FAQ: Sea-Level Rise and California
Human-caused climate change is pushing global temperatures and global sea levels to new heights. In the last 100 years, average global sea level rose roughly 15-18 centimeters (6-7 inches). In California, average sea levels measured by a tide gauge near San Diego reflect this global trend, with a rise of around six inches.
Additionally, the global rate of sea-level rise is accelerating. Satellite measurements of sea surface heights over the last 30 years suggest that the global rate of sea-level rise has more than doubled since 1993.
Researchers have projected that sea levels along the coastal United States will rise by an average of 25-30 centimeters (10-12 inches) between 2020 and 2050. The precise amount of sea-level rise will vary from place to place due to an array of factors including rising and sinking coastal land heights and local differences in ocean height.
This local variation means every coastal state in the U.S. needs to individually assess the unique risks and challenges posed by Earth’s rising seas to prepare for the future.
The California Ocean Protection Council recently published a draft report updating sea-level rise projections for California and providing guidance for how to make use of their findings.
In the Q&A below, Laura Engeman, one of the authors of the report and a coastal resilience specialist with the Center for Climate Change Impacts and Adaptation at Scripps, answers some common questions about sea-level rise in California.
How does climate change cause sea-level rise?
The accumulation of greenhouse gas emissions from human activities in the atmosphere has been responsible for approximately 1.1°C (2°F) of warming since 1900. The main greenhouse gas driving this warming is carbon dioxide. Rising global average temperatures increase sea levels in two main ways.
First, the oceans are getting hotter because they are absorbing 90% of the extra heat created by global warming. This has the effect of making sea levels higher because water expands as it heats up – a phenomenon known as thermal expansion.
Second, higher temperatures are melting ice stored on land in mountain glaciers and in the Greenland and Antarctic ice sheets. As this ice melts, fresh water is added to the oceans which increases sea levels. It is important to note that melting of floating sea ice does not contribute directly to sea-level rise.
How much sea-level rise can California expect by 2050?
California sea levels are expected to increase between 0.15 and 0.37 meters (0.5 feet and 1.2 feet) relative to baseline 2000 levels by the year 2050, according to the California Sea Level Rise report.
But not all California coastal communities will experience the same amount of sea-level rise. This is because sea levels are a relative measurement of where the ocean meets the land. In some locations the shoreline is sinking while sea levels are rising. In a process known as vertical land motion, shoreline lands can subside or uplift which affects their rates of sea-level rise. We see this in Humboldt Bay, where the land is sinking, and by consequence, sea-level rise is projected to be between 0.24 meters (0.8 feet) to 0.5 meters (1.63 feet).
A one foot increase in sea level might sound modest, but the impacts will be very apparent. With a one-foot increase, California will see more frequent flooding during high tides, big waves, and storm activity. Research suggests that mild coastal flooding seen once or twice a year today in La Jolla is projected to occur nearly 50 times a year with around one foot of sea-level rise. This means many of our communities are likely to see “sunny day” mild to moderate flooding on typical high tides by 2050. Higher sea levels can also make coastal storms more destructive because waves will reach higher land elevations and flood areas further inland.
A graph showing projections of the number of days per year that will feature high-tide flooding that meets or exceeds NOAA's minor flooding threshold from a 2021 study. The orange line represents the high end of the projections (90th percentile), the blue line represents the median projections (50th percentile), and the blue shading shows the 10-90th percentile range of the projections. Thompson et al. Nature Climate Change (2021).
How severe will sea-level rise get in California in the next century?
Future sea-level rise is highly dependent on the severity of the global warming we see between now and 2100. Therefore, it is difficult to accurately predict sea-level rise in 2100 because it isn’t clear how much the global population will limit greenhouse gas emissions between now and then. If humanity can limit these emissions, we can make a real difference in slowing sea-level rise. In this sense, how high sea levels get by 2100 depends a lot on human choices, which are difficult to predict.
Because of this uncertainty, sea-level rise projections account for a range of outcomes associated with different global warming scenarios. For example, the new California Sea Level Rise report projects that if future emissions drive an increase in average global temperatures of 2°C Celsius (3.6°F) above pre-industrial levels, San Diego’s sea level is likely to increase 0.9 meters (3 feet) by 2100 and it is less likely but possible that sea level could increase by up to 2 meters (6.6 feet) in that same time frame.
How do we expect sea-level rise to impact California?
The predominant sea-level rise impacts for the California coast are more flooding and erosion.
As sea levels rise, we can expect California’s coastal flooding to occur more frequently, to last longer, and to extend farther inland. Eventually, some areas will flood permanently. The most severe and damaging flooding is expected to occur during storms or large swell events. With sea levels higher, storm waves can reach farther onto dry land, which has the potential to damage California’s roads, property and infrastructure.
During calm weather, communities will also experience more frequent high tide flooding and groundwater flooding. In low-lying areas, increasing sea levels will push the high-tide water level farther up beaches or over today’s flood barriers. Some areas of California will also experience infiltration of sea water into freshwater coastal aquifers, groundwater, and below-ground drainage infrastructure. When this happens, freshwater sources can become salinated and unusable, underground drainages can back up causing flooding on streets, and buried contaminants may be mobilized and leach into nearby soils or waters.
The same basic fact of sea-level rise that facilitates coastal flooding also accelerates coastal erosion: As seas rise, the waves can reach farther up California’s beaches and cliffs where they remove sand and sediment. The higher water levels get, the more beach loss occurs and the more cliff erosion we’re likely to see.
What are some research tools helping California prepare for and adapt to sea-level rise?
The Coastal Processes Group (CPG) at Scripps is working on a number of tools to improve our ability to track and prepare for sea-level rise impacts.
The group is testing a coastal flood alert system in seven coastal communities in Southern California. The system uses a combination of data including tides, wave forecasts, and local coastal measurements such as beach slope to produce alerts whenever conditions might produce coastal flooding in the coming three to five days.
To help communities understand how erosion is impacting their coastline, Scripps researchers developed the Coastal Change Database, which monitors beach and coastal cliff erosion in California with aerial LiDAR.
To help communities with adaptation, Scripps scientists and California Sea Grant’s Coastal Dune Science Network are monitoring beach nourishment and dune restoration sites to find out how well they respond to storm and high-tide events and if they can mitigate flooding and erosion. This research will help refine the toolkit for sea-level rise adaptation and guide future investments.
Finally, in collaboration with San Diego State University researchers, Scripps researchers are also monitoring coastal groundwater in Imperial Beach. Groundwater samples are used to determine how shallow the water table is. As sea levels rise, groundwater tables rise and this can contribute to greater flooding on streets.
Expert Reviewer:
- Mark Merrifield, Director, Center for Climate Change Impacts and Adaptation
References:
- Sweet, W.V., B.D. Hamlington, R.E. Kopp, C.P. Weaver, P.L. Barnard, D. Bekaert, W. Brooks, M.Craghan, G. Dusek, T. Frederikse, G. Garner, A.S. Genz, J.P. Krasting, E. Larour, D. Marcy, J.J. Marra, J. Obeysekera, M. Osler, M. Pendleton, D. Roman, L. Schmied, W. Veatch, K.D. White, and C. Zuzak, 2022: Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines. NOAA Technical Report NOS 01. National Oceanic and Atmospheric Administration, National Ocean Service, Silver Spring, MD, 111 pp.
- Griggs, G, Árvai, J, Cayan, D, DeConto, R, Fox, J, Fricker, HA, Kopp, RE, Tebaldi, C, Whiteman, EA (California Ocean Protection Council Science Advisory Team Working Group). Rising Seas in California: An Update on Sea-Level Rise Science. California Ocean Science Trust, April 2017.
- Philip R. Thompson et al, Rapid increases and extreme months in projections of United States high-tide flooding, Nature Climate Change (2021)
- Sangsefidi, Y., K. Bagheri, H. Tavakol-Davani, and M. Merrifield (2023), Data analysis and integrated modeling of compound flooding impacts on coastal drainage infrastructure under a changing climate, J. Hydrology, 616, 12823.
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