Regional Sea-Level Projection

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Abstract
Projections of global sea-level rise by 2100 C.E. range from 20 cm ([ 1 ][1]) to as much as 2 m ([ 2 ][2], [ 3 ][3]), and sea level will not stop rising then. For civilization, the stakes are high. Without adaptation, a rise by 0.5 m would displace 3.8 million people in the most fertile part of the Nile River Delta ([ 4 ][4]). A rise by 2 m could displace 187 million people globally ([ 5 ][5]). Credible projections of sea-level rise in the 21st century are essential for devising adaptation or mitigation measures. Yet, present estimates of future sea-level rise are too imprecise to inform such decisions. Earth system models used in the last Intergovermental Panel on Climate Change report ([ 1 ][1]) project a total global sea-level rise of 20 to 60 cm by 2100 C.E. from thermal expansion of the ocean, glacier melt, and the balance between melting, snowfall, and the regular outflow of glaciers from the ice sheets. However, potential contributions from accelerated ice-sheet outflow or the rapid collapse of glaciers that are in direct contact with the oceans are not currently included in these models. Furthermore, complex regional patterns lead to a larger rise in some regions and to considerable but poorly quantified interannual and decadal variability of sea level. ![Figure][6] All change. Interactions between ice sheets, ocean, and atmosphere affect the balance of mass of the Greenland and Antarctic ice sheets. The dynamic response of the ocean, which affects regional sea level, may bring warmer waters in contact with marine glaciers, leading to the decay of ice shelves. Rapid changes at the boundary of the ice sheets can be communicated far into the interior of the ice sheets by ice streams, leading to unloading of the continent and changes in the global gravitational field and thus sea level. Changes in atmospheric temperatures and circulation may bring more precipitation to the Antarctic, offsetting ice loss at the boundaries. Global climate models approximately reproduce the thermal expansion of ocean waters over recent decades ([ 6 ][7]). However, the observational record is short, the deep ocean is poorly observed, and discrepancies between observations and simulations remain. Large discrepancies between regional projections from different models suggest that the ocean's dynamic response to climate forcing is poorly understood ([ 1 ][1]). Local geological influences in important urban settings like New Orleans ([ 7 ][8]) could result in tens of centimeters of additional rise by 2100 C.E. Critical comparisons between models and observations are needed to better constrain regional projections. However, adequate ocean salinity and temperature data have only become available in the past decade, since the completion of the Argo array of profiling floats ([ 8 ][9]). Before the 2000s, large regions were undersampled, particularly in the Southern Ocean, making ocean warming estimates uncertain. Glaciers are projected to make an increasing contribution to sea-level rise. A new glacier inventory (location, elevation, areas, and volumes) has just been completed ([ 9 ][10]), but fewer than 100 glaciers have observations that allow mass balance to be estimated over decades. Addressing these shortfalls will be critical to improving 21st-century projections. The global sea-level rise observed in recent decades can be approximately attributed to thermal expansion and glacier ice loss ([ 10 ][11]), but contributions from the ice sheets in Greenland and Antarctica over this period are poorly constrained. Mass loss from these ice sheets also remains the largest uncertainty in projections of sea-level rise for the 21st century and beyond. The problem is not purely a glaciological one, with ice, ocean, air, and land each playing a role (see the figure) ([ 11 ][12]). Accurate representation of ice-sheet response and ice-ocean interactions in models remains an enormous scientific challenge. Upper bounds on sea-level rise have been estimated from kinematic constraints on glacier outflow ([ 3 ][3]) and paleoclimate analogs ([ 12 ][13]), but these provide no information about the likelihood of such scenarios. Projections with semi-empirical models, which depend on historical statistical relationships between sea level (mostly since about 1850) and surface temperature ([ 13 ][14]) or radiative forcing ([ 14 ][15]), have been developed. These models project much higher sea level (more than twice) than do current Earth system models (fig. S1). Is this discrepancy a result of incomplete understanding of the physical processes, or are the semi-empirical models unable to accurately project future sea-level rise? Earth system models have significant uncertainties in abyssal ocean heat uptake, snowfall on Antarctica, ocean circulation beneath the ice shelves and the resulting basal melting, and glacier and ice-sheet response. In the case of the semi-empirical models, it is unclear whether the statistical relationships are robust in a changing climate. These models are sensitive to the formulation and training data used, they may be improperly scaling contributions from terrestrial water storage and non–climate change–related glacier and ice-sheet loss, and they do not account for reduced ocean heat uptake and reduced glacier contributions (from reduced area) during the 21st century ([ 15 ][16]). Also, semi-empirical projections provide no information on regional variations. To advance sea-level projections, particularly at the regional level, it is crucial to understand the dynamical interactions between the ice sheets and the oceans, quantify changes in precipitation over Antarctica, advance understanding of the ocean's dynamic response to climate change, and continually monitor the ongoing changes. Given the difficulties and time required to develop fully coupled Earth system models that include full-stress ice-sheet models ([ 11 ][12]) that can reliably project sea-level rise, it is important to...

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