ABSTRACT Watson, P.J., 2020. Updated mean sea-level analysis: Australia. Journal of Coastal Research, 36(5), 915–931. Coconut Creek (Florida), ISSN 0749-0208. As an island nation with 60,000 km of open coastline and extensive margins of increasingly urbanised intertidal estuarine foreshores, Australia is critically exposed to the global threat posed by rising sea levels into the future. This study provides a contemporary assessment of sea-level rise around Australia to the end of 2018, based on all available tide gauge records and satellite altimetry. The study provides the first national assessment of vertical land motion (VLM) around the coast, identifying margins more prevalent to subsidence, which in turn exacerbate the localised effects of a rising global mean sea level. These areas include coastlines between Townsville and Coffs Harbour, Burnie to Port Pirie, and Fremantle to Wyndham. State-of-the-art time-series analysis techniques applied to all high-quality tide gauge records exceeding 75 years in length (four sites) enabled improved insights into the temporal resolution of current rates of rise and accelerations in mean sea level around Australia than were previously available. Averaged across these four records in 2018, approximately 40% of the “relative” velocity observed (∼2.2 ± 1.8 mm/y, 95% confidence limit [CL]) is attributable to VLM. When corrected for VLM, only the Fort Denison site exhibits “geocentric” mean sea-level velocity in 2018 exceeding 2 mm/y. The average geocentric velocity across all four sites in 2018 equates to 1.3 ± 2.0 mm/y (95% CL). Interestingly, each long record exhibits similar temporal characteristics, whereby a low point in the velocity time series occurs sometime in the period from 1970 to 1990, after which velocity increases over time to a peak occurring sometime after ca. 2010, suggesting the presence of a small acceleration (albeit not statistically different to zero at the 95% CL) in the record.

ABSTRACT Watson, P.J., 2019. An assessment of the utility of satellite altimetry and tide gauge data (ALT-TG) as a proxy for estimating vertical land motion. Journal of Coastal Research, 35(6), 1131–1144. Coconut Creek (Florida), ISSN 0749-0208. Policy, planning, and adaptation responses to sea-level rise are being developed at increasingly localised scales, placing greater emphasis on sea-level studies to more accurately account for all vertical land motions (VLM) at tide gauge sites using technologies such as global navigation satellite systems (GNSSs). Although the spatial coverage and length of GNSS data records continues to increase in proximity to tide gauges, large tracts of the world's coastlines contain limited or no GNSS data. Various studies have investigated proxy methods of estimating VLM by trends from differenced altimetry–tide gauge techniques (ALT-TG). This study has concentrated on investigating the utility of ALT-TG techniques using so-called ‘off the shelf’ gridded satellite altimetry products (U.S. National Aeronautics and Space Administration Jet Propulsion Laboratory [NASA JPL] and Copernicus Climate Change Service [C3S]) for use in regional and local scale sea-level studies to estimate VLM. Twenty locations across the globe were analysed; these locations met specific data constraints designed to ensure the longest overlapping coverage of GNSS, altimetry, and tide gauge data, with the proximity of the GNSS record limited to within 1 km of the tide gauge. The utility of ALT-TG estimates was significantly improved by using gridded altimetry products no closer than 30 km from the open coast. When compared directly with measured GNSS solutions from NASA JPL, Systeme d'Observation du Niveau des Eaux Littorales (SONEL), and Nevada Geodetic Laboratory (NGL), both ALT-TG VLM estimates agree with the NGL GNSS solution for 19 of the 20 locations (95% confidence interval). This is significant given that the NGL estimates are derived over the longest timeframe available from all three key GNSS data repositories considered. If one considers the limitations of altimetry sea surface height measurements in the coastal zone, the results using gridded sea surface height anomaly products for ALT-TG derived estimates of VLM are extremely encouraging for sea-level research.

ABSTRACT Watson, P.J., 2019. Updated mean sea-level analysis: South Korea. Journal of Coastal Research, 35(2), 241–250. Coconut Creek (Florida), ISSN 0749-0208. The threat of sea-level rise to the heavily populated Korean Peninsula has profound and far-reaching implications. This study updates and extends the several previous works undertaken to analyse tide-gauge records and satellite altimetry around South Korea using enhanced time-series analysis techniques to detect coastal vertical land motion and current rates of rise and accelerations in mean sea level to augment planning, design, and risk management activities. Although the longest tide-gauge records available only date back to 1960, every effort has been made to separate the mean sea-level trend from the more dynamic influences with improved precision using state-of-the-art analytical techniques. The analysis identified general trends of subsidence observed around the margins bounded by the East China Sea and East Sea (Sea of Japan) below 36°N, whereas uplift was a more prevalent feature along the margins bounded by the Yellow Sea. All tide-gauge records longer than 50 years exhibited ‘relative’ mean sea-level rise increasing marginally over the length of the record, suggesting the presence of an acceleration; however, the estimated time-varying accelerations (albeit predominantly positive) are small and not statistically different from zero (95% confidence interval). Although the average trend of sea-surface height from satellite altimetry across this region was 3.2 mm/y, key spatial variations were evident, with the highest rates of rise centred in two discrete areas east and west of South Korea around 37.5°N, each exceeding 8 mm/y.

ABSTRACT Watson, P.J., 2017. Acceleration in European mean sea-level? A new insight using improved tools. Research into sea-level rise has taken on particular prominence in more recent times owing to the global threat posed by climate change and the fact that mean sea level and temperature remain the key proxies by which we can measure changes to the climate system. Under various climate change scenarios, it has been estimated that the threat posed by the effects of sea-level rise might lead to annual damage costs across Europe on the order of €25 billion by the 2080s. European mean sea-level records are among the best time series data available globally by which to detect the presence of necessary accelerations forecast by physics-based projection models to elevate current rates of global sea-level rise (≈3 mm/y) to anywhere in the vicinity of 10–20 mm/y by 2100. The analysis in this paper is based on a recently developed analytical package titled “msltrend,” specifically designed to enhance estimates of trend, real-time velocity, and acceleration in the relative mean sea-level signal derived from long annual average ocean water level time series. Key findings are that at the 95% confidence level, no consistent or compelling evidence (yet) exists that recent rates of rise are higher or abnormal in the context of the historical records available across Europe, nor is there any evidence that geocentric rates of rise are above the global average. It is likely a further 20 years of data will distinguish whether recent increases are evidence of the onset of climate change–induced acceleration.

ABSTRACT Watson, P.J., 2016. Acceleration in U.S. mean sea level? A new insight using improved tools. The detection of acceleration in mean sea level around the data-rich margins of the United States has been a keen endeavour of sea-level researchers following the seminal work of Bruce Douglas in 1992. Over the past decade, such investigations have taken on greater prominence given mean sea level remains a key proxy by which to measure a changing climate system. The physics-based climate projection models are forecasting that the current global average rate of mean sea-level rise (≈3 mm/y) might climb to rates in the range of 10–20 mm/y by 2100. Most research in this area has centred on reconciling current rates of rise with the significant accelerations required to meet the forecast projections of climate models. The analysis in this paper is based on a recently developed analytical package titled “msltrend,” specifically designed to enhance estimates of trend, real-time velocity and acceleration in the relative mean sea-level signal derived from long annual average ocean-water-level time series. Key findings are that at the 95% confidence level, no consistent or substantial evidence (yet) exists that recent rates of rise are higher or abnormal in the context of the historical records available for the United States, nor does any evidence exist that geocentric rates of rise are above the global average. It is likely that a further 20 years of data will identify whether recent increases east of Galveston and along the east coast are evidence of the onset of climate change induced acceleration.

ABSTRACT Watson, P.J., 2016. How to improve estimates of real-time acceleration in the mean sea level signal. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research , Special Issue, No. 75, pp. 780–784. Coconut Creek (Florida), ISSN 0749-0208. One of the most critical environmental issues confronting mankind into the foreseeable future remains the ominous spectre of climate change, in particular the pace at which impacts will occur and our capacity to adapt. Sea level rise is one of the key artefacts of climate change that will have profound impacts on global coastal populations. Although extensive research has been undertaken into this issue, there remains considerable conjecture and scientific debate about the temporal changes in mean sea level and the climatic and associated physical forcings responsible for them. Over recent years, significant debate has centered around the issue of a measurable acceleration in mean sea level, a feature central to projections based on the current knowledge of climate science. To reduce this uncertainty, it is necessary to determine the better performing analytical approaches for isolating the mean sea level signal from long, individual ocean water level time series with improved temporal resolution. This paper summarises the testing and development of an analytical tool designed specifically to enhance real-time estimates of velocity and acceleration in mean sea level derived from contemporary ocean water level data sets. The long ocean water level record at San Francisco, USA has been used to highlight the application and utility of the improved approach.

ABSTRACT Watson, P.J., 2015. Development of a unique synthetic data set to improve sea-level research and understanding. One of the most fundamental, critical environmental issues confronting mankind into the foreseeable future remains the ominous spectre of climate change, in particular the pace at which impacts will occur and our capacity to adapt. Sea-level rise is one of the key artefacts of climate change that will have profound effects on global coastal populations. Although extensive research has been undertaken into this issue, there remains considerable conjecture and scientific debate about the temporal changes in mean sea level and the climatic and associated physical forcings responsible for them. In particular, over recent years, significant debate has centred around the issue of a measurable acceleration in ocean water-level records, a feature central to projections based on the current knowledge of climate science. The complexity of the dynamic influences and noise embedded within ocean water-level data sets has led sea-level research toward successively more sophisticated time series analytical techniques to estimate the trend. In the absence of an absolute knowledge of the mean sea-level signal (or trend) for a particular record, the accuracy of the trend has increasingly been inferred from the assumed sophistication of the underpinning analytical approach applied to the data record. An innovative and transparent process by which to identify the most efficient analytical technique for isolating the mean sea-level signal is to test such approaches against “synthetic” (or custom built) data sets with a known mean sea-level signal. This paper details the development of a monthly average data set to meet the aforementioned objective.