Global mean sea level (GMSL) rise is a key indicator of climate change, with continuous, accurate measurements available since the launch of satellite altimeters in 1993. This study investigates the rate of GMSL rise over the past three decades, focusing on the acceleration observed in recent years. Understanding this acceleration is crucial for coastal communities that need to plan for the consequences of sea level rise. The research aims to quantify the observed increase in GMSL, analyze the underlying causes, and project future sea level rise based on the observed trend. The importance of this study lies in its ability to provide observation-based insights into the near-term trajectory of sea level rise, informing adaptation strategies and improving the accuracy of future projections. The current rate of sea level rise significantly impacts coastal communities, infrastructure, and ecosystems worldwide. The accuracy and reliability of sea level projections rely on a robust understanding of both the total amount and rate of sea level rise and their connection to climate processes and greenhouse gas increases. The study seeks to determine the current trajectory of sea level rise which is crucial for informing effective coastal management and adaptation strategies.

Previous research has established a link between increasing greenhouse gases and rising sea levels. Studies using satellite altimetry data have shown a general increase in GMSL, but the acceleration in recent years has been a focus of recent studies. The impact of El Niño-Southern Oscillation (ENSO) on short-term GMSL fluctuations has also been investigated, highlighting the need to distinguish between natural variability and long-term trends. Models used to project sea level rise vary in their accuracy and their accounting of factors like ice sheet instability. This study builds upon these previous investigations by using an extended dataset to better assess the long-term trend, and quantify the acceleration of the GMSL rise in recent years. The authors review previous work on GMSL rise, including studies quantifying the contributions of thermal expansion and ice melt, and those analyzing the impacts of climate variability, such as ENSO, on short-term GMSL fluctuations. They note that while the overall trend is toward increased sea levels, the rate of this increase is not constant and exhibits periods of acceleration. This previous work forms the basis for the analysis presented in this paper, providing a context for understanding the significance of the newly observed acceleration in GMSL rise.

The study utilizes a global mean sea level (GMSL) time series derived from integrated multi-mission ocean altimeter data, including measurements from TOPEX/Poseidon, Jason-1, OSTM/Jason-2, Jason-3, and Sentinel-6 Michael Freilich. The time series is corrected for glacial isostatic adjustment (GIA) and the seasonal cycle is removed. A quadratic fit is applied to the data to estimate the rate and acceleration of GMSL rise. Uncertainty estimates are calculated by considering three sources of error: serially-correlated residuals, uncertainties in the GIA correction, and measurement errors associated with satellite altimetry. These errors are assessed using a Monte Carlo simulation with 10,000 iterations. The quadratic fit, representing the trajectory of GMSL rise, is extrapolated to project future sea level rise to 2050. This extrapolation is then compared to model-based projections from the IPCC 6th Assessment Report (AR6), considering various Shared Socioeconomic Pathways (SSPs). The choice to extrapolate to 2050 is based on the expectation that major ice sheet instabilities unlikely to significantly influence near-term sea level rise within the next three decades, which is consistent with the methodologies of the IPCC AR6 and supports a comparison of the observed rise with model-based projections.

The main finding is the significant acceleration in GMSL rise. The rate of GMSL rise more than doubled over the 31-year satellite altimeter record, increasing from 2.1 ± 1.0 mm/year in 1993 to 4.5 ± 1.0 mm/year in 2023. The total GMSL rise from 1993 to 2024 was 111 mm. A quadratic fit to the data reveals an acceleration of 0.08 ± 0.06 mm/year². Extrapolating this trajectory, the authors project an additional 169 mm rise by 2050. This projection is largely consistent with the mid-range scenarios of the IPCC AR6, suggesting the observation-based estimate adds confidence to these model-based projections. Furthermore, the study finds that the acceleration is robust to the influence of short-term climate variability, such as ENSO, shown by the consistency of results from slightly different periods within the total dataset. The analysis of shorter altimetry record lengths shows that the estimated rate and acceleration have relatively little variation as the record lengthens, thus strengthening the confidence in the projection. The comparison of the observation-based extrapolation with the IPCC AR6 model-based projections shows substantial overlap, lending further credibility to the study's findings. Specifically, the observation-based projection closely aligns with the median estimate and likely range of the SSP2-4.5 scenario in IPCC AR6. The study demonstrates that the acceleration in GMSL is not significantly affected by short-term climate variations.

The findings confirm the significant and accelerating rise in global sea levels, driven by ocean warming and ice melt, both linked to increasing greenhouse gas concentrations. The doubling of the rate of sea level rise over three decades highlights the urgency of addressing climate change and the need for enhanced adaptation strategies. The consistency between the observation-based extrapolation and the mid-range IPCC AR6 projections strengthens the confidence in these projections and provides a valuable independent confirmation. The observation-based projection serves as a useful complement to model-based projections, adding a level of independent verification and potentially reducing uncertainties in the near-term estimates of sea level rise. The robustness of the acceleration estimate to short-term climate variability underscores the validity of the findings and enhances the reliability of the projections for near-term sea-level rise. This has important implications for coastal communities, requiring them to adapt more rapidly and potentially implement more aggressive mitigation efforts.

This study provides compelling evidence of the accelerating rate of global sea level rise, confirming a more than doubling of the rate over the past three decades. The projection of a substantial further rise in the coming decades emphasizes the urgency of addressing climate change and implementing proactive adaptation measures. The consistency between observation-based estimates and model-based projections strengthens the overall confidence in future sea level rise scenarios. Future research should focus on refining regional sea level projections, improving understanding of ice sheet dynamics, and developing more sophisticated climate models that can better capture the complex interactions driving sea level change. Continued monitoring of GMSL through satellite altimetry is critical to track the progression of sea level rise and refine our ability to predict the trajectory of future changes.

The study's projections are limited to the near term (2050) due to uncertainties associated with potential future ice sheet instabilities that could lead to larger contributions to sea level rise beyond that timeframe. The analysis relies on global mean sea level data, which does not fully capture the regional variations in sea level rise. Local factors can influence sea level rise at specific locations, exceeding global averages. While efforts were made to account for several error sources in the data analysis, minor uncertainties may still exist, and future improvements in satellite data and modeling could refine the projections further. The assumption of a constant acceleration in the future is a simplification; future changes in greenhouse gas emissions and climate dynamics could impact the rate of sea level rise.