Watson, C.S., White, N.J., Church, J.A., King, M.A., Burgette, R.J., Legresy, B., 2015, Unabated global mean sea-level rise over the satellite altimeter era, Nature Climate Change 5, 565-568, doi:10.1038/nclimate2635.
Measuring sea level on a global scale is an extremely complicated task, especially in the context of monitoring sea level rise, which demands a high level of precision. Satellite altimetry revolutionized the study of sea-level rise (and more broadly, Oceanography as a whole) by allowing for very precise (within several centimeters), continuous measurements of sea surface height over the globe. Since the TOPEX/Poseidon mission began in 1993, snapshots of global sea surface height have been reliably delivered every ten days. Measurements of sea surface height are made possible by radar pulses emitted from an orbiting satellite. The precise position of the satellite as well as the round trip travel time (satellite to ocean surface and back to satellite) of the radar pulse are measured which can be used to determine sea surface height. Additionally, satellite altimetry provides information about wave height as well as other meteorological information. After the data is collected, corrections are required to account for distortions that arise due to differing atmospheric and sea state conditions. As the current Jason-2 satellite makes its rounds, it provides global coverage between 66° N and 66° S resulting in 95% global coverage of the ice-free oceans (NASA.gov).
Prior to satellite altimetry, tide gauges were primarily used to measure sea level, with some in operation since the mid to late 19th century. Tide gauges are devices that record sea level height with respect to a known elevation. Measurements of sea level from tide gauges offered the first look at how sea level has risen over the 20th century. Despite their usefulness in providing over a century of sea level measurements, tide gauges have several issues. Most tide gauges are located very near the coast, providing limited spatial coverage and leading to sea level measurements that are biased to the nearshore. Additionally, tide gauges do not account for changes in vertical land movements. For example, some locations may experience land subsidence (sinking), relative to sea level, leading to an apparent rise in sea level that is not observed by a tide gauge. The limited spatial coverage and biasing towards coastal areas gives an incomplete picture of sea level, especially when compared to the technologically advanced satellite altimetry.
From 1993 to 2014, sea level has risen 3.2 ± 0.4 mm per year based on global average measurements from satellite altimetry. This rate of sea level rise is nearly twice as high as the average rate over the twentieth century (reported as 1.5-1.9 mm per year in the most recent IPCC assessment report), suggesting acceleration in sea level rise. However, when looking at just the last decade, the rate of sea level rise has been lower than 3.2 mm per year, suggesting a recent slowing in sea level rise. A slowing of sea level rise is not consistent with other global trends such as increasing rates of melt in the Greenland ice sheet. Another unusual finding from satellite altimetry is that estimations of the summed contributions to global sea level rise (e.g. melting of land ice and expansion of seawater) do not add up to the observed 3.2 mm per year rise. These two confounding factors led scientists to question the long-term precision of the satellite altimetry observations. In this study, Watson et al. estimate the error in sea surface height measurement for each of the four satellite missions: TOPEX A, TOPEX B, Jason-1 and Jason-2. These errors are referred to as bias drift, which the authors define as errors specific to each satellite mission, The authors then used the estimates in bias drift to correct the rate of sea level rise over the satellite altimetry era (1993 onwards).
Watson et al. calculated bias drift for each satellite mission by comparing sea surface height from tide gauges located worldwide to observations of sea surface height measured by satellite altimetry at the corresponding location. To ensure the quality of the tide gauge data, vertical land movements were accounted for at the location of the tide gauge by using nearby Global Positioning Satellite (GPS) stations. These GPS stations helped determine changes in land elevation. Of the 96 tide gauges, 69% had a GPS station located within 100km. At tide gauges where there was no nearby GPS station, a suboptimal determination of vertical land movements was achieved through modeling.
Estimates of bias drift were most significant during the first six years of the first satellite altimetry mission (TOPEX A), and the smallest during the most recent mission (Jason-2), suggesting technological improvement over time (Fig. 1). The calculated bias drift from all satellite missions creates an overestimation of global mean sea level rise from 1993 to 2014, revising the rate of sea level rise to 2.6 ± 0.4 to 2.9 ± 0.4 mm per year, down from 3.2 ± 0.4 mm per year. This lowered rate from 1993 to 2014 means that sea level rise has continued to accelerate over the most recent decade, which is significantly different than the previous deceleration finding (Fig. 2). Additionally, the revised rate of sea level rise compares more closely with the rates estimated from the summed contributions (melting ice and seawater expansion).. The authors suggest that more attempts at estimating bias drifts should be employed before using their results to correct the satellite altimetry record.
Sea level rise is one of the most challenging problems arising from climate change due to the world’s population preference for the coast. Sea level rise is projected to continue accelerating over the next century with a low-end projection of about 1 foot of globally averaged rise by 2100. The upper projection of globally averaged rise is as much as 3 feet of rise, with regions such as the U.S. East coast seeing closer to 4 feet, as sea level rise is not a globally uniform process (See 5th IPCC assessment report for more information). This increases the vulnerability of low-lying coastal communities for coastal flooding from storm surge or even high tidal cycles. Satellite altimetry is the most accurate method for monitoring sea level rise on a global scale. It is of utmost importance to understand the rates of sea level rise since post industrialization. Projections of future sea level rise rely on accurate observational data. This study brings to focus the need to understand errors in satellite altimetry observation, considering that such small errors can be the difference between accelerating or decelerating sea level rise.
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