Delta tunnel — Inconvenient truths about sea level rise and salinity intrusion

The Delta tunnel Draft Environmental Impact Report (DEIR) has a new Sea Level Rise Study in Technical Appendix 5A-F. The study was done with Bay-Delta SCHISM, a 3-D hydrodynamic model of the San Francisco Bay and Delta.

The new SCHISM study has limitations, in that it assumes that Delta levees are raised to keep up with sea level rise. However, the study is useful in showing how salinity will increases with sea level rise, and how much additional outflow would be needed to repel salinity.

The Emmaton station at Sherman Island in the western Delta is the furthest downstream compliance point on the Sacramento River for salinity. The reproduced below (5AF-7) shows the monthly average increase in salinity at Emmaton with 7 different levels of sea level rise, if Delta inflows and exports are unchanged.

The description states:

The area of light gray shading shows the range of historical extreme EC value observed from the last two decades. These extreme values largely occurred in droughts and outside the periods with D-1641 agricultural salinity standard (later in the season than August). The area of dark gray area shading represents simulated EC values greater than recent historical experience.

What if one increases Delta inflows and reduces exports to maintain current D1641 salinity standards at Emmaton?  Figure 5AF-8 (reproduced below) shows the associated water costs. There is a nonlinear increase.

For low values of sea level rise, the new  study estimates less outflow to repel salinity than a 2008 study by William Fleenor et. al. That study estimated that one foot of sea level rise would require at least 475,000 acre-feet a year of additional outflow.

How probable are the higher values of sea level rise by 2100? The current best available science is arguably NOAA’s March 2022 publication, Global and Regional Sea Level Rise Scenarios for the United States (Sweet et. al. 2022.) NOAA’s guidance is more recent than the 2018 Ocean Protection Council guidelines cited in DWR’s Sea Level Rise Study. The high sea level rise estimates are 2 meters by 2100 – 6.9 feet, and intermediate-high estimates are 1.5 meters by 2100 – 4.9 feet.

Sweet et. al. explain:

Current and future emissions will determine the amount of additional rise in the future: the greater the emissions, the greater the warming, and the greater the likelihood of higher sea levels.

Above 5.5°F (3°C) of global warming, much greater sea level rise becomes possible for the U.S. and globally because of the potential for rapid melting of ice sheets in Greenland and Antarctica. The amount of additional warming required to trigger this is unknown because ice sheet instability is difficult to model and there is great variability in current modeling approaches.

The table below shows mean projections of sea level rise by Sweet et. al., for different projected increases in mean temperature.  While the probability of the intermediate-high scenario is low, it is not negligible. The probability of the high sea level rise increases with the very high emissions scenario.Sweet et. al. do not include the extreme scenario of 2.5 meters (10.2 feet) of sea level rise by 2100 because it is “now viewed as less plausible in the coming decades before potentially becoming a factor toward the end of the 21st century and beyond.” But they also note that this threshold could potentially be reached “in the decades immediately following 2100 (and continue rising).”

In planning for adaptation to global increases in temperature above 3 degrees Centigrade, one must consider the deep uncertainties about potential global tipping points. Kemp et. al (2022) published a letter today in the Proceedings of the National Academy of Sciences calling for an IPCC study of catastrophic climate change scenarios. The authors state:

Prudent risk management requires consideration of bad-to-worst-case scenarios. Yet, for climate change, such potential futures are poorly understood. Could anthropogenic climate change result in worldwide societal collapse or even eventual human extinction? At present, this is a dangerously underexplored topic. Yet there are ample reasons to suspect that climate change could result in a global catastrophe…

Catastrophic impacts, even if unlikely, have major implications for economic analysis, modeling, and society’s responses. For example, extreme warming and the consequent damages can significantly increase the projected social cost of carbon.

As we commented in 2019, the State Water Project is currently using power from a natural gas plant in Lodi. The social cost of using fossil fuels to ship water hundreds of miles could potentially be extremely high.

Further reading

[DWR] California Department of Water Resources. 2022. Delta Conveyance Project Draft Environmental Impact Report, Appendix 5A, Section F, Modeling Technical Appendix – Sea Level Rise And Delta Water Quality Modeling.

Des Jardins, D. 2019. California’s Water Resilience Portfolio should reduce GHG emissions. California Water Research blog, April 29, 2019.

Fleenor, W., Hanak, E., Lund, J., Mount, J. 2008. Delta Hydrodynamics and Water Salinity with Future Conditions Technical Appendix. Public Policy Institute of California.

Kemp, L., Xu, C, Depledge, J., Ebi, K. L., Gibbins, G., Kohler, T. A., Rockström, J., Scheffer, M., Schellnhuber, H., Steffen, W., Lenton, T.M. 2022. Climate Endgame: Exploring catastrophic climate change scenarios. Proceedings of the National Academy of Sciences 119(34).

Sweet, W.V., Hamlington, B.D., Kopp, R.E., Weaver, C.P., Barnard, P.L., Bekaert, D., Brooks, W., Craghan, M., Dusek, G., Frederikse, T. et. al., 2022: Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines. 2022. National Oceanic and Atmospheric Administration, National Ocean Service. (Note: Delta Independent Science Board member Jayantha Obeysekera is a co-author.)

Water.ca.gov. c2022. SCHISM. [accessed August 1, 2022.]