Quaternary Sea-Level Research

by

Mark Schuerch

• LAST REVIEWED: 28 February 2017
• LAST MODIFIED: 28 February 2017
• DOI: 10.1093/obo/9780199874002-0149

Introduction

Sea level variations have been, are, and will always be taking place on local, regional, and global scales as well as on all temporal scales. Ranging from changes with frequencies of less than 1 Hz, such as capillary and small wind waves, to changes on time scales of several million years, regional and global sea-level changes have an enormous impact on coastal processes as well as on the coastal population. Sea level changes have been part of the earth history and, on geological time scales (millions of years), have been related to the geophysical processes forming the earth surface, such as plate tectonics and associated changes in the land-sea distribution. On shorter time scales, changes in the earth’s climate and associated sea-ice distributions seem to be responsible for much of the observed variations. With the discussion about modern climate change and the impact of the anthropogenic contribution, studies about past sea-level changes, driven by climate change, have gained huge interest. A specific focus is, thereby, put on past periods, when sea levels have been higher or rising faster than what we currently experience. Both situations have previously occurred, although it should be noted that during those times the world’s coasts had not yet been populated. During the last c. 7000 years, however, global sea levels have been remarkably stable as compared to previous time periods. The currently observed rapid increase in global sea levels is strongly related to anthropogenic emissions of greenhouse gases and will, in the future, lead to rates of sea level rise (SLR) that make it necessary for the coastal population to better adapt and protect themselves against marine hazards, such as storm surges. However, uncertainties related to the projection of future sea levels are high, with the largest uncertainties related to the nonlinear, threshold-driven behavior of the Greenland and Antarctic ice sheets. Hence, the analysis of Quaternary sea-level changes is particularly relevant, as these are primarily driven by glacial cycles, with extensive glaciers and ice sheets during cold climate periods (glacials) and small extents of glacier and ice sheets during warm climate periods (interglacials). This article gives an overview on the currently available literature about the causes and magnitudes of Quaternary sea-level changes as well as the potential impacts of current and future sea-level changes on the world’s coastlines.

Causes for Global Sea-Level Variations

Understanding the causes of global sea-level variations is one of the major aims of current sea-level research. While causes may vary depending on time scales under consideration, this section focuses on literature about Quaternary sea-level changes, which are primarily driven by global temperature variations (Miller, et al. 2005). These induce changes in the density of ocean water (thermosteric effect) and are responsible for the extension and melting of glaciers and ice sheets (barystatic effect); both processes affect the ocean’s volume and sea level. Based on the data of Levitus, et al. 2005, Antonov, et al. 2005 delivered some to first global estimates on the thermosteric effect and suggested it to lie between 0.33 mm yr⁻¹ and 1.2 mm yr⁻¹ for the time periods 1955–2003 and 1993–2003, respectively. Three years later Domingues, et al. 2008 presented significantly different values, presumably due to a systematic instrumental bias in previous estimates. They argue that recent thermosteric rates were much lower (i.e., 0.79 mm yr⁻¹ between 1993 and 2003) and historic rates were much higher (0.53 mm yr⁻¹ between 1961 and 2003). Their data are supported by modeling outputs that suggest the thermosteric effect to be reduced after major volcanic eruptions as a consequence of negative radiative forcing (Domingues, et al. 2008). Gregory, et al. 2012, however, claims that the employed atmosphere–ocean general circulation models (AOGCMs) generally underestimate ocean heat uptake as their equilibrium states do not include the effects of volcanic eruptions. They further point out that global loss of glaciers and ice sheets have been the primary driver for SLR in the 20th century. Interestingly, no significant increase for that contribution could be identified despite increasing temperatures. Meanwhile, Shepherd and Wingham 2007 consider the current glacial and ice sheet contribution to be rather small (0.35 mm yr⁻¹) compared to the total global average (3 mm yr⁻¹). In very recent years (since about 2000), however, this contribution has increased dramatically (e.g., 1.4 ± 0.2 mm yr⁻¹ between 2001 and 2005), as reported by Cazenave and Llovel 2010. In the coming century, it is expected to become even larger, due to ice sheet instabilities (Shepherd and Wingham 2007). And indeed, the increased flow rates of glaciers in parts of Antarctica (i.e., West Antarctica) were found to dominate the ice sheet’s mass balance as opposed to their surface mass balance (SMB) (Rignot, et al. 2008).

• Antonov, J. I., S. Levitus, and T. P. Boyer. “Thermosteric Sea Level Rise, 1955–2003.” Geophysical Research Letters 32.12 (2005): L12602.

  DOI: 10.1029/2005GL023112

  Based on the data on the ocean’s heat uptake, delivered by Levitus, et al. 2005, this study estimates the thermosteric contribution to global SLR between 1955 and 2003. A long-term contribution of 0.4 mm yr⁻¹ (1955–1998) and a recent acceleration to 1.6 mm yr⁻¹ (1993–2003) are reported.

• Cazenave, A., and W. Llovel. “Contemporary Sea Level Rise.” Annual Review of Marine Science 2.1 (2010): 145–173.

  DOI: 10.1146/annurev-marine-120308-081105

  This article constitutes a comprehensive review on the understanding of processes contributing to global mean sea level (GMSL) rise and summarizes the estimates of their relative contributions. It is concluded that between 1993 and 2007 about 30 percent of GMSL rise could be attributed to ocean thermal expansion, whereas 55 percent was caused by melting ice sheets. Since 1993, however, the ice sheet contribution has increased to up to 80 percent.

• Domingues, C. M., J. A. Church, N. J. White, et al. “Improved Estimates of Upper-Ocean Warming and Multi-Decadal Sea-Level Rise.” Nature 453.7198 (2008): 1090–1093.

  DOI: 10.1038/nature07080

  The study presents new calculations for thermal expansion rates, correcting the biases originating from an instrumental error, related to non-consistent fall velocities of the profiling devices (XBTs). Other than previous datasets, the new data is capable of reproducing the decreased ocean heat uptake in periods of reduced radiative forcing after volcanic eruptions.

• Gregory, J. M., N. J. White, J. A. Church, et al. “Twentieth-Century Global-Mean Sea Level Rise: Is the Whole Greater than the Sum of the Parts?” Journal of Climate 26.13 (2012): 4476–4499.

  DOI: 10.1175/JCLI-D-12-00319.1

  The contributions of all global sea-level affecting processes are revisited. In previous studies, the sum of each contribution has overestimated the total GMSL, whereas this study shows that the budget can be closed, when accounting for observations that GMSL has not risen much faster in the second half of the 20th century compared to the first half.

• Levitus, S., J. Antonov, and T. Boyer. “Warming of the World Ocean, 1955–2003.” Geophysical Research Letters 32.2 (2005): L02604.

  DOI: 10.1029/2004GL021592

  After their initial estimates, published in the year 2000, this study presents recalculated and improved estimates for global ocean heat uptake after incorporating some additional two million temperature profiles that became available with the establishment of the World Ocean database in 2001. It is concluded that during 1955 and 1998 the mean ocean temperature (0–3000 m) had increased by 0.037°C.

• Miller, K. G., M. A. Kominz, J. V. Browning, et al. “The Phanerozoic Record of Global Sea-Level Change.” Science 310.5752 (2005): 1293–1298.

  DOI: 10.1126/science.1116412

  The study analyzes the variations of sea level changes on different time scales ranging from ten thousands to millions of years. It reviews the previously published reconstructions of sea levels during the past 543 million years and presents a new sea level curve, which is based on the backstripping method and aims to better understand the drivers for sea level variations on the analyzed time scales.

• Rignot, E., J. L. Bamber, M. R. van den Broeke, et al. “Recent Antarctic Ice Mass Loss from Radar Interferometry and Regional Climate Modelling.” Nature Geoscience 1 (2008): 106–110.

  DOI: 10.1038/ngeo102

  Data retrieved from interferometric synthetic-aperture radar (InSAR) provide some highly improved estimates of the glacial flow velocities in Antarctica. Dramatic losses of ice sheet area were recorded in West Antarctica as compared to negligible losses in East Antarctica. The yearly ice loss rates in West Antarctic increased by 59 percent between 1996 and 2006.

• Shepherd, A., and D. Wingham. “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets.” Science 315.5818 (2007): 1529–1532.

  DOI: 10.1126/science.1136776

  As a major contribution to GMSL rise, the dynamics of the earth’s ice sheets are reviewed. A large range of mass balance estimates, which considerably improved over the last decades by the use of satellite imagery, are presented, and it is concluded that glacier flow has considerably accelerated during the last decade and could continue do so in the future.