Sea level science and applications support coastal resilience
Known for precise, planetary-scale measurements, NASA is improving its decades-long effort to observe and understand sea levels to help communities prepare for the effects of Earth’s rising ocean.
Planetary warming is increasing the height, and thus the volume, of Earth’s ocean. Today, the ocean gains about 300 trillion gallons (almost 1,150 trillion liters) of liquid water every year. If you put this water into 1-gallon milk jugs and lined them up end to end, they would extend across the whole solar system and back. Spreading this volume over the ocean’s surface translates into about 3.4 millimeters of global mean sea level rise per year, a more familiar metric of climate change.
As the rising ocean gradually encroaches on land, it increases the severity and frequency of flooding, threatening populated coastal communities, damaging property and infrastructure, posing risks to national security, and endangering coastal ecosystems and biodiversity. The ocean’s expansion will continue for millennia: In the present climate, we are already committed to a global sea level increase of 2–6 meters over the next 2 millennia [Intergovernmental Panel on Climate Change (IPCC), 2021].
Two thousand years might sound too far into the future to be relevant, but even the single meter of sea level rise projected for major coastal cities by the end of this century would be life changing (e.g., Figure 1). For example, an additional meter of sea level rise could be catastrophic for energy storage and distribution facilities located on coasts or at major inland ports—as they are predominantly in the United States—and would become unsustainable with frequent flooding and eventual inundation.
Recent power outages caused by inclement weather, as in Texas in 2021, have proven traumatic on local to regional scales, but such impacts would pale in comparison with those of a large-scale collapse of coastal energy and economic infrastructure.
The need to secure our existing and future energy infrastructure amid a rapidly changing climate to ensure public safety, maintain economic stability, and reduce energy dependence on aggressive autocratic petrostates with geopolitical expansion ambitions provides urgency to improve projections of future sea levels and communicate this information to decisionmakers in a timely manner.
Recognizing this urgency, efforts to increase the resilience of coastal communities and economies are gaining traction on an international scale, as demonstrated by discussions at the recent United Nations Climate Change Conference of the Parties (COP26). Such efforts are also growing at local, state, and national levels in the United States—in the form of bipartisan infrastructure legislation, for example—and abroad. The Biden administration has identified building national resiliency against climate change impacts as one of its priorities. This climate agenda is supported in part by the White House National Climate Task Force and various interagency programs aimed at accelerating progress toward coastal resilience. One of these, now nearly a decade old, is NASA’s Sea Level Change Team, or N-SLCT (pronounced “NASA select”).
Integrated Sea Level Science at NASA
In the early 1990s, NASA, together with national and international partners, pioneered the collection of direct, accurate measurements of ocean height from space, starting with the TOPEX/Poseidon ocean altimeter satellite. In the 3 decades since, the agency has continued providing global views of rising seas, with accuracies down to a few centimeters, using observations from multiple altimetry missions, including the Jason series satellites and now the Sentinel-6 missions (Figure 2; see NASA’s Ocean Physicsand Ocean Surface Topography pages for more information).
With these observations and data from other NASA and partner platforms, we now know how fast and why sea level is changing near coastal cities around the world. The accuracy and longevity of NASA’s sea level observatory have deepened our understanding of the physics of the Earth system. They have also allowed scientists to improve climate models and physics-based projections of processes and changes in the ocean and atmosphere and on land, including those in the IPCC’s recent Sixth Assessment Report (AR6) [IPCC, 2021].
In 2014, as NASA faced increasing demand for sea level information, the agency assembled the centralized, multidisciplinary N-SLCT, consisting of leading experts in the fields of ocean physics, geodesy, cryosphere, hydrology, modeling, statistics, and science communication. The initial goals of the team were to transcend barriers among different geoscience disciplines and to develop integrated views of sea level both today and in the future.
Since then, the team has made significant scientific advances [Hamlington et al., 2020], resulting in a better understanding of the dynamics of land ice and ocean-ice interactions that contribute to sea level rise [e.g., Nowicki and Seroussi, 2018] and how the ocean’s intrinsic variability drives coastal sea level changes [e.g., Dangendorf et al., 2021], as well as improved representations of vertical land motion related to coastal subsidence, tectonics, and postglacial rebound [e.g., Hammond et al., 2021]. In addition, N-SLCT has made significant strides toward integrated assessments of ongoing and future sea level rise [e.g., Larour et al., 2020; Thompson et al., 2021]. This work has served as the foundation to develop various Web-based capabilities that provide access to and visualizations of sea level change on local levels over the past 3 decades.
N-SLCT continues to provide observational evidence of sea level change for use by team members and by other researchers around the world. To address pressing scientific unknowns, we are diving deeper into the underlying drivers of changing ocean thermodynamics in a warming climate, as well as of disappearing ice sheets and mountain glaciers and their impacts on terrestrial hydrology and the ocean. And as N-SLCT approaches its 10-year anniversary, its ambition and scope of work have expanded. Our goal is not only to use our improved understanding of sea level gained through observations and modeling to advance predictions and projections of sea level over a range of time horizons but also to make this information useful to decisionmakers facing difficult tasks of planning and communicating adaptation and mitigation responses in their communities. N-SLCT is working to do this through our leadership within sea level science and through collaboration and engagement with practitioners and national and international partners.
Improved Assessments of Future Sea Level
N-SLCT’s strengths as a multidisciplinary team with expertise in both observations and modeling relevant to sea level rise are allowing us to make advancements on three important fronts: integrating observations into projections of changing regional relative sea levels (i.e., the height of the sea relative to land), improving the representation of connections between different processes in these projections, and producing assessments of future sea level effects that integrate processes across time and space scales.
To project regional relative sea levels, researchers contributing to AR6 used a probabilistic framework known as the Framework for Assessing Change to Sea Level, or FACTS [IPCC, 2021]. FACTS, which was developed in part through N-SLCT efforts, combines input from models of the different processes that contribute to sea level, such as projections of ice sheet melt and projections of changing ocean dynamics. Direct sea level observations can help improve this input either through their incorporation into these models or by serving as input themselves. Such observations are also useful in evaluating output from a projection framework. Decades-long satellite altimetry and tide gauge records, for example, are now long enough that they can be used to assess near-term trajectories of sea level rise and provide comparisons with model-based projections [Sweet et al., 2022].
Examining and improving projection frameworks themselves are another focus of N-SLCT. In addition to advancing IPCC’s FACTS, we are developing a new framework to improve representations of some of the physical connections between different processes that act across the ocean, land, atmosphere, and solid Earth, recognizing that many of these processes are linked [e.g., Adhikari et al., 2016; Larour et al., 2020].
And beyond improving sea level rise projections, N-SLCT has several ongoing efforts to put the pieces together to generate integrated assessments of current and future sea levels and how they will combine with other factors to affect humanity. Although future sea level projections are foundational in many of these assessments, projections alone do not capture the impacts that are ultimately felt at the coast. In a recent example of these efforts, N-SLCT members combined data on tides and internal sea level variability with sea level projections to identify a potential rapid increase in high-tide flooding for much of the U.S. coastline by the mid-2030s [Thompson et al., 2021]. The framework used in this work and an accompanying interactive tool, which lets users explore potential changes in future flooding at more than 90 coastal sites around the country, are provided via the NASA sea level portal.
Open Access to Critical Information
A major goal of N-SLCT is to find ways to translate scientific information so it’s useful and available to those who need it, from national, state, and local planners to interested individuals. To meet this challenge, N-SLCT is developing a variety of tools to deliver societally relevant sea level information.
One example is the flooding projection tool mentioned above. Another relates to NASA’s focus on evaluating regional sea level budgets (in addition to the global sea level budget), which quantify contributions, on the basis of observational satellite and tide gauge data, of the different physical mechanisms driving sea level change on more local scales. This evaluation was first done for U.S. coastal sites [Harvey et al., 2021] and was then expanded internationally with an accompanying tool that provides detailed, observation-based information about sea level trends over the past 3 decades for hundreds of sites around the world.
Timed with the release of AR6, NASA partnered with the IPCC to launch a data delivery and visualization platform that provides users timely, open, and easy access to the critical climate information and sea level projections—out to 2150 following a half dozen different climate scenarios—shared in the report (e.g., Figure 1). This tool provides a bookend to the technical expertise and analysis that N-SLCT members contributed to the report itself, showing how the team supported the flow of information from observation to scientific understanding to dissemination.
N-SLCT also recently produced its first scenarios projecting mean sea level rise around the U.S. coastline from decades to a century in the future (e.g., Figure 3). These hot-off-the-press scenarios, observational comparisons, and the science behind them conducted by N-SLCT have already demonstrated their impact by forming the basis of a recent national assessment of regional sea level rise [Sweet et al., 2022]. This effort was supported by multiple federal agencies (NOAA, NASA, the U.S. Geological Survey, and EPA) as part of the Interagency Sea Level Rise Taskforce convened jointly by the White House Subcommittee on Ocean Science and Technology and the U.S. Global Change Research Program. As in the case of the partnership with IPCC, NASA developed a similar tool focused on national sea level assessments.
These efforts and open-access tools are examples of NASA’s commitment to environmental justice and equity, bringing actionable state-of-the-art climate information to those who need it, including vulnerable coastal communities around the world. For example, the AR6 visualization platform has users from more than 200 counties around the world, with 80% of users located outside the United States. To further increase the reach and value of N-SLCT science advances, we have convened a Practitioner Consultation Board consisting of representatives from boundary organizations like NOAA Sea Grant and other federal agencies, coastal resilience officials from several U.S. cities and municipalities, and others. With N-SLCT, this board codesigns sea level tools and products to make information more accessible and useful for the intended audiences. Both the IPCC and national sea level assessment tools were codeveloped jointly with the board.
N-SLCT is also engaged in other outreach efforts. For example, the team was recently selected as an official program of the United Nations’ Decade of Ocean Science for Sustainable Development, in recognition of N-SLCT’s approach to interdisciplinary science and outreach. With its participation in the U.N.-organized effort, N-SLCT will offer seminars, workshops, and other educational outreach to share its foundational science and approach to making sea level science actionable and useful on the global stage, serving as an exemplar for similar efforts in the near future.
Better Projections for Better Preparation
Like sea level itself, sea level science is dynamic and rapidly evolving. Even the most recent global climate consensus, AR6, is already out of date to some degree as new data, modeling, and discoveries outpace our abilities to synthesize available knowledge about the climate system. Since the release in 2022 of the agency’s first projected scenarios for sea level rise, N-SLCT members have revealed several new physical mechanisms and interactions in the climate system, suggesting the potential limitations and incompleteness of our projections.
For example, Piecuch et al. [2022] recently quantified the effects of atmospheric rivers on coastal sea level and storm surges on the U.S. West Coast, and Larour et al. [2021] uncovered how ice mélange (a mixture of sea ice and glacial ice) can control rifting of Antarctic ice shelves. Meanwhile, Wood et al. [2021] helped illuminate the ocean’s dominant role in driving Greenland glacier dynamics, finding a nearly direct response between subsurface ocean temperatures and glacier retreat that is not accounted for in many climate models, which may thus underestimate mass loss by at least a factor of 2.
Continued improvement of our knowledge of Earth’s climate system will lead to better projections of its future. This improvement requires diligent assessment of the planet’s potential climate trajectories. As Earth’s climate responds to natural and anthropogenic forcings, the only way to narrow the projected trajectories (e.g., note their divergence in Figure 3) so we can plan for the most likely scenario is to observe where we are headed in real time.
This need is why NASA and its global partners are committed to sustaining and enhancing sea level observations, both through our current observing satellite altimetry missions, like Sentinel-6, and with investments in new observing capabilities, like those of the Surface Water and Ocean Topography (SWOT) mission. These capabilities, combined with the multidisciplinary expertise of N-SLCT and other scientists, will enable us to track Earth’s rising oceans today and tomorrow and to synthesize knowledge into open, accurate, timely, and actionable information that will help humanity plan and prepare for the changes to come as Earth’s climate—and coastlines—changes.