Minimal snow was found at the Phillips Station meadow before the start of the first snow survey of 2018, conducted by the California Department of Water Resources. The survey site is approximately 90 miles east of Sacramento off Highway 50 in El Dorado County. Photo taken January 3, 2018. Kelly M. Grow/ California Department of Water Resources, FOR EDITORIAL USE ONLY

DR. GEETA PERSAD: Climate change and the future of California’s water

Minimal snow was found at the Phillips Station meadow at the first snow survey of 2018. (Photo by DWR)

Climate change is already transforming California’s water cycle, putting stress on the state’s rigidly engineered water infrastructure as well as our unique ecosystems. Current water management strategies will need to be fundamentally rethought as climate change depletes the state’s natural snowpack storage, concentrates rainfall into increasingly extreme events, brings salty ocean water farther inland, and dries out vegetation and soils. The San Francisco Estuary, the heart of the state’s natural water infrastructure, will be ground zero for the confluence of these stresses.

Dr. Geeta Persad is a senior climate scientist with the Climate and Energy Program at the Union of Concerned Scientists.  In her role, Dr. Persad conducts research and analysis related to climate change in the Western US.  Her work focuses on the intersection of climate change with water and air quality issues with the goal of innovating science-based solutions to these interlinked problems.  In this presentation from the 2019 State of the Estuary conference, Dr. Persad discussed the ways in which climate change is going to fundamentally transform how, when, and where California gets its water and how those changes will have profound impacts for the state and for the San Francisco estuary in particular.

Dr. Persad began by showing historical and future projections of average temperature and average precipitation, noting that climate scientists have been using computer models of the climate system for decades to try to understand what the rising heat trapping gasses mean for the planet and for the places that we live and love.

Annual average temperatures in the state have already warmed by a degree Fahrenheit and parts of the state have warmed by more than 2 degrees,” she said.  “The graph on the left is showing future projections of maximum temperature throughout the state, and the climate model projections show that the annual maximum temperatures in the state could warm by another ten degrees, depending on the choices that society makes about how much to keep emitting.”

The graph on the above right shows the projections of future precipitation in California; she acknowledged that it doesn’t have as much of an upward or downward trend as the graph on the left.  Some climate models show increased precipitation while others show decreased precipitation.

It’s easy to look at a graph like this and think that either California doesn’t have much to worry about when it comes to water, or that we don’t know enough to start planning for what’s coming,” she said.  “Unfortunately, climate change gives us a lot to worry about in California water, but fortunately we do know enough to start planning for those impacts and planning for those impacts and planning for what’s to come.


Dr. Persad then summed up the impacts of climate change.  “We know that as the air warms, more of that precipitation comes as rain, rather than snow, and coupled with warming in the mountains, that will mean less snowpack.  As late winter and early spring temperatures get warmer and warmer, we know that that snowmelt will run off earlier, and that will change the timing and amount of the streamflow and the river flows that people, plants, and animals have depended on.  As the planet warms, sea level rise will encroach on our freshwater, both above ground and below ground.  All of these changes mean that we’re going to have to fundamentally rethink the way that we manage water here in California.

The San Francisco Estuary is at the very heart of the state’s natural water infrastructure and as a result of that, it’s going to see the pressures from climate change from all sides, she said.  “Rising sea levels will make it harder to maintain the needed balance between salty and fresh water in the estuary and will push the estuary’s vibrant wetlands farther inland into areas where they actually may not be able to move.  As California’s snowpack declines and changes, freshwater flows into the estuary are going to become less reliable, and climate change is going to transform how and when water can come into the estuary from upstream, and it will increase demands for water coming out of the estuary from users downstream.  The estuary itself will also see dramatic changes in the local climate that will change the way the estuary and the ecosystem operates.”

Dr. Persad then went through each of these impacts in turn, discussing what we know about them and what they mean for the San Francisco estuary.


As ice sheets and glaciers melt, that water has ended up in the oceans and as the ocean has taken up more heat, it’s also expanded; the combination of those two things have increased sea levels and pushed oceans farther on to the coasts.  Sea level rise along the central and southern California coast has already risen by almost half of a foot over the last hundred years.  In the Bay Area, it’s increased by about 8 inches already which is already having impacts on infrastructure, and it’s only the beginning of those impacts, she said.

The graph shows projections of future sea level rise at the mouth of the Golden Gate from the California Ocean Protection Council report under scenarios of future climate change.  The blue line shows the sea level rise that we are committed to even if we do make drastic cuts in our heat trapping gases due to the inertia of the ocean and the built-up carbon dioxide and heat that is already present in the earth system.  The red line is where we’re headed based on our current global emissions trajectory under a best-case scenario of how the Antarctic ice sheet that holds a huge amount of our freshwater will react to warming that’s created by our emissions.  The purple line, which is this H++ line, is how high sea level rise could go if some of these recently identified feedbacks in the Antarctic ice sheet that can cause it to destabilize much more quickly than we first thought do actually manifest.

To illustrate what this means, Dr. Persad presented three maps.  The map on the upper left is a map of downtown Oakland with a red dot showing the location of the Scottish Rite Theater (the location of the conference).  The map in center shows what Oakland would look like with two and a half feet of sea level rise which is roughly where we’re headed in the next 80 years if those ice sheet feedbacks don’t manifest.  The light blue areas are showing the areas that would be underwater, including parts of the airport; the green areas which include Lake Merritt would be subject to chronic flooding at high time or during storms.

The map on the upper right shows what it would look like if the feedbacks do materialize.  “You can see that we would be underwater where we sit here today, so a very different location for the 100th biennial of the State of the Estuary conference,” she said.

For the San Francisco Estuary, it means that both salinity and the coastline itself will move farther inland.  As the oceans rise, salty water will move further upstream into the estuary and maintaining the water quality necessary for a healthy ecosystem will become more difficult.  Tidal wetlands will also need to move farther inland with the coastline in order to sustain themselves, however in many cases, those places are already developed, she noted.

Dr. Persad then presented another series of slides showing the impact of sea level rise on the coastline, the Bay, and Suisun Marsh, the upper left showing current conditions, the middle showing the current trajectory in the next 80 years without ice sheet feedbacks, and the upper right showing what it would look like if those feedbacks do manifest.

This means really drastic changes for the oceans influence on the estuary, and this is not even considering things like ocean acidification and the rising temperatures of the ocean which themselves have huge impacts on these ecosystems,” she said.


The San Francisco Estuary, much like the state’s engineered water system, is adapted to California’s winter snowpack and the natural reservoir that it provides for the state’s water.  The spring pulse of snowmelt and the source of freshwater throughout the late spring and early summer has historically been a huge part of maintaining healthy flows and water quality conditions in the estuary.

California’s snowpack is an aspect of our water system that is going to be most strongly transformed by climate change,” said Dr. Persad.

She presented a slide from a recent study out of UCLA that looked at how California’s snowpack will react to climate change.  The graph is showing how the springtime temperatures are projected to change on our current emissions trajectory in the next 80 years.  The projections show that temperatures could increases up to 10 degrees Fahrenheit in the areas shown in the darkest red across much of the middle elevations of the Sierra Nevada.

The warming in those areas means that by the end of the century on our current trajectory, we would have almost complete loss of spring snowpack across most of the lower and middle elevations,” she said.

She next presented a slide (lower, left) showing the projected change in the April 1st Snow Water Equivalent, showing a total loss of snowpack in the foothills, shown in darker brown colors.

The loss of spring snowpack means earlier runoff of that snowmelt.  The graphic (upper, right) shows how much earlier the center of mass of snowmelt runoff will occur on our current trajectory of emissions at the end of the century.

For much of the regions responsible for supplying snowmelt flows into the estuary, the peak of those flows may come up to 3 months earlier,” she said.  “That’s up to 3 months earlier than these wetlands that our water management systems are adapted to.”

California has always gone through dry and wet years, through high snowpack and low snowpack years, and that will continue, but for our snowpack, climate change will mean that our dry years will be more taxing and our wet years will be less beneficial.

The graph shows the evolution of our snowpack during the 2012-2015 drought.  The gray line shows what the observed snowpack was.  The purple line is showing what that snowpack would have looked like without the climate change that’s already been created in the system.  The orange line shows what the 2012-2015 drought would look like by the end of the century on the current emissions trajectory.  The blue line shows what it would look like with some climate action but not aggressive climate action.

The warming that we have already generated made those snowpack droughts much worse than they would have been otherwise,” she said.  “And with that additional warming, the droughts of the future become much worse.”

Dr. Persad then presented a graph showing similar information for snowpack during the wet winter of 2016-2017.  The difference between the purple and the gray lines shows how much more snowpack there would have been without the climate warming that we’ve already put into the system.  The blue and orange lines show how much less water those years will give us with future warming, particularly if global emissions don’t decline substantially from their current trajectories.

All of these changes in addition to directly affecting the snowmelt supplies to the estuary are going to fundamentally transform the state’s water management system and that will have impacts on the estuary as well,” she said.  “The Delta and the estuary is one of the key transit points for the state’s water system, and it’s going to feel the pressure from those changes and how the state’s engineered water system will need to be operated in order to get much more unreliable water supply to evermore thirsty water demands.”


In addition to the warming, the snowpack changes, and sea level rise, the current generation of climate models are projecting that the fundamentals of how, when, and where California gets its water will change dramatically.  Those changes include a 75% increase in precipitation coming as rain, which will need to be stored versus being naturally stored in the snowpack; a 15% increase in the concentration of rainfall into the winter months meaning longer and harsher dry summers; and an increase by more than 50% of extreme flood risk events meaning more of our water will come in intense events that will stress reservoirs and be harder to manage and store.

To look at what this means for the state’s water management system, earlier this year, the Department of Water Resources conducted a climate vulnerability assessment that looked at a range of metrics of how the water system would operate under future climate conditions.  The graph shows the assessment of future system shortages for the State Water Project under a range of changes in future annual temperature and future annual precipitation.  The orange colors show shortages that are more than historic conditions, and the bubble shows the range of average temperature and precipitation changes that the current generation of models are projecting.

For the vast majority of potential future climate conditions, the State Water Project will have substantially more system shortages than what we’ve seen historically,” said Dr. Persad.  “This is just looking at the impact of average shifts.  It’s not including shifts such as the compression of precipitation into the winter months or the extreme events that will make water even harder to manage and store.  So the challenges are likely to be even larger than what is reflected here.”


As the state comes to terms with how it’s going to continue to supply water to meet all of California’s needs, as our demands increase, and as our water comes in these harder to manage forms, the challenges that we’ve already seen with ensuring that the needs of ecosystems like the estuary are met will only intensify,” said Dr. Persad.

The estuary’s local climate is changing and these changes will mean new conditions for the estuary’s ecosystem.  The two figures on the slide show temperature and precipitation changes projected for the San Francisco Estuary.

On our current trajectory, the hottest day of the year in the estuary could be up to ten degrees hotter and even with some climate action, it will still be four to five degrees hotter,” she said.  “And depending on the trajectory of future emissions, the wettest day of the year in the estuary could be around 30% wetter than it is now.  There remains a lot of research to be done on how these changes will affect the pulse of the estuary – how changes in temperature will affect the alignment of bird migrations and the availability of food, what increased precipitation extremes like this mean for decreased water quality in the estuary, or for improved flood flow conditions for the estuary, and how the estuary can continue to survive and thrive.”


Despite all the impacts she just described, Dr. Persad said there is a lot of reason for hope.

The hopeful thing that I see about having these numbers and data to show you about the things that are coming down the pike for the estuary and the state is that it means that we know enough to act,” she said. Imagine how much worse this would be if all of these changes were coming and we had no idea.  So we can continue and accelerate the actions that we’re taking to reduce our heat trapping emissions so that the worst of these projected impacts never manifest, and we can take action to adapt and prepare for the impacts that we know are coming before they get here.”

I encourage all of you to think at every step about how these climate change impacts are going to affect the issues and your decisions,” she continued.  “Think about how you can bring the data and information that is available to us now into the decisions that you’re making to enhance the resilience of the system.  Think about the partnerships you can build with other scientists, with stakeholders, with practitioners, to center these impacts in how you’re making decisions going forward and create more resilience for all of us.”

Explore climate change at the California Water Library

Click here to view these documents at the California Water Library website.


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