We assess the relationship between temperature and global sea-level (GSL) variability over the Common Era through a statistical metaanalysis of proxy relative sea-level reconstructions and tide-gauge data. GSL rose at 0.1 ± 0.1 mm/y (2σ) over 0–700 CE. A GSL fall of 0.2 ± 0.2 mm/y over 1000–1400 CE is associated with ∼0.2 °C global mean cooling. A significant GSL acceleration began in the 19th century and yielded a 20th century rise that is extremely likely (probability P≥0.95) faster than during any of the previous 27 centuries. A semiempirical model calibrated against the GSL reconstruction indicates that, in the absence of anthropogenic climate change, it is extremely likely (P=0.95) that 20th century GSL would have risen by less than 51% of the observed 13.8±1.5 cm. The new semiempirical model largely reconciles previous differences between semiempirical 21st century GSL projections and the process model-based projections summarized in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report.

The processes that control the formation, intensity and track of hurricanes are poorly understood. It has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones, but this proposal has been challenged on the basis that the instrumental record is too short and unreliable to reveal trends in intense tropical cyclone activity. Storm-induced deposits preserved in the sediments of coastal lagoons offer the opportunity to study the links between climatic conditions and hurricane activity on longer timescales, because they provide centennial- to millennial-scale records of past hurricane landfalls. Here we present a record of intense hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon that contain coarse-grained deposits associated with intense hurricane landfalls. The record indicates that the frequency of intense hurricane landfalls has varied on centennial to millennial scales over this interval. Comparison of the sediment record with palaeo-climate records indicates that this variability was probably modulated by atmospheric dynamics associated with variations in the El Niño/Southern Oscillation and the strength of the West African monsoon, and suggests that sea surface temperatures as high as at present are not necessary to support intervals of frequent intense hurricanes. To accurately predict changes in intense hurricane activity, it is therefore important to understand how the El Niño/Southern Oscillation and the West African monsoon will respond to future climate change.

A 4500-year record of hurricane-induced storm surges is developed from sediment cores collected from a coastal sinkhole near Apalachee Bay, Florida. Recent deposition of sand layers in the upper sediments of the pond was found to be contemporaneous with significant, historic storm surges at the site modeled using SLOSH and the Best Track, post-1851 A.D. dataset. Using the historic portion of the record for calibration, paleohurricane deposits were identified by sand content and dated using radiocarbon-based age models. Marine-indicative foraminifera, some originating at least 5 km offshore, were present in several modern and ancient storm deposits. The presence and long-term preservation of offshore foraminifera suggest that this site and others like it may yield promising microfossil-based paleohurricane reconstructions in the future. Due to the sub-decadal (~ 7 year) resolution of the record and the site's high susceptibility to hurricane-generated storm surges, the average, local frequency of recorded events, approximately 3.9 storms per century, is greater than that of previously published paleohurricane records from the region. The high incidence of recorded events permitted a time series of local hurricane frequency during the last five millennia to be constructed. Variability in the frequency of the largest storm layers was found to be greater than what would likely occur by chance alone, with intervals of both anomalously high and low storm frequency identified. However, the rate at which smaller layers were deposited was relatively constant over the last five millennia. This may suggest that significant variability in hurricane frequency has occurred only in the highest magnitude events. The frequency of high magnitude events peaked near 6 storms per century between 2800 and 2300 years ago. High magnitude events were relatively rare with about 0-3 storms per century occurring between 1900 to 1600 years ago and between 400 to 150 years ago. A marked decline in the number of large storm deposits, which began around 600 years ago, has persisted through present with below average frequency over the last 150 years when compared to the preceding five millennia.

A number of past studies have attempted to place modern Atlantic TC activity in a longer-term context using regional proxy evidence of past landfalling Atlantic hurricane activity [6][7][8] . Some studies 4 have sought to infer past changes in activity from plausible local conditioning factors such as wind strength and Sea Surface Temperature (SST), though the interpretations of these studies have been contested 5 . Qualitative comparisons between paleo-hurricane reconstructions appear to show some temporal coherence 8-9 .However, no past studies have attempted to synthesize multiple records from distinct regions into a basin-integrated reconstruction of Atlantic hurricane activity. Moreover, no past studies have sought to quantitatively relate estimated variations in hurricane or TC activity to reconstructions of the key large-scale climate factors known to have a 3 significant influence on modern Atlantic TC activity. Here we produce an empirical record of past landfalling Atlantic hurricane activity by combining information from multiple sedimentary records of TC-induced overwash. Further we compare these resulting estimates to independent statistical model predictions of past TC activity driven by proxy-based large-scale climate reconstructions.Sediment-based overwash reconstructions of TC landfall are limited in number, but span a wide geographic area across the North Atlantic basin impacted by hurricanes. Our compilation includes (Figure 1) a site from the Caribbean (Vieques, PR 6,9,10 ), one from the U.S. Gulf Coast 11 , one from the southeastern U.S. coast 8 , three from the mid-Atlantic coast (one from New York 9 and two from New Jersey 12,13 ) and two from southeastern New England (one from Rhode Island 14 and another from Massachusetts 15 ) yielding 5 distinct regional series. We obtained a probabilistic estimate of past basin-wide landfalling hurricane activity using an appropriately weighted combination of the information from these 5 regional series, and incorporating radiocarbon age model uncertainties.An independent estimate of past tropical cyclone activity was obtained using a statistical model for Atlantic TC counts. This previously developed and validated 16,3 statistical model conditions annual Atlantic TC counts on three key large-scale climate state variables tied to historical variations in Atlantic TC counts: (i) Sea Surface Temperature (SST) over the main development region (MDR) for tropical Atlantic TCs, which reflects favorability of the local thermodynamic environment, (ii) the El Niño/Southern Oscillation (ENSO) which influences the amount of (unfavorable) vertical wind shear, and (iii) the North Atlantic Oscillation (NAO) phenomena which affects the tracking of storms (which influences how favorable an environment they 4 encounter). The statistical model was driven by proxy-based reconstructions 17,18 of these three state variables (Figure 2), yielding a predicted history of Atlantic TC counts for past centuries We compared the sediment-based record against the above statistical e...

The distribution of New England salt marsh communities is intrinsically linked to the magnitude, frequency, and duration of tidal inundation. Cordgrass (Spartina alterniflora) exclusively inhabits the frequently flooded lower elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) typically dominate higher elevations. Monitoring plant zonal boundaries in two New England salt marshes revealed that low-marsh cordgrass rapidly moved landward at the expense of higher-marsh species between 1995 and 1998. Plant macrofossils from sediment cores across modern plant community boundaries provided a 2,500-year record of marsh community composition and documented the migration of cordgrass into the high marsh. Isotopic dating revealed that the initiation of cordgrass migration occurred in the late 19th century and continued through the 20th century. The timing of the initiation of cordgrass migration is coincident with an acceleration in the rate of sea-level rise recorded by the New York tide gauge. These results suggest that increased flooding associated with accelerating rates of sea-level rise has stressed high-marsh communities and promoted landward migration of cordgrass. If current rates of sea-level rise continue or increase slightly over the next century, New England salt marshes will be dominated by cordgrass. If climate warming causes sea-level rise rates to increase significantly over the next century, these cordgrass-dominated marshes will likely drown, resulting in extensive losses of coastal wetlands. R ecent studies indicate that both climate warming (1, 2) and increases in the rate of sea-level rise (SLR) in New England (3) over the last 150 years are unprecedented in at least the last 1,000 years. The possibility that emission of greenhouse gases is influencing and will continue to influence global climate and potentially SLR has prompted considerable research into the possible implications for plant and animal communities (4). The distribution of salt-marsh communities is mechanistically linked to the duration of tidal inundation. As a result, coastal wetlands may be particularly sensitive to changes in sea level (5). Wetland loss has been documented in areas of the Mississippi River Delta (6) and Chesapeake Bay (7), where rates of local SLR exceed marsh accretion.In New England salt marshes, cordgrass (Spartina alterniflora) exclusively dominates daily f looded low-marsh elevations, whereas a mosaic of marsh hay (Spartina patens), spike grass (Distichlis spicata), and black rush (Juncus gerardi) dominates higher marsh elevations (8). Lower species borders are controlled by physical stress tolerance to flooding and soil anoxia, whereas upper species borders are controlled by interspecific plant competition (9, 10). Marsh hay and spike grass are excluded from the low marsh by low substrate oxygen levels. The ability of cordgrass to oxygenate substrates (11) allows this species to dominate frequently flooded lower-marsh elevations, where...

How climate controls hurricane variability has critical implications for society is not well understood. In part, our understanding is hampered by the short and incomplete observational hurricane record.

We present new sea-level reconstructions for the past 2100 y based on salt-marsh sedimentary sequences from the US Atlantic coast. The data from North Carolina reveal four phases of persistent sea-level change after correction for glacial isostatic adjustment. Sea level was stable from at least BC 100 until AD 950. Sea level then increased for 400 y at a rate of 0.6 mm/y, followed by a further period of stable, or slightly falling, sea level that persisted until the late 19th century. Since then, sea level has risen at an average rate of 2.1 mm/y, representing the steepest century-scale increase of the past two millennia. This rate was initiated between AD 1865 and 1892. Using an extended semiempirical modeling approach, we show that these sea-level changes are consistent with global temperature for at least the past millennium.climate | ocean | late Holocene | salt marsh C limate and sea-level reconstructions encompassing the past 2,000 y provide a preanthropogenic context for understanding the nature and causes of current and future changes. Hemispheric and global mean temperature have been reconstructed using instrumental records supplemented with proxy data from natural climate archives (1, 2). This research has improved understanding of natural climate variability and suggests that modern warming is unprecedented in the past two millennia (1). In contrast, understanding of sea-level variability during this period is limited and the response to known climate deviations such as the Medieval Climate Anomaly, Little Ice Age, and 20th century warming is unknown. We reconstruct sea-level change over the past 2100 y using new salt-marsh proxy records and investigate the consistency of reconstructed sea level with global temperature using a semiempirical relationship that connects sea-level changes to mean surface temperature (3, 4). The new sea level proxy data constrain a multicentennial response term in the semiempirical model. Results and DiscussionSea-Level Data. Salt-marsh sediments and assemblages of foraminifera record former sea level because they are intrinsically linked to the frequency and duration of tidal inundation and keep pace with moderate rates of sea-level rise (5, 6). We developed transfer functions using a modern dataset of foraminifera (193 samples) from 10 salt marshes in North Carolina, USA (7). Transfer functions are empirically derived equations for quantitatively estimating past environmental conditions from paleontological data (8). The transfer functions were applied to foraminiferal assemblages preserved in 1 cm thick samples from two cores of salt-marsh sediment (Sand Point and Tump Point, North Carolina; Fig. 1) to estimate paleomarsh elevation (PME), which is the tidal elevation at which a sample formed with respect to its contemporary sea level (9). Unique vertical errors were calculated by the transfer functions for each PME estimate and were less than 0.1 m. Composite chronologies were developed using Accelerator Mass Spectrometry (AMS) 14 C (conventional, high-precision, and bo...

Over the past century, many of the world's major rivers have been modified for the purposes of flood mitigation, power generation and commercial navigation. Engineering modifications to the Mississippi River system have altered the river's sediment levels and channel morphology, but the influence of these modifications on flood hazard is debated. Detecting and attributing changes in river discharge is challenging because instrumental streamflow records are often too short to evaluate the range of natural hydrological variability before the establishment of flood mitigation infrastructure. Here we show that multi-decadal trends of flood hazard on the lower Mississippi River are strongly modulated by dynamical modes of climate variability, particularly the El Niño-Southern Oscillation and the Atlantic Multidecadal Oscillation, but that the artificial channelization (confinement to a straightened channel) has greatly amplified flood magnitudes over the past century. Our results, based on a multi-proxy reconstruction of flood frequency and magnitude spanning the past 500 years, reveal that the magnitude of the 100-year flood (a flood with a 1 per cent chance of being exceeded in any year) has increased by 20 per cent over those five centuries, with about 75 per cent of this increase attributed to river engineering. We conclude that the interaction of human alterations to the Mississippi River system with dynamical modes of climate variability has elevated the current flood hazard to levels that are unprecedented within the past five centuries.