The acceleration of sea level rise is dramatic under climate change (Chen et al. 2017). Figure 1 shows actual measurement of historic global sea level change. According to IPCC reports
Global mean sea level (GMSL) increased by 0.20 [0.15 to 0.25] m over the period 1901 to 2018 with a rate of rise that has accelerated since the 1960s to 3.7 [3.2 to 4.2] mm yr-1 for the period 2006–2018 (high confidence, which means that experts in IPCC have high confidence in these results). Human activity was very likely the main driver of observed GMSL rise since 1970, and new observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components contributing to sea level rise, including expansion due to ocean warming and melting of glaciers and ice sheets (high confidence)” (IPCC 2022).
In the future, the likely range between 1 and 1.7m rise in sea levels by 2150 under Shared Socioeconomic Pathways (SSPs) scenarios, shown in Figure 2.
Rising seas affect homes, businesses, and habitats, particularly for low-lying highly populated, highly productive, and environmentally sensitive areas. Figure 3 shows a satellite imagery of New Orleans, Louisiana, along the U.S. Gulf Coast. The Satellite data from decades of observations help researchers tease apart natural and human-caused drivers of sea level rise, to prepare for the future.
Prior to the advent of satellite observation technology, the monitoring of global sea level changes could only be based on observations from tide stations around the world. The spatial coverage of tidal stations is limited, and tidal stations can only observe sea level changes near the coast. Since the mid-1970s, scientists have begun to use satellite altimetry to study global sea level changes. In 1992, the modern high-precision ocean altimeter satellite (TOPEX/Poseidon) jointly funded by the French National Center for Space Science (CNES) and NASA was launched, and since then, a series of modern high-precision ocean altimeter satellites have been launched, and the observation and research of global sea level change has entered a new historical period (Cole 2019).
Unabated global mean sea-level rise over the satellite altimeter era
TOPEX/Poseidon and Jason series (Jason-1, Jason-2, and Jason-3) of satellite radar altimeter missions have allowed estimates of global mean sea level, as shown in Figure 4. The trend in the graph makes it obvious that global mean sea-level rises over the satellite altimeter era (Watson et al. 2015). At the regional scale, ocean thermal expansion is the main cause of the spatial trend patterns observed by satellite altimetry (Cazenave, Palanisamy, and Ablain 2018) and the loss of ice from the Greenland and Antarctic ice sheets and from terrestrial glaciers is now the main contributor to sea level rise (Pörtner et al. 2019). Since 1970, the global ocean has absorbed more than 90% of the excess heat in the climate system and since 1993, the rate of ocean warming has increased. A record was reached in 2020 (Cheng et al. 2021) with measured impacts on the ocean heat content to at least 2000 m.
Improvement and advancement of Sentinel-6
The next generation of high-precision altimeters, Jason-CS/Sentinel-6 includes two identical satellites with the first Sentinel-6 Michael Freilich (S6-MF) launched in November 2020 and the second, satellite B scheduled for launch in 2025. The Jason-CS/Sentinel-6 satellites will extend this legacy through at least 2030, providing a nearly 40-year record of satellite data, along with changes in ocean currents and conditions. The Sentinel-6 mission has a specific focus on sea level rise measurements and sea state measurements. Figure 5 presents a summary overview of the Sentinel-6 system. Figure 6 shows several views of S6-MF. Dr. Donlon, a Sentinel-6 mission scientist at the European Space Agency (ESA) said, “Building on a long line of European heritage dual-frequency altimeter missions, Sentinel-6’s Poseidon-4 altimeter was designed to bring new high-resolution Ku-band synthetic aperture radar measurements into the altimetry reference time series” (EUMETSAT 2021). Figure 7 shows the improvement of Sentinel-6 range noise with respect to Jason-3.
Figure 8 shows image of the Ozero Nayval peninsula, Russia, from different satellites. The Ozero Nayval peninsula is surrounded by a mountainous region and lies on the eastern part of the Bering Strait. It has a unique low-lying land-bound lagoon, various river and lake features which are clearly visible in the image and marked together with the ground track of S6MF as it crosses the region, shown in Figure 8(a). Unlike the Sentinel-1 image, the Sentinel-6 Poseidon-4 radar is illuminating the scene from top to bottom of the image. In this case, ocean wave structures and wave refraction at the southern coastline can be clearly seen. Unavoidable range ambiguities are visible in the upper part of the image. The low noise performance of Poseidon-4 measurements is clearly revealed in this striking result.
Dr. Donlon (ESA 2021) explained, “Establishing the differences between Sentinel-6 and Jason-3 is important if stability in the sea-level rise time series from satellite altimetry is to be maintained with low uncertainties.
Measurements show that the absolute difference between measurements from Sentinel-6 and Jason-3 is less than 2 mm, which is remarkable for two independent satellites operating at an altitude of 1330 km. Dr. Donlon (ESA 2021)
Applications and challenges of satellite altimetry technology
The complexity of global sea level change in spatial and temporal domains exceeds the cognitive scope based on observation data of surface tide stations in the past by far. Sea level data observed by tide stations is discontinuous in space, and missing in the middle of the ocean. The start time and interval time of observation from different tide stations are different. Observation by satellite altimetry provides spatial-temporal datasets that are continuous and can cover the wider range of the ocean. The application of satellite altimetry, as facilitated for example by Sentinel-6, is mainly to measure sea surface topography with high accuracy and reliability and further to support ocean forecasting systems, environmental monitoring and climate monitoring. Large-scale continuous satellite observations complement detailed (but sparse) field observations and provide measurements of unsurpassed volume and content for theoretical modeling and data assimilation.
An increase in the value of the data and the efficiency of satellite remote sensing applications will likely be related to successes in:
- Increasing both coverage and resolution in spatial and temporal records of observations;
- Increasing information content of observations by deploying the satellite instruments with enhanced capabilities and exploring synergies of complimentary observations;
- Achieving continuity in consistent satellite observations and long-term data sets by assuring sufficient compatibility and agreement of past, present and future data sets that are required for accumulating climate records.
Sea-level rise is a key indicator of climate change. Therefore, accurately monitoring the changing height of the sea surface over decades is essential for climate science, for policy-making and, ultimately, for protecting the lives of those in low-lying regions at risk.
In the past decades satellite remote sensing has become one of most efficient tools for surveying sea level change at local, regional, and global spatial scales. Past Jason missions and current missions such as Copernicus Sentinel-6 are unique solutions to provide accurate information on this trend, observing and monitoring it accurately, as well revealing the alarming acceleration of the rise.
It is important and urgent that the international community takes action to combat climate change and its impacts. Preparing our resilience and adaptive capacity to sea-level rise as an effect of climate change is a top priority for the decades to come, as also reflected by Sustainable Development Goals (SDG 13). From SSPs scenarios we can observe that the sea-level rise phenomenon is accelerating faster than expected. Advanced space-based technologies are crucial to strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.