The article explores the implications of the highest and lowest Intergovernmental Panel on Climate Change (IPCC) emissions pathways for climate change and associated impacts in California. Using two state-of-the-art climate models (Parallel Climate Model and Hadley Centre Climate Model, version 3), the study finds that annual temperature increases nearly double from the lower B1 to the higher A1f1 emissions scenarios before 2100. Summer temperatures show greater increases compared to winter in three out of four simulations. Under the B1 scenario, heatwaves and extreme heat in Los Angeles quadruple in frequency by the end of the century, with heat-related mortality increasing two to three times. Alpine/subalpine forests are reduced by 50–75%, and Sierra snowpack is reduced 30–70%. Under A1f1, these impacts are more severe, with heatwaves in Los Angeles increasing six to eight times more frequently, heat-related excess mortality increasing five to seven times, and snowpack declines 73–90%. The study highlights the significant impacts of different emissions pathways on California's climate and suggests that adaptation options are limited for impacts not easily controlled by human intervention. The findings support the conclusion that climate change and its impacts scale with the quantity and timing of greenhouse gas emissions.The article explores the implications of the highest and lowest Intergovernmental Panel on Climate Change (IPCC) emissions pathways for climate change and associated impacts in California. Using two state-of-the-art climate models (Parallel Climate Model and Hadley Centre Climate Model, version 3), the study finds that annual temperature increases nearly double from the lower B1 to the higher A1f1 emissions scenarios before 2100. Summer temperatures show greater increases compared to winter in three out of four simulations. Under the B1 scenario, heatwaves and extreme heat in Los Angeles quadruple in frequency by the end of the century, with heat-related mortality increasing two to three times. Alpine/subalpine forests are reduced by 50–75%, and Sierra snowpack is reduced 30–70%. Under A1f1, these impacts are more severe, with heatwaves in Los Angeles increasing six to eight times more frequently, heat-related excess mortality increasing five to seven times, and snowpack declines 73–90%. The study highlights the significant impacts of different emissions pathways on California's climate and suggests that adaptation options are limited for impacts not easily controlled by human intervention. The findings support the conclusion that climate change and its impacts scale with the quantity and timing of greenhouse gas emissions.