Theme for 2019:
The Impacts of Global Warming on California — A 30-Year Retrospective and Future Projections
University of California, Davis
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About the Theme
The theme is based on the June 1989 report prepared by the California Energy Commission titled: The Impacts of Global Warming on California [PDF* 2.9 MB].
Using the report to look back at what was known in 1989 and the projections made then, topics included:
- What were the projections 30 years ago regarding global warming impacts on California?
- How good were the projections compared to what has happened?
- Compare the climate modeling science in 1989 with today's modeling.
- What are today's future projections for global warming impacts on California?
The 1989 report is significant for many reasons. Consider the Introduction:
In mid-1988, the "greenhouse effect" burst onto the national scene with a flurry of articles in magazines and newspapers and a series of hearings in the U.S. Congress. Yet for decades scientists and policy makers have been researching and discussing what might happen if increased greenhouse gases in the atmosphere caused substantial warming of the world's climate.
Early in 1988, before the issue capitivated [sic] the national media, California's Legislature introduced AB 4420 (Sher) calling for the California Energy Commission to study how global warming trends may affect the state's energy supply and demand, economy, environment, agriculture, and water supplies. The bill, which was signed by the Governor in September 1988, specifies that the study include recommendations for avoiding, reducing, and addressing the impacts which are identified.
Special Recognition Award
The 2019 Special Recognition Award was presented to Timothy N. Washburn. See the award language, biographical information, and hear the presentation on the Symposium's Tim Washburn award page.
Climate Projections for California: What We Knew Then (1989), What We Know Now (2019)
Thirty years ago, the California Energy Commission published a report on The Impacts of Global Warming on California in consultation with other state agencies representing climate-sensitive sectors. This report was the first to consider whether the state would face significant risks from anthropogenic climate change and what types of impacts might occur if warming did in fact occur. The report named "Water Resources" as being at significant risk due to warming-induced reductions in snowpack storage, increased wintertime stream flows, and (coupled with increased evapotranspiration) decreased water deliveries. "Electrical Energy" was deemed to be at moderate risk due to increased demand and reduced hydroelectric resources.
At the time of the 1989 report, projections were limited by coarse resolution and other limitations of those early global climate models. In 2018, California's Fourth Climate Change Assessment was released, including climate and hydrological scenarios at 1/16th degree (ca. 6km x 6km), daily resolution as well as probabilistic sea level rise projections and wildfire scenarios. While some aspects of projected precipitation have remained uncertain, a number of key elements related to timing, distribution, variability, extremes, and hydrology have been clarified. Our understanding of projected temperature-both mean and extremes-has also evolved.
This talk will explore what we knew then (1989), what we know now (2019), and close with a few observations regarding outstanding questions related to California's climate (e.g., coastal low cloudiness, wind fields, fog, ARs) and consideration of what we might soon know (next 5-6 years) when the next generation (CMIP6) of global climate models is downscaled for California.
Past and Current State of Modeling and Downscaling California's Climate
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Over the past thirty years, advances in computational performance and model design have drastically improved our forecast accuracy on the short time scales characteristic of numerical weather prediction. At time scales more characteristic of climate, similar advances in model performance have been observed as models are pushed to higher and higher spatial resolutions and physical processes captured with greater fidelity. This talk examines past performance of global models, with a focus on what has improved, what has not, and the reasons for some of these persistent uncertainties. We conclude by looking forward to the future of global modeling, particularly with a growing focus on subseasonal-to-seasonal prediction.
Drivers of Large Wildfire Occurrence in Northern California
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Large wildfire occurrence appears to be increasing in frequency throughout northern California, particularly in the last several years. Climate more conducive to fire spread and changing vegetation (fuel) complexes are readily cited causes of this. This presentation describes the historical context for large wildfire occurrence in northern California using a newly discovered dataset, and compares that to contemporary occurrence. Additionally, it explores the contribution of climate, fuels, and ignitions on observed patterns of large wildfire occurrence.
Drought, Warming, and Environmental Flows: Implications for Native Aquatic Species
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Climate change is expected to result in warming temperatures and fewer, more intense storms during California's winter months, leading to earlier snowmelt and lower summer stream flows. However, 95% of rivers in California show evidence of human alteration to stream flow, in most cases dampening natural patterns of variability by decreasing high flow events and increasing low summer flows. An understanding of natural flow patterns over time, current flow regimes, and flows needed to achieve ecological objectives are needed to manage stream flows for ecological outcomes under current and future climate conditions.
We predicted natural monthly flows for all stream reaches in California from 1950 to 2015 and compared observed flows measured at 540 stream gages with modeled natural flows at the same locations to quantify flow alteration. Patterns of flow alteration during California's most recent drought (2011-2015) illustrate that hydrology and water management interact to create streamflow conditions that vary across rivers and do not always reflect the assumption of low summer flows under drought conditions.
The Nature Conservancy is working with partners to develop statewide tools to assess natural streamflow, flow alteration, priority areas for conservation of freshwater biodiversity and groundwater dependent ecosystems, and ecological flow recommendations. These tools can be integrated to provide a holistic framework for managing surface flow and groundwater for ecological outcomes under current conditions and into the future.
Climate Change Trends and Impacts on California's Agricultural Sector
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California is a global leader in agricultural production that produces more than 400 types of commodities. With 77,500 farms and 3.8 million hectares of irrigated cropland, California generates an overall agricultural production value of $50.5 billion. Current and future changes in climate change pose a significant threat to the state\'s highly productive agricultural industry.
In this presentation, we synthesized the current understanding of climate change trends in temperature, precipitations, snowpack, and extreme events including drought and heat waves and consequent impacts on California\'s agriculture. The range of studies on trends and impacts of climate change on California agriculture justifies the importance of enhancing the adaptive capacity of agriculture to climate change impacts, increased research effort to agricultural adaptation to water shortages, and effective stakeholder engagements.
Panel Discussion: Climate Modeling Advances and Global Warming Impacts on Droughts
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Projections of Climate Change Effects on Atmospheric Rivers
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Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. These "rivers in the sky" have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds.
This study takes a uniform, global approach that is used to quantify how ARs change between climate simulations of the present versus the future. The projected changes indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports. These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes. The above study and findings will be presented in the context of California winter water and weather extremes.
Using Decision-Scaling and Stochastic Weather Generation in Tuolumne River Basin Demonstration Study of Climate Change Vulnerabilities to Water Systems
Over the past couple of decades, acknowledgment of climate change based risks to water resource systems has led to a large number of impact studies. These studies have traditionally been limited to the evaluation of a relatively narrow set of possible future climate states. As a result, a trend towards vulnerability-based, bottom-up study frameworks has emerged. These frameworks emphasize a broader exploration of possible future climate scenarios. However, these bottom-up evaluations are often limited by their inability to provide plausible future flood and drought scenarios. Thus, both traditional and bottom-up analytical frameworks often leave water resource managers with incomplete information about either the range of possible future climate states or how future climate states will impact the extreme flood and drought events that their systems are designed to mitigate.
In this presentation, we demonstrate how stochastic weather generation, in conjunction with a structured set of flood and drought relevant climate change pathways, are being used to evaluate the climate based vulnerabilities of the Tuolumne River Watershed water resources system. The application of this approach to other water resource systems is also discussed.
Using Forecasts in Reservoir Operations
The U.S. Army Corps of Engineers prescribes rules for flood control operations and oversees those operations for reservoirs having a flood control purpose.
This presentation will discuss how forecasts have been used historically, recent updates to how forecasts are being used, and how they may be used in the future in reservoir operations.
New operating rules for Folsom Dam-Lake and potential operating rules for Coyote Dam-Lake Mendocino will be discussed.
Exploring Flooding and Water Supply Risks from Sea Level Rise in the Delta
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The Sacramento-San Joaquin Delta is considered the hub of California's water supply system that transfers water from the wetter northern part of the state to agriculturally productive and more populous central and southern parts of the state.
This talk explores climate change risks and impacts to the Delta from increasing temperatures, shifting precipitation patterns and rising sea levels. In the Delta, saline tidal flows from San Francisco Bay in the west meet managed freshwater flows from the Sacramento, San Joaquin and other tributary rivers. Water management practices, such as upstream reservoir releases and Delta exports, will be adjusted for changes in precipitation, runoff, and salinity intrusion from sea level rise. Pressure on Delta levees will increase due to the combined impacts of subsidence and sea level rise. Sea level rise combined with flood events will raise Delta water levels even higher at times. Recent research improves our understanding of these impacts and can be used to inform decision making.
Attribution of Extreme Weather Events to Climate Variability and Change
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The field of attribution science aims to quantify the contributions of natural climate variability and anthropogenic climate change to the likelihood and magnitude of extreme weather events. I will review three attribution methodologies, including their strengths and assumptions, and present research applications of each.
The first uses observationally-driven statistical models to estimate observed changes in the return periods and return values of extreme events. This method can be applied to various types of observable extreme events and assumes that substantial physical influences on extremes are accounted for as covariates in the statistical model.
The second uses global Earth system models to estimate changes in the characteristics and statistics of extreme events. This method produces century-long simulations and therefore can quantify changes in the probability of occurrence of rare events. However, its suitability for intense storms depends on model resolution, which is limited by supercomputing power.
Finally, we developed a novel attribution method that uses regional climate models to evaluate changes in the intensity of specific observed extreme events. This "story-line approach" uses a hindcast-based framework to assess how a specific event could change if a similar event occurred in a different climate scenario, and is therefore conditional on the occurrence of the event. We implemented this approach using high-resolution (3 km) simulations suitable for investigating the most intense weather events, such as extreme precipitation and major hurricanes.
Altogether, research using these different approaches indicates confidence in an attributable human influence on precipitation.
Panel Discussion: Managing Flood Risk and Water Resources Impacted by Global Warming
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Leadership: A Look at DWR's Climate Change Program
Since the 1987 report, "Possible Changes in California Snowpack Runoff Patterns," by Maury Roos, the CA-DWR has been leading adaptation and mitigation strategies for the water sector. The Climate Change Program has full-time engineering and scientific staff dedicated to planning and research. The Program has won several national and local awards for its Climate Action Plan, and conducts engaging public outreach. The poster reviews not only Program accomplishments, but several milestones and key legislation the State has achieved over the years. In addition, there is a look ahead toward the future of climate adaptation. Our colorful poster is available for review, if you'd like it emailed.
Want to be a Citizen Scientist?
Promote the Community Collaborative Rain Hail Snow Network (CoCoRaHS) and its usefulness in capturing extreme precipitation events in California. Since its start in 2008 network has collected over 1,500,000 daily precipitation records in California. Rain gauges will be on display and staff will be available for questions and demonstration.
Changes in Extreme IVT on the U.S. West Coast in NA-CORDEX, and Relationship to Mountain and Inland Extreme Precipitation
Western U.S. rainfall and snowpack has large interannual and decadal variability, and this, paired with its importance to water resources, makes future projections of these variables extremely societally relevant. Previous studies have shown that precipitation events in the western U.S. are influenced by the timing, positioning, and duration of extreme integrated water vapor transport events (IVT, e.g., atmospheric rivers) at the coast, and also by the pathways which this moisture-rich air takes through the complex terrain of the western U.S.
We investigate projections of western U.S. precipitation and IVT in a collection of regional climate models (RCMs) forced by several global climate models (GCMs) from the North American Coordinated Regional Downscaling Experiment (NA-CORDEX). We briefly explore how well present-day precipitation and IVT are represented by NA-CORDEX. We then document projected changes in precipitation and extreme IVT statistics at the end of the 21st century in the RCP8.5 CMIP5 emissions pathway. Several of the NA-CORDEX RCMs project a decrease in precipitation at high elevation (e.g., across the Sierra Nevada) with a corresponding increase in the Great Basin of the U.S. We explore the causes of this terrain-related precipitation change in a subset of the NA-CORDEX RCMs using diagnostics such as the lifting condensation level and drying ratio.
Projected Climate Change Impacts in the Tahoe Basin: Recent Findings from Global Climate Models
The output from four General Circulation Models (GCMs) downscaled to a 6 km grid scale was used to drive the Variable Infiltration Capacity (VIC) model and derive a suite of 24 hydrologic and climate variables covering the Tahoe Basin. Here we focus on trends in the return levels of maximum daily discharge of six basin streams, kinetic energy (KE) of raindrops falling on snow-free ground, basin-wide climatic water deficit, and wind speed.
To analyze time trends in historic and modeled future extreme values, we applied the program extremes, based on the Generalized Extreme Value (GEV) distribution. Values of KE on snow-free ground were derived by statistically disaggregating daily rainfall to hourly, and using literature values to convert rainfall intensity (mm/hr) for days without snowpack to KE (Joules/m2/hr).
We found strong upward trends in extreme values of annual maximum and total KE, with most of the effect due to loss of snowpack, but a significant effect of increasing rainfall for one model. The GEV results for the six streams, averaged across the four models, indicate an increase in the 20-year flood of 65-117 percent. Climatic water deficit showed strong upward trends for three of the four models, with a maximum at mid-century for one model. Averaged across the basin and across the four models, maximum seasonal winds are projected to decrease slightly in all seasons. These trends in averages and extreme values will have important effects on vegetation, wildfire severity, flood hazards and the clarity of Lake Tahoe.
Impact of Extreme Wet and Dry Years on the Persistence of Microcystis Harmful Algal Blooms in San Francisco Estuary
Cyanobacteria harmful algal blooms became a concern in the upper San Francisco Estuary, California beginning in 1999, when yearly blooms of Microcystis began in the Delta region. Subsequent research identified the increase in the magnitude, duration, and toxicity of Microcystis blooms was associated with drought related conditions of elevated water temperature and low streamflow.
The 2014 and 2017 water years provided a unique opportunity to determine the effect of climatic "whiplash" produced by the occurrence of extreme wet conditions following extreme dry conditions on the Microcystis bloom. We hypothesized that the period of record wet conditions in 2017 would revert the estuary phytoplankton community back to pre-bloom conditions following the peak Microcystis bloom during the 2014 drought.
Field sampling was conducted at 2-week or 4-week intervals between July and November at stations throughout the Delta for both years and included a suite of physical, chemical, and biological factors. Using PRIMER-e DISTLM we determined that residence time in the upper estuary and water temperature were key environmental drivers. The period of record high streamflow in 2017 was not sufficient to eliminate the Microcystis bloom, even though it had a low magnitude, late initiation, short duration, narrow distribution, and low toxin production. Optimum warm water temperature enabled the bloom to flower in late summer despite high streamflow.
We concluded that CHABs established during drought conditions are likely to be resistant to reversals from extreme wet years, as long as water temperature or other key variables reach optimum threshold levels.
Investigating Geometric Complexity of Precipitation in California via the Fractal-Multifractal Method
This work employs a deterministic geometric approach named the fractal-multifractal (FM) method to encode highly intermittent daily rainfall records, in order to investigate the intrinsic complexity of rainfall in various stations within the State of California. To this effect, 60+ years of daily precipitation records gathered, from South to North, at Cherry Valley, Merced, Sacramento and Shasta Dam are studied.The analysis reveals that:
- The FM approach results in several faithful equifinal encodings of all records, by years, with mean square errors in accumulated rain that do not exceed 3%,
- The evolution of the corresponding FM parameters while reflecting the implicit variability in the records do not exhibit discernible trends in time at every station that may be attributed to global change, and
- A comparison of FM parameters for the four sites confirms the expected notion that all stations are equally complex
New Perspectives on ENSO, the Winter of 2016/17, and California Hydroclimate Predictably and Variability
Abstract:ENSO is commonly regarded as the largest control of California hydroclimate. However, recent winters have challenged that view including the extremely dry winter of 2013/14, the failure of the 2015/16 El Niño to drive an extremely wet winter, and the extremely wet winter of ENSO-neutral 2016/17.
- First, we show that by describing ENSO from a location of tropical convection perspective rather than by a fixed location index (e.g., Niño 3.4), we can explain why California did not experience the expected hydrometeorological response to the 2015/16 El Niño and further, increase the value of ENSO as a predictor of western U.S. hydroclimate.
- Second, we show that the unusual wet winter of 2016/17, which was characterized by ENSO-neutral conditions, seems to have largely been the result of internal atmospheric variability. We demonstrate that the storms which contributed to the failure of the Oroville dam were neither remarkable individually or collectively; however, using a potential temperature metric as a rain-on-snow (ROS) indicator, we show that the unprecedented overtopping of the dam may have had a significant contribution from an ROS event.
- Third, the extreme drought year of 2013/14 was the result of the simultaneous combination of both low precipitation and high temperatures. To that end, we introduce a framework for understanding how large-scale modes of variability alter the probabilities for experiencing multivariate extremes.
Using this framework, we quantify ENSO-driven temperature and precipitation variability and show how a large-scale planetary wave pattern is able to force the co-occurrence of both low precipitation and high temperatures.
Recent Changes in Southern Sierra Snowpack
Earlier studies had shown contrasting trends in Sierra snowpack with decreases with time in the northern Sierra and increases in the southern Sierra. A recheck of these trends in 2017 after the 2012-16 drought showed that the southern Sierra snowpack also is now showing a decrease, although not as much as the north. A similar trend is noted in the decreasing portion of water year runoff during the April-July snowmelt season, again more noticeable in the lower elevation Sacramento River north than the San Joaquin River south.
Addressing Uncertainties in Climate Change Assessments
Warming temperatures, shifting hydrology and rising sea levels will challenge management of California's water resources. This study uses the California Department of Water Resources (DWR) newly developed water planning model, CalSim 3.0, as a risk assessment tool to quantify potential impacts of climate change on State Water Project. Impacts were assessed for 20 climate change scenarios (10 global climate models and two representative concentration pathways (RCP4.5 and RCP8.5)). Water supply and water quality metrics evaluated include Delta exports, North of Delta Carryover storage, reservoir dead storage, and Delta salinity.
Additionally, to address uncertainties with the impact assessment based on 20 climate change scenarios, a series of sensitivity tests were implemented to assess individual impacts of four climate change factors on the State Water Project and Central Valley Project operations:
- Flow seasonal pattern shift
- Sea level rise
- Annual flow volume change
- Water demand change
It was found that flow seasonal pattern shift is a major climate change factor in half million acre feet of Delta export reduction and a dominating factor in about 25% decrease of North-of-Delta carryover storage in the middle of this century around 2060. It also shows that extra runoff, from early snow melting and higher percentage of rain in the precipitation in the winter and early spring, is not able to be conserved in reservoirs to meet higher demand in the summer in the current SWP/CVP system. The extra water is released as flood water in the winter and early spring to become Delta outflow.
Quantifying the Impacts of Climate Extremes on Watershed Hydrology with Integrated Models and High Performance Computing
In the western U.S., as in other parts of the world, the effects of climate change have recently been observed as fluctuations between extended periods of droughts followed by extreme precipitation events such as atmospheric rivers. These oscillations are expected to intensify due to anthropogenic induced climate warming with unknown consequences on watershed hydrodynamics or water management decisions.
State-of-the-practice approaches utilize empirical models and rely on highly calibrated observations for hydrologic predictions, thus lacking the physics to understand fluctuations based on future climate scenarios and extremes for which there is no historical precedent. Because the mechanisms and feedbacks involved in the water-energy balance at and near the land surface are highly non-linear, understanding the partitioning of water across the earth's critical zone from a mechanistic standpoint is imperative for water resilience in the future.
In this work, watershed-scale simulations utilizing an integrated hydrologic model are used in conjunction with high performance computing to strengthen our scientific understanding of how watersheds respond to perturbations from climate change and its subsequent impacts (for example, an increased occurrence and scale of wildfires). Case studies include a rare un-dammed watershed straddling California's Sierra Nevada Mountain range and Central Valley interface, which allows for the impacts of climate change to be isolated from water management schemes. Simulation results highlight regions of the watershed most sensitive to climate extremes and anthropogenic activities, and under what conditions (e.g., meteorological forcing, water management practice, or antecedent moisture) the above and below-ground hydrodynamics might be most impacted.
The Chart that Defines our Warming World: Temperature Changes Around the World (1901-2018)
Is this the simplest way to show what is meant by global warming? The chart organises all the countries of the world by region, time and temperature. The trend is unmistakeable.
Each line of colored pixels is the temperature record of an individual nation within its region, stacked one atop the other. Blues are cooler years; the reds are warmer. The far left is 1900; the far right is 2018. The entire planet has got hotter, increasingly so in recent decades.
This global "Climate Stripes" graphic is the work of Prof. Ed Hawkins at Reading University who has sought clearer ways to communicate the issues around climate change.
[Article by Jonathan Amos, BBC Science Correspondent, as published at https://www.bbc.com/news/science-environment-48678196, retrieved on 21 June 2019]
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