Sea level is rising: Do we know why?

The gradual rise of sea level is one of the most troubling aspects of global change, especially because it is likely to accelerate in the future as global warming progresses. Understanding the linkage between warming climate and sea-level rise therefore is important and has been the subject of much study (e.g., refs. 1–5). Two processes are involved: an increase of the mass of water in the oceans (the eustatic component), derived largely from the melting of ice on land, and an increase of the volume of the ocean without change in mass (the steric component), largely caused by the thermal expansion of ocean water. Neither of these components is understood fully, and observations are not sufficient yet to develop a precise assessment of the causes of present-day sea-level rise let alone a projection of future rise. In fact many of the analyses produce conflicting results. The article by Munk (6) in this issue of PNAS enlightens and sharpens our understanding of the enigma before us by examining a number of geophysical constraints on the process.

[1]  W. Munk Twentieth century sea level: An enigma , 2002, Proceedings of the National Academy of Sciences of the United States of America.

[2]  Bruce C. Douglas,et al.  The Puzzle of Global Sea-Level Rise , 2002 .

[3]  Niels Reeh,et al.  Thinning of the ice sheet in northwest Greenland over the past forty years , 2001, Nature.

[4]  A. Cazenave,et al.  Sea Level Rise During Past 40 Years Determined from Satellite and in Situ Observations , 2001, Science.

[5]  W. Krabill,et al.  Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning. , 2000, Science.

[6]  M. Meier,et al.  Twentieth century climate change: evidence from small glaciers. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[7]  M. Richards,et al.  Polar wandering in mantle convection models , 1999 .

[8]  Paul Johnston,et al.  Sea‐level change, glacial rebound and mantle viscosity fornorthern Europe , 1998 .

[9]  William C. Clark,et al.  Climate Change 1995: The Science of Climate Change , 1997 .

[10]  B. Steinberger,et al.  Changes of the Earth's rotation axis owing to advection of mantle density heterogeneities , 1997, Nature.

[11]  L. V. Morrison,et al.  Long-term fluctuations in the Earth’s rotation: 700 BC to AD 1990 , 1995, Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences.

[12]  G. Spada,et al.  Mountain building and earth rotation , 1994 .

[13]  Andrew J. Plater,et al.  Book reviewSea-level change: Roger Revelle; Studies in Geophysics, National Research Council, National Academy Press, Washington, DC, 1990; xii + 246 pp.; USD 29.95, GBP 25.75; ISBN 0-309-04039 , 1992 .

[14]  W. Peltier Global Sea Level and Earth Rotation , 1988, Science.

[15]  W. Harrison,et al.  Elevation, volume and terminus changes of nine glaciers in North America , 1998, Journal of Glaciology.

[16]  W. Harrison,et al.  Elevation and volume changes on the Harding Icefield, Alaska , 1998, Journal of Glaciology.