The precise timing and sequence of human arrival into the tropical South Pacific islands is contested and there is ongoing debate around the drivers of migration over the past three thousand years. Recent evidence supports the role of changing climate through palaeoclimate evidence that suggests that the South Pacific has experienced shifts between dry and wet periods throughout the human occupation of the region. Here we focus on the importance of relative drought, as islands are space and resource limited with human populations that depend upon agricultural crops that are rainfall dependent. As such, the occurrence of significant drought events likely threatened early Pacific societies, and such an impact proffers a potential driver of migration. Using lake sediment cores from the island of Mangaia in the Southern Cook Islands, this study utilises stable isotopes, geochemistry and diatoms to create a hydroclimate record extending back 2500 years. We show that prior to the arrival of Polynesians into the Southern Cook Islands there was a significant dry period dating to approximately 885-1075 CE and a further dry period from 1320 to 1460 CE coincident with human settlement of the Southern Cook Islands coinciding with agricultural intensification.

The migration of humans into the eastern Pacific was relatively rapid and focused around 900-1250 CE. Although the causes for this migration are varied, we put forward evidence to suggest that a change in the mean state of the tropical South Pacific from La Nina like to El NinAo like conditions accompanied by an increase in climate "shocks" around the period of migration, could have created conditions to promote migration east into the Pacific. We use a range of sediment archives and hydroclimate proxies located in sites within the region of migration, to reconstruct climate conditions in the 'sending' islands, and 'receiving' archipelagos. Climate in the period immediately prior to the eastward migration was characterised by a drier southwest Pacific. During the period following settlement of the central region of eastern Polynesia, the mean climate state in receiving islands became wetter, with fewer climate "shocks".Results from socio-hydrological models highlight the sensitivity of growing populations to droughts within a drying climate. Using these lines of evidence, we suggest that as populations grew, particularly in drought-sensitive islands, people with some knowledge of eastern "Gateway islands", chose to move east, fortuitously at a time when wetter conditions supported their long-term settlement.

Abstract Understanding how precipitation responds to temperature change is crucial for anticipating future climate variability. The geological past provides a unique window into rainfall responses to large-scale climate shifts, yet regional responses in the South–Central Pacific remain poorly constrained due to the lack of continuous long-term palaeoclimate records extending beyond the Holocene. To address this gap, we reconstruct temperature and precipitation using biomarker proxies from a 50,000-year peat core from Nuku Hiva, French Polynesia. During the Last Glacial period, increased hydrogen isotopic values of plant waxes indicate drier conditions. Pollen data show increased abundances of drought-adapted herbaceous taxa and reduced cloud forest species. Temperature-sensitive bacterial lipids indicate substantially cooler glacial conditions relative to the Holocene. Notably, reconstructed temperature and precipitation changes are strongly correlated, consistent with data assimilation studies. These results highlight the sensitivity of the tropical Pacific to background climate state and provide important constraints on regional climate dynamics.

In the 21st century tsunamis have claimed the lives of over 250,000 individuals, and have caused extensive damage to vulnerable coastal ecosystems. This vulnerability continues to increase in many areas as human activity further degrades the coastal forests that once provided a degree of protection against storms and tsunamis, collectively known as high energy marine inundation events. This work presents a case study of the design and implementation of a forested bioshield established to protect a vulnerable wetland on Maui's south east coast. Although subject to coastal inundation, this ecosystem provides high quality habitat for numerous endangered species. Anthropogenic modifications around the wetlands, particularly the loss of the protective forest, have made this ecosystem vulnerable to future inundation events. Establishing an effective bioshield requires in-depth knowledge of both the frequency and intensity of inundation events, as well as effective tree species selection and their proper configuration within the bioshield. Here, we present palynological and archaeobotanical data from the studied wetlands, and combine this with local paleotsunami data, previously published data on forested bioshields, and traditional ecological knowledge to design, optimize and install an 8,000 m2 forested bioshield, and review the wider benefits and limitations of this bioshield approach.

The South Pacific was one of the last regions on earth to be colonised by humans and offers a unique opportunity to study early climate-human interactions in environments previously untouched by people. Palaeoclimate evidence suggests the South Pacific has experienced shifts between dry and wet periods throughout the past three thousand years, the broad period of colonisation, with extremes in both modes being prevalent. Drought has significant repercussions for small Pacific islands, affecting water and food resources, with potential consequences on the viability of life on these islands leading to internal stress, conflict, collapse or migration. Previously, socio-ecological models have been developed to test mechanisms of change within prehistoric societies worldwide that can lead to migration or societal change, but thus far the connections between past climatic change and prehistoric island life within the tropical South Pacific have not been fully explored. This study utilises palaeoclimatic data alongside a new system dynamics socio-ecological model to explore the relationship between climate, agricultural carrying capacity and population dynamics on the Polynesian island of Mangaia (Cook Islands) in the tropical South Pacific. Model results suggest that as the population density of the island increases, the impact of drought events on population dynamics increases. We also show that the severity of the drought rather than the return frequency drove the largest changes in carrying capacity and population dynamics. Changes in long-term rainfall leading to persistent dry conditions impacted the timing and rate of population growth due to its role as a limiting factor for agricultural productivity. We compare our modelled results with the known history of population stress and societal change from Mangaia and found these corresponded with drought periods and low food availability. We demonstrate the potential for droughts to have impacted on the early colonisation and societal change on Eastern Polynesian islands.

The South Pacific Convergence Zone is a band of intense austral summer rainfall in the tropical Pacific. Changes in the South Pacific Convergence Zone are linked to Pacific sea surface temperatures on decadal timescales, but its behaviour and impacts over longer timescales remain poorly understood due to limited proxy records and model uncertainties. We combine new plant wax hydroclimate records with existing proxy evidence and climate model simulations to investigate South Pacific Convergence Zone changes over the past 1500 years. Our findings indicate that between 1000 and 200 years ago, the eastern South Pacific Convergence Zone became wetter while the western part became drier. Model simulations suggest that these centennial-scale changes were driven by Pacific sea surface temperature gradients. This eastward shift coincides with Polynesian colonisation, implying hydroclimate shifts both 'pushed' migration eastward and 'pulled' successful eastern settlement.

Understanding when gravel moves in river beds is essential for a range of different applications, but is still surprisingly hard to predict. The critical shear stress at which a grain will move depends on its relative size and structure within the bed, and spatial and temporal changes in grain-scale structure are likely to be a major driver of changes in critical shear stress. Consequently grain-structure metrics such as protrusion, pivot angle and contact with any surrounding fine grained matrix are used as parameters in models to predict critical shear stress, and so there is an increasing demand for measurements of these parameters in order to improve our predictive ability. However, we do not have established methods for measuring these parameters, nor do we know whether different methods provide consistent results. Here we present and compare new datasets of sediment structure metrics collected from eight locations in a small gravel-bed stream using three different methods: direct field-based measurements, terrestrial laser scanning (TLS), and computed tomography (CT) scanning. Using each method, we measure metrics including grain size distribution, grain protrusion and fine matrix content. We find that distributions of grain size are consistent between field-based and TLS data, but smaller in CT data. All three methods produce similar distributions of protrusion relative to grain size. There is also some consistency between field and CT measures of fine-grained matrix. However, the identification of similarity also depends on the type of analysis, and an alternative analysis shows less similarity in protrusion and fine-grained matrix between the different methods. Of the three methods, TLS-based approaches have potential to be most easily applied, and our analysis suggests that for grain-size and protrusion they perform as well as the alternative methods. However, they cannot currently be used for measuring fine-grained matrix content.

The South Pacific Convergence Zone (SPCZ) is a critical region of intense precipitation, particularly during the austral summer (November—March), situated in the tropical Pacific Ocean. This region is known to be highly sensitive to variations in sea surface temperatures (SSTs) across the Pacific. Previous research has demonstrated that decadal-scale shifts in the SPCZ are influenced by SST variability, but our understanding of longer-term changes—spanning multi-decadal to millennial timescales—remains limited. These constraints arise from the scarcity of high-resolution hydroclimate proxy records and systemic biases in even state-of-the-art coupled climate models.To address this knowledge gap, we combine newly developed hydroclimate proxies (from Nuku Hiva and Tahiti, French Polynesia) with previously established proxies and novel climate model simulations to explore SPCZ dynamics over the last 1500 years. Our findings provide new insights into centennial- and millennial-scale precipitation variability and its potential drivers.Our proxy data indicate a spatial shift in SPCZ precipitation patterns from 1000 yrs BP to 200 yrs BP. During this interval, the eastern SPCZ experienced a significant increase in precipitation, whilst the western SPCZ underwent notable drying. This north-eastward shift in precipitation was likely driven by changes in Pacific SST gradients, as shown by our climate model simulations and proxy SST reconstructions. Our modelling results show that a previously hypothesised weakening of the tropical Pacific zonal SST gradient is consistent with our new proxy reconstructions and offer a plausible mechanism for the observed hydroclimatic shifts.An intriguing corollary of our study is the coincidence of this millennial-scale hydroclimatic changes with key events in human history. The eastward shift in SPCZ precipitation overlaps temporally with the Polynesian colonization of the eastern SPCZ region, including islands such as the Marquesas, and eventually Hawaii. This suggests a potential linkage between climate-driven changes in the SPCZ and patterns of human migration and settlement. Enhanced precipitation in the eastern SPCZ would have likely improved freshwater availability, agricultural potential, and overall habitability of these islands, potentially facilitating successful colonization. Conversely, drier conditions in the western SPCZ may have influenced resource pressures, encouraging exploration and eastward movement.Ultimately, these findings emphasize the need for continued development of high-resolution proxies and improvements in coupled climate models to deepen our understanding of long-term Pacific region climate dynamics and their societal impacts.

The South Pacific Convergence Zone (SPCZ) is the most prominent precipitation feature in the southern hemisphere, extending southeast from Papua New Guinea to French Polynesia. Changes in SPCZ precipitation dynamics can have major impacts on local communities and ecosystems, as well as the global hydrologic balance and ocean circulation. Variability in SPCZ precipitation can be characterized as changes in precipitation intensity throughout the entire rainfall band, or as changes in its mean annual position. Proxy reconstructions of precipitation rates from single sites within the SPCZ region cannot distinguish changes in SPCZ intensity from changes in SPCZ location, and the low density of proxy-based precipitation records from the pre-instrumental era makes it challenging to characterize past SPCZ dynamics. To address this gap, we present quantitative records of rainfall rates derived from sediment cores collected from five freshwater lakes in the western portion of the SPCZ (from Tetepare and Rendova Islands in Solomon Islands, and from Thion Island in northern Vanuatu), spanning the past 500 to 1000 years, depending on the site. Our records are based on the hydrogen isotope composition of the dinoflagellate biomarker dinosterol, which is quantitatively related to mean annual precipitation. Our dinosterol records are complemented by analyses of magnetic susceptibility, pollen, and leaf wax hydrogen isotopes. We pair our new dinosterol-based precipitation reconstructions with previously published, comparable records from lakes in Samoa, Wallis, and southern Vanuatu to demonstrate that precipitation rates were systematically lower throughout the western and central SPCZ during the Little Ice Age (1450 – 1850 CE), indicating a decrease in precipitation intensity. The earlier Medieval Climate Anomaly (950 – 1250 CE) is also characterized by a tendency to drier conditions than in the modern period, but with more spatial heterogeneity. This networked reconstruction of precipitation rates in the SPCZ region provides the opportunity to better assess how rainfall dynamics in the region have changed through time, and how modes of variability within the SPCZ are related to global climate change.

Hydroclimate in the tropical South Pacific is dominated by the South Pacific Convergence Zone (SPCZ), a region of low-level atmospheric convergence responsible for providing fresh water to 11 million people. The SPCZ is known to change in orientation and intensity in response to interannual climate phenomena, including El Niño Southern Oscillation (ENSO) and the interdecadal Pacific Oscillation (IPO), principally through modulation of trade wind strength (i.e., Walker circulation strength), and the resultant moisture inflow. Understanding how the orientation and intensity of the SPCZ changed under past climate states is important to predict future SPCZ changes, currently poorly represented in existing GCM’s. However, our knowledge of the dynamics of the SPCZ beyond the last 1000 years is limited by a lack of proxy archives and a large spread in climate model ensembles. We present a 60 ka plant wax record of paleoprecipitation collected from a peat sediment core from the island of Nuku Hiva, French Polynesia, located in the northeastern margin of the SPCZ. We demonstrate that Nuku Hiva was drier during the last glacial maximum (LGM) and wetter during the early Holocene compared to modern conditions. This indicates that the SPCZ was located further to the south during the LGM and further to the north during the early Holocene. We find a strong correlation between our SPCZ precipitation record and foraminifera based reconstructions of western Pacific warm pool thermocline depth. Given that both modern western Pacific thermocline depth and Nuku Hiva precipitation are influenced by easterly trade wind speed, we deduce that trade wind speeds were likely lower during the LGM and higher during the early Holocene, highlighting the long term dependence of SPCZ orientation on Walker circulation strength. This study, will help constrain future predictions of SPCZ precipitation change.

Historical climate change in southern England was investigated using ostracod oxygen-isotope (δ 18 O) records from two anthropogenic lakes in Hampshire, southern England. A strong relationship is observed between δ 18 O ostracod , δ 18 O precipitation and δ 18 O lake_water in the contemporary environment and therefore δ 18 O ostracod from the sedimentary record of these systems has the potential to reflect past climate variability. The possibility of these sites to act as archives of climate change through δ 18 O ostracod analysis is explored through the study of lake sediment cores that cover the period from the early 20th century onwards. Both lakes showed similar directionality of shifts in δ 18 O ostracod over this period, suggesting common driving mechanisms. Comparing δ 18 O ostracod timeseries to meteorological data is challenging in part because of the complexity with which climate parameters are recorded in the δ 18 O lake_water and consequently within lacustrine carbonates. Our findings highlight the potential of sediments from anthropogenic lakes to act as archives of past climate and indicate they may be an important resource for generating climatic reconstructions across the medieval to instrumental period, which the sediments of many anthropogenic lakes cover. Such climate reconstructions would greatly improve our spatial and temporal understanding of climate variability where instrumental data are unavailable and other natural archives are scarce.

In Hawai`i, tsunamis are often described in orally transmitted legends (mo`olelo). This study examines sedimentary evidence of a possible local submarine landslide-generated tsunami, described in a legend from the south east coast of Maui which originated between the 15th Century CE and the first arrival of Europeans in 1778 CE. Physical evidence for a tsunami, found at the Nu'u Refuge, Maui, is primarily comprised of an extensive coral clast deposit (found 8.5 m above msl and 251 m inland from the shoreline) together with waterworn cobbles which form fracture-embedded wedge clasts in a local basalt escarpment (at up to 8 m above msl). U/Th dating of the coral clasts gives a maximum tsunami deposit age of 1671 CE for the event that may have inspired the local mo`olelo. This depositional sequence is used to characterize the nature of the assumed tsunami in terms of inundation distance, maximum wave runup and minimum flow velocities. A numerical model developed using GeoClaw matches well with the physical evidence. The data and modeling presented here suggest that locallygenerated tsunamis from submarine landslides warrant further research attention as sources of destructive high energy marine inundation events.

Elevated fine sediment accumulation in a river system's gravel bed is known to cause detrimental ecological impacts. Current sediment targets and approaches to mitigation have failed due to the oversimplification of geomorphological processes controlling fine sediment accumulation and the lack of relevant scientific knowledge underpinning them. This is particularly apparent in chalk streams (groundwater-dominated systems) which regularly exhibit high rates of sediment accumulation despite low suspended sediment yields. A necessary first step is to better characterise their sedimentology; thus, the novelty of this study was to determine the sedimentological characteristics of chalk stream gravel beds, specifically the quantity and distribution of fine sediment with depth. We collated published and unpublished freeze-core data, encompassing 90 sites across 11 UK chalk streams. Results showed average quantities of fine sediment (<2 mm) in chalk stream gravel beds were 25% by weight, with >75% of beds exceeding thresholds for ecological degradation. Quantities of fine sediment increased with increasing depth into the bed, with an average increase between surface and subsurface layers of 54%, and 89% of the gravel bed over-saturated with fine sediment. Regional differences were attributed to differences in stream power and local sediment sources, including surficial geology and catchment land use. Additionally, a major contrast was identified between experimental conditions in flume studies used to establish models describing interactions/mechanisms of fine sediment infiltration into immobile gravel beds and the natural conditions observed in chalk streams. As such, the use of such models as a basis to explore sediment management scenarios is unlikely to predict the outcome of such management techniques correctly in a real-world situation.

Understanding when gravel moves in river beds is essential for a range of different applications but is still surprisingly hard to predict. Here we consider how our ability to predict critical shear stress (tau c) is being improved by recent advances in two areas: (1) identifying the onset of bedload transport; and (2) quantifying grain-scale gravel bed structure. This paper addresses these areas through both an in-depth review and a comparison of new datasets of gravel structure collected using three different methods. We focus on advances in these two areas because of the need to understand how the conditions for sediment entrainment vary spatially and temporally, and because spatial and temporal changes in grain-scale structure are likely to be a major driver of changes in tau c. We use data collected from a small gravel-bed stream using direct field-based measurements, terrestrial laser scanning (TLS) and computed tomography (CT) scanning, which is the first time that these methods have been directly compared. Using each method, we measure structure-relevant metrics including grain size distribution, grain protrusion and fine matrix content. We find that all three methods produce consistent measures of grain size, but that there is less agreement between measurements of grain protrusion and fine matrix content. We review recent advances in monitoring bedload transport and in quantifying sediment structure and show how they offer new opportunities to improve predictions of critical shear stress. As illustration, we compare three methods to measure a range of parameters that describe grain-scale sediment structure: direct field measurements, terrestrial laser scan data and CT scan data. Measurements of grain size are comparable between methods, but measurements of grain protrusion and fine-grained matrix are more variable. image

Estimates of current and future population exposure to both coastal and inland flooding do not exist consistently in all Small Island Developing States (SIDS), despite these being some of the places most at risk to climate change. This has primarily been due to a lack of suitable or complete data. In this paper, we utilise a similar to 30 m global hydrodynamic flood model to estimate population exposure to coastal and inland flood hazard in all SIDS under present day, as well as under low, intermediate, and very high emissions climate change scenarios (SSP1-2.6, SSP2-4.5 and SSP5-8.5). Our analysis shows that present day population exposure to flooding in SIDS is high (19.5% total population: 100 year flood hazard), varies widely depending on the location (3%-66%), and increases under all three climate scenarios-even if global temperatures remain below 2 C-degrees warming (range in percentage change between present day and SSP1-2.6:-4.5%-44%). We find that levels of flood hazard and population exposure are not strongly linked, and that indirect measures of exposure in common vulnerability or risk indicators do not adequately capture the complex drivers of flood hazard and population exposure in SIDS. The most exposed places under the lowest climate change scenario (SSP1-2.6) continue to be the most exposed under the highest climate change scenario (SSP5-8.5), meaning investment in adaptation in these locations is likely robust to climate scenario uncertainty.

In the South-East Pacific, understanding how precipitation will respond to changes in temperature is crucial in anticipating future climate changes. However, our understanding of the regional climate temperature sensitivity is limited by a lack of any long-term paleoclimate reconstructions which extend before the climatically stable Holocene. To address this knowledge gap,we reconstruct temperature and precipitation using biomarker and pollen proxies sampled from a 50\,ka year-long sediment core located in Nuku Hiva (French Polynesia). The hydrogen isotopic composition of plant waxes is more positive during the last glacial period indicating drier conditions. Pollen analysis indicates that allwetter cloud forest species decreased in abundance in the glacial period, whilst drier-adapted herbaceous taxa increased. Temperature-sensitive bacterial lipids indicate much cooler conditions in the glacial period than in the Holocene. Crucially, changes in reconstructed temperature and precipitation are highly correlated over this time period, which is at odds with state-of-the-art modelling studies. These results willaid models in predicting future changes in precipitation in a region already experiencing the devastating effects of anthropogenic climate change.

The increasing similarity of plant species composition among distinct areas is leading to the homogenization of ecosystems globally. Human actions such as ecosystem modification, the introduction of non-native plant species and the extinction or extirpation of endemic and native plant species are considered the main drivers of this trend. However, little is known about when floristic homogenization began or about pre-human patterns of floristic similarity. Here we investigate vegetation trends during the past 5,000 years across the tropical, sub-tropical and warm temperate South Pacific using fossil pollen records from 15 sites on 13 islands within the biogeographical realm of Oceania. The site comparisons show that floristic homogenization has increased over the past 5,000 years. Pairwise Bray–Curtis similarity results also show that when two islands were settled by people in a given time interval, their floristic similarity is greater than when one or neither of the islands were settled. Importantly, higher elevation sites, which are less likely to have experienced human impacts, tended to show less floristic homogenization. While biotic homogenization is often referred to as a contemporary issue, we have identified a much earlier trend, likely driven by human colonization of the islands and subsequent impacts.

Resilience, which can also be described as absorbing capacity, describes the amount of change that a system can undergo in response to disturbance and maintain a characteristic, self-sustaining regime of functions, processes, or sets of feedback loops. Rivers exhibit varying levels of resilience, but the net effect of industrialized anthropogenic alteration has been to suppress river resilience. As changing climate alters the inputs to rivers and human modification alters the morphology and connectivity of rivers, restoration increasingly considers how to enhance resilience. Characteristics that underpin river absorbing capacity include natural regimes, connectivity, physical and ecological integrity, and heterogeneity. River management emphasizing channel stabilization and homogenization has reduced river absorbing capacity. We propose that the paths to restoring rivers include defining relevant measures of absorbing capacity and understanding the scales of restoration and the sociopolitical elements of river restoration. We provide a conceptual framing for choosing measures that could be used to assess river absorbing capacity.