Hydroclimatic Shifts Research Articles

The Holocene precipitation dipole pattern in the Asian drylands: Mechanisms and processes from PMIP4 simulations and paleo-proxy evidence

Asian drylands encompass Arid Central Asia (ACA) and West Asia (WA), where water vapor transport is consistently governed by the westerlies. Recent research has identified a dipole pattern in Holocene hydroclimate changes between the ACA and WA, challenging previous assumptions of uniform hydroclimate shifts across the westerlies-dominated mid-latitudes of the Northern Hemisphere. However, the mechanisms behind the dipole pattern remain largely unknown. Our findings demonstrate that PMIP4 models accurately reproduced the dipole pattern, attributing it mainly to contrasting spring precipitation changes. From the middle to late Holocene, strengthened westerlies led to increased precipitation in the ACA. In contrast, rising spring solar insolation intensified the subtropical high, leading to reduced water vapor and increased descending air movement in WA, which decreased precipitation in the region. Our study suggests that WA is a distinctive region within the westerlies-dominated mid-latitudes, where low-latitude processes led to inconsistent precipitation variations from the middle to late Holocene.

Hydroclimatic impacts of the abrupt cooling event 8200 years ago in the western Indo-Pacific Warm Pool

The most prominent abrupt climate event during the Holocene, the ‘8.2 ka event’, was characterized by severe cooling at high northern latitudes causing diverse hydroclimate shifts globally. A precise understanding of the hydroclimate response of the Indo-Pacific Warm Pool (IPWP) region to the 8.2 ka cooling, remains elusive. Here we present sub-centennial stable carbon isotope record on benthic foraminifera Asterorotalia pulchella and X-ray fluorescence elemental data of a marine sediment core from the Kallang River Basin, Singapore. We show that the rainfall in the western IPWP weakened for ~200 years at ~8.15 ± 0.03 ka BP, consistent with other regional and Southern Hemispheric records, however there is a lag of ~100 years from the North Atlantic cooling. A north-south signal propagation from the North Atlantic possibly via oceanic along with atmospheric routes operating on centennial scales led to southward location of Intertropical convergence zone causing droughts in Southeast Asia.

Identification of the Campanian Ignimbrite in the Dead Sea and consequent time-transgressive hydroclimatic shifts in the Eastern Mediterranean.

Robust chronologies and time equivalent tephra markers are essential to better understand spatial palaeoenvironmental response to past abrupt climatic changes. Identification of well-dated and widely dispersed volcanic ash by tephra and cryptotephra (microscopic volcanic ash) provides time synchronous tie-points and strongly reduces chronological uncertainties. Here, we present the major, minor and trace element analyses of cryptotephra shards in the Dead Sea Deep Drilling sedimentary record (DSDDP 5017-1A) matching the Campanian Ignimbrite (CI). This geochemical identification expands the known dispersal range of the CI to the southeastern Mediterranean, over 2300km from the volcanic source. Due to the CI eruption occurring near-synchronous with North Atlantic ice surge of Heinrich Event 4 (HE4), this tephra provides insights into regional responses to large-scale climatic change in the Mediterranean. In the Dead Sea, the CI layer is associated with wetter climatic conditions. This contrasts with the contemporaneous occurrence of the CI deposition and dry conditions in the central and eastern Mediterranean suggesting a possible climate time-transgressive expansion of HE4. Our finding underscores the temporal and spatial complexity of regional climate responses and emphasises the importance of tephra as a time marker for studying large-scale climatic changes verses regional variations.

Holocene hydroclimate in highland Costa Rica: new evidence from hydrogen and carbon isotopes in n‐alkanes of terrestrial leaf waxes in a 10 000‐year sediment profile

ABSTRACTWe conducted compound‐specific stable hydrogen (δD) and carbon (δ13C) isotope analysis on n‐alkanes from terrestrial leaf waxes preserved in a 10 000‐year sediment profile from Lago de las Morrenas 1 (9.4925° N, 83.4848° W, 3480 m), a glacial lake on the Chirripó massif of the Cordillera de Talamanca in Costa Rica. Our results demonstrate millennial‐scale variations in hydroclimate across the Holocene, with drier than average conditions in the highlands during the early Holocene, but with gradually increasing precipitation; mesic conditions during the middle Holocene with a gradual drying trend; and highly variable conditions during the late Holocene. This general pattern is punctuated by several centennial‐scale manifestations of global climate events, including dry conditions during the 8200, 5200 and 4200 cal a bp events and the Terminal Classic Drought (1200–850 cal a bp). Our δ13C analyses demonstrate that carbon isotope signals are responding to changes in hydroclimate at the site and reinforce prior interpretations of a stable páramo plant community that established following deglaciation and persisted throughout the Holocene. The shifts in hydroclimate inferred from analyses of n‐alkanes in Lago de las Morrenas 1 sediments show correspondence with charcoal records in multiple lakes, with fires most common during drier intervals.

Postglacial environmental change inferred from carbonate‐ and organic‐rich sediments of groundwater‐fed Kelly Lake, Kenai Peninsula, south‐central Alaska

ABSTRACTMajor shifts in hydroclimate have been documented during the last deglacial period and the Holocene in south‐central Alaska. Rare freshwater calcium carbonate (marl) deposits in lakes on the Kenai Peninsula can be used to reconstruct past changes in hydroclimate, including the influence of groundwater inflow to lakes. Here, the postglacial sediment sequence from groundwater‐fed Kelly Lake (60.514°N, 150.374°W) was analyzed for multiple proxies including isotopes of carbon and oxygen in marl calcite (δ13Cmarl and δ18Omarl), and isotopes of carbon (ẟ13COM) and abundances of C and N in organic matter. Bulk sediment analyses include organic matter and calcium carbonate (CaCO3) contents, visual stratigraphy and sediment flux. These analyses extend those of a previous paleoenvironmental reconstruction from Kelly Lake, which focused on sedimentary diatom oxygen isotopes and mass balance modeling over the past 10 000 years. Here, we show that Kelly Lake was deglaciated prior to 14.6 ka, and that by 14.0 ka marl dominated the sediments, with CaCO3 precipitation probably driven by groundwater input and mediated by shallow‐water charophytes. Marl accumulation decreased as organic and clastic inputs increased between ~12.2 and 11.5 ka. This shift, together with an increase in both δ13Cmarl and δ18Omarl values and a decrease in CaCO3 content, indicates an increase in the influence of meteoric water on the hydrologic budget under wet conditions, possibly driven by a strengthened Aleutian Low atmospheric pressure cell. A shift to lower δ13Cmarl and δ18Omarl values at ~11.5 ka is interpreted as an increase in the proportion of groundwater relative to meteoric water in the lake. Beginning around 9 ka, the proportion of meteoric water input continued to increase, the surrounding coniferous forest was established, and by 8 ka, CaCO3 accumulation ended. Our results elucidate the environmental conditions under which marl was deposited during the Lateglacial and early Holocene in this part of Alaska, and demonstrate how a variety of synoptic‐ and local‐scale climatic variables can converge to influence sedimentation in a groundwater‐fed lake.

A new 500-year reconstruction of Truckee River streamflow

The Truckee River Basin, located on the Nevada-California border, is an area of extreme hydrologic variability, being subject to both prolonged multi-decadal droughts and devastating floods; however, due to the brief instrumental record, understanding of the full range of this variability is limited. To assist local water managers assess the post-2000 drought in the context of historic droughts, this study revisits the first tree-ring reconstruction of Truckee River runoff: Hardman and Reil (1936). Incorporating their original 1930s tree cores as well as newly sampled material, three new site chronologies were developed and combined with other regional chronologies to produce a 1491–2003 reconstruction of Truckee River streamflow, an over 400-year extension of the instrumental record and 230 years longer than the previous reconstruction, providing new insights into the basin’s natural variability. In addition to evidence of extended droughts and extreme high streamflow years, this reconstruction shows a marked hydroclimatic shift centered around the 1850s. Prior to then, the Truckee River experienced decadal to multi-decadal periods of higher than average streamflow; subsequently, these periods have been decreasing in length with only two instances above three consecutive years of high streamflow since 1900. Whether this represents fundamental shift to a new hydroclimatic regime remains unclear. However, as global temperatures continue to rise, fewer long-term high streamflow episodes may have lasting impacts on water availability in the basin, raising the question further of whether the post-2000 drought is a new megadrought or a sign of aridification.

East Asian hydroclimate responses to the Eocene-Oligocene transition in the Weihe Basin, central China

The Eocene-Oligocene transition (EOT) at ∼34 Ma marked a significant global cooling on Earth when the ice cover in the Antarctic was formed. How the regional hydroclimate responded to this global cooling event has been extensively investigated, but the specific processes in the Asian monsoon dominated regions are still elusive, due to the lack of independent and detailed timescales. Moreover, previous sedimentary records are mainly from the Asian interior and the influence of tectonic deformation/uplifts in northeastern Tibetan Plateau on the deposit records has not been effectively separated. Here, we present a high-resolution hydroclimate record from a fluvial-lacustrine sequence in the Weihe Basin, central China, which is dated from ∼34.9 to 31.5 Ma by a high-resolution magnetostratigraphy with age controls of mammal fossil assemblages and apatite fission track (AFT) chronometry. This record shows a remarkable hydroclimatic shift occurred at ∼33.7 Ma, as manifested by a sedimentary environment change from lakeshore to lake-delta. The associated significant changes in sediment grain size and magnetic susceptibility exhibit responses of regional drying to the EOT and the Early Oligocene Glacial Maximum (EOGM). In addition, spectral analyses of the time series of paleoenvironmental proxies reveal that the hydroclimate also varied at orbital frequencies with relatively persistent eccentricity cycles (∼100 kyr), accompanied by precession (∼20 kyr) and obliquity (∼ 40 kyr) cycles after the formation of the Antarctic ice sheet at ∼33.2 Ma. Our results suggest that the monsoon-like paleoclimate variations in the mid-latitude northern Hemisphere during the EOT were partly modulated by high-latitude temperature and insolation variations.

Concordant changes in late Holocene hydroclimate across southern Patagonia modulated by westerly winds and the El Niño–Southern Oscillation

AbstractThe southern westerly winds influence weather patterns and water resources across the southern high-latitude regions, with important socioeconomic impacts. The strengthening and poleward migration of these winds since the late 20th century also have implications for regional environmental change, including drought, wildfire, and sea-ice loss. However, it is challenging to recognize the natural variability of the westerlies and predict their future behavior, as those recent changes have been influenced by anthropogenic factors. We present a 4200-yr-long record from a southern Patagonian peatland in a location that is sensitive to changes in the position and/or strength of the westerlies. Our δ13C record shows a 6‰ increasing trend from 4200 to 1200 cal. yr B.P., indicating a progressive, millennial-scale increase in peatland moisture. This long-term trend is attributed to an increase in moisture induced by strengthening southern westerly winds associated with a change in the mean state of the El Niño–Southern Oscillation (ENSO) system. Superimposed on this millennial trend, centennial-scale shifts in hydroclimate persist into modern times. We suggest that a “paleo”–Southern Annular Mode, which is linked to tropical Pacific climate, with dry events contemporaneous with positive phases and La Niña–like conditions, is responsible for this enhanced hydroclimate variability. Overall, our results point to millennial- and centennial-scale changes in hydroclimate during the late Holocene that link tropical Pacific climate variability with the Southern Annular Mode and the southern westerlies, with far-reaching implications for future changes in the southern high latitudes, including CO2 ventilation from the Southern Ocean.

Late Holocene hydroclimatic changes inferred from a karst peat archive in the western Guizhou Plateau, SW China

The paleoenvironmental implications of Ca-related indices of peat from karst regions are poorly understood. In this study, we analyze a 450-cm peat core drilled from a sub-alpine karst mountain in the western Guizhou Plateau, SW China. This core is analyzed for carbonate contents and loss on ignition (LOI). High-resolution X-ray fluorescence (XRF) core scanning was also performed to see the variation of chemical compositions. Based on these measurements and 12 accelerator mass spectrometry (AMS)14C dates, we reconstructed the history of hydroclimatic shifts in SW China and explored the significance of Ca-related indices. The reconstructed hydroclimatic conditions were consistent with the stalagmite, lacustrine sediment, and peat records from the regions controlled by the Indian monsoon. Abrupt decreases in precipitation and temperature were clearly shown during the 4.2 ka and Little Ice Age (LIA) cold events. High carbonate contents in the Yejiping peat during the Medieval Warm Period (MWP) were linked to a warm and humid climate. Additionally, wavelet analysis shows that variations in Ca content have 500, 125, 103, 80, 58, 43, 20, and 12-a quasi-periods, which correspond to the Gleissberg, Pacific Decadal Oscillation (PDO), and Schwabe cycles. Our results highlight the importance of Ca-related indices in peat deposits from karst depressions and provide a detailed description of the MWP and the LIA.

Large-scale climatic drivers of bison distribution and abundance in North America since the Last Glacial Maximum

As the dominant large herbivore in midcontinent North America since the terminal Pleistocene, bison (Bison spp.) have been a fundamental component of ecosystems and economies. Despite the importance of bison in late Quaternary North America, large-scale (regional to continental) patterns of bison biogeography are not well understood. Here we integrate archaeological and paleontological bison occurrence data with simulated climate data to better understand long-term drivers of bison distribution and abundance in North America. We used these records to model bison distribution and abundance over the past 20 thousand years at 1-thousand-year intervals. Our results show that late Quaternary changes in the distribution and abundance of bison were influenced by large-scale trends in temperature and precipitation. The distribution of bison since the Bølling–Allerød Interstadial (ca. 14 ka) is primarily explained by seasonal temperature patterns (mean temperature of the coldest quarter is the most important variable for 12 of the 14 1-thousand-year intervals). The modeled climate of bison distributions progressively narrowed since the Last Glacial Maximum (ca. 20 ka) as bison populations retracted from disjunct Pleistocene refugia and congregated in midcontinent rangelands. Through the Pleistocene-Holocene transition, bison experienced rapidly warming summer temperatures that increased faster in midcontinent North America than other regions and the continent as a whole. Model results suggest that Holocene bison abundance was influenced by hydroclimatic shifts that affected the quality and availability of forage. Bison abundances decreased through the dry early and mid-Holocene and increased when moisture availability improved in the late Holocene. We infer that bison have thrived under a broad range of environmental conditions since the Last Glacial Maximum and that the climatic and biogeographic space occupied by bison narrowed in recent millennia.

Abrupt hydroclimatic changes in southern China during the transition from the Little Ice Age to Current Warm Period

The variations in Indian summer monsoon (ISM) precipitation during the last millennium are key to understanding ISM dynamics and predicting extreme hydroclimatic changes under global warming scenarios. Here, we reconstructed the history of ISM precipitation during the past ca. 800 yr based on multiple proxies, including sediment grain size, organic matter content, biogenic silica content, magnetic susceptibility, and major element data (e.g., Fe, Ti, and Ca), obtained for a core from Lake Mahu in southwestern China. Relatively humid climate conditions prevailed during the Little Ice Age (LIA; 1400–1700 CE), whereas a relatively dry climate has characterized the Current Warm Period (CWP; 1800 CE–present). A significant dry event occurred during the transition from the LIA to CWP. The hydroclimatic record for the last millennium from Lake Mahu is generally similar to those of other areas affected by the ISM, including southwestern China, South China, the South China Sea, and northern India. We propose that southward migration of the Intertropical Convergence Zone may have been responsible for the abrupt hydroclimatic change during the transition from the LIA to CWP. • Humid LIA and dry CWP climates occurred in southern China and adjacent areas. • A notably drying event occurred during the LIA-CWP transition in southern China. • A rapidly southward shifted ITCZ may be responsible for such a hydroclimatic shift.

Air-sea coupling shapes North American hydroclimate response to ice sheets during the Last Glacial Maximum

The Western U.S. is vulnerable to hydrological stress, and insights from past climate periods are helpful for providing historical benchmarks for future climate projections. Myriad evidence from coupled models and paleoclimatic proxies suggests a major reorganization of west coast hydroclimate during the Last Glacial Maximum (LGM, ∼17–25 ka), such that the Southwest U.S. was wetter than modern day and the Pacific Northwest was drier. Yet the fundamental mechanisms underlying these hydroclimatic shifts remain unclear. Here, we employ a suite of targeted model simulations to probe the influence of LGM Northern Hemisphere ice sheets on west coast atmospheric dynamics. Whereas previous modeling studies have suggested that the southward shift of LGM west coast precipitation was driven only by the mechanical steering of atmospheric circulation by elevated ice sheet topography, we find this to be an artifact of earlier simulations that neglected realistic air-sea interaction. Instead, our simulations indicate that ice sheet albedo induced a pattern of North Pacific sea surface temperatures, reinforced by ocean-atmosphere feedbacks, that shifted the large-scale atmospheric circulation as well as the latitudinal distribution of west coast precipitation southward during the LGM. Crucially, we find that atmosphere-ocean feedbacks that sustained this ice sheet albedo-induced temperature pattern in the LGM could drive similar hydroclimatic changes today.

Abrupt Common Era hydroclimate shifts drive west Greenland ice cap change

Ice core archives are well suited for reconstructing rapid past climate changes at high latitudes. Despite this, few records currently exist from coastal Greenlandic ice caps due to their remote nature, limiting our long-term understanding of past maritime and coastal climate variability across this rapidly changing Arctic region. Here, we reconstruct regionally representative glacier surface mass balance and climate variability over the last two thousand years (~169–2015 ce) using an ice core collected from the Nuussuaq Peninsula, west Greenland. We find indications of abrupt regional hydroclimate shifts, including an up to 20% decrease in average snow accumulation during the transition from the Medieval Warm Period (950–1250 ce) to Little Ice Age (1450–1850 ce), followed by a subsequent >40% accumulation increase from the early 18th to late 20th centuries ce. These coastal changes are substantially larger than those previously reported from interior Greenland records. Moreover, we show that the strong relationship observed today between Arctic temperature rise and coastal ice cap decay contrasts with that of the last millennium, during which periods of warming led to snowfall-driven glacial growth. Taken together with modern observations, the ice core evidence could indicate a recent reversal in the response of west Greenland ice caps to climate change. Coastal west Greenland ice caps fluctuated strongly compared to the interior in response to rapid Common Era changes in snow accumulation, according to modelling of proxy records developed from a Nuussuaq Peninsula ice core covering the last 2,000 years.

Recent evidence for warmer and drier growing seasons in climate sensitive regions of Central America from multiple global datasets

AbstractSmallholder livelihoods throughout Central America are built on rain‐fed agriculture and depend on seasonal variations in temperature and precipitation. Recent climatic shifts in this highly diverse region are not well understood due to sparse observations, and as the skill of global climate products have not been thoroughly evaluated. We examine the performance for several reanalysis and satellite‐based global climate data products (CHIRPS/CHIRTS, ERA5, MERRA‐2, PERSIANN‐CDR) as compared to the observation‐based GPCC precipitation dataset. These datasets are then used to evaluate the magnitude and spatial extent of hydroclimatic shifts and changes in aridity and drought over the last four decades. We focus on water‐limited regions that are important for rain‐fed agriculture and particularly vulnerable to further drying, and newly delineate those regions for Central America and Mexico by adapting prior definitions of the Central American Dry Corridor. Our results indicate that the CHIRPS dataset exhibits the greatest skill for the study area. A general warming of 0.2–0.8°C·decade−1 was found across the region, particularly for spring and winter, while widespread drying was indicated by several measures for the summer growing season. Changes in annual precipitation have been inconsistent, but show declines of 20–25% in eastern Honduras/Nicaragua and in several parts of Mexico. Some regions most vulnerable to drying have been subject to statistically significant trends towards summer drying, increases in drought and aridity driven by precipitation declines, and/or a lengthening of the winter dry season, highlighting areas where climate adaptation measures may be most urgent.

Adaptive hybrid architecture for enhancement of the complex hydroclimatic system and assessment of freshwater security

Abstract Future freshwater security relies on hydroclimatic (HC) shifts and regimes for sustainable development. The approximation of the HC system faces major uncertainties and complexities due to the incorporation of heavy datasets, characteristics, and constraints. The proposed study focused on the parallel computing of emulator modeling-based spatial optimization to enhance the HC systems with the perspective of future freshwater security in the Upper Chattahoochee River basin (UCR). Here, the framework compiles both physical and machine learning concepts with adaptive technology for the replication of real-world scenarios. Besides, it contains 2Emulator Model Fitting, Spatial Optimization, Parallel Computing, and Initial and Adaptive sampling to upgrade model efficiency, while UCR has inadequate groundwater and the assessment of freshwater security in UCR is more necessary for varying future climatic conditions. The results displayed that the proposed spatial optimization algorithm proved to be an effective and efficient approach in the approximation of HC models. The assessment of water security in UCR was showed in terms of scarcity and vulnerability indicators for median and low-level conditions, respectively. Moreover, this study provides the potential framework for the enhancement of physical model predictions with the incorporation of hybrid concepts for problem-solving technology which can provide significant information on HC issues.

Drivers of river reactivation in North Africa during the last glacial cycle

North African greening phases, during which large rivers ran through the Sahara Desert, occurred repeatedly during the Quaternary and are regarded as key periods for the development of past human populations. However, the timing and mechanisms responsible for the reactivation of the presently dormant fluvial systems remain highly uncertain. Here we present hydroclimate changes over the past 160,000 years, reconstructed from analyses of the provenance of terrestrial sediments in a marine sediment record from the Gulf of Sirte (offshore Libya). By combining high-resolution proxy data with transient Earth system model simulations, we are able to identify the various drivers that led to the observed shifts in hydroclimate and landscapes. We show that river runoff occurred during warm interglacial phases of Marine Isotope Stages 1 and 5 due to precession-forced enhancements in the summer and autumn rainfall over the entire watershed, which fed presently dry river systems and intermittent coastal streams. In contrast, shorter-lasting and less-intense humid events during glacial Marine Isotope Stages 3 and 4 were related to autumn and winter precipitation over the Libyan coastal regions driven by Mediterranean storms. Our results reveal large shifts in hydroclimate environments during the last glacial cycle, which probably exerted a strong evolutionary and structural control on past human populations, potentially pacing their dispersal across northern Africa. The presence of large rivers in North Africa critical for Quaternary human migrations were controlled by a combination of orbital forcing and Mediterranean storminess, according to terrestrial proxy records from a marine core off Libya integrated with paleoclimate modelling.

A gradual transition into Greenland interstadial 14 in southeastern China based on a sub-decadally-resolved stalagmite record

Greenland stadials and interstadials (GS and GIS) are prominent features in ice core records of the last glacial period and are characterized by millennial-scale climate oscillations with sub-decadal- to decadal-scale hydroclimatic shifts. Over the past two decades, studies of Chinese stalagmite records have revealed corresponding Chinese Stadials/Interstadials (CS/CIS). However, the CS/CIS shifts and their corresponding forcings across the Asian monsoon region are still not fully understood. In this study, we present a 230Th-based sub-decadally-resolved stalagmite δ18O record from Xianyun Cave in southeastern China covering the period 58.2–50.8 kyr BP (before 1950 AD) and including GIS (and CIS) 16–14. We used GIS (CIS) 15.1 (peak point) and 15.2 (first point) to chronologically link the high and low-latitude records. We found that the onset of GIS 14 in our record occurred earlier than in an ice core record from Greenland and stalagmite records from northern China. This finding implies that low-latitude tropical climate likely played a key role in triggering abrupt millennial-scale events. Moreover, the transition into GIS 14 lasted 1.49 kyr in our record, which is in contrast to the abrupt temperature rise observed in NGRIP (the North Greenland Ice Core Project, 0.02 kyr). Compared with previous stalagmite records, we identified two transitional patterns into GIS 14 in monsoonal China: 1) a rapid transition in northern China and 2) a gradual transition in southeastern China. The gradual transition in our Xianyun record is analogous to a sea surface temperature (SST) record in the western tropical Pacific, highlighting a possible inherent connection between the changes in western tropical Pacific SSTs and East Asian summer monsoon variability in southeastern China. Paleoclimate records in southeastern China are therefore critical for understanding the mechanisms of abrupt millennial-scale climate events, as they can act as a bridge between high- and low-latitude records.

Inter-model agreement on projected shifts in California hydroclimate characteristics critical to water management

Shifts away from the historical hydroclimate in populated regions can have dire consequences for water management. Regions like the state of California—where highly engineered, geographically interconnected, and inflexible water management systems are predicated on particular spatiotemporal patterns of water availability—are particularly vulnerable to hydroclimate shifts. However, much of the analysis of hydroclimate sensitivity to anthropogenic climate change has focused on gross metrics like annual mean precipitation, which is highly uncertain at the regional scale. This perceived uncertainty has deterred adaptation investments and quantitative integration of climate projection data into regional water management. Here, we assess projected future shifts in the state of California in a range of hydroclimate metrics critical to water management, using data from 10 statistically downscaled global climate model and two emissions scenarios currently used by the state. We find substantial inter-model agreement under both emissions scenarios—and > 80% inter-model agreement under the more severe climate change scenario—across metrics that collectively point toward an increasingly volatile, temporally concentrated, and extreme precipitation future for the state. We show, via hydrologic and operations modeling, that accounting for shifts in these more nuanced metrics reduces the projected reliability and sustainability of current water management practices to a greater degree than would be inferred from changes in total annual precipitation alone. These results highlight both the viability and critical importance of incorporating climate change projections quantitatively into water management decisions in California and other regions vulnerable to hydroclimate shifts, and underscore the need to develop integrated climate-hydrologic-operations models and decision-making protocols capable of accounting for all projected hydroclimate shifts.

Stormflows Drive Stream Carbon Concentration, Speciation, and Dissolved Organic Matter Composition in Coastal Temperate Rainforest Watersheds

AbstractStream water carbon concentrations can be highly dynamic on the time scales of both individual storm events and seasonal hydroclimatic shifts. We collected stream water daily over a 6‐day storm from three headwater subcatchments of varying landcover (poor fen, forested wetland, and upland forest) and the catchment outlet to evaluate how precipitation events impact the concentration and speciation of carbon (organic vs. inorganic) as well as the composition of dissolved organic matter (DOM) exported laterally from coastal temperate rainforest catchments. Dissolved and particulate organic carbon concentrations increased during the storm at all sites, while dissolved inorganic carbon concentrations were diluted during peak flows. These results highlight the importance of quantifying all forms of lateral carbon export when evaluating the role of storms in catchment‐scale carbon cycling. Isotopic hydrograph separation using stream water δ18O showed that percent new water was significantly related to carbon concentration and form providing a clear link between stream water sources (i.e., recent event water) and soil carbon source areas that become connected to surface water during storms. Furthermore, ultrahigh‐resolution mass spectrometry showed that stream water DOM exported from the upland forest contained the greatest molecular diversity of the three landscape types and had the largest changes in composition over the storm suggesting that the wetland‐dominated subcatchments were less compositionally diverse with regard to soil DOM pools active during the storm. Overall, this study provides insight into hydro‐biogeochemical drivers that control lateral carbon export from forested catchments in a region where an increasing fraction of precipitation is falling as rain.

High-resolution geochemical records of extreme drought in southeastern Africa during the past 7000 years

The magnitude and frequency of extreme events is projected to increase under future climate change and understanding the major drivers of global climate variability is dependent on the development of detailed records from critical regions. Subtropical Africa is known to have undergone phases of significant change resulting from variations in broad-scale atmospheric and oceanic circulation patterns, but the nature and cause for these variations remain intensely debated. Here we present a highly resolved sedimentary record from the eastern margin of South Africa to reconstruct hydroclimate in southeast Africa over the past 7000 years. Stratigraphic and inorganic proxies preserve a highly sensitive record that document dramatic shifts in moisture balance, with phases of severe drought recorded at 4700–4200 and 3700–2600 cal yr BP. We suggest that pronounced hydroclimate variability observed in the Mkhuze record was likely triggered by changes in the activity of El Niño-Southern Oscillation (ENSO), which acted to control the transport of moisture from the Indian Ocean across southeast Africa. We show that rapid shifts in moisture availability were a characteristic feature of mid-late Holocene climate across the summer rainfall region of southern Africa, with these events broadly anti-phased with hydroclimatic shifts in East Africa and northwest Madagascar. These findings suggest that evaluating drought frequency, ecological risk and food security within the region is likely to be closely tied to ENSO and its response to future global warming.