Evaluating model simulations of twentieth-century sea-level rise. Part II: regional sea-level changes

Twentieth-century regional sea level changes are estimated from 12 climate models from phase 5 of the Climate Model Intercomparison Project (CMIP5). The output of the CMIP5 climate model simulations was used to calculate the global and regional sea level changes associated with dynamic sea level, atmospheric loading, glacier mass changes, and ice sheet surface mass balance contributions. The contribution from groundwater depletion, reservoir storage, and dynamic ice sheet mass changes are estimated from observations as they are not simulated by climate models. All contributions are summed, including the glacial isostatic adjustment (GIA) contribution, and compared to observational estimates from 27 tide gauge records over the twentieth century (1900–2015). A general agreement is found between the simulated sea level and tide gauge records in terms of interannual to multidecadal variability over 1900–2015. But climate models tend to systematically underestimate the observed sea level trends, particularly in the first half of the twentieth century. The corrections based on attributable biases between observations and models that have been identified in Part I of this two-part paper result in an improved explanation of the spatial variability in observed sea level trends by climate models. Climate models show that the spatial variability in sea level trends observed by tide gauge records is dominated by the GIA contribution and the steric contribution over 1900–2015. Climate models also show that it is important to include all contributions to sea level changes as they cause significant local deviations; note, for example, the groundwater depletion around India, which is responsible for the low twentieth-century sea level rise in the region

The Framework for Assessing Changes To Sea-level (FACTS) v1.0: a platform for characterizing parametric and structural uncertainty in future global, relative, and extreme sea-level change

Future sea-level rise projections are characterized by both quantifiable uncertainty and unquantifiable structural uncertainty. Thorough scientific assessment of sea-level rise projections requires analysis of both dimensions of uncertainty. Probabilistic sea-level rise projections evaluate the quantifiable dimension of uncertainty; comparison of alternative probabilistic methods provides an indication of structural uncertainty. Here we describe the Framework for Assessing Changes To Sea-level (FACTS), a modular platform for characterizing different probability distributions for the drivers of sea-level change and their consequences for global mean, regional, and extreme sea-level change. We demonstrate its application by generating seven alternative probability distributions under multiple emissions scenarios for both future global mean sea-level change and future relative and extreme sea-level change at New York City. These distributions, closely aligned with those presented in the Intergovernmental Panel on Climate Change Sixth Assessment Report, emphasize the role of the Antarctic and Greenland ice sheets as drivers of structural uncertainty in sea-level change projections

Modelling regional sea-level changes in recent past and future

Sea-level change is one of the most important consequences of a warming climate, affecting many densely populated coastal communities. To improve coastal management and the planning of flood defences, information on the future development of sea-level rise is needed. However, sea-level rise is not uniform around the world. It is therefore not sufficient to know how much the global mean sea level will rise in the future. Instead, there is a pressing need for information on a regional scale. Making sea-level projections, both globally and locally, requires understanding of the processes that contribute to sea-level change. The research in this thesis focuses on modelling these processes, and in particular on their regional patterns in sea-level change. Firstly, sea level can rise or fall due to the addition or removal of water. Water may be added when glaciers or ice sheets shrink or due to groundwater pumping, which may be partly compensated by storage of water in newly constructed dams in rivers. Apart from the direct effect of addition or removal of water, a gravitational effect needs to be considered when large masses of water are displaced. This gravitational effect causes a very distinctive pattern in sea-level change, with a sea-level fall close to the water input source, and sea-level rise further away from the source. Apart from the gravitational effect, there is a reaction of the solid earth to changes in the redistribution of mass on the Earth’s surface. This is an effect that occurs immediately, the ‘elastic’ response, as well as on time scales of thousands of years, the ‘viscous’ response. The latter process, termed ‘Glacial Isostatic Adjustment’, results for instance in an uplift of up to a centimetre per year in parts of Scandinavia, as a result of ice melt after the Last Ice Age. Another cause of sea-level change are density variations, which occur when temperature or salinity changes. An increasing temperature causes expansion and thus sea-level rise, while increasing salinity causes densification and thus sea-level fall. Since these changes are not the same everywhere on earth, this causes a very irregular pattern of sea-level change. The research described in this thesis combines the knowledge on these processes to produce regional patterns of sea-level change. These patterns have been computed for the period 1961-2003, and compared to sea-level measurements over the same period in order to see whether we understand the regional patterns in sea-level change. The same approach is also used for future projections. These projections clearly show that regional variations are expected to be substantial. We find that 10% of the ocean surface will experience a change that differs more than 25% from the global mean. Our research also shows that each process may locally dominate sea-level change, and hence it is very important to include all these processes when considering regional variations

An assessment of uncertainties in using volume-area modelling for computing the twenty-first century glacier contribution to sea-level change

A large part of present-day sea-level change is formed by the melt of glaciers and ice caps (GIC). This study focuses on the uncertainties in the calculation of the GIC contribution on a century timescale. The model used is based on volume-area scaling, 5 combined with the mass balance sensitivity of the GIC. We assess different aspects that contribute to the uncertainty in the prediction of the contribution of GIC to future sea-level rise, such as (1) the volume-area scaling method (scaling constant), (2) the choice of glacier inventory, (3) the imbalance of glaciers with climate, (4) the mass balance sensitivity, and (5) the climate models. Additionally, a comparison of the model 10 results to the 20th century GIC contribution is presented. We find that small variations in the scaling constant cause significant variations in the initial volume of the glaciers, but only limited variations in the glacier volume change. If two existing glacier inventories are tuned such that the initial volume is the same, the GIC sea-level contribution over 100 yr differs by 0.027 m. It appears that the mass 15 balance sensitivity is also important: variations of 20% in the mass balance sensitivity have an impact of 17% on the resulting sea-level projections. Another important factor is the choice of the climate model, as the GIC contribution to sea-level change largely depends on the temperature and precipitation taken from climate models. Combining all the uncertainties examined in this study leads to a total uncertainty of 4.5 cm or 30% 20 in the GIC contribution to global mean sea level. Reducing the variance in the climate models and improving the glacier inventories will significantly reduce the uncertainty in calculating the GIC contributions, and are therefore crucial actions to improve future sea-level projections

Zeespiegelveranderingen (regionaal) in de eenentwintigste eeuw

Hoewel je het niet zou vermoeden op een windstille dag is de zee helemaal niet vlak als een spiegel: het zeeniveau is een heuvellandschap met hoogteverschillen van honderden meters. Ook de toekomstige veranderingen als gevolg van de opwarming van het klimaat zullen niet overal even groot zijn. In een gezamenlijke studie presenteren onderzoekers van de Universiteit Utrecht, KNMI en TU Delft daarom een klimaatscenario voor de regionale zeespiegelverandering in de eenentwintigste eeuw. Zoals verwacht laten de uitkomsten grote regionale verschillen zien. Een goede schatting voor de toekomstige bijdrage van de ijskappen op Groenland en Antarctica blijkt cruciaal voor een betrouwbaar resultaat

River deltas and sea-level rise

Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation.▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories.▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression.▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet.▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections.▪ River deltas globally might lose 5% (∼35,000 km2) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario

BRICK v0.2, a simple, accessible, and transparent model framework for climate and regional sea-level projections

Simple models can play pivotal roles in the quantificationand framing of uncertainties surrounding climatechange and sea-level rise. They are computationally efficient,transparent, and easy to reproduce. These qualities also makesimple models useful for the characterization of risk. Simplemodel codes are increasingly distributed as open source,as well as actively shared and guided. Alas, computer codesused in the geosciences can often be hard to access, run,modify (e.g., with regards to assumptions and model components),and review. Here, we describe the simple modelframework BRICK (Building blocks for Relevant Ice andClimate Knowledge) v0.2 and its underlying design principles.The paper adds detail to an earlier published modelsetup and discusses the inclusion of a land water storagecomponent. The framework largely builds on existing modelsand allows for projections of global mean temperatureas well as regional sea levels and coastal flood risk. BRICKis written in R and Fortran. BRICK gives special attention tothe model values of transparency, accessibility, and flexibilityin order to mitigate the above-mentioned issues while maintaininga high degree of computational efficiency.We demonstratethe flexibility of this framework through simple modelintercomparison experiments. Furthermore, we demonstratethat BRICK is suitable for risk assessment applications byusing a didactic example in local flood risk management

Northern North Atlantic sea level in CMIP5 climate models - evaluation of mean state, variability and trends against altimetric observations

The northern North Atlantic comprises a dynamically complex area with distinct topographic features, making it challenging to model oceanic features with global climate models. As climate models form the basis for assessment reports of future regional sea level rise, model evaluation is important. In this study, the representation of regional sea level in this area is evaluated in 18 climate models that contributed to the Coupled Model Intercomparison Project Phase 5.Modeled regional dynamic height is compared to observations from an altimetry-based record over the period 1993–2012 in terms of mean dynamic topography, interannual variability, and linear trend patterns. As models are expected to reproduce the location and magnitude but not the timing of internal variability, the observations are compared to the full 150-yr historical simulations using 20-yr time slices. This approach allows to examine modeled natural variability versus observed changes and to assess whether a forced signal is detectable over the 20-yr record or whether the observed changes can be explained by internal variability.The models perform well with respect to mean dynamic topography. However, model performances degrade when interannual variability and linear trend patterns are considered. The modeled region-wide average steric and dynamic sea level rise is larger than estimated from observations and the marked observed increase in the subpolar gyre is not consistent with a forced response but rather a result of internal variability. Using a simple weighting scheme, it is shown that the results can be used to reduce uncertainties in sea level projections

Comparing tide gauge observations to regional patterns of sea-level change (1961–2003)

Although the global mean sea-level budget for the 20th century can now be closed, the understanding of sea-level change on a regional scale is still limited. In this study we compare observations from tide gauges to regional patterns from various contributions to sea-level change to see how much of the regional measurements can be explained. Processes that are included are land ice mass changes and terrestrial storage changes with associated gravitational, rotational and deformational effects, steric/dynamic changes, atmospheric pressure loading and glacial isostatic adjustment (GIA). The study focuses on the mean linear trend of regional sea-level rise between 1961 and 2003. It is found that on a regional level the explained variance of the observed trend is 0.87 with a regression coefficient of 1.07. The observations and models overlap within the 1? uncertainty range in all regions. The main processes explaining the variability in the observations appear to be the steric/dynamic component and the GIA. Local observations prove to be more difficult to explain because they show larger spatial variations, and therefore require more information on small-scale processes.Space EngineeringAerospace Engineerin