The magnitude of future climate change depends significantly on the greenhouse gas emission pathways we choose. This study explores the implications of the highest and lowest Intergovernmental Panel on Climate Change (IPCC) emissions scenarios for climate change and its impacts in California. Using climate projections from two advanced climate models with low and medium sensitivity, it is found that annual temperature increases nearly double from the lower B1 to the higher A1fi scenario by 2100. Three of four simulations show greater increases in summer temperatures compared to winter. Extreme heat and its impacts on temperature-sensitive sectors are significantly greater under the higher emissions scenario, with some differences apparent before mid-century. By the end of the century under the B1 scenario, heatwaves in Los Angeles quadruple in frequency, and heat-related mortality increases two to three times. Alpine/subalpine forests are reduced by 50–75%, and Sierra snowpack is reduced by 30–70%. Under A1fi, heatwaves in Los Angeles are six to eight times more frequent, with heat-related mortality increasing five to seven times. Alpine/subalpine forests are reduced by 75–90%, and snowpack declines by 73–90%, with cascading impacts on runoff and streamflow that could disrupt California's water rights system. California, with its diverse climate zones, limited water supply, and economic dependence on climate-sensitive industries, is a challenging test case for evaluating regional-scale climate change under different emissions pathways. The study examines a range of potential climate futures that represent uncertainties in both the physical sensitivity of climate models and greenhouse gas emissions pathways. Two global climate models, the low-sensitivity National Center for Atmospheric Research/Department of Energy Parallel Climate Model (PCM) and the medium-sensitivity U.K. Met Office Hadley Centre Climate Model, version 3 (HadCM3), are used to calculate climate change resulting from the SRES B1 and A1fi scenarios. These scenarios bracket a large part of the IPCC non-intervention emissions futures with atmospheric CO2 concentrations reaching approximately 550 ppm (B1) and 970 ppm (A1fi) by 2100. The study finds that temperature increases are more pronounced under the A1fi scenario, with significant impacts on snowpack, runoff, and water supply. Snowpack in the Sierra Nevada Mountains is projected to decline substantially, with cascading effects on California's winter recreation, streamflow, and water storage. The distribution of vegetation types also changes significantly, with alpine/subalpine forests reduced and desert cover expanded under the A1fi scenario. Climate change is expected to impact California's agriculture, with higher temperatures affecting dairy production and wine grape quality. Adaptation options are limited for impacts not easily controlled by human intervention, such as the decline in snowpack and loss of alpine and subalpine forests. The study concludes that climate change and its impacts scale with the quantityThe magnitude of future climate change depends significantly on the greenhouse gas emission pathways we choose. This study explores the implications of the highest and lowest Intergovernmental Panel on Climate Change (IPCC) emissions scenarios for climate change and its impacts in California. Using climate projections from two advanced climate models with low and medium sensitivity, it is found that annual temperature increases nearly double from the lower B1 to the higher A1fi scenario by 2100. Three of four simulations show greater increases in summer temperatures compared to winter. Extreme heat and its impacts on temperature-sensitive sectors are significantly greater under the higher emissions scenario, with some differences apparent before mid-century. By the end of the century under the B1 scenario, heatwaves in Los Angeles quadruple in frequency, and heat-related mortality increases two to three times. Alpine/subalpine forests are reduced by 50–75%, and Sierra snowpack is reduced by 30–70%. Under A1fi, heatwaves in Los Angeles are six to eight times more frequent, with heat-related mortality increasing five to seven times. Alpine/subalpine forests are reduced by 75–90%, and snowpack declines by 73–90%, with cascading impacts on runoff and streamflow that could disrupt California's water rights system. California, with its diverse climate zones, limited water supply, and economic dependence on climate-sensitive industries, is a challenging test case for evaluating regional-scale climate change under different emissions pathways. The study examines a range of potential climate futures that represent uncertainties in both the physical sensitivity of climate models and greenhouse gas emissions pathways. Two global climate models, the low-sensitivity National Center for Atmospheric Research/Department of Energy Parallel Climate Model (PCM) and the medium-sensitivity U.K. Met Office Hadley Centre Climate Model, version 3 (HadCM3), are used to calculate climate change resulting from the SRES B1 and A1fi scenarios. These scenarios bracket a large part of the IPCC non-intervention emissions futures with atmospheric CO2 concentrations reaching approximately 550 ppm (B1) and 970 ppm (A1fi) by 2100. The study finds that temperature increases are more pronounced under the A1fi scenario, with significant impacts on snowpack, runoff, and water supply. Snowpack in the Sierra Nevada Mountains is projected to decline substantially, with cascading effects on California's winter recreation, streamflow, and water storage. The distribution of vegetation types also changes significantly, with alpine/subalpine forests reduced and desert cover expanded under the A1fi scenario. Climate change is expected to impact California's agriculture, with higher temperatures affecting dairy production and wine grape quality. Adaptation options are limited for impacts not easily controlled by human intervention, such as the decline in snowpack and loss of alpine and subalpine forests. The study concludes that climate change and its impacts scale with the quantity