2. C LIMATE C HANGE IN O RINDA

2.1 Climate Change and Climate Stressors

Climate change is a long-term change in the average meteorological conditions in an area. The global climate is changing due to an increase in greenhouse gas (GHG) emissions that trap heat near the Earth’s surface. While some levels of these gases are necessary to maintain a comfortable temperature on Earth, an increased concentration of these gases due to human activity traps additional heat, changing Earth’s climate system in several ways. These effects of climate change are experienced in the form of climate stressors. Climate stressors are a condition or trend related to climate variability and change such as decreased precipitation or warmer temperatures that can exacerbate natural hazards. Climate stressors fall into two categories: primary climate stressors and secondary climate stressors.

As GHG emissions build in the atmosphere and global temperatures continue to rise, primary climate stressors at the local level are likely to become more severe, which in Orinda are changes in air temperature or precipitation levels. These primary climate stressors can lead to secondary climate stressors or hazards, which are events or physical conditions that have the potential to cause fatalities, injuries, property damage, infrastructure damage, agricultural losses, damage to the environment, interruption of business, and other types of harm or loss. Such secondary stressors may include wildfires, landslides, and flooding and intense precipitation. These hazards can harm populations and assets in Orinda, and in many cases, already cause harm. Populations and assets may already be subject to non-climate stressors, or trends unrelated to climate that can exacerbate climate change hazards. These non-climate stressors may include lack of funding for infrastructure maintenance, chronic health conditions or disabilities, or lack of access to lifelines.

2.2 Climate Change Modeling Considerations

The Intergovernmental Panel on Climate Change (IPCC), an organization that represents the global scientific consensus about climate change, identified four climate scenarios in the Fifth Assessment Report, also called Representative Concentration Pathways (RCPs), that can be used to project future conditions. i RCPs are labeled with different numbers (e.g., RCP 2.6, RCP 6) that refer to the increase in the amount of energy that reaches each square meter of Earth’s surface under that scenario. The four RCPs are: i The IPCC recently released “The Physical Science Basis” of the Sixth Assessment Report that updates global climate change projections for the near-term, mid-term, and long-term based on GHG emission trends from the past decade. It moves away from using RCPs, instead using five different scenarios called “shared socioeconomic pathways,” which consider socioeconomic trends underlying each scenario. This Vulnerability Assessment does not use these updated projections because at time of writing, they are not available at a local scale. However, the IPCC report does reaffirm the use of projections comparable to RCP 8.5 as the suggested emission scenario to use for Cal-Adapt data.

• RCP 2.6: Global GHG emissions peak around 2020 and then decline quickly.

• RCP 4.5: Global GHG emissions peak around 2040 and then decline.

• RCP 6: Global emissions continue to rise until the middle of the century.

• RCP 8.5: Global emissions continue to increase at least until the end of the century.

Projections of climate hazards from Cal-Adapt and other sources rely on climate models, which are computer simulations that forecast future climate conditions under these different RCP scenarios. It is critical for the City to account for all reasonably plausible future conditions, including the most severe of plausible conditions, which will help ensure greater resiliency from climate change. Therefore, the projections in the Vulnerability Assessment use the RCP 8.5 scenario, following guidance in the APG and to be consistent with the IPCC Sixth Assessment Report. No model can project future conditions perfectly, but current models are heavily reviewed by climate scientists and can accurately reproduce observed climate conditions.

The Vulnerability Assessment also relies on the understanding that “weather” and “climate” are two different things. “Weather” describes the conditions at a particular time and place, and “climate” describes the long-term average of conditions. Because there are large variations in the weather, it is difficult to accurately project weather conditions more than a few days in advance. However, because climate is a long-term average, it can be projected out for years or decades with a high degree of accuracy. It is important to remember that, because climate is an average, it does not say whether an event will or will not occur, only how likely it is. For example, extreme heat is likely to become more frequent in Orinda, but a year with few heat waves does not mean that this projection is wrong, because the projection only says that extreme heat days are expected to occur, on average, more often than in the past.

2.3 Climate Change in California

The most accurate data for California-specific projections is available for the RCP 4.5 and RCP 8.5 scenarios. These scenarios help generate climate models, which are meant to simulate conditions across the globe. The model divides Earth’s surface into cells using a grid, and then forecasts the conditions in each square of the grid. The size of these squares makes them suitable for projecting global conditions, but they are too big to accurately model the difference in climate across smaller areas. 7 Per State guidance, these models have been “downscaled” to much finer grids, which means that they have grids that are less than four miles on each side to show projections on a county or city level. The California Fourth Climate Change Assessment and Cal-Adapt provide a foundation of climate change science and downscaling for the state. The State of California has also developed a comprehensive set of reports and tools that local jurisdictions can use to assess climate change hazards and prepare for these hazards. The State-provided reports and tools used in the Vulnerability Assessment include:

• Cal-Adapt.org

• California 4th Climate Change Assessment (statewide and regional reports) (2018)

• California Adaptation Clearinghouse

• California Adaptation Planning Guide (2020)

• California Climate Adaptation Strategy (2021)

• California Building Resilience Against Climate Effects (2018)

• California’s Wildfire and Forest Resilience Action Plan (2021)

• Defining Vulnerable Community in the Context of Climate Adaptation (2018)

• Department of Transportation Climate Change Vulnerability Assessments, District 4 (2018)

• Planning and Investing for a Resilient California (2018)

• Draft California Extreme Heat Action Plan (2022)