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Twentieth century sea level: an enigma
1Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA. wmunk@ucsd.edu
Summary
Global sea level rise is not fully explained by ocean warming. Historical tide records and greenhouse warming data do not account for the observed 20th-century sea level increase, suggesting other factors are at play.
Area of Science:
- Climate Science
- Oceanography
- Geophysics
Background:
- Sea level changes result from ocean volume (thermal expansion) and mass (melting/storage) variations.
- Greenhouse warming has increased ocean heat content, contributing to sea level rise.
Purpose of the Study:
- To reconcile discrepancies between observed sea level rise and attributed causes.
- To investigate the contributions of ocean warming, ice melt, and other factors to historical sea level changes.
Main Methods:
- Analysis of global ocean temperature compilations (Levitus et al.).
- Comparison of tide records with climate model outputs and Intergovernmental Panel on Climate Change (IPCC) estimates.
- Evaluation of polar ice sheet melt contributions and Earth rotation data.
Main Results:
- 20th-century ocean heat content increase corresponds to 3 cm sea level rise, with an accelerating rate of ~6 cm/century.
- Historical tide records show a linear rise of ~18 cm/century with no measurable acceleration until the latter half of the 20th century.
- A residual sea level rise of ~12 cm/century remains unexplained by current ocean warming and IPCC-attributed melting estimates.
Conclusions:
- Ocean warming alone cannot account for the observed historical sea level rise due to timing and trend discrepancies.
- Polar ice melt, within IPCC upper limits, may explain the residual rise but faces constraints from Earth rotation data.
- Resolutions may involve revising traditional sea level rise estimates, increasing ocean heat storage calculations, or reinterpreting astronomical records.
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