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Climate Change

Rise Up! Overestimated sea-level rise during the 20th century.

Article:

Hay, C.C., Morrow, E., Kopp, R.E., Mitrovica, J.X., (2015). Probabilistic reanalysis of twentieth-century sea-level rise. Nature 517, 481-484, doi:10.1038/nature14093.

 

Background:

Estimating the rate of sea-level rise in the 20th-century is a complicated matter. Only over the past couple of decades has it been possible to measure the ocean height from satellites, providing a global snapshot of sea-level. Since the late 19th-century, tide gauges have been the primary means of recording sea-level. As one could imagine, the global number of tide gauges has grown significantly since the turn of the century.

Several problems arise when using tide gauge data to estimate global average sea-level rise. The first problem has to do with the location and distribution of tide gauges. The vast majority of tide gauges are found along the coasts, in the northern hemisphere, and away from more polar regions. This distribution means that sea-level data is very sparse, and very biased towards coastal areas. Additionally, there are gaps in sea-level data due to tide gauges failing over time, or some being replaced or relocated creating discontinuities in the data. Estimations of global average sea-level rise must attempt to work around all of these problems.

Previous rate estimates of global sea-level rise range from 1.5 to 1.9 millimeters (mm) per year over the 20th-century. The problem with these estimates become evident when summing the individual contributors to sea-level rise. Quite simply, the amount of sea-level rise due to all contributing processes, such as melting ice sheets and the thermal expansion of water, add up to 1.2 mm of rise per year, which is lower than the estimated 1.5-1.9 mm per year. Many scientists believed that this lowered estimation is due to one of the contributing processes had been underestimated.

 

Results:

This study tests the hypothesis that estimations of calculated average global sea-level rise over the 20th-century have been overestimated due to the inherent problems associated with inconsistent data from tide gauges. The researchers applied a new statistical method that performs well with spatially sparse data. A total of 622 tide gauges were incorporated into the new method and new estimates of globally averaged sea-level rise were calculated for the 20th century. The resulting calculated rise in sea-level was 1.2 mm per year (Figure 1), which is essentially identical to the summed contributions. This suggests that the processes, such as melting ice sheets and thermal expansion of water, were not underestimated, but rather that the estimations from tide gauges overestimates twentieth-century sea-level rise.

 

Figure 1. Comparison of twentieth-century sea-level rise estimates. The rates of twentieth-century global sea-level rise from this study (KS, blue line) versus previous studies (Ref. 3 and 4, red and purple lines, respectively).  This study estimates a lower rate of 1.2 mm per year, when compared to previous studies which range from 1.5 to 1.9 mm per year.  The lower rate from this study from 1901-1990 and a similar rate of 3.0 mm per year from 1993-2010 means that this study shows a greater acceleration in sea-level rise over the twentieth century.

Figure 1. Comparison of twentieth-century sea-level rise estimates.
The rates of twentieth-century global sea-level rise from this study (KS, blue line) versus previous studies (Ref. 3 and 4, red and purple lines, respectively). This study estimates a lower rate of 1.2 mm per year, when compared to previous studies which range from 1.5 to 1.9 mm per year. The lower rate from this study from 1901-1990 and a similar rate of 3.0 mm per year from 1993-2010 means that this study shows a greater acceleration in sea-level rise over the twentieth century.

 

The researchers vigorously tested the new methodology by comparing reconstructions of sea-level rise against observational data. The new method passed with flying colors, such that only reconstructions of 8 tide gauges (out of the 622) were found to greatly deviate from observational data. Using the new methodology, it was also determined that sea-level rise from 1993 – 2010 was approximately 3.0 mm per year, which was identical to estimates from the previous studies. These estimates are thought to be very accurate due to the use of satellites to measure sea-level. The lower initial rate of rise, determined by the new methodology and the identical rate from 1993 – 2010, yields a much greater acceleration in sea-level rise over the 20th-century (Figure 1).

 

Significance:

A critical step required for making future projections of sea-level is to have accurate estimates of global sea-level rise during the 20th-century. Sea-level projections are the result of climate models, which are only as strong as the observational data used as the input. This study challenges the estimated rate of global sea-level rise over the 20th-century, preferring a lower rate of 1.2 mm per year. This number is consistent with the sum of the individual contributions to sea-level rise, and thus closes a gap that exists with previous estimates of 1.5-1.9 mm per year. The consistent rise rate of 3.0 mm per year over the last couple of decades suggests that the acceleration in sea-level rise is larger than previously thought. This larger acceleration necessitates revisiting future projections of sea-level, especially since there is a possibility of greater global sea-level rise occurring over the next century.

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