OceanTEFFH O iciaL MAGAZINEog OF the OCEANOGRAPHYraphy SOCIETY CITATION Tamisiea, M.E., and J.X. Mitrovica. 2011. The moving boundaries of sea level change: Understanding the origins of geographic variability. Oceanography 24(2):24–39, doi:10.5670/ oceanog.2011.25. COPYRIGHT This article has been published inOceanography , Volume 24, Number 2, a quarterly journal of The Oceanography Society. Copyright 2011 by The Oceanography Society. All rights reserved. USAGE Permission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: [email protected] or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. do WNLOADED FROM WWW.tos.org/oceanography SPECIAL ISSue on Sea LEveL BY MARK E. TamISIea anD JERRY X. MITRovICA TheMoving Boundaries of Sea Level Change Understanding the Origins of Geographic Variability ABSTRACT. As ice sheets gain or lose mass, and as water moves between the continents and the ocean, the solid Earth deforms and the gravitational field of the planet is perturbed. Both of these effects lead to regional patterns in sea level change that depart dramatically from the global average. Understanding these patterns will lead to better constraints on the various contributors to the observed sea level change and, ultimately, to more robust projections of future changes. In both of these applications, a key step is to apply a correction to sea level observations, based on data from tide gauges, satellite altimetry, or gravity, to remove the contaminating signal that is due to the ongoing Earth response to the last ice age. Failure to accurately account for this so-called glacial isostatic adjustment has the potential to significantly bias our understanding of the magnitude and sources of present-day global sea level rise. This paper summarizes the physics of several important sources of regional sea level change. Moreover, we discuss several promising strategies that take advantage of this regional variation to more fully use sea level data sets to monitor the impact of climate change on the Earth system. 24 Oceanography | Vol.24, No.2 InTRODUCTION averages. However, there are at least is called glacial isostatic adjustment Sea level displays complex variability three reasons for moving beyond esti- (GIA), or post-glacial rebound. We will in both space and time that reflects mates of global means. First, and most use both terms, although the former the broad suite of geophysical forcings importantly, local rather than global is preferred because it is more general, that act upon Earth. When considering variations in sea level have the greatest encompassing regions of crustal subsid- this variability, people often think of and most immediate impact on society. ence as well as rebound. While there dynamic processes, such as changing Woodworth et al. (2011, in this issue) is a rich literature on the time history tides, winds, currents, temperature, and show the dramatic geographic variability of sea level change caused by GIA (Wu salinity. However, few consider how the in recent sea level rates observed in the and Peltier, 1983; Nakada and Lambeck, ground moving beneath them or changes ocean. Understanding this variability, 1989; Mitrovica, 1996; Peltier, 1998; in gravity can impact sea level. Modern now and into the future, is the prin- Milne et al., 1999; Kendall et al., 2005), mass loss from glaciers and ice sheets, cipal concern for coastal communities. this paper focuses only on the contribu- for example, as well as changes in the Second, until recently, our sampling of tion to present-day sea level observa- water stored on continents, cause crustal the ocean has been incomplete. Thus, tions, such as tide-gauge and altimetry motions that perturb Earth’s gravity. In if the underlying processes giving records as well as mass changes derived addition, Earth is still adjusting to the rise to regional sea level trends were from the Gravity Recovery and Climate collapse of the large ice sheets from the not well understood, this incomplete Experiment (GRACE) satellite mission. last ice age. Both of these processes— sampling could lead to significant biases As we will demonstrate, if these observa- associated with recent and ancient in attempts to infer global averages. tions were interpreted as being due only changes in the Earth system—introduce Finally, without understanding these to present-day changes in polar ice mass large-scale regional variations into processes, which ultimately requires a flux or the thermosteric contribution, sea level change. full accounting of regional variation, any resulting inference would be biased. In tabulating the various contributions efforts to project future sea level rise will In fact, in the case of GRACE gravity to sea level rise, the focus has frequently be profoundly hampered. observations, any inferred ocean mass been on changes to the total ocean mass This paper explores two causes of changes are of the same magnitude as the associated with freshwater flux from regional sea level change in detail. The contribution from GIA. We emphasize grounded ice sheets, and on volume first is the ongoing response of Earth that, for the purpose of this paper, the changes linked, for example, to tempera- and the ocean to the collapse of the term GIA will be specifically associated ture and salinity variations (e.g., Willis Pleistocene ice sheets since the Last with the response to ice sheet changes et al., 2008). This focus makes sense Glacial Maximum of the most recent ice associated with the last glacial cycle. when the goal is to understand global age, about 20,000 years ago. This process The second cause of regional sea level Oceanography | June 2011 25 variability we explore is ongoing water dynamic processes and simply refer to viscoelastic behavior. In this case, an exchange between the continents and the top of the ocean as the sea surface. initial elastic response to loading (or the ocean. This water can come from This terminology is adopted because the unloading) is followed by viscous flow. melting ice sheets and glaciers, or from value of the equipotential that defines In geophysics, the canonical example of the hydrological cycle over the conti- the sea surface will actually change with such behavior is post-glacial rebound— nents, and in either case the time scale time as Earth deforms and/or water the adjustment of the crust in areas like of sea level variability will match the enters and leaves the ocean. Canada and Sweden associated with the fluctuations in the source. Frequently, The assumption that the two bound- deglaciation of these regions at the end of these ongoing fluxes are expressed in aries are time-invariant intrinsically the last ice age. terms of an equivalent, globally averaged assumes that the solid Earth is rigid and When measuring present-day sea level change in sea level, such as, for example, that the water (mass) moving around in change, we often rely on three different 0.3 mm yr –1 from Greenland. However, the ocean and on the continents does observation systems: tide gauges, satellite this method of reporting often contrib- not generate any gravitational forces. altimetry, and gravity changes inferred utes to the mistaken impression that However, Earth is far from rigid, and its from GRACE. Although temperature the processes lead to a geographically behavior depends upon the time scale and salinity changes derived from the uniform sea level change. In contrast, we of the forcing that is applied to it. For Argo float system provide an impor- will show that the patterns of sea level example, the motions of Earth’s tectonic tant fourth observation set, we will change show dramatic geographic vari- plates are ultimately driven by thermal not include it because this paper does ability and that each source, whether it is convective fluid flow in Earth’s mantle. not consider thermosteric contribu- a melting ice sheet or a varying ground- Similarly, the ellipticity of Earth’s figure tions to sea level. Tide gauges, for water reservoir, will be characterized is due to rotation and it can be accu- example, measure the change of the sea by a distinct variability. The so-called rately predicted by treating the planet surface relative to a nearby benchmark “fingerprints” of sea level change allow as a rotating fluid (Nakiboglu, 1982). connected to the solid Earth, leading to us to gain more information from Note that mantle convection results in a the term relative sea level. Thus, a sea historical records and provide a more small but important deviation from this level rise can result from either crustal societally relevant prediction of regional form. We will return to this issue later. subsidence or a rise in the sea surface. sea level change. In contrast to this fluid behavior, on very The model predictions described below short time scales, hours to decades, Earth include global-scale estimates of both BaCKGRounD responds nearly elastically to applied vertical crustal motion and changes Oceanographers generally consider the forcing. Commonly cited examples of to the height of the sea surface. The ocean’s two bounding surfaces, the solid this behavior are propagation of seismic difference between these two changes, crust and the geoid, to be time-invariant. waves from the source of an earthquake sea surface minus crustal height, is the The geoid is typically defined as the and deformation of the solid Earth due change in thickness of the ocean at any time-averaged equipotential surface to tidal forcing, both discussed a century given point, which is directly comparable that corresponds to the sea surface if no ago by Love (1911).