Global Mean Sea Level Change Research Articles

Projection of the Zhujiang (Pearl) River Delta’s potential submerged area due to sea level rise during the 21st century based on CMIP5 simulations

Projections of potential submerged area due to sea level rise are helpful for improving understanding of the influence of ongoing global warming on coastal areas. The Ensemble Empirical Mode Decomposition method is used to adaptively decompose the sea level time series in order to extract the secular trend component. Then the linear relationship between the global mean sea level (GMSL) change and the Zhujiang (Pearl) River Delta (PRD) sea level change is calculated: an increase of 1.0 m in the GMSL corresponds to a 1.3 m (uncertainty interval from 1.25 to 1.46 m) increase in the PRD. Based on this relationship and the GMSL rise projected by the Coupled Model Intercomparison Project Phase 5 under three greenhouse gas emission scenarios (representative concentration pathways, or RCPs, from low to high emission scenarios RCP2.6, RCP4.5, and RCP8.5), the PRD sea level is calculated and projected for the period 2006–2100. By around the year 2050, the PRD sea level will rise 0.29 (0.21 to 0.40) m under RCP2.6, 0.31 (0.22 to 0.42) m under RCP4.5, and 0.34 (0.25 to 0.46) m under RCP8.5, respectively. By 2100, it will rise 0.59 (0.36 to 0.88) m, 0.71 (0.47 to 1.02) m, and 1.0 (0.68 to 1.41) m, respectively. In addition, considering the extreme value of relative sea level due to land subsidence (i.e., 0.20 m) and that obtained from intermonthly variability (i.e., 0.33 m), the PRD sea level will rise 1.94 m by the year 2100 under the RCP8.5 scenario with the upper uncertainty level (i.e., 1.41 m). Accordingly, the potential submerged area is 8.57×103 km2 for the PRD, about 1.3 times its present area.

Geographic Variability of Sea-Level Change

Local sea-level changes differ significantly from global-mean sea-level change as a result of (1) non-climatic, geological background processes; (2) atmosphere/ocean dynamics; and (3) the gravitational, elastic, and rotational “fingerprint” effects of ice and ocean mass redistribution. Though the research communities working on these different effects each have a long history, the integration of all these different processes into interpretations of past changes and projections of future change is an active area of research. Fully characterizing the past contributions of these processes requires information from sources covering a range of timescales, including geological proxies, tide-gauge observations from the last ~3 centuries, and satellite-altimetry data from the last ~2 decades. Local sea-level rise projections must account for the different spatial patterns of different processes, as well as potential correlations between different drivers.

Sea level rise for India since the start of tide gauge records

The global mean sea level (GMSL) changes derived from modelling do not match actual measurements of sea level and should not be trusted. Compilations of individual tide gauges of sufficient quality and length provide much more reliable information. The present work is a contribution towards a better understanding of the observed of sea levels in India and its relation to worldwide observations. The latest average relative rate of rise of worldwide sea levels from a compilation of 170 stations with more than 60 years of data returns an average relative rate of rise +0.25 mm/year. The individual rates of rise are about constant in between subsequent updates suggesting the absence of any acceleration. Observation in key sites suggests a similarly stable pattern. Along the coastline of India, the average rate of rise of sea level is +1.06 mm/year computed by considering the 11 longest tide gauges of average length 51 years. Shorter records may overrate the sea level rate of rise because of the local phasing of the quasi-60-year oscillation. In the longest records, the rates of rise are decreasing since 1955. The lack of any GPS monitoring of the vertical position of the tide gauge does not permit the determination of the absolute rates of rise.

Closing the sea level budget at the Last Glacial Maximum

Establishing the volume of excess ice contained in the global ice sheets during the Last Glacial Maximum (LGM; ∼26,000–19,000 y ago) remains a longstanding problem in Ice Age climate dynamics. Expressed as the equivalent lowering of global mean sea level (GMSL), estimates of this value have varied from 105 (1) to 163 m (2), with many estimates suggesting ∼120 m (3). This wide range introduces substantial uncertainties in LGM global boundary conditions (ice sheet height and extent and continental shelf exposure) that strongly influence climate through their impacts on atmospheric and ocean circulation and global temperature. More recently, this uncertainty has influenced our understanding of present sea level change, whereby the ongoing glacial isostatic adjustment (GIA) of the solid Earth to the redistribution of mass since the LGM must be accounted for in satellite measurements of sea level and of current mass changes from shrinkage of glaciers and ice sheets. In PNAS, Lambeck et al. (4) present a comprehensive analysis of nearly 1,000 paleo-sea level markers to reconstruct GMSL change over the last 35,000 y, with their estimate for the LGM GMSL of 134–140 m being significantly larger than most published estimates but similar to a recent reassessment of the Barbados sea level record (130 m) that had been the basis for many estimates of ∼120 m (5). If it stands, this reconstruction will reduce uncertainties in our understanding of Ice Age climate and modern sea level change. At the same time, however, it raises a significant challenge: is it possible to explain this total sea level change from our best estimates of the contributions from individual LGM glaciers and ice sheets?

Global and regional sea level change during the 20th century

Sea level variations prior to the launch of satellite altimeters are estimated by analyzing historic tide gauge records. Recently, a number of groups have reconstructed sea level by applying EOF techniques to fill missing observations. We complement this study with alternative methods. In a first step gaps in 178 records of sea level change are filled using the pattern recognition capabilities of artificial neural networks. Afterward satellite altimetry is used to extrapolate local sea level change to global fields. Patterns of sea level change are compared to prior studies. Global mean sea level change since 1900 is found to be 1.77±0.38 mm yr−1 on average. Local trends are essentially positive with the highest values found in the western tropical Pacific and in the Indian Ocean east of Madagascar where it reaches about +6 mm yr−1. Regions with negative trends are spotty with a minimum value of about −2 mm yr−1 south of the Aleutian Islands. Although the acceleration found for the global mean, +0.0042 ± 0.0092 mm yr−2, is not significant, local values range from −0.1 mm yr−2 in the central Indian Ocean to +0.1 mm yr−2 in the western tropical Pacific and east of Japan. These extrema are associated with patterns of sea level change that differ significantly from the first half of the analyzed period (i.e., 1900–1950) to the second half (1950–2000). We take this as an indication of long period oceanic processes that are superimposed to the general sea level rise.

The land-ice contribution to 21st-century dynamic sea level rise

Abstract. Climate change has the potential to influence global mean sea level through a number of processes including (but not limited to) thermal expansion of the oceans and enhanced land ice melt. In addition to their contribution to global mean sea level change, these two processes (among others) lead to local departures from the global mean sea level change, through a number of mechanisms including the effect on spatial variations in the change of water density and transport, usually termed dynamic sea level changes. In this study, we focus on the component of dynamic sea level change that might be given by additional freshwater inflow to the ocean under scenarios of 21st-century land-based ice melt. We present regional patterns of dynamic sea level change given by a global-coupled atmosphere–ocean climate model forced by spatially and temporally varying projected ice-melt fluxes from three sources: the Antarctic ice sheet, the Greenland Ice Sheet and small glaciers and ice caps. The largest ice melt flux we consider is equivalent to almost 0.7 m of global mean sea level rise over the 21st century. The temporal evolution of the dynamic sea level changes, in the presence of considerable variations in the ice melt flux, is also analysed. We find that the dynamic sea level change associated with the ice melt is small, with the largest changes occurring in the North Atlantic amounting to 3 cm above the global mean rise. Furthermore, the dynamic sea level change associated with the ice melt is similar regardless of whether the simulated ice fluxes are applied to a simulation with fixed CO2 or under a business-as-usual greenhouse gas warming scenario of increasing CO2.

Global sea level trend during 1993–2012

Projection of future sea level change relies on the understanding of present sea-level trend and how it has varied in the past. Here we investigate the global-mean sea level (GMSL) change during 1993–2012 using Empirical Mode Decomposition, in an attempt to distinguish the trend over this period from the interannual variability. It is found that the GMSL rises with the rate of 3.2±0.4mm/yr during 1993–2003 and started decelerating since 2004 to a rate of 1.8±0.9mm/yr in 2012. This deceleration is mainly due to the slowdown of ocean thermal expansion in the Pacific during the last decade, as a part of the Pacific decadal-scale variability, while the land-ice melting is accelerating the rise of the global ocean mass-equivalent sea level. Recent rapid recovery of the rising GMSL from its dramatic drop during the 2011 La Niña introduced a large uncertainty in the estimation of the sea level trend, but the decelerated rise of the GMSL appears to be intact.

Antarctic ice-mass balance 2003 to 2012: regional reanalysis of GRACE satellite gravimetry measurements with improved estimate of glacial-isostatic adjustment based on GPS uplift rates

Abstract. We present regional-scale mass balances for 25 drainage basins of the Antarctic Ice Sheet (AIS) from satellite observations of the Gravity and Climate Experiment (GRACE) for time period January 2003 to September 2012. Satellite gravimetry estimates of the AIS mass balance are strongly influenced by mass movement in the Earth interior caused by ice advance and retreat during the last glacial cycle. Here, we develop an improved glacial-isostatic adjustment (GIA) estimate for Antarctica using newly available GPS uplift rates, allowing us to more accurately separate GIA-induced trends in the GRACE gravity fields from those caused by current imbalances of the AIS. Our revised GIA estimate is considerably lower than previous predictions, yielding an estimate of apparent mass change of 53 ± 18 Gt yr−1. Therefore, our AIS mass balance of −114 ± 23 Gt yr−1 is less negative than previous GRACE estimates. The northern Antarctic Peninsula and the Amundsen Sea sector exhibit the largest mass loss (−26 ± 3 Gt yr−1 and −127 ± 7 Gt yr−1, respectively). In contrast, East Antarctica exhibits a slightly positive mass balance (26 ± 13 Gt yr−1), which is, however, mostly the consequence of compensating mass anomalies in Dronning Maud and Enderby Land (positive) and Wilkes and George V Land (negative) due to interannual accumulation variations. In total, 6% of the area constitutes about half the AIS imbalance, contributing 151 ± 7 Gt yr−1 (ca. 0.4 mm yr−1) to global mean sea-level change. Most of this imbalance is caused by ice-dynamic speed-up expected to prevail in the near future.

On the recent global mean sea level changes: Trend extraction and El Niño's impact

The spatial sampling offered by TOPEX and Jason series of satellite radar altimeters and its continuity during the last twenty years are major assets to provide an improved vision of the global mean sea level (GMSL). The objective of this paper is to examine the recent GMSL variations (1993–2012) and to investigate the correlation between the GMSL and ENSO (El Niño-southern oscillation) episodes. For this purpose, a mean sea level anomalies time series, obtained from TOPEX, Jason-1 and Jason-2 measurements, is used to determine the trend of GMSL changes by using a simplified form of an unobserved components model (namely UCM). Then, to investigate the impact of the ENSO phenomenon on the GMSL changes, we considered the sea surface temperature anomalies (SSTA) index over the Niño3 region (5N–5S 150W–90W). Cross wavelet transform and wavelet coherence analysis are performed to expose common power between the GMSL changes and the SSTA index and their relative phase in the time–frequency space. The results indicate that there are in the estimated GMSL's trend a number of fluctuations over short periods that are least partly related to the El Niño/La Niña episodes. Cross wavelet transform and wavelet coherence analysis indicate that a significant correlation between GMSL and ENSO occurred during 1997–1998, 2006–2007, 2009–2010 El Niño events and 2007–2008 and 2010–2011 La Niña ones. All these areas show in-phase relationship, suggesting that GMSL and SSTA index vary synchronously.

Evaluating the ability of process based models to project sea-level change

We evaluate the ability of process based models to reproduce observed global mean sea-level change. When the models are forced by changes in natural and anthropogenic radiative forcing of the climate system and anthropogenic changes in land-water storage, the average of the modelled sea-level change for the periods 1900–2010, 1961–2010 and 1990–2010 is about 80%, 85% and 90% of the observed rise. The modelled rate of rise is over 1 mm yr−1 prior to 1950, decreases to less than 0.5 mm yr−1 in the 1960s, and increases to 3 mm yr−1 by 2000. When observed regional climate changes are used to drive a glacier model and an allowance is included for an ongoing adjustment of the ice sheets, the modelled sea-level rise is about 2 mm yr−1 prior to 1950, similar to the observations. The model results encompass the observed rise and the model average is within 20% of the observations, about 10% when the observed ice sheet contributions since 1993 are added, increasing confidence in future projections for the 21st century. The increased rate of rise since 1990 is not part of a natural cycle but a direct response to increased radiative forcing (both anthropogenic and natural), which will continue to grow with ongoing greenhouse gas emissions.

Constraining Wind Stress Products with Sea Surface Height Observations and Implications for Pacific Ocean Sea Level Trend Attribution*

Abstract A number of global surface wind datasets are available that are commonly used to examine climate variability or trends and as boundary conditions for ocean circulation models. However, discrepancies exist among these products. This study uses observed Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) sea surface height anomalies (SSHAs) as a means to help constrain the fidelity of these products in the tropical region. Each wind stress product is used to force a linear shallow water model (SWM) and the resulting hindcast thermocline depth anomalies are converted to SSHAs. The resulting SSHAs are then assessed to see how well they reproduce the dominant EOF modes of observed variability and the regional (global mean removed) sea level trend (1993–2007) in each of the three ocean basins. While the results suggest that all wind datasets reproduce the observed interannual variability with reasonable fidelity, the two SWM hindcasts that produce the observed linear trend with the highest fidelity are those incorporating interim ECMWF Re-Analysis (ERA-Interim) and Wave- and Anemometer-Based Sea Surface Wind (WASWind) forcing. The role of surface wind forcing (i.e., upper ocean heat content redistribution) versus global mean sea level change (i.e., including the additional contributions of glacier and ice sheet melt along with ocean thermal expansion) on the recent dramatic increase in western equatorial Pacific island sea level is then reassessed. The results suggest that the recent sea level increase cannot be explained solely by wind stress forcing, regardless of the dataset used; rather, the global mean sea level signal is required to fully explain this observed recent abrupt sea level rise and to better explain the sea level variability of the last 50–60 years.

Modeled steric and mass-driven sea level change caused by Greenland Ice Sheet melting

Meltwater from the Greenland Ice Sheet (GIS) has been a major contributor to sea level change in the recent past. Global and regional sea level variations caused by melting of the GIS are investigated with the finite element sea-ice ocean model (FESOM). We consider changes of local density (steric effects), mass inflow into the ocean, redistribution of mass, and gravitational effects. Five melting scenarios are simulated, where mass losses of 100, 200, 500, and 1000Gt/yr are converted to a continuous volume flux that is homogeneously distributed along the coast of Greenland south of 75°N. In addition, a scenario of regional melt rates is calculated from daily ice melt characteristics. The global mean sea level modeled with FESOM increases by about 0.3mm/yr if 100Gt/yr of ice melts, which includes eustatic and steric sea level change. In the global mean the steric contribution is one order of magnitude smaller than the eustatic contribution. Regionally, especially in the North Atlantic, the steric contribution leads to strong deviations from the global mean sea level change. The modeled pattern mainly reflects the structure of temperature and salinity change in the upper ocean. Additionally, small steric variations occur due to local variability in the heat exchange between the atmosphere and the ocean. The mass loss has also affects on the gravitational attraction by the ice sheet, causing spatially varying sea level change mainly near the GIS, but also at greater distances. This effect is accounted for by using Green's functions.

Estimating Mean Sea Level Change from the TOPEX and Jason Altimeter Missions

The Jason-2 satellite altimeter mission was launched in June 2008, extending the record of precision sea level measurements that was initiated with the launch of TOPEX/Poseidon in 1992 and continued with the launch of Jason-1 in December 2001. We have used the measurements from these three missions to construct a seamless record of global mean sea level change from 1993 to the present. We present the results of our calibration activities, including data comparisons during the “tandem period” of the missions, during which we solve for biases between the missions, as well as comparisons to independent tide gauge sea level measurements. When the entire record is assembled, the average rate of sea level rise from 1993–2009 is 3.4 ± 0.4 mm/year. There is considerable interannual variation due to ENSO-related processes, which include the period of lower sea level rise over the last three years of the time series during the recent La Nina event.

Identifying the causes of sea-level change

Global mean sea-level change has increased from a few centimetres per century over recent millennia to a few tens of centimetres per century in recent decades. A review of the latest work shows that global mean sea-level rise is unlikely to exceed one metre over the twenty-first century, but regional departures from this global mean could reach several decimetres.

GRACE observes small‐scale mass loss in Greenland

Using satellite gravity data between February 2003 and January 2008, we examine changes in Greenland's mass distribution on a regional scale. During this period, Greenland lost mass at a mean rate of 179 ± 25 Gt/yr, equivalent to a global mean sea level change of 0.5 ± 0.1 mm/yr. Rates increase over time, suggesting an acceleration of the mass loss, driven by mass loss during summer. The largest mass losses occurred along the southeastern and northwestern coast in the summers of 2005 and 2007, when the ice sheet lost 279 Gt and 328 Gt of ice respectively within 2 months. In 2007, a strong mass loss is observed during summer at elevations above 2000 m, for the first time since the start of the observations.

Relationship between global mean sea-level and global mean temperature in a climate simulation of the past millennium

The possibility of using global mean near-surface temperature, its rate of change or the global mean ocean heat-flux as predictors to statistically estimate the change of global mean sea-level is explored in the context of a long climate simulation of the past millennium with the climate model ECHO-G. Such relationships have recently been proposed to by-pass the difficulty of estimating future sea-level changes based on simulations with coarse-resolution climate models. It is found that, in this simulation, a simple linear relationship between mean temperature and the rate of change of sea level does not exist. A regression parameter linking both variables, and estimated in sliding 120-year windows, varies widely along the simulation and, in some periods, even attains negative values. The ocean heat-flux and the rate-of-change of mean temperature seem to better capture the rate-of-change of sea level due to thermal expansion.

Sea level is not level: the case for a new approach to predicting UK sea-level rise

ABSTRACTCurrent predictions for future sea-level rise along the UK coasts are based on IPCC values for global mean sea-level change combined with information on land movements, changes in tidal range and storm surges. In this article we argue that the global IPCC sea-level values should be replaced by a suite of regional values, to take into account known processes that cause regional changes in sea-surface topography. Quantification of these processes, which include gravitational adjustments of the ocean surface and nearshore ocean-density changes, requires the use of numerical ocean and geophysical models. ‘Ocean siphoning’ and ‘continental levering’ are two additional mechanisms that are not included in IPCC assessments, but can be quantified using a modelling approach. Data of vertical land movements based on geological information alone, as presently used by UKCIP (Defra) in UK sea-level rise scenarios, are potentially unreliable as they represent, in essence, values of relative land-level (or sea-level) change. These data can be improved by including geophysical model predictions and GPS measurements in assessments of vertical land motion. A combined modelling/geological approach will produce more robust regional sea-level predictions for the UK that are of real practical value to agencies responsible for coastal defence and flood protection.

Comment on "A Semi-Empirical Approach to Projecting Future Sea-Level Rise"

Rahmstorf (Reports, 19 January 2007, p. 368) presented an approach for predicting sea-level rise based on a proposed linear relationship between global mean surface temperature and the rate of global mean sea-level change. We find no such linear relationship. Although we agree that there is considerable uncertainty in the prediction of future sea-level rise, this approach does not meaningfully contribute to quantifying that uncertainty.

Steric Sea-level Variations Inferred from Combined Topex/Poseidon Altimetry and GRACE Gravimetry

In this study, we propose to estimate the steric sea-level variations over a < 2-year period (April 2002 through December 2003) by combining global mean sea level (GMSL) based on Topex/Poseidon (T/P) altimetry with time-variable geoid averaged over the oceans, as observed by the GRACE (Gravity Recovery and Climate Experiment) satellite. In effect, altimetry-derived GMSL changes results from two contributions: Steric (thermal plus salinity) effects due to sea water density change and ocean mass change due to water exchange with atmosphere and continents. On the other hand, GRACE data over the oceans provide the ocean mass change component only. The paper first discusses the corrections to apply to the GRACE data. Then the steric contribution to the GMSL is estimated using GRACE and T/P data. Comparison with available thermal expansion based on in situ hydrographic data is performed.

Estimation of steric sea level variations from combined GRACE and Jason-1 data

We estimate the mean steric sea level variations over the 60°S–60°N oceanic domain for the recent period (from August 2002 to April 2006), by combining sea level data from Jason-1 altimetry with time-variable gravity data from GRACE. The observed global mean sea level change from satellite altimetry results in total from steric plus ocean mass change. As GRACE measurements averaged over the ocean represents the ocean mass change component only, the difference between GRACE and altimetry observations provides an estimate of the mean steric sea level. Two different sets of GRACE geoid solutions (the GRGS EIGEN-GL04 and the GFZ EIGEN-GRACE04S products) have been used. Each GRACE data set ranges over approximately 3 yr or more (August 2002–April 2006 for the GRGS geoids and February 2003–February 2006 for the GFZ geoids).We first focus on the seasonal variations. The two GRACE data sets agree very well in terms of mean annual mass variation, both in amplitude (approximately 7.3 mm equivalent sea level) and phase (maximum on October 14). For both time spans, Jason-1 sea level minus GRACE ocean mass has an annual amplitude of approximately 5.8 mm with a maximum on March 11. The latter signal compares well with the steric annual sea level estimated from the World Ocean 2004 climatology and the Ishii et al. [M. Ishii, M. Kimoto, K. Sakamoto, and S.I. Iwasaki, Steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses, Journal of Oceanography, 62 (2) (2006) 155–170.] ocean temperature data. We also examine the interannual fluctuations of the Jason-1 minus GRACE sea level. The two resulting steric sea level time series (based on the two GRACE data sets) agree well. The inferred steric sea level curve exhibits an increasing trend during the last 3.5 yr (August 2002–April 2006), of the same order of magnitude as the 1993–2003 steric sea level trend computed with in situ hydrographic data. However, over the last 3.5 yr, we note a strong discrepancy between altimetry minus GRACE and in situ-based steric sea level trend, the latter exhibiting a negative slope. The cause for such a discrepancy is yet unknown but may be related to inadequate sampling of in situ ocean temperature measurements.