroclimatic conditions were relatively warm/dry during the latter Marine Isotope Stage (MIS) 3 with a high abundance of conifers. This was followed by a generally cold/wet MIS 2 with expansion of ferns and herbs, and a warm/dry MIS 1 with more evergreen species. The sporadic occurrence of evergreen species and drier conditions since about 19.0 cal kyr BP coincides with the beginning of the last deglaciation. An increase in sedimentary accumulation rates and changes in vegetation components at the onset of the Holocene are likely related to maximum transgression. Wet conditions coincide with several cold intervals, including Heinrich events and the Last Glacial Maximum. The overall warm/dry and cold/wet climatic patterns agree well with the palaeoclimatic records elsewhere in the middle Yangtze region, showing similar hydroclimatic forcing mechanisms related to the migration of the Meiyu Front modulated by the El Niño-Southern Oscillation (ENSO) activities. Cold/wet conditions probably correspond to enhanced ENSO and reduced insolation while warm/dry conditions correspond to dampened ENSO and increased insolation. The extremely humid conditions that occur during cold events may result from the combined effects of the Atlantic Meridional Overturning Circulation and ENSO. These results expand our understanding of the monsoonal hydroclimate in eastern China during the late Quaternary period.

Implications of sea level variability on the formation and evolution of subtropical Rainbow Beach patterned fen complexes, Queensland, Australia

The Great Sandy National Park [K’gari (Fraser Island) and Cooloola] contains the largest subtropical patterned fen complexes in the world. These globally significant, groundwater-dependent ecosystems have been previously studied in relatively undisturbed areas on K’gari and were suggested to be resilient to changes in hydrology, sea level and wildfires. The Rainbow Beach patterned fens are under-studied systems thought to be formed in local perched aquifers. The palaeoenvironmental conditions required for the formation and continuation of these peatlands, and how they react to changes in hydroclimate, sea level and human activities are uncertain. We attempt to resolve this ambiguity using proxies for vegetation and environmental changes over the last ~12,770 cal yr BP from a sediment core located in the Rainbow Beach patterned fen complex. We infer the formation of an aquitard layer and Empodisma minus mire development at ~12,770 cal yr BP, with conditions conducive for patterning ~12,000–10,000 cal yr BP. Paludification occurred in the early Holocene, coincident with increased sea levels, which expanded the mire inland. Increased salt marsh taxa during this period coincides with decreased E. minus values, while further peatland development occurred ~4200 cal yr BP, suggesting that marine influences greatly effect these coastal peatlands. Evidence of vegetation thickening associated with post-European fire suppression was observed. Compared to those on K’gari, the Rainbow Beach complex appears to have initiated through different processes and show greater sensitivity to changes in sea levels. Therefore, subtropical patterned fens should be assessed independently to identify individual trajectories and sensitivities.

Elevation dependence of projected hydro-climatic change in eastern Siberia

Over the last several decades, eastern Siberia has experienced some of the largest temperature increases worldwide. We use the RCP8.5 simulation of the Community Climate System Model version 4 to examine how projected monthly changes in temperature and hydro-climatic variables in eastern Siberia depend on latitude and elevation. Temperature increases are largest at the highest latitudes in winter and late fall and are smaller at higher elevations. For precipitation and snowfall, there is a latitudinal dependence in autumn and spring, with precipitation, snowfall, and snow depth mostly increasing between 60 and 70° N. Although snow cover extent (SCE) decreases almost everywhere, the largest changes occur during the transition seasons which we define as spring and autumn, and the timing of the changes depends on latitude, elevation, and the specific month within seasons. The decreases in SCE are larger at lower latitudes and lower elevations in April and November and larger at higher elevations and higher latitudes in June and September. For the highest latitudes, snow depth actually increases, and increases more at higher elevations. These projections are generally consistent with those of four other climate models. For precipitation, all models project increases in non-summer seasons, but they are not consistent with respect to the direction of the elevation dependence of precipitation. We discuss the complex interactions among the projected changes in all the variables.

A structured framework to interpret hydro-climatic and water quality trends in Mediterranean climate zones

Many regions of the world have experienced rapid hydro-climatic changes in recent decades. Mediterranean areas have especially experienced prolonged droughts, changes to runoff, and low-flow periods. Many methods and assumptions for assessing trends in rainfall and runoff are available, however, a comprehensive framework is required that can consider variations at large temporal and spatial scales, long-term persistence (LTP), seasonal patterns, and multiple change points. An analysis framework has been developed in R and applied to a network of 107 rainfall gauges (with about 100 yrs data) and 90 streamflow-water quality gauges (30 – 50 yrs data) across South-west Western Australia, a region with a typical Mediterranean climate. Trends were estimated using the Original Mann–Kendall (OMK) family of tests including using the Mann-Kendall Test under the Scaling Hypothesis (MKLTP) and Seasonal Mann–Kendall (SMK). Single and multiple change points were calculated by the non-parametric Lanzante’s test (LAT), E-Agglomerative algorithm (ECP), and Energy Divisive (EDP). Trend tests show annual rainfall has decreased by 2.6 mm/yr in the Western areas and increased by 0.9 mm/yr in the Eastern areas with corresponding change points in the 1940 s-1960 s and from 1995 to 2000 although 30 % of the annual rainfall and 100 % of the standardized precipitation index (SPI) time series were found to be under LTP. A regional flow trend analysis shows catchments are yielding much less runoff in more than 90 % of gauged stations. The runoff in perennial and ephemeral streams has shown a decline with the rate of 6 (54 %) and 2 (50 %) mm/yr, respectively. Runoff and baseflow decline occurred as step changes lagged after rainfall, and the rate of decline in baseflow was more than for runoff, suggesting that groundwater connectivity with stream inverts has fallen substantially. Stream salinities increased by 64 mg/L/yr (or 15 %) overall which was highly significant. The framework provides a structured approach to analyse changes in rainfall, runoff and stream salinity for a Mediterranean climate that is projected to become even warmer and drier.

Biomarker and Pollen Evidence for Late Pleistocene Pluvials in the Mojave Desert

AbstractThe climate of the southwestern North America has experienced profound changes between wet and dry phases over the past 200 Kyr. To better constrain the timing, magnitude, and paleoenvironmental impacts of these changes in hydroclimate, we conducted a multiproxy biomarker study from samples collected from a new 77 m sediment core (SLAPP‐SRLS17) drilled in Searles Lake, California. Here, we use biomarkers and pollen to reconstruct vegetation, lake conditions, and climate. We find that δD values of long chain n‐alkanes are dominated by glacial to interglacial changes that match nearby Devils Hole calcite δ18O variability, suggesting both archives predominantly reflect precipitation isotopes. However, precipitation isotopes do not simply covary with evidence for wet‐dry changes in vegetation and lake conditions, indicating a partial disconnect between large scale atmospheric circulation tracked by precipitation isotopes and landscape moisture availability. Increased crenarchaeol production and decreased evidence for methane cycling reveal a 10 Kyr interval of a fresh, productive, and well‐mixed lake during Termination II, corroborating evidence for a paleolake highstand from shorelines and spillover deposits in downstream Panamint Basin and Death Valley during the end of the penultimate (Tahoe) glacial (140–130 ka). At the same time brGDGTs yield the lowest temperature estimates (mean months above freezing = 9°C ± 3°C) of the 200 Kyr record. These limnological conditions are not replicated elsewhere in the 200 Kyr record, suggesting that the Heinrich stadial 11 highstand was wetter than the last glacial maximum and Heinrich 1 (18–15 ka).

Resistance of grassland productivity to hydroclimatic changes in the Tibetan Plateau

An increasing trend of hydroclimatic disturbances, such as droughts, which are projected to become more frequent and intense with global warming and climate change. Droughts adversely affect the structure and function of terrestrial ecosystems, damage vegetation growth, and even increase mortality. In this study, we assess the drought resistance (Rd) of ecosystems in the Tibetan Plateau. Ecosystem water use efficiency (WUE) was used as an indicator of change in carbon and water cycles, and then ecosystem resistance in terms of change in WUE in drought years was analyzed. Our results suggested that the WUE in the Tibetan Plateau exhibited significant spatial heterogeneity and drastically reduced under dry and cool conditions. The evaluation of ecosystem drought resistance indicated that the grassland in most areas of the Tibetan Plateau is resistant to hydroclimatic fluctuations. In particular, the grassland ecosystem in areas with lower temperatures or higher precipitation exhibited greater drought resistance. Our results facilitate the identification of drought-vulnerable areas in the Tibetan Plateau to inform grassland ecosystem management and climate policy-making and sustain ecosystem stability under future climate conditions.

Last millennium hydroclimate and atmospheric circulation change in Northeast China: A dual δ13C and δ18O approach from a mountaintop Sphagnum bog

Northeast China receives most moisture through warm air masses from the western Pacific Ocean and some moisture from the Eurasian Continent predominantly through the westerlies. Changes in the contribution of these different moisture sources affect the amount and isotopic composition of precipitation, which would in turn influence peatland surface moisture and peat moss (Sphagnum) growth. Recently a dual δ13C and δ18O approach from peat-core archive has been used not only as a proxy for peatland moisture/wetness but also as a constraint for interpreting isotope in precipitation and moisture trajectories. Here we used the coupled δ13C and δ18O measurements of peat moss cellulose, along with macrofossil and other proxy data, from a well-dated peat core retrieved from a Sphagnum-dominated ombrotrophic (meteoric water-fed) bog in Northeast China to reconstruct regional climate and moisture sources of precipitation over the last millennium.Our results show no sign of extremely negative δ13C signals that would have been caused by the uptake of respired CO2 by peat moss for photosynthesis, thus we argue that the cellulose δ13C signatures document the peatland moisture conditions, owing to water-film effect. We found a positive correlation between δ13C and δ18O, suggesting that δ18O primarily reflects the δ18O signal in precipitation rather than evaporative 18O-enrichment, because the evaporative enrichment would result in a negative correlation between these isotopes as documented in modern process studies along the hummock-hollow moisture gradients. We propose that the positive δ13C-δ18O correlation is a general feature in regions that are influenced by both summer monsoon and the westerlies, as warm Pacific moisture that has high δ18O values also brings abundant precipitation, resulting in wet conditions that induce high δ13C values, and vice versa. The δ13C results show a decrease of ca. 6‰ from −23 to −29‰, with large-magnitude fluctuations, over the last 1000 years, suggesting a long-term drying trend. An abrupt decrease at 1370 CE of 4‰ in δ18O from >21‰ to ca. 17‰ represents the transition from the warm and wet Medieval Climate Anomaly (MCA) at 815–1370 CE to the cold and dry Little Ice Age (LIA) at 1370–1940 CE, reaching the lowest δ18O value of ca. 15‰ at around 1850 CE. The increase in δ18O to 19‰ after 1900 CE reflects post-LIA climate warming. Enhanced moisture transport from the western Pacific might have contributed to the warm and wet MCA. Three short intervals with pronounced low δ13C and δ18O values occurred at 1370–1450 CE, the 19th Century, and the 1990s, suggesting extreme dry periods with reduced Pacific moisture inputs. The driest period with consistently low δ13C value of < -25‰ (Suess effect-corrected value of −23‰) occurred after 1990 CE, with a corresponding increase in dry-adapted moss Polytrichum.Variability in the regional atmospheric circulation—the western Pacific subtropical high (WPSH)—might have explained the change in moisture sources. The westward extension of the WPSH—often corresponding with the El Niño-like conditions—would block the Pacific moisture from transporting to Northeast China. Our record, together with other paleo-records over monsoon-influenced regions in East Asia, shows synchronous but opposite shifts of summer rainfall anomalies in North or Northeast China and South China on decadal to centennial scales, substantiating a coherent atmospheric circulation pattern associated with WPSH and ENSO variability. We argue that there were major shifts in synoptic-scale atmospheric circulation and associated hydroclimate change during the MCA, LIA, and Current Warming Period in Northeast China.