Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
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Sea level instrumental observations are some of the oldest and longest running records pertaining to climate that we have; many going back well into the 1700's. While long by human standards, they are still somewhat insufficient to most accurately project out future sea level change out to 2100 and beyond.
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Perhaps not critical, but the lack of graphical representation of longer sea level records is somewhat frustrating. Although for short term projections (2100) extensive records (millions of years) may be less important, the chapter still describes them in some detail because understanding the long term variability of the system is important (Fig. 1). This figure is not from this chapter but is an example of a long-term record of sea level change that the chapter reviews but provides no graphical support for.
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Modern computing technology allows for the compilation of worldwide measurements to produce the observed global mean change over the instrumental record.
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Although we only add a few years worth of observational data to AR5 since AR4, we are now seeing increasing change in the recent past. The chapter could perhaps emphasize recent change in the context of the longer records available.
Observations
Projections
Chapter Assessment
While the chapter is an assessment of the state of knowledge of sea level change, here we provide short assessment of the chapter itself, the content it did or did not supply, and a summary review of the model projections used.
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Given our knowledge of the volume of the oceans and how changing densities and thermal properties of the seas change its volume, the calculations for changing sea level are fairly straightforward. However, the accuracy of those changes is dependent on our knowledge of the heat uptake of the ocean and input of rivers and ice. Therefore, the uncertainty in the sea level projections is linked to the uncertainty in the estimate of heat uptake by the ocean (and how heat circulates) and the glacier and ice sheet input. Projections of future input are based on projected climate models, which (in the long term) are largely dependent on future GHG concentrations, which are a product of the future socio-economic and policy choices.
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Therefore the largest uncertainty in projections of future sea level rise is linked back to the future socio-economic and policy decisions we make. I don't think this point is made clear enough in the chapter and would like to see it more explicitly stated.
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Given this uncertainty, the models give reasonable future sea level projections with the information available.
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Although the chapter does point out areas of uncertainty, the reader is left wanting more. Perhaps it is not the main purpose of this assessment to provide directions for future research, but I also feel that the authors of this chapter (after reading all the relavant literature) are the best positioned to determine what areas of the field need more work.
Figure 1: An example of a long term sea level record derived from Oxygen-18 isotopes and Mg/Ca ratios (Sosdian and Rosenthal, 2009).