Sea Level Rise - The State of the Science

Outline:

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General Background

  • Two primary processes contribute to sea level rise (SLR):
  1. Thermal expansion—the increase in water volume resulting from heat uptake;
  2. Mass inputs—the transfer of freshwater from land to the ocean. Large ice sheets on Greenland and Antarctica represent the largest potential source of additional water mass (#2 below).
  • There are two general mechanisms by which such ice sheets transfer water to the ocean:
  1. Surface melting of ice on land with subsequent runoff into the ocean;
  2. Ice dynamics, wherein ice actually flows from land into the ocean
  • New evidence indicates that ice flow is accelerating around Greenland and Antarctica, making this process the greatest source of uncertainty for predicting future SLR as warming proceeds.

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Ancient Sea Level Change

  • Over the past four cycles, each lasting about 100,000 years, sea level rose and fell by about 400 feet, with ice ages having lower sea level and warm interglacial periods having higher sea level. Contributions from land-based ice sheets dominated SLR during interglacial warming.
     
  • During the warmest part of the last interglacial period (about 130,000 years ago), global average temperature was 2-3 °F warmer than today and global sea level was 13-20 feet higher. During the Middle Pliocene (3 million years ago), global temperature was 3.5-5.5 °F warmer than today and sea level was 80-115 feet higher.
     
  • If emissions of man-made greenhouse gases continue without abatement, the earth could warm by 5.5 °F (3 °C) within the next century.
     
  • The complete loss of large ice sheets would take from several centuries to millennia, but warming could cross a threshold within decades that could permanently destabilize large ice sheets.

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Recent Sea Level Rise

  • Based on tide gauges, sea level rose by an average of 0.7 inches per decade and accelerated by 0.05 inches per decade over the 20th century.
     
  • More accurate satellite measurements indicate that global sea level has risen by 1.2 inches over the past decade, about 70% faster than the 20th century average.
     
  • Scientists assumed that thermal expansion dominated contemporary SLR, but recent progress reveals that freshwater contributions from land dominate, consistent with recent acceleration of ice loss from glaciers.

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Projections of Future SLR

  • Because the ocean has an enormous thermal inertia, it takes many decades for sea level to adjust to a quantity of heat that it absorbs. This delay means that even if man-made greenhouse gas emissions were halted today, sea level rise would be committed to an additional 0.4-1.0 foot by the end of this century.
     
  • In 2001 the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) projected that sea level would rise by 0.3-2.9 feet by the end of the 21st century.
     
  • In 2007 the Fourth Assessment Report of the IPCC projected that sea level would rise by 0.6-1.9 feet by the end of the 21st century.
     
  • The lower ends of the projected SLR ranges from the Third and Fourth IPCC reports are similar to the sea level rise commitment described above, leaving little or no room for additional SLR generated by continued greenhouse gas emissions. The upper end of both omits the uncertainty associated with the future ice dynamical changes in Greenland and West Antarctica.
     
  • Current SLR models undershoot the sea level change observed during the 20th century, possibly because they do not adequately simulate freshwater mass contributions from land. As an alternative, a recent study extended the statistical relationship between temperature rise and SLR observed during the 20th century, using models of future temperature change to drive SLR. This empirical approach estimated 21st century SLR to be in the range of 1.6-4.5 feet, if manmade greenhouse gas emissions continue to grow.

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Summary

  • Both long-term tide gauge measurements and recent satellite measurements suggest that SLR accelerated during the 20th century, concomitant with increasing global temperature.
     
  • One of the most significant developments of recent years is the realization that mass contributions from land ice have dominated contemporary SLR.
     
  • Large polar ice sheets are more sensitive to surface warming than previously realized, and large dynamical changes are now being observed on the Greenland and West Antarctic ice sheets.
     
  • New understanding of how these ice sheets behaved in the past suggests that they could add water mass to the oceans much more quickly than previously assumed.
     
  • The Fourth Assessment Report of the IPCC projects a 21st century sea level rise of 0.6-1.9 feet by 2100, but notes that accelerated glacial melt could contribute another 0.7 feet, raising the upper end to 2.6 feet.
     
  • A new empirical projection of SLR for the 21st century indicates that sea level could rise by 1.6-4.5 feet this century.
     
  • The potential for the rapid collapse of large ice sheets seems more plausible than in the recent past, and some scientists therefore warn that continued anthropogenic warming could result in the triggering of abrupt sea level rise within the current century.

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