Climate change in California – why is this region especially vulnerable?
It is very likely that global warming has already been affecting the California region., and global model projections indicate that much larger changes will unfold over the coming decades. In this talk we review results from two recent State-sponsored assessments of prospective climate change scenarios for California, which indicate that impacts in this region may be particularly challenging. Among the rest of the United States, the annual delivery of precipitation in this region is remarkably volatile, being prone to multi- year droughts and occasional wet spells and large storms—climate change may exacerbate this. An important part of the water supply that historically has come in the form of snow in mountain watersheds will probably shift to rain, which is harder to manage and save for dry summer irrigation and other forms of consumption. Furthermore, much of the water supply is conveyed through the San Franciso Bay/Delta, a complex estuary that will be impacted by bigger floods and rising sea levels.
California agriculture in a changing climate
How will California crops respond to a changing climate, and what adaptations would be most effective in reducing negative or enhancing positive impacts? The answers to these questions hinge on a series of climatic, biological, and human management processes which we only partially understand. This work aims to project impacts on some of the key crops in California with a thorough assessment of uncertainties. We develop models of crop response to changes in average monthly climatic conditions using historical weather and crop data, and integrate these models with projections of future temperature and precipitation from multiple climate models. The results are probabilistic estimates of yield impacts out to 2050 in the absence of adaptation. Some crops, such as cherries, appear particularly vulnerable to warming while many others will likely see small effects. We also consider the impact of extreme events that are too rare to be modeled reliably based on historical data but may nonetheless be important in the future. Implications for adaptation and future research will be discussed.
The Impacts of Sea Level Rise on California's Coast
Climate change will have a wide range of impacts on California, but among the most severe will be the implications of sea level rise for coastal ecosystems, developments, and human populations. As part of a comprehensive set of studies done for the State of California, we present here the results of a detailed analysis of the risks of climate-induced sea-level rise for economic structures, populations, and natural ecosystems. Using a combination of high-resolution digital mapping and data sets of the value and type of property and land uses, we develop quantitative estimates of the value of property at risk of inundation over the next century under different sea-level rise scenarios. We also evaluate the number of people at risk of flooding, among other indices of vulnerability. The final phase of the project also looks at the costs of building different levels of coastal protection. The approaches developed here can be applied in any coastal region, but our assessment also points out the need for improvements in data collection and mapping.
Application of the Water Evaluation and Planning (WEAP) System for Integrated Hydrologic and Scenario-based Water Resources Systems Modeling in the Western Sierra Nevada
Rivers draining western slopes of the Sierra Nevada provide critical water supply, hydropower, fisheries and recreation services to California. Coordinated efforts are under way to better characterize and model the possible impacts of climate change on Sierra Nevada hydrology. Research suggests substantial end-of- century reductions in Sierra Nevada snowpack and a shift in the center of mass of the snowmelt hydrograph. Management decisions, land use change and population growth add further complexity, necessitating the use of scenario-based modeling tools. The Water Evaluation and Planning (WEAP) system is one of the suite of tools being employed in this effort. Unlike several models that rely on perturbation of historical runoff data to simulate future climate conditions, WEAP includes a dynamically integrated watershed hydrology module that is forced by input climate time series. This allows direct simulation of water management response to climate and land use change. This paper presents ABY2008, a WEAP application for the Yuba, Bear and American River (ABY) watersheds of the Sierra Nevada. These rivers are managed by water agencies and hydropower utilities through a complex network of reservoirs, dams, hydropower plants and water conveyances. Historical watershed hydrology in ABY2008 is driven by a 10 year weekly climate time series from 1991-2000. Land use and soils data were combined into 12 landclasses representing each of 324 hydrological response units. Hydrologic parameters were incorporated from a calibration against observed streamflow developed for the entire western Sierra. Physical reservoir data, operating rules, and water deliveries to water agencies were obtained from public documents of water agencies and power utilities that manage facilities in the watersheds. ABY2008 includes 25 major reservoirs, 39 conveyances, 33 hydropower plants and 14 transmission links to 13 major water demand points. In WEAP, decisions for transferring water at diversion points from rivers to facilities are based on assigned priorities. Priorities in ABY2008 follow Federal Energy Regulatory Commission license requirements and power purchase agreements between licensees and water/power contractors. These generally allocate water according to the following priorities - (i) maintaining minimum instream flows below diversions;(ii) irrigation and domestic consumptive water demands; and (iii) power generation. ABY2008 simulations compared well with historical annual and monthly hydropower generation. Annual hydropower for 31 hydropower plants was simulated with r2=0.85 and ste=58 GWh. Monthly hydropower for 21 power plants owned by three water agencies were simulated with r2= 0.74 and ste= 7.4 GWh. We also present early results on how climate change, manifest by increasing weekly average temperatures, translates into changes in the projected timing of runoff and patterns of snow accumulation. Consequent changes in met water supply demands and hydropower generated are discussed. Further, stakeholders in the northern Sierra seek to use ABY2008 to investigate management scenarios geared towards increased conservation flows for fish populations, and the possible tradeoffs thereof with hydropower and water supply. These applications with ABY2008 illustrate the substantial utility of scenario-based modeling with the WEAP system.
Anthropogenic reduction of Santa Ana winds
The frequency of Santa Ana wind events is investigated within a high-resolution downscaling of the ERA40 Reanalysis to 6-km resolution over Southern California. In this climate reconstruction, the number of Santa Ana days per winter season declines significantly over the 44-year reanalysis period, resulting in nearly 20% fewer events per year between 1992 and 2002 than between 1959 and 1969. We further investigate this observed signal in late-20th and mid-21st century realizations of the NCAR CCSM3 global climate change scenario run downscaled to 12-km resolution over California. The reduction in events per year in the mid- 21st century compared with the late-20th century is similar to that seen in the ERA40 downscaling, suggesting the cause is a change in the climate due to anthropogenic forcing. A regression model is used to reproduce the Santa Ana time series based on two forcing mechanisms: synoptically-forced strong offshore winds at the mountain tops which transport offshore momentum to the surface, and a local desert-ocean temperature gradient causing katabatic-like winds as the cold desert air pours down the coastal topography. Both datasets have a large reduction in the contribution of the local temperature gradient to the Santa Ana time series. This reduction is due to the differential warming that occurs during transient climate change conditions, with more warming in the desert interior than over the ocean.
Climate Change Effects on Annual Average Concentrations of Fine Particulate Matter (PM2.5) in California
California has one of the worst particulate air pollution problems in the nation with some estimates predicting more than 5000 premature deaths each year attributed to air pollution. Climate change will modify weather patterns in California with unknown consequences for PM2.5. Previous down-scaling exercises carried out for the entire United States have typically not resolved the details associated with California's mountain-valley topography and mixture of urban-rural emissions characteristics. Detailed studies carried out for California have identified strong effects acting in opposite directions on PM2.5 concentrations making the net prediction for climate effects on PM2.5 somewhat uncertain. More research is needed to reduce this uncertainty so that we can truly understand climate impacts on PM2.5 and public health. The objective of this research is to predict climate change effects on annual average concentrations of particulate matter (PM2.5) in California with sufficient resolution to capture the details of California's air basins. Business-as-usual scenarios generated by the Parallel Climate Model (PCM) will be down-scaled to 4km meteorology using the Weather Research Forecast (WRF) model. The CIT/UCD source-oriented photochemical air quality model will be employed to predict PM2.5 concentrations throughout the entire state of California. The modeled annual average total and speciated PM2.5 concentrations for the future (2047-2049) and the present-day (2004-2006) periods will be compared to determine climate change effects. The results from this study will improve our understanding of global climate change effects on PM2.5 concentrations in California.
Projections of Climate Extremes in California
In the next few decades, it is likely that California must face the challenge of coping with increased impacts from extreme events such as heatwaves, wildfires, droughts, and floods. Such events can cause significant damages, and are responsible for a large fraction of near-term climate-related impacts every year. Some extreme events have already very likely changed in frequency and intensity over the past several decades, and these changes are expected to continue with relatively small changes in average conditions. We synthesize existing research to characterize current understanding of the direct impacts of extreme events across sectors, as well as the interactions between sectors as they are affected by extreme events. We also produce new projections of changes in the frequency and intensity of extreme events in the future across climate models, emissions scenarios, and downscaling methods for producing regional climate information, for each county in California. We evaluate historical and projected changes for a suite of temperature and precipitation-based climate indicators, and we conduct a return level analysis to investigate projected changes in extreme temperatures. Finally, we include an analysis of the future likelihood of events similar in magnitude to specific historical events, such as the July 2006 heat wave. Consistent with other studies, we find significant increases in the frequency and magnitude of both maximum and minimum temperature extremes in many areas, with the magnitude of change dependent on the magnitude of projected emissions and overall temperature increase. For example, in many regions of California, at least a ten-fold increase in frequency is projected for extreme temperatures currently estimated to occur once every 100 years, even under a moderate emissions scenario (SRES B1). Under a higher emissions scenario (SRES A2), these temperatures are projected to occur close to annually in most regions. Also consistent with other studies, we find that projections of precipitation extremes are less spatially coherent and statistically significant compared to temperature extremes at a county-by-county scale, and are more sensitive to the choice of climate model and downscaling methodology.
Adaptation Challenges and Emerging Efforts in Adaptation Planning in California
Following Governor Schwarzenegger's Executive Order (S-03-05) of 2005, numerous researchers have been engaged in an ongoing assessment effort to support the state's mitigation and adaptation efforts. Under the sponsorship and coordination of the California Energy Commission's Public Interest Energy Research (PIER) Program, a wide range of climate change impacts and adaptation studies are being conducted and summarized on a biannual basis to assess the latest climate change science, potential impacts on critical sectors, and the state's efforts to manage its climate change risks. In the past, adaptation needs assessments in the state have primarily used a hazards-based (i.e., climate scenario-driven, top-down) approach, while vulnerability-based, bottom-up studies are only now emerging. They are increasingly viewed as complementary and necessary to adequately inform adaptation strategies. This paper briefly highlights this assessment history and then focuses on the planning efforts currently underway to prepare California's first state-wide adaptation plan. As the science and policy/management evolve in tandem, this paper will suggest future policy- or use-inspired research areas, and offer recommendations on how to improve interaction between researchers and practitioners at the science-policy interface, in order to build the state's decision support capacity in the face of a rapidly changing climate.